DISCLOSURE / FILE
A March 2010 DIA Defense Intelligence Reference Document, DIA-08-1003-002, surveying positron (antimatter) propulsion concepts for air-breathing engines, single-stage reusable launch vehicles, and manned Mars missions.
DIA AATIP Defense Intelligence Reference Documents (38 DIRDs), FOIA Release, DIRD_08-DIRD_Positron_Aerospace_Propulsion.pdf
A March 2010 DIA Defense Intelligence Reference Document, DIA-08-1003-002, surveying positron (antimatter) propulsion concepts for air-breathing engines, single-stage reusable launch vehicles, and manned Mars missions.
Produced under the AAWSA Program and dated 2 March 2010 with an ICOD of 1 December 2009, this 35-page UNCLASSIFIED/FOUO reference document analyzes positrons as an aerospace fuel, citing a specific energy of 180 MJ/μg, ten orders of magnitude above chemical energy. It describes positron turbojet/ramjet engines (PTRE), unmanned aerial vehicles that could circumnavigate Earth on 150 micrograms of positrons, a BOMARC-derived ramjet-assisted missile extended to 2,000 km range on 1 milligram of positrons, and a single-stage reusable vehicle requiring 86.9 mg of positrons with a gross liftoff weight 43 percent lower than its chemical counterpart. It also surveys solid-core, gas-core, and Sänger photon positron rockets, Mars-mission trajectories, positron production via undulator sources for the International Linear Collider, and Penning-trap and positronium-in-aerogel storage approaches.
Antimatter is considered an extremely attractive fuel for aerospace propulsion because of its enormous advantage in energy density over all other known sources of energy. However, because antimatter does not occur naturally and is unstable in the presence of matter, no vehicles have ever flown using it.p.5
Antimatter is appealing for aerospace uses because its specific energy by annihilation is 180 MJ/μg, or 10 orders of magnitude larger than chemical energy, as shown in Figure 1.p.6
Hence, when Ps self-annihilates, there is 100 percent conversion of mass into electromagnetic energy given by Einstein's famous equation, E = mc², where c is the speed of light.p.6
Figure 6 shows range versus positron mass for a 60-kg UAV with lift/drag of four that can circumnavigate Earth on 150 μg of positrons.p.10
The BOMARC range could be increased to 2,000 km with 1 milligram (mg) of positrons. In addition, positrons could act as ordnance to destroy electronics on missiles or aircraft by electromagnetic pulse (EMP) by detonation of micrograms of positrons up to hundreds of meters from the target.p.13
The GLOW of the positron SSRV is 43 percent less than that of the chemical SSRV owing to reduced propellant mass. The PTRE will dramatically increase the affordability of space transportation by increasing the useful payload.p.14
To illustrate, a human behind such a shield at a distance of 10 meters from a 100-mg source of annihilating positrons would experience a whole-body radiation dose of 2 rem (roentgen equivalent man), which is within the annual tolerance for radiation workers in the United States.p.16
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UNCLASSIFIED/ FOR OFFICIAL USE ONLY Defense Intelligence Reference Document Acquisition Threat Support 2 March 2010 ICOD: 1 December 2009 DIA-08-1003-002 Positron Aerospace Propulsion UNCLASSIFIED/ FOR OFFICIAL USE ONLY
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UNCLASSIFIED/ FOR OFFICIAL USE ONLY Positron Aerospace Propulsion Prepared by: (b)(3):10 USC 424 Defense Intelligence Agency Author: (b)(6) Administrative Note COPYRIGHT WARNING: Further dissemination of the photographs in this publication is not authorized. This product is one in a series of advanced technology reports produced in FY 2009 under the Defense Intelligence Agency, (b)(3):10 USC 424 Advanced Aerospace Weapon System Applications (AAWSA) Program. Comments or questions pertaining to this document should be addressed to (b)(3):10 USC 424:(b)(6) AAWSA Program Manager, Defense Intelligence Agency, ATTN: (b)(3):10 USC 424 Bldg 6000, Washington, DC 20340-5100. ii UNCLASSIFIED/ FOR OFFICIAL USE ONLY
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UNCLASSIFIED/FOR OFFICIAL USE ONLY 67 G. A. Smith, "Long-Term Confinement of Dense Positron Plasmas", Final Technical Report, AFRL Contract F08630-02-C-0017, Eglin AFB, FL, January 23 (2007). 68 Ibid. 69 G. A. Smith, "Positron Energy Conversion (PEC)", 53th JANNAF Propulsion Meeting, Subcommittee on Future Technologies for Spacecraft Propulsion (2005 0356 EA), CPIAC JSC CD43 December (2005). 70 G. A. Smith, "Stabilization and Long Term Confinement of Atomic Positronium", Final Technical Report, AFRL Contract F08630-02-C-0018, Eglin AFB, FL, January 23 (2007). 71 S. J. Hoffman and D. I. Kaplan, eds. "Human Exploration of Mars: The Reference Mission of the NASA Mars Exploration Study Team," NASA Special Publication 6107, JSC (1997). 72 G. A. Smith, "Stabilization and Long Term Confinement of Atomic Positronium", Final Technical Report, AFRL Contract F08630-02-C-0018, Eglin AFB, FL, January 23 (2007). 73 J. Lu, "Classical Trajectory Monte Carlo Simulation of Ion-Rydberg Atom Collisions", Ph. D. Dissertation, University of Bielefeld, Germany, May (2003). 74 Ibid. 75 K. Sudarshan et al., J. Phys: Condens. Matter 19, 386 (2007). 76 G. A. Smith, "Stabilization and Long Term Confinement of Atomic Positronium", Final Technical Report, AFRL Contract F08630-02-C-0018, Eglin AFB, FL, January 23 (2007). 30 UNCLASSIFIED/FOR OFFICIAL USE ONLY
Concatenated page-by-page transcript. Born-digital pages came through pdf.js; scanned pages were transcribed by Claude vision OCR. Pages marked unreadable failed multiple OCR retries (heavy redaction, microfilm artifacts, or blank separators) and are kept in place for audit.
UNCLASSIFIED/ FOR OFFICIAL USE ONLY Defense Intelligence Reference Document Acquisition Threat Support 2 March 2010 ICOD: 1 December 2009 DIA-08-1003-002 Positron Aerospace Propulsion UNCLASSIFIED/ FOR OFFICIAL USE ONLY
UNCLASSIFIED/ FOR OFFICIAL USE ONLY Positron Aerospace Propulsion Prepared by: (b)(3):10 USC 424 Defense Intelligence Agency Author: (b)(6) Administrative Note COPYRIGHT WARNING: Further dissemination of the photographs in this publication is not authorized. This product is one in a series of advanced technology reports produced in FY 2009 under the Defense Intelligence Agency, (b)(3):10 USC 424 Advanced Aerospace Weapon System Applications (AAWSA) Program. Comments or questions pertaining to this document should be addressed to (b)(3):10 USC 424:(b)(6) AAWSA Program Manager, Defense Intelligence Agency, ATTN: (b)(3):10 USC 424 Bldg 6000, Washington, DC 20340-5100. ii UNCLASSIFIED/ FOR OFFICIAL USE ONLY
UNCLASSIFIED/ FOR OFFICIAL USE ONLY
Contents
Introduction.......................................................................................................v
Antimatter......................................................................................................... 1
Positron Air-Breathing Propulsion............................................................... 2
PTRE Applications ........................................................................................ 5
Unmanned Aerial Vehicle (UAV)................................................................ 5
Ramjet-Assisted Missile (RAM) ................................................................. 7
Single-Stage Reusable Vehicle (SSRV) ..................................................... 8
Positron-Powered Rockets .......................................................................... 11
The Solid-Core Positron Rocket............................................................... 12
The Gas-Core Positron Rocket ................................................................ 14
The Sänger Photon Positron Rocket ....................................................... 16
Positron Rocket System Comparison...................................................... 17
Positron Energy Conversion for Onboard Power.................................... 18
Positrons for a Manned Mars Mission ...................................................... 19
Positron Production...................................................................................... 22
Positron Costs ............................................................................................... 23
Positron Storage ........................................................................................... 24
Formation of Positronium in Porous Media ............................................ 25
Long-Term Storage of Positronium ......................................................... 25
Conclusions .................................................................................................... 27
Figures
Figure 1. Specific Energy for Chemical, Nuclear and Antimatter Materials.. ........... 1
Figure 2. Tory-IIC Ready for Testing....................................................................... 3
Figure 3. PTRE Turbojet and Turbo-Ramjet Modes ................................................ 3
Figure 4. Details of the PTRE Engine. ................................................................... 4
Figure 5. Combustion Turbo-Ramjet...................................................................... 4
Figure 6. UAV Range vs. Positron Mass ................................................................ 5
Figure 7. LOCAAS Turbojet Engine ....................................................................... 6
iii
UNCLASSIFIED/ FOR OFFICIAL USE ONLYUNCLASSIFIED/ FOR OFFICIAL USE ONLY
Figure 8. BOMARC and Talos Ramjet-Assisted Missiles......................................... 8
Figure 9. Positron SSRV Flight Profile .................................................................. 10
Figure 10. Artist's Rendition of a Four-Engine Positron SSRV.............................. 11
Figure 11. Solid-Core Positron Rocket Engine With a Hot-Bleed Configuration..... 12
Figure 12. Fluid Systems ...................................................................................... 15
Figure 13. Modified Sänger Photon Rocket Concept ............................................ 17
Figure 14. Closed Brayton Cycle Using Positron Annihilation ............................... 18
Figure 15. Conceptual 110 Watt Positron Closed Cycle Generator Based on the
NASA Glenn Research Center Stirling Radioisotope Generator (SRG).. 18
Figure 16. Mars Trajectories (X-Coordinates Defined in Direction of Aries) ......... 20
Figure 17. Spacecrafts Using Positron Engines..................................................... 21
Figure 18. Proposed Undulator-Based Positron Source for the International
Linear Collider ...................................................................................... 22
Figure 19. Penning Trap....................................................................................... 24
Figure 20. A Positron Forms Ps ............................................................................ 25
Figure 21. Computer Simulation of Ps Atoms ...................................................... 26
Figure 22. TEM of Silica Aerogel .......................................................................... 26
Tables
Table 1. GLOW for Chemical SSRV ....................................................................... 9
Table 2. GLOW for Positron SSRV ........................................................................ 9
Table 3. Total Positron Requirement for SSRV With a Dry Mass of 60,500 kg....... 10
Table 4. Comparison of Space Propulsion and Power Systems – Solid Core .......... 13
Table 5. Comparison of Three Positron Propulsion Concepts for Mars Mission ..... 17
Table 6. Positron and Antiproton Expected Costs in the Next 10 Years ................ 23
iv UNCLASSIFIED/ FOR OFFICIAL USE ONLYUNCLASSIFIED// FOR OFFICIAL USE ONLY Positron Aerospace Propulsion Introduction Antimatter is considered an extremely attractive fuel for aerospace propulsion because of its enormous advantage in energy density over all other known sources of energy. However, because antimatter does not occur naturally and is unstable in the presence of matter, no vehicles have ever flown using it. After a short overview of the various aerospace applications of antimatter, this paper provides a detailed analysis of air-breathing turbojets and turbo-ramjet missiles, as well as rockets for manned interplanetary missions. It discusses new methods of producing and storing large numbers of antielectrons, or positrons, and compares their costs with those of antiprotons. Finally, the paper considers the prospects for the first, modest demonstration of positron propulsive flight within the next 10 years. Interplanetary missions on positron-propelled spaceships are described in detail, with estimates of positron requirements for each mission. Standalone positron power systems are described briefly. Studies of positrons as a fuel for aerospace propulsion applications have been sponsored by the Air Force Research Laboratory, Eglin Air Force Base, Florida, and the NASA Institute for Advanced Concepts, Atlanta, Georgia. This paper is an anthology of that work and not a general review of antimatter propulsion. The positron was predicted by Dirac in 1929¹ and discovered by Anderson in 1932.² Along with the antiproton, which was discovered in 1954,³ the positron has the largest specific energy of any known material. Because aerospace propulsion performance is ultimately limited by specific power, this advantage was immediately appreciated. However, compared with chemical sources of energy, positrons presented new and serious production and storage challenges. Antimatter has a long history of appearing in science fiction literature, dating to a 1942 short story in Astounding Science Fiction and the 1949 book Seetee. It later appeared in the Star Trek television and film series and continues to be an appealing subject for contemporary books and films, such as Angels and Demons. Positron aerospace propulsion is now entering a critical period owing to new technologies that bear on production and storage issues. To their advantage, positrons, unlike nuclear fission and antiprotons, present no radiation or environmental safety problems. v UNCLASSIFIED// FOR OFFICIAL USE ONLY
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Antimatter
Antimatter appears in the form of fundamental particles that have their sign of electric
charge reversed from their matter counterpart. For example, the positron, e⁺, is the
antiparticle to the electron, e⁻. According to the CPT theorem,⁴ properties of matter and
their counterpart antimatter particles are identical. This has been tested in the
laboratory to an accuracy of roughly 1 part in 10 million.
Antimatter is appealing for aerospace uses because its specific energy by annihilation is
180 MJ/μg, or 10 orders of magnitude larger than chemical energy, as shown in Figure
1.
1.00E+03
1.00E+02
1.00E+01
1.00E+00
1.00E-01
Energy Density 1.00E-02
MJ/μg 1.00E-03
1.00E-04
1.00E-05
1.00E-06
1.00E-07
1.00E-08
Chemical Fission Fusion Positron
Annihilation
Figure 1. Specific Energy for Chemical, Nuclear, and Antimatter Materials
In the presence of matter, the positron binds with an electron to form a short-lived
atom called positronium (Ps). Depending on the relative spin orientations of the
positron and electron, Ps has a mean lifetime in a vacuum of 125 picoseconds (para-Ps,
spins antiparallel), or 142 nanoseconds (ortho-Ps, spins parallel).
From quantum number conservation, para-Ps decays into two gamma rays of equal
energy, 511 kiloelectronvolts (keV), whereas ortho-Ps decays into three gamma rays
whose energies add up to 1022 keV. Hence, when Ps self-annihilates, there is 100
percent conversion of mass into electromagnetic energy given by Einstein's famous
equation, E = mc², where c is the speed of light.
Although the energies of positron annihilation gamma rays are on the nuclear scale,
they have none of the undesirable features associated with nuclear energy. First, the
annihilation evolves rapidly (nanoseconds) and is controllable under predictable
electromagnetic forces. There is no long-term inertia as with nuclear reactors.
Consequently, positron-generated thrust can be throttled.
1
UNCLASSIFIED// FOR OFFICIAL USE ONLYUNCLASSIFIED/ FOR OFFICIAL USE ONLY Second, low-energy gamma rays from positron annihilation cannot make residual radioactivity in surrounding air and containment vessels. In contrast, antiprotons annihilate into a host of high-energy particles, including π-mesons and gamma rays that can induce residual radioactivity in nearby materials. Finally, low-energy gamma rays from positron annihilation can be readily converted into useful forms of energy, including heat and electricity required for propulsion systems. This contrasts with large, complex systems required for conversion of antiproton annihilation and nuclear fission/fusion energy. There are two reasons why positrons have yet to be used for aerospace applications. First, it has not been possible to produce them in the numbers required. However, recent developments in high-energy physics research are resulting in expanding levels of positron production. Second, methods for storing positrons for basic research do not hold enough positrons long enough for propulsion applications. Recent developments in storage techniques may significantly improve the situation, with lifetimes up to months and possibly years. Positron Air-Breathing Propulsion Aeronautical engines burn a mixture of aviation fuel and oxygen in air to heat a working fluid. To keep engines small, the combustion rate in the engine needs to be high.⁵ At sea level for a fuel-air mass ratio of 0.068, it is 500,000 kJ/m³-s. To maintain speed, the thrust specific fuel consumption (TSFC) for turbojets and turbo-ramjets is in the range of 0.075 - 0.11 and 0.17 - 0.26 kilogram/hour-Newton (kg/hr-N), respectively. All aeronautical engines are limited in range and flight duration by the fuel on board. Because of the aforementioned performance bounds of combustion engines, the aeronautic industry has worked diligently to increase the range and payload of aircraft by maximizing the performance of combustion engines and optimizing aerodynamic design. Beyond this, the only way a combustion-powered aircraft can extend its range and endurance is by in-flight refueling. Two projects investigated nuclear power as a way to increase performance. In 1946, the U.S. Air Force established the Nuclear Energy for Propulsion of Aircraft program. However, this program was disbanded in 1951 in favor of the joint Atomic Energy Commission-Air Force Aircraft Nuclear Propulsion program. Implementing nuclear fission to power an aircraft required two approaches. One was direct cycle, whereby air was heated by passing it through a nuclear reactor; the other was indirect cycle, whereby the reactor heated a liquid metal that in turn heated air in a secondary heat exchanger. The program never produced a prototype and was canceled in 1961. In 1957, the Pentagon started development of a nuclear ramjet missile (SLAM, Supersonic Low-Altitude Missile) to fly below Soviet defenses. The Lawrence Livermore National Laboratory Pluto program successfully tested two engines, Tory-IIA and Tory- IIC (Figure 2), at the Nevada Test Site. The program was canceled in 1964.⁶, ⁷ 2 UNCLASSIFIED// FOR OFFICIAL USE ONLY
UNCLASSIFIED// FOR OFFICIAL USE ONLY
[FIGURE: Black and white photograph of Tory-IIC engine at test site]
Figure 2. Tory-IIC Ready for Testing (courtesy LLNL)
In a positron turbojet/ramjet engine (PTRE),⁸ tungsten shells are heated by gamma
rays, with heat transferred to air by convection.⁹, ¹⁰
turbojet turbine tungsten shells
used for heating
positron trap
path of gamma ray
turbojet compressor nozzle
turbojet heating
chamber turbojet mode of operation
turbo-ramjet mode of operation
Figure 3. PTRE Turbojet (green) and Turbo-Ramjet (red) Modes (courtesy Positronics Research LLC)¹¹
3 UNCLASSIFIED// FOR OFFICIAL USE ONLYUNCLASSIFIED/ FOR OFFICIAL USE ONLY
Concentric Annular Tungsten Heating Turbine Powered Electrical
Surfaces Transfer Positron Conversion Generator is Power Source
Heat Energy To Airflow For Positron Trap and Electric
Motor for Compressor
Electric Motor
Driven Exhaust
Compressor Airflow
Positron
Trap
Heated Air expands thru
turbine and nozzle
Intake Positron Conversion
Airflow Chamber
Compressor and turbine can be feathered to convert
turbo jet to ramjet
Figure 4. Details of the PTRE Engine (courtesy Positronics Research LLC)¹²
A comparison with the combustion turbo-ramjet engine (Figure 5) shows that volume
used for combustion heating is used by PTREs for convection heating.
turbojet
combustion chamber nozzle
turbojet
compressor ramjet fuel injectors
turbojet
turbine
ramjet combustion
chamber
Figure 5. Combustion Turbo-Ramjet (courtesy Positronics Research LLC)¹³
4 UNCLASSIFIED// FOR OFFICIAL USE ONLYUNCLASSIFIED/ FOR OFFICIAL USE ONLY
PTRE Applications
Three uses for PTREs have been investigated: unmanned aerial vehicles (UAVs),
ramjet-assisted missiles (RAMs), and single-stage reusable vehicles (SSRVs).
UNMANNED AERIAL VEHICLE (UAV)
Figure 6 shows range versus positron mass for a 60-kg UAV with lift/drag of four that
can circumnavigate Earth on 150 μg of positrons.¹⁴ It is modeled after the LOCASS
turbojet (Figure 7) at the Air Force Research Laboratory (AFRL), Eglin Air Force Base,
Florida.¹⁵
350
300
250
Range (kNM)
200
150
100
50
0
0.00 0.50 1.00 1.50 2.00
Positron Mass (mg)
Figure 6. UAV Range Versus Positron Mass (courtesy Positronics Research LLC)¹⁶
5 UNCLASSIFIED// FOR OFFICIAL USE ONLYUNCLASSIFIED/ FOR OFFICIAL USE ONLY [FIGURE: Black and white photograph of LOCAAS Turbojet Engine] Figure 7. LOCAAS Turbojet Engine (courtesy of AFRL, Eglin AFB, FL)¹⁷ A UAV fueled with positrons would allow for an ultralong-endurance platform with many "dual-use" applications, including the following: Military • Intelligence, surveillance, and reconnaissance. • Real-time battlefield command observation. • Monitoring and early warning of nuclear, biological, and chemical weapons. • Ordnance delivery. • Target laser illumination/covert target acquisition. • Coastal patrol. • Drug interdiction. • Search and rescue. • Geomagnetic and atmospheric surveys. Civil, State, and Local Government • Airborne early warning (storms, terrorist attacks, chemical/biological/nuclear). 6 UNCLASSIFIED// FOR OFFICIAL USE ONLY
UNCLASSIFIED/ FOR OFFICIAL USE ONLY • Environmental/pollution monitoring. • Fire detection and monitoring. • Aerial surveying. • Weather and atmospheric monitoring. • Border patrol. • Emergency communications relay. • Drug interdiction. • Law enforcement. • Highway/road monitoring. • Search and rescue. Commercial • Iceberg patrol/tracking in shipping lanes. • Railway/pipeline/power line monitoring. • Commercial fishing reconnaissance/sea-life monitoring. • Environmental/pollution monitoring. • Livestock monitoring. • Mineral exploration. • Weather sensing. • Agriculture. Additional advantages to the commercial airline industry: • Less propellant means increased payload/passenger per aircraft. • Global nonstop flights are possible. • Increase in structural mass allows more electronics/passenger amenities. RAMJET-ASSISTED MISSILE (RAM) The BOMARC 440-kilometer (km)-range antiaircraft missile was developed by the U.S. Air Force in the 1950s to counter Soviet bombers. In the 1960s, the Talos, a long-range surface-to-air missile, was developed by the U.S. Navy and later converted to the Vandal missile. Both ramjets (Figure 8) activated at mach 1. 7 UNCLASSIFIED// FOR OFFICIAL USE ONLY
UNCLASSIFIED/ FOR OFFICIAL USE ONLY [FIGURE: Two photographs — BOMARC aircraft (left) and Talos missile on launcher (right)] Figure 8. BOMARC (left) and Talos (right) Ramjet-Assisted Missiles (courtesy Boeing and the U.S. Navy) The BOMARC range could be increased to 2,000 km with 1 milligram (mg) of positrons. In addition, positrons could act as ordnance to destroy electronics on missiles or aircraft by electromagnetic pulse (EMP) by detonation of micrograms of positrons up to hundreds of meters from the target.¹⁸ SINGLE-STAGE REUSABLE VEHICLE (SSRV) The Department of Defense, NASA, and the aerospace industry are working to eliminate multistage rockets for access to low Earth orbit (LEO) to lower launch costs. An SSRV would integrate components and allow more rapid turnaround on missions. The combustion SSRV has a lower payload ratio than conventional rockets. Current designs use combined-cycle or combination engines for the endoatmospheric phases of the mission. Use of air-breathing engines significantly reduces propellant mass and gross liftoff weight (GLOW). Improving payload mass requires reducing propellant and structural mass, both of which can be accomplished using positrons as fuel. A PTRE eliminates propellant mass during the endoatmospheric phase of operation. Any reduction of propellant mass has serious positive effects for three reasons. First, it allows an increased payload/number of passengers per aircraft. Second, adding positrons on the milligram level does not change the lift requirements for the aircraft, resulting in longer range flights, perhaps exceeding 100 kilonautical miles (kNM). Third, the structural mass may be increased. Tables 1 and 2 provide a mass budget comparison between a chemically fueled SSRV¹⁹ and a positron-powered SSRV.²⁰ 8 UNCLASSIFIED// FOR OFFICIAL USE ONLY
UNCLASSIFIED/ FOR OFFICIAL USE ONLY
Table 1. GLOW for Chemical SSRV²¹
Vehicle Component Mass
Structure 25,700 kg
Thermal Protection 12,300 kg
Propulsion (4 engines) 14,900 kg
Electronics 7,600 kg
TOTAL DRY MASS 60,500 kg
15% Margin + Unused Propellants 11,400 kg
Payload 11,340 kg (24,948 lbs)
BURNOUT MASS 83,240 kg
TOTAL PROPELLANT 368,300 kg
GLOW 451,540 kg (993,388 lbs)
Table 2. GLOW for Positron SSRV²²
Vehicle Component Mass
Structure 25,700 kg
Thermal Protection 12,300 kg
Propulsion (4 engines) 14,900 kg
Electronics 7,600 kg
TOTAL DRY MASS 60,500 kg
15% Margin + Unused Propellants 11,400 kg
Payload 11,340 kg (24,948 lbs)
BURNOUT MASS 83,240 kg
TOTAL PROPELLANT 176,000 kg
GLOW 259,240 kg (590,328 lbs)
The GLOW of the positron SSRV is 43 percent less than that of the chemical SSRV
owing to reduced propellant mass. The PTRE will dramatically increase the affordability
of space transportation by increasing the useful payload.
9 UNCLASSIFIED// FOR OFFICIAL USE ONLYUNCLASSIFIED// FOR OFFICIAL USE ONLY
1000
t = 1530 sec
v = 7600 m/sec,140 km
t = 1200 sec
M = 8, 38 km
t = 1650 sec
100 300 km
t = 146 sec
M = 1.8, 10000m
ROCKET MODE
Altitude (km)
10
RAMJET MODE
1
0.1
0 500 sec 1000 sec 1500 sec
Time
Figure 9. Positron SSRV Flight Profile (courtesy Positronics Research LLC)²³
The SSRV takes off horizontally (Figure 9) from Edwards Air Force Base, accelerates to
mach 1.8 as a turbojet, and then goes to the ramjet mode. At mach 8 the rocket is
ignited and takes the vehicle into LEO.²⁴ The positron mass budget for a 60,500-kg dry
mass for ascension to LEO is 86.9 mg (Table 3).
Table 3. Total Positron Requirement for SSRV With a Dry Mass of 60,500 kg²⁵
Mission Mode Mass Of e⁺
Turbojet – Launch 2.5 mg
Ramjet 76.5 mg
10% margin (turbojet mode upon 7.9 mg
landing, inclination changes, etc.)
TOTAL 86.9 mg
An artist's rendition of the SSRV is provided in Figure 10.
10 UNCLASSIFIED// FOR OFFICIAL USE ONLYUNCLASSIFIED/ FOR OFFICIAL USE ONLY [FIGURE: Black silhouette artist's rendition of a four-engine positron SSRV aircraft] Figure 10. Artist's Rendition of a Four-Engine Positron SSRV (courtesy of Positronics Research LLC)²⁶ Positron-Powered Rockets A positron rocket offers significant advantages over nuclear fission and antiproton rockets.²⁷ Nuclear fission reactors contain enormous amounts of highly toxic radioactive material, and antiproton rockets produce radioactivity in surrounding materials by interactions of high-energy mesons and gamma rays from antiproton annihilation. The proposed nuclear fission gas core rocket and antiproton adaptations would release radioactive fission fragments into the atmosphere. In contrast, positron rockets are totally radioactivity free. Second, in the event of an accidental detonation of the positron fuel, the prompt (nanoseconds) burst of gamma rays can be shielded from humans on spacecraft. The 1/e absorption length of a 511-keV gamma ray in lead is 5.6 millimeters. A 13- centimeter (cm)-thick shield thus reduces the gamma ray flux by a factor 8.3 x 10⁻¹¹. To illustrate, a human behind such a shield at a distance of 10 meters from a 100-mg source of annihilating positrons would experience a whole-body radiation dose of 2 rem (roentgen equivalent man), which is within the annual tolerance for radiation workers in the United States. The following sections investigate three positron rockets: solid core, gas core, and photon.²⁸ In a solid-core positron rocket, positrons heat a hydrogen working fluid through an attenuating solid such as tungsten. In a gas-core positron rocket, gamma rays directly heat propellant through a one- or two-fluid process. In a photon positron rocket, solid propellant is ablated from a surface bombarded by gamma rays. 11 UNCLASSIFIED// FOR OFFICIAL USE ONLY
UNCLASSIFIED/ FOR OFFICIAL USE ONLY
THE SOLID-CORE POSITRON ROCKET
Figure 11 depicts a solid-core positron-powered rocket, similar in many regards to the
NERVA nuclear-thermal concept.²⁹
H₂ Flow Turbopump
H₂ Tank
Attenuating
HX
Positron Trap
Hot Bleed Inlet Plenum
Line Turbine &
Exhaust Nozzle
Figure 11. Solid-Core Positron Rocket Engine With a Hot-Bleed Configuration (courtesy Positronics
Research LLC)³⁰
The cryogenic hydrogen propellant is supplied from a storage tank through a high-
pressure pump and routed to cool the regenerative nozzle, the casing of the heat
exchanger, and the central positron target tubes. Ps enters the inlet plenum to the
attenuator that is heated by gamma rays to high temperature. Hydrogen propellant
passes through the attenuating matrix and is heated and exhausted through a nozzle to
generate thrust.
A small fraction of the hot exit propellant is bled off to a turbine that drives the high-
pressure feed pump. The high-temperature bleed can either be mixed with cold
hydrogen to reduce its temperature or directly fed to the turbine. If it is directly fed to
the turbine, it must be made of materials that can withstand high temperatures. The
bleed flow is exhausted from a turbine exit nozzle to space after driving the turbine.
As with the NERVA system, the positron solid-core concept is thermally limited by
materials in the heating chamber. The difference is that the fission system requires a
reactor and complex machinery, whereas the positron system relies on Ps atoms
injected upstream from a storage unit. This has two advantages. First, a reduction in
the engine mass for a given thrust is realized; second, there is greater choice in
materials to be used in the heating chamber.
A thermal-fluids analysis was conducted to predict performance. A specific impulse of
920 seconds is attainable with chamber temperatures at 3,000 Kelvin. The
corresponding thrust and power emulate fission systems. Mars trip burn times are on
12 UNCLASSIFIED// FOR OFFICIAL USE ONLYUNCLASSIFIED/ FOR OFFICIAL USE ONLY
the order of 30 minutes, indicating that a spacecraft employing three 72-kN solid-core
engines would require 6-9 mg of positrons per mission.
Solid-core fission and positron systems are compared in Table 4.
Table 4. Comparison of Space Propulsion and Power
Systems – Solid Core
Fission-Based Positron Powered
Technology • NERVA/Rover • Conceptual
demonstrated • Must demonstrate
• Never flight tested positron storage and
controlled injection
• Near-term technology
demonstration for positron
storage needed
Performance • Isp ≈ 950 sec • Isp - as in fission systems
• Thrust ≈ 72 - 123 kN • Thrust - variable, similar to
• Power ≈ 367- 5320 MW fission systems
(matched to thrust) • Power - matched to thrust
• Lifetime ≈ 2 hours total • Lifetime - set by material
operation considerations
Operation • Design dictated by • No criticality, burn up or
neutronics, fuel burn up poison accumulation
and fission poisoning issues
• High neutron & gamma • Design based only upon
radiation during operation heat transfer and gamma
• Requires active, accurate attenuation issues
and massive control • Does not require
• Requires shutdown shutdown cooling
cooling to remove heat • Simple on-off control,
from nuclear waste power controlled by rate of
• Radiation after shutdown positron utilization
due to fission products • Not a radiation source
Materials • Material dictated by • Material choices dictated
neutronics by temperature
• Propellant:-heated H₂ • Propellant-heated H₂
• Working fluids for power • Working fluids for power
systems - inert gas systems - inert gas
• Uranium in graphite media
• Corrosion issues require
complex fuel
Payload • Requires massive shield • Shield required around
Integration from reactor manned area separated
• Requires separation from from positron storage
reactor • Propulsion and power
• Complex design issues sources can be integrated
due to neutron scattering into vehicle
Post Operation • Not able to return to earth • Able to return to Earth or
or inhabited surface inhabited surface
• Not reusable or refuelable • Reusable and refuelable
13 UNCLASSIFIED// FOR OFFICIAL USE ONLYUNCLASSIFIED// FOR OFFICIAL USE ONLY THE GAS-CORE POSITRON ROCKET The gas-core positron concept follows nuclear gas-core concepts,³¹, ³², ³³ which are different from the solid-core concept in that gamma rays directly heat a fluid under pressure. The limit of the solid-core approach is melting temperatures of the solid matrix gamma ray attenuator. By direct heating, temperatures can increase significantly as long as the gas does not appreciably heat the walls. Four versions of the gas-core concept are illustrated in Figure 12. Synchronous with pulsed Ps injection are (a-c) pulses of LN₂ or LNe or LH₂ with LXe gamma ray attenuator and (d) pulses of LH2 where Ps is encapsulated in lead, a gamma ray converter. For fluid injection, a turbo-pump (not shown) is located upstream, with power obtained from a positron Brayton cycle system described later in this paper. Results from computational fluid dynamics codes reveal that high-density regions of the fluid move away from the gamma ray source when the power in the system exceeds 300 megawatts. Under these conditions, the propellant does not efficiently absorb gamma rays. Furthermore, calculations of heat required for continuous operation suggest the vortex configuration of (b) breaks down and reverts to two-fluid flow. However, both the two-fluid, flow-through model and the Ps lead-cartridge concepts show promise if the mass flow rate of the hydrogen propellant exceeds that of the xenon or lead by a factor of five. By operating in a pulsed mode, one should be able to control the positron delivery into the chamber core. With complete absorption of gamma rays in the 2-cm lead casing, performance of the system matches that of previously examined systems.³⁴, ³⁵ Thrusts of 130 kN (1,000 atmospheres) are predicted for a single-engine system with an efficiency of 85 percent. Burn times are 30 minutes for ΔV = 3.7 km/second (sec) with 25 mg of positrons consumed for a 50,000-kg burnout mass. The limit of the gas- core concept occurs near the thresholds for ionization of hydrogen, corresponding to Isp of ~ 2,500 sec. 14 UNCLASSIFIED// FOR OFFICIAL USE ONLY
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LN2 or Ne HEATING CHAMBER NOZZLE
Positron Trap γ
INLET
(a)
H2
LXe
Positron Trap
vortex
(b)
H2
LXe
Positron Trap
(c)
H2
Expanding
Positron Trap Plasma
Pb ENCASED e+
(d)
Figure 12. Fluid Systems. (a) One-fluid system of LN₂ or Ne propellant; (b) Two-fluid system of LH₂ and LXe; (c)
Two-fluid flow-through system with LH₂ at higher mass flow rates than LXe; and (d) One-fluid LH₂ system with lead
encapsulated Ps (courtesy Positronics Research LLC)³⁶
15 UNCLASSIFIED// FOR OFFICIAL USE ONLYUNCLASSIFIED//FOR OFFICIAL USE ONLY THE SÄNGER PHOTON POSITRON ROCKET In 1953, German engineer Eugen Sänger proposed the photon rocket. One means of providing thrust was to shower a parabolic mirror with positron annihilation gamma rays.37 Unfortunately, there are no materials that reflect gamma rays at large angles. A schematic of a modified Sänger photon rocket is shown in Figure 13. Ps is emitted in 'pellets' from several storage banks located behind the engine. Positrons are programmed by supporting fields to annihilate in front of a stiffened pressure plate, the shape of which was assumed to be parabolic for this work.38 To make the Sänger photon rocket practical, solid propellant that can be ablated from the plate is added. Gamma rays deposit energy on the surface of the ablation material and jettison high-energy particles with large Isp. As the solid material recedes from the target, the location of the positron annihilation can be correspondingly moved inward to preserve focal properties of the system. A thickness of a few meters of material is sufficient for a planetary mission. [FIGURE 13: Diagram of Modified Sänger Photon Rocket Concept with labels: e+ TRAP BANKS, PRESSURE PLATE, PAYLOAD, PEC SYSTEM (OPTIONAL), SOLID ABLATION MATERIAL] Figure 13. Modified Sänger Photon Rocket Concept (courtesy Positronics Research LLC)30 The predicted Isp for this system results in fast transit times to Mars, warranting a positron power plant. Unlike the solid-core or gas-core concepts, there are no means to draw off some of the ablated material to provide power. A separate Brayton-cycle positron energy conversion system provides power to the pellet mass driver and other 16 UNCLASSIFIED//FOR OFFICIAL USE ONLY
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components. Alternatively, solar collectors or closed-loop nuclear reactors could be
employed.
To improve ablation efficiency, annihilation gamma rays must be wavelength shifted
(WLS) by passing them through a high-Z WLS material. The material of choice, lead,
also serves as the shell of the Ps pellet. The pellet vaporizes into high-energy plasma,
and the WLS photons propagate to the pressure plate. Silicon carbide ablation material
has been adopted from the antiproton catalyzed microfission/fusion concept40, 41
developed at Penn State University by the author and coworkers. Photon energy
distributions are shifted through 2 cm of lead to 1-10 keV from 511 keV with 85 percent
efficiency.
Performance depends primarily on the energy of the WLS photons and the energy per
pellet. At 8 keV, Isp is in the range 1,200-3,000 sec. Thrust is 40-145 kN, the latter at a
pellet injection rate of 1 hertz. The total quantity of positrons consumed for a one-way
trip to Mars over this range of Isp is 15-40 mg with 50 percent of gamma rays striking
the plate and a 85 percent WLS efficiency.
POSITRON ROCKET SYSTEM COMPARISON
A side-by-side comparison of three positron rocket propulsion concepts is presented in
Table 5 for a one-way transit to Mars using ΔV = 3.7 km/sec.
Table 5. Comparison of Three Positron Propulsion
Concepts for Mars Mission
Solid-Core Gas-Core Sänger Ablation
Isp 650 - 920 sec 1000 - 2500 sec 1200 - 3000 sec
Thrust 72 kN, small class 130 kN (1000 atm) 40 -145 kN (1 Hz)
Limits • Wall and nozzle • Wall and nozzle • Positron density
temperature temperature per pellet
• H Ionization
e+ mass • 6-9 mg (100% • < 25 mg (85% • 15 - 40 mg
efficiency) efficiency) (42.5% efficiency)
Special • Continuous burn • Pulsed burn • Pulsed burn
Notes • Multiple engines • Multiple engines • Multiple engines
may be possible may be possible may be possible
• Hot-bleed line • Hot-bleed line • No direct onboard
possible possible power
• Engine can be • Engine can be
throttled throttled
Future work • Efficiency study • Efficiency study • Further WLS and
• Lower pressure radiation transport
possible? study
17
UNCLASSIFIED//FOR OFFICIAL USE ONLYUNCLASSIFIED//FOR OFFICIAL USE ONLY POSITRON ENERGY CONVERSION FOR ONBOARD POWER Research was conducted on positron utilization in a standalone, closed-loop, high-power system.42 A Brayton cycle engine (Figure 14) was investigated with output power of 100 kW, consistent with Mars Reference Mission specifications.43, 44 Results show efficiencies of 25-30 percent and positron consumption of 7 μg/hour. Such a power system would have practical meaning for fast transits to Mars where positron consumption does not dominate rocket positron consumption. [FIGURE 14: Diagram of Closed Brayton Cycle Using Positron Annihilation with labels: Regenerator, e+, Heater-Attenuator, Turbine, Work (Alternator), Compressor, Radiator, and numbered components 1-6] Figure 14. Closed Brayton Cycle Using Positron Annihilation (courtesy Positronics Research LLC)45 In addition, a small, 110-watt, positron-driven generator (Figure 15) for small, onboard tasks was designed around the NASA Glenn Research Center Stirling Radioisotope Generator,46 with heat provided by Ps gamma rays. [FIGURE 15: Diagram with labels: Stirling Convertor 1(55 We), Stirling Convertor 2 (55 We), Ps Storage Heat, Cryocooler Units, Electric Power Outlet, Magnetic Source, and partially illegible labels on right side] Figure 15. Conceptual 110-Watt Positron Closed-Cycle Generator Based on the NASA Glenn Research Center Stirling Radioisotope Generator47 (courtesy Positronics Research LLC)48 18 UNCLASSIFIED//FOR OFFICIAL USE ONLY
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Positrons for a Manned Mars Mission
Positron propulsion systems improve engine performance, making them an attractive
substitute for chemical and nuclear systems for manned exploration of the planets. One
of the boldest challenges is a manned mission to Mars. Onboard propellant requires an
overall interplanetary system mass that prohibits use of any type of existing launch
vehicle, including the Saturn V. The need to protect astronauts from radiation hazards
in space inhibits use of low-impulse interplanetary trajectories to reduce propellant
mass. Missions must be established that can transport astronauts to Mars in less than
180 days.
Demands on a positron engine to get from LEO to Mars are based on two parameters:
mass of the spacecraft after burnout and the ΔV provided by orbital mechanics. Efforts
to minimize burnout mass for a positron-based rocket spacecraft prompted examination
of previously designed systems. The NASA Mars Exploration Study Team studied such
systems in 1997-98.49, 50
Conclusions reached by NASA and adopted for this study include:
• To make the Mars mission economically feasible, multiple payloads should be
launched to Mars instead of a single, "all-in-one" vehicle. This keeps payload masses
within reach of existing chemical launch systems.
• A solid-core nuclear-thermal rocket (NTR) was studied. The study adopted existing
NERVA rockets with Isp = 900 seconds and a core temperature near 2,800 °C. The
1993 study examined 15 kilopound-force klbr and 20 klbr rockets.51
• Each launch had a payload consisting of the NTR with its Mars payload.
• Unpiloted cargo was sent on a low-energy ("C3") Hohmann-type transfer, generally
the slowest means of reaching Mars.
• The Mars excursion vehicle should be sent on a "fast transit" to Mars from LEO. A
fast, 180-day mission would not require artificial gravity on the spacecraft to protect
astronauts from weightlessness.
• The Earth return vehicle (ERV) sits in Mars orbit at 250-km periapsis and waits until
astronauts have docked from Mars using a liquid oxygen (LOX)/methane propulsion
system. The ERV uses a chemical propulsion system to return home to avoid use of
a fission-based propulsion plant in the atmosphere.
• Minimization of ΔV to Mars is performed by launching during estimated planetary
conjunctions (every 778 days) and by using aerobraking.
• Aerobraking uses the chemical propulsion system of the cargo vessel or lander.
Payload is jettisoned from the NTR system (called the trans-Mars insertion system
[TMI]) sometime during the trip to Mars.
• To reduce the probability of impact with Earth, an additional ΔV is given to the TMI
stage after the payload has separated.
19
UNCLASSIFIED//FOR OFFICIAL USE ONLYUNCLASSIFIED//FOR OFFICIAL USE ONLY Benefits of using positrons for a Mars mission include: • The "disposable ΔV" used to propel TMI stages into low-probability Earth or Mars intercepts can be eliminated, reducing total propellant mass. • Reduction in shielding and engine mass give lower initial mass low Earth orbit for launch vehicles or faster transits for piloted missions. • The ERV uses a positron engine instead of LOX/CH4. This gives significant mass savings or an equivalent reduction in Mars-to-Earth return time for astronauts. • The improvement in Isp translates to either a reduced launch payload mass for cargo missions or reduced transit times for piloted missions to Mars. • More chemical propellant can be stored on the lander to improve aerobraking or landing strategies that reduce hazards for astronauts. Launch dates are set for around 2030. Assuming minimum ΔV for Mars opposition-class missions, interplanetary scenarios are illustrated in Figure 16. The ΔV for an insertion trajectory into Mars for the manned mission (Figure 16b) is ΔV = 3.7 km/sec. Each manned trajectory assumes a 180-day transit time. [FIGURE 16: Four orbital trajectory diagrams labeled (a), (b), (c), (d)] Figures 16. Mars Trajectories (X-coordinates defined in direction of Aries): (a) 2029 cargo mission; (b) 2031 manned lander to Mars; (c) 2033 manned return to Earth; (d) 2035 manned lander to Mars, if necessary (courtesy Positronics Research LLC)52 20 UNCLASSIFIED//FOR OFFICIAL USE ONLY
UNCLASSIFIED//FOR OFFICIAL USE ONLY The Mars reference mission53, 54 considered payload masses of 60,000 kg for 2015 missions. This can be reduced to 45,000 kg assuming technological advances by 2031 with a complete interplanetary spacecraft mass of 90,000 kg. In summary, the mission scenario for a positron spacecraft is similar to that for existing studies, but with the use of less costly launch vehicles. Every 778 days, two 45,000-kg payloads are launched from Earth using a Saturn V or equivalent chemical rocket. One payload is the unmanned system or manned crew lander sent to Mars; the other contains the positron propulsion system and the propellant tank. They are assembled as a complete unit in LEO. Unmanned systems are launched in advance of the crewed system in order to ensure that the Martian habitat is well established. The crew arrives at Mars in late 2033, performs research for 1 year, and then returns home in a smaller positron spacecraft using a shorter trajectory. Artists' renditions of two possible positron spaceships previously described in this study are shown in Figure 17. [FIGURE 17: Two images labeled (a) and (b)] Figure 17. Spacecraft Using Positron Engines. (a) Solid-core system enters Mars orbit; (b) Modified Sänger photon rocket system burns for landing on Mars (courtesy Positronics Research LLC).55 Architectures for Mars exploration using a positron SSRV are summarized below: • Before humans leave for Mars on initial flights, cargo ships precede them to Mars on low-energy trajectories to take the components of a Mars space station (MSS) and necessary supplies, including a Mars surface lander (MSL). The MSS will be similar to an Earth space station (ESS). The cargo ships will utilize positron rocket engines. • Manned positron SSRVs launched from Earth rendezvous in LEO with the ESS. The SSRV is a horizontal-takeoff, horizontal-landing winged-body, manned vehicle in which the first stages of flight use air-breathing engines with positrons heating the air. It switches to the rocket engine to complete the final ascent phase to LEO. • Once ready for interplanetary flights at the ESS, including refueling, the SSRV flies to Mars on a fast, high-energy trajectory, carrying a crew of five or six astronauts and powered by positron rocket engines. The SSRV conducts a rendezvous with the 21 UNCLASSIFIED//FOR OFFICIAL USE ONLY
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MSS, and the astronauts descend to the Mars surface on the MSL using a high-
thrust variant of the positron rocket engine.
Positron Production
Positrons are currently produced at particle accelerators worldwide for basic and
applications research. For example, the positron-emitting radioisotope Na22 (2.7 year
mean lifetime) is made by bombarding targets with neutrons from a high-energy proton
accelerator in the reaction Al27(n,x)Na22. Capture of these positrons is used to form
beams with keV (slow) to MeV (fast) energies. Handling of large radioactive sources
results in limits of 106 slow positrons/sec.
For intensities up to 1010/sec, bombardment of metal targets with electron beams in the
10- to 100-MeV range is used, followed by collection and acceleration (deceleration) of
positrons to form fast or slow beams. In addition, it has recently been shown that slow
positron beams of up to 1011/sec can be realized by converting neutrons in reactors to
electron-positron pairs in thin metal foils.
Much higher positron currents are being sought in a variety of proposed solutions.
Illustrated below are a few of the more promising concepts.
First, in 1996, the U.S. Naval Research Laboratory56 proposed developing an intense
source of fast positrons (1016/sec) utilizing compact electron betatron accelerators.
Second, tabletop femtosecond laser-driven positron sources currently under
development at the National Ignition Facility (Lawrence Livermore National Laboratory),
the Rutherford-Appleton Laboratory (United Kingdom), and the Max Planck Institute
(Munich) look promising, although more must be done to demonstrate efficient
collection of positrons into beams.
Finally, a most important step forward is multi-gigaelectronvolt (GeV) energy electron
storage rings being developed for the high-energy physics International Linear Collider
(ILC) project that uses undulators in electron beams to create intense photon beams
that produce intense (1014-16/sec) positron beams by pair production.57 A schematic
drawing from one proposal for the ILC is shown in Figure 18.58
Layout of ILC Positron Source
[FIGURE 18: Schematic diagram of proposed undulator-based positron source with labels: Positron Linac, IP, Helical Undulator, In By-Pass Line, e- source, DR, Photon Target, e- Dump, e- Dump, Photon Dump, e- pre-accelerator ~5GeV, Auxiliary e- Source, e- Target, e- DR]
Figure 18. Proposed Undulator-Based Positron Source for the International Linear Collider (courtesy
KEK, Japan)59
22
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Assuming that 1016 positrons/sec can be realized in the next 10 years, then 150
micrograms could be produced in 6 months to enable a globe-encircling flight of a small
air-breathing turbojet UAV as discussed earlier.
Positron Costs
An independent study has been done to determine future costs of positrons and, for
comparison, antiprotons as well. The results, shown in Table 6, are based on data for
existing sources and proposals for future sources.60, 61, 62
Table 6. Positron and Antiproton Expected Costs in the Next 10 Years
Source Trap Injection Filling Rate When $/JOULE
Energy (MeV) (sec-1) (annihilation)
CERN AD (pbar)25 0.01 – 0.1 4 x 105 Now ?
Fermilab (pbar)26 <0.002 2.8 x 104 Now 333*
e+/14 MeV e- linac27/ILC24 0.1 5 x1012/1016 2011/19 0.4/0.004**
* $100 million/year (est. op. cost).
** $5 million/year (est. op. cost, adjusted for inflation)/$100 million/year (est. op. cost).
Two clear results of the study should be noted.
First, measured on a scale of dollars per joule of annihilation energy, positrons cost less
than antiprotons by a factor of 1,000-100,000. Because each antiproton produces 1,836
times more energy per annihilation than a positron, this result appears to defy logic.
However, the laboratory energy threshold for producing antiprotons is 6,000 times
greater than for positrons, requiring a relatively complex proton synchrotron that is
costly to construct and run. In addition, antiprotons are made at much higher
laboratory energy than positrons and require costly apparatuses to decelerate them to
trapping energies.
On the other hand, because electrons and positrons are relativistic at very low energy,
their electron production and secondary systems are comparatively simple and less
costly to operate and maintain than proton systems. These factors, combined with the
absence of radioactive residue associated with positron annihilation, make positrons the
obvious choice over antiprotons.
Second, the cost of positrons is projected to be $0.004/J x 180 MJ/μg = $720K/μg.
Hence, the cost of 1 gram is $0.72T, or 5 percent of the 2008 U.S. gross domestic
product (GDP). A 2000 NASA study63 on which this author collaborated placed the cost
of antiprotons at $64T/g, consistent with the $333/J figure in the second line of Table
5, and roughly six times the 2000 GDP. Unfortunately, this is still being quoted in U.S.
scientific and government communities. The dramatic reduction in the unit cost of
antimatter since 2000 is due to a new emphasis on positrons by the physics
community, and hopefully this paper will help spread that good news.
Earlier, a nonstop flight around the globe by a small positron UAV was described as
equivalent to the 1927 Spirit of St. Louis transatlantic flight of Charles Lindbergh. From
23
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Table 5, the required 150 μg for this epic flight could be manufactured in 6 months for
$96 million, or 0.0007 percent of U.S. GDP.
Positron Storage
Confinement of antimatter has been reviewed extensively in the literature.64
Historically, the first approach was the Penning trap.65 Stores of 109 positrons for 1
hour have been achieved.66 Electric potentials are required to overcome space charge
forces.67 To illustrate, confinement of 1015 positrons in a 10-cm-radius sphere in a
perfect vacuum requires an electric potential of 240 kilovolts. Laboratory control of such
large potentials restricts stores to < 1 picogram (1015).
[Photograph of laboratory equipment]
Figure 19. Penning Trap With Trapping Volume of 1,000 Cubic Centimeters (center), Injection
Apparatus (left) and Controls (right) (courtesy Positronics Research LLC)68
In addition, with a magnetic field there are magnetic energy density restrictions on
confinement of positron plasmas. The Brillouin Density Limit is:
nB = εoB2/2me (1)
For a practical magnetic field of 1 Tesla, nB = 9.7 x 1012/cubic centimeters. In a 10-cm
radius sphere, the Brillouin Number Limit is 4 x 1016 (40 picograms). Therefore, by
24
UNCLASSIFIED/FOR OFFICIAL USE ONLYUNCLASSIFIED/FOR OFFICIAL USE ONLY either space charge or magnetic energy considerations, the storage limit is tens of picograms, 7-8 orders of magnitude short of 100 micrograms, where practical uses of positrons begin to emerge, as illustrated earlier. The second approach is confinement of neutral Ps atoms in manufactured porous media of either regular lattices of atoms, such as polymers, or irregular strands of insulator material encapsulating voids, such as silica aerogel.69, 70 Regardless of void size, Ps atoms ultimately annihilate with electrons attached to atoms on the boundaries of voids by the so-called "pickoff" process. Therefore, large, observable lifetimes require materials with extraordinarily large voids. FORMATION OF POSITRONIUM IN POROUS MEDIA Positrons are injected into a porous material at low energy (~100 keV) to ensure that they stop and form a Ps atom over a distance of a few millimeters.71 The positron rapidly loses its energy by collisions with electrons attached to atoms in the material. As it nears 6.8 electronvolts (eV)—the binding energy of the ground state of Ps—it captures a weakly bound electron and forms Ps. It diffuses through the material, and over about 1 nanosecond, its energy is rendered to the room temperature of the material, 0.025 eV. This is called thermalization. The quantum mechanical model of Ps is remarkably similar to the hydrogen atom. The major difference is that Ps spontaneously annihilates, whereas hydrogen is stable. The "self-annihilation" of Ps due to overlap of electron and positron wave functions results in extremely short lifetimes, as noted earlier. Figure 20. A Positron Forms Ps on the Edge of a Void, Thermalizes, and Becomes Trapped in a Void Before Annihilating (courtesy University of Michigan) Lifetimes against "self-annihilation" can be demonstrably increased if the following two conditions are met: (1) a way is found to isolate the electron wave function from the positron wave function, and (2) materials provide voids large enough to allow detection of lifetimes well beyond 142 nanoseconds (ns). A high vacuum is required to avoid Ps annihilation on gas molecules within the voids. The following describes how Positronics Research LLC has approached these issues in the laboratory.72 LONG-TERM STORAGE OF POSITRONIUM Under crossed magnetic and electric fields, Ps assumes a doubly oblate shape (Figure 21), with the electron and positron separated by hundreds of nanometers to tens of micrometers, depending on the size of the fields.73 Computation of lifetimes against quantum mechanical barrier penetration reveals lifetimes in excess of 1 year over a large range of magnetic and electric fields. 25 UNCLASSIFIED/FOR OFFICIAL USE ONLY
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Lifetime shortening owing to cyclotron
radiation of the electron and positron
gyrating in the magnetic field are not
included in this model. Because this
process is proportional to B2, magnetic
fields should be small—less than 0.1 Tesla
based on our computations of the effect.
This, in turn, renders the atom very large,
with up to 1-micrometer elongation. If
this can be verified in the laboratory, the
first of the two conditions laid out in the
previous section will be satisfied.
Next, it is important to identify a storage
medium with the largest possible voids.
Silica aerogel is a promising material that
was developed by NASA because of its
extremely low density and excellent
thermal insulating properties. It is
composed of strands of SiO2 (silica)
grains suspended in a gel that has been
dried and expanded by injection of gases
to a very-low-density configuration of
large voids within an irregular lattice of
silica strands (Figure 22). Silica aerogel is
available commercially with typically 20-nanometer average voids. Recent research has
produced silica aerogel with up to 1-micrometer void sizes.
Figure 21. Computer Simulation of Ps Atoms in a
5-Tesla Magnetic Field and 100 V/cm Electric Field
(1 a.u. = 0.052 nm) (courtesy University of
Bielefeld, Germany)74
Experiments in Japan and at Positronics
Research LLC33 with low-energy positrons
in silica aerogel show a high efficiency
(~35 percent) for making Ps through the
interaction of the positron with silica
grains. High radiation exposure from
positrons implanted in the material result
in it becoming "paramagnetic,"
permanently at low temperatures, with a
high density of "dangling bonds"
containing very loosely bound electrons
that explains the high efficiency for Ps
formation. It therefore serves a dual role
as source and storage medium for Ps.
What lifetimes might be expected working
with this material?
20 nm
Figure 22. TEM of Silica Aerogel (courtesy
Lawrence Berkeley Laboratory)
The Ps decay rate in a porous material is given by:75
λ = k'/(R – r') + λT + λq (2)
26
UNCLASSIFIED/FOR OFFICIAL USE ONLYUNCLASSIFIED/FOR OFFICIAL USE ONLY where k' is characteristic of the electron density for silica strands = 0.0164 nm-ns-1, R is the void radius (nm), r' is the silica strand thickness (0.68 nm), λT is the self- annihilation rate, and λq is the quenching rate for air (0.00427 ns-1 at 0.1 torr vacuum, typical of a standard rotary pump). To illustrate, without magnetic and electric fields (λT = 1/142 ns = 0.007 ns-1) and for R = 20 nm, λ= 0.0121 ns-1, or τ = λ-1 = 82.5 ns. This is consistent with our measurements.76 Assuming self-annihilation is suppressed (λT ~ 0) by crossed magnetic and electric fields, a high vacuum (10-5 torr) is maintained (λq ~ 0) and R = 20 nm, then λ = 8.5 x 10-4 ns-1, or τ = 1.2 μs. Proprietary experiments at Positronics Research LLC show that Ps atoms in crossed magnetic and electric fields in silica aerogel with 20-nm voids live up to 10 μs. This is somewhat longer than the predicted lifetime owing to severe radiation damage induced in the silica aerogel by positrons that alters k' from the values quoted above. Measured lifetimes in the present experiments at Positronics Research LLC are within limits set by diffusion to the trap walls. This is consistent with the expectation that Ps atoms "stabilized" in crossed magnetic and electric fields will be "delocalized" and drift freely across magnetic field lines. Future experiments at Positronics Research LLC will use super-dilute media (R = 1,000 nm), large container volumes (10 cm), weak magnetic fields (< 0.1 Tesla), and electric fields. Substitution of R = 1,000 nm into Equation 2, assuming again a high vacuum and crossed magnetic and electric field suppression of self-annihilation, predicts a lifetime of 61 milliseconds. Assuming the following results are consistent with this prediction, it will be demonstrated beyond any reasonable doubt that Ps can be stabilized against "self-annihilation" in crossed fields. To ultimately reach lifetimes of months and years required by aerospace propulsion, oscillating electric-gradient-field-confinement forces will be required to keep Ps atoms off the walls of a high-vacuum trap. Ps atoms will be produced by a beam of low-energy positrons intercepting silica aerogel and be stabilized in the trap using crossed magnetic and electric fields. Ps in crossed magnetic and electric fields has an enormous electric dipole moment, and confinement using the classical p•∇E force looks very encouraging at this time. Conclusions Conceptual designs and missions for turbojets, turbo-ramjet missiles, and interplanetary rockets powered by positron annihilation have been presented. Positron requirements range from 150 micrograms for a globe-encircling UAV turbojet flight to 100 milligrams for a mission to Mars. A positron-powered SSRV could take off from Earth horizontally, go to LEO, launch to Mars for a 1-year exploration of the Red Planet, and return to LEO and then Earth with a horizontal landing without refueling. Within 10 years, the 150 μg of positrons required for a globe-encircling, nonstop turbojet flight could be made in 6 months at a cost of $69 million. This first-ever antimatter voyage, approximately 90 years after Lindbergh's 1927 Spirit of St. Louis transatlantic flight, would stir the public's imagination and eventually lead to positron- powered exploration of the solar system in the 21st century. 27 UNCLASSIFIED/FOR OFFICIAL USE ONLY
UNCLASSIFIED/FOR OFFICIAL USE ONLY New developments in stabilizing and storing positronium atoms in a matrix of dilute materials are encouraging. Lifetimes of 10 μs have been achieved, and tens-of- millisecond lifetimes are expected in the next round of experiments. Present limits are due to interactions on the walls of the trap container. Application of oscillating gradient electric fields to the huge electric dipole moment of the stable Ps atom should mitigate this problem. With the issue of stabilization in crossed fields now settled, very long lifetimes are but a matter of engineering! 1 P. A. M. Dirac, Proc. Roy. Soc. A .126, 360 (1930). 2 C. D. Anderson, Phys. Rev. 43, 491 (1933). 3 O. Chamberlain, E. Segre, C. Wiegand and T. Ypsilantis, Phys. Rev. 100, 947 (1955). 4 G. Luders, Ann. Phys. 2, 1-15 (1957). 5 P. Hill and C. Peterson, Mechanics of Thermodynamics of Propulsion, 2nd ed., Addison - Wesley Publishing Co. (1992). 6 G. Herken, "The Flying Crowbar", Air and Space Magazine, Vol. 5 (#1), p. 28, April/May (1990) (http://www.merkle.com/pluto/pluto.html). 7 R. W. Bussard and R. D. DeLauer, Fundamentals of Nuclear Flight, McGraw-Hill Book Co. (1965) 8 G. A. Smith et al., "Revolutionary Positron Conversion", Final Technical Report, AFRL Contract F08630-00-C- 0010, Eglin AFB, FL, March (2002). 9 O. Chamberlain, E. Segre, C. Wiegand and T. Ypsilantis, Phys. Rev. 100, 947 (1955) 10 G. Luders, Ann. Phys. 2, 1-15 (1957). 11 G. A. Smith et al., "Revolutionary Positron Conversion", Final Technical Report, AFRL Contract F08630-00-C- 0010, Eglin AFB, FL, March (2002). 12 Ibid. 13 Ibid. 14 Ibid. 15 Ibid. 16 Ibid. 17 Ibid. 18 Ibid. 19 Northrop-Grumman Corp: Report to Kaiser-Marquardt for HTHL blended-body SSTO engine, "Vision Vehicle Final Report," April 30 (1998). 20 G. A. Smith et al., "A Revolutionary Positron Based SSRV Vehicle for Application to Human Exploration and Development of Space", The Advanced Space Propulsion Workshop, NASA Marshall Space Flight Center, Huntsville, AL, April 2-6 (2001). 21 Northrop-Grumman Corp: Report to Kaiser-Marquardt for HTHL blended-body SSTO engine, "Vision Vehicle Final Report," April 30 (1998). 22 G. A. Smith et al., "A Revolutionary Positron Based SSRV Vehicle for Application to Human Exploration and Development of Space", The Advanced Space Propulsion Workshop, NASA Marshall Space Flight Center, Huntsville, AL, April 2-6 (2001). 23 Ibid. 24 Ibid. 25 Ibid. 26 Ibid. 27 G. A. Smith, "Positron Propelled and Powered Space Transport Vehicle for Planetary Missions", NIAC Phase I Final Report, Research Subaward No. 07605-003-048, September 1, 2005 – March 31, 2006. 28 Ibid. 29 D. R. Koenig, "Experience Gained from the Space Nuclear Rocket Program (Rover)," LA-10062-H, Los Alamos National Laboratory, Los Alamos, NM., May (1986). 30 G. A. Smith, "Positron Propelled and Powered Space Transport Vehicle for Planetary Missions", NIAC Phase I Final Report, Research Subaward No. 07605-003-048, September 1, 2005 – March 31, 2006. 31 L. E. Thode et al., J. Propulsion and Power 14, 4 (1998). 32 D. I. Poston and T. Kammash, Nuclear Science and Engineering 122, 32 (1996). 28 UNCLASSIFIED/FOR OFFICIAL USE ONLY
UNCLASSIFIED/FOR OFFICIAL USE ONLY 33 S. K. Borowski et al., "Nuclear Thermal Rocket/Vehicle Design Options for Future NASA Missions to the Moon and Mars," AIAA-93-4170 (NASA Tech Memorandum 107071), (1993). 34 D. I. Poston and T. Kammash, Nuclear Science and Engineering 122, 32 (1996). 35 S. K. Borowski et al., "Nuclear Thermal Rocket/Vehicle Design Options for Future NASA Missions to the Moon and Mars," AIAA-93-4170 (NASA Tech Memorandum 107071), (1993). 36 G. A. Smith, "Positron Propelled and Powered Space Transport Vehicle for Planetary Missions", NIAC Phase I Final Report, Research Subaward No. 07605-003-048, September 1, 2005 – March 31, 2006. 37 E. Sänger, Ing. Arch. 21, 213 (1953). 38 G. A. Smith, "Positron Propelled and Powered Space Transport Vehicle for Planetary Missions", NIAC Phase I Final Report, Research Subaward No. 07605-003-048, September 1, 2005 – March 31, 2006. 39 Ibid. 40 G. Gaidos et al., "Antiproton-Catalyzed Microfission/fusion Propulsion Systems for Exploration of the Outer Solar System and Beyond," AIAA-98-3589, Presented at the 34th AIAA/ASME/SAE/ASEE Joint Propulsion Conference & Exhibit (1998). 41 W. Lance Werthman, "Antiproton-Catalyzed Microfission/fusion Space Propulsion," MS Thesis, Dept. of Aerospace Engineering, Penn State University (1995). 42 G. A. Smith, "High Density Storage of Antimatter", Advanced High Energy Storage Conference, MITRE Corp., McLean, VA, Aug. 1 (2005). 43 S. J. Hoffman and D. I. Kaplan, eds. "Human Exploration of Mars: The Reference Mission of the NASA Mars Exploration Study Team," NASA Special Publication 6107, JSC (1997). 44 B. G. Drake, ed. "Reference Mission Version 3.0: Addendum to the Human Exploration of Mars: The Reference Mission of the NASA Mars Exploration Study Team," http://ares.jsc.nasa.gov/ HumanExplore/ Exploration/ EXLibrary/docs/ MarsRef/addendum/index.htm, June (1998). 45 G. A. Smith, "High Density Storage of Antimatter", Advanced High Energy Storage Conference, MITRE Corp., McLean, VA, Aug. 1 (2005). 46 J. Dion, "Stirling Radioisotope Generator", NASA Glenn Research Center, ME 388R.2, Spring (2005). 47 Ibid. 48 G. A. Smith, "High Density Storage of Antimatter", Advanced High Energy Storage Conference, MITRE Corp., McLean, VA, Aug. 1 (2005). 49 S. J. Hoffman and D. I. Kaplan, eds. "Human Exploration of Mars: The Reference Mission of the NASA Mars Exploration Study Team," NASA Special Publication 6107, JSC (1997). 50 B. G. Drake, ed. "Reference Mission Version 3.0: Addendum to the Human Exploration of Mars: The Reference Mission of the NASA Mars Exploration Study Team," http://ares.jsc.nasa.gov/ HumanExplore/ Exploration/ EXLibrary/docs/ MarsRef/addendum/index.htm, June (1998). 51 S. K. Borowski et al., "Nuclear Thermal Rocket/Vehicle Design Options for Future NASA Missions to the Moon and Mars," AIAA-93-4170 (NASA Tech Memorandum 107071), (1993). 52 G. A. Smith, "Positron Propelled and Powered Space Transport Vehicle for Planetary Missions", NIAC Phase I Final Report, Research Subaward No. 07605-003-048, September 1, 2005 – March 31, 2006. 53 S. J. Hoffman and D. I. Kaplan, eds. "Human Exploration of Mars: The Reference Mission of the NASA Mars Exploration Study Team," NASA Special Publication 6107, JSC (1997). 54 B. G. Drake, ed. "Reference Mission Version 3.0: Addendum to the Human Exploration of Mars: The Reference Mission of the NASA Mars Exploration Study Team," http://ares.jsc.nasa.gov/ HumanExplore/ Exploration/ EXLibrary/docs/ MarsRef/addendum/index.htm, June (1998). 55 G. A. Smith, "Positron Propelled and Powered Space Transport Vehicle for Planetary Missions", NIAC Phase I Final Report, Research Subaward No. 07605-003-048, September 1, 2005 – March 31, 2006. 56 C.A. Kapatanakos, J. Synchrotron Rad. 3, 268-271 (1996). 57 ILC undulator-based source, www.ippp.dur.ac.uk/~gudrid/source/BCD-source. ILC capitalization is estimated at $9B (see www.linearcollider.org). 58 Ibid. 59 Ibid. 60 R. Landua, CERN, "Precision Experiments with Antiprotons", Feb. 24 (2003). 61 S. Howe et al., AIP Conf. Proc. 746, 520 (2005). 62 14 MeV Electron Linac @ $5M capitalization cost and $5M/yr operating cost (A. Herer et al., "Applications of High Voltage High Powered Electron Beams", IBA, Belgium, 1997). 63 G. R. Schmidt et al., J. Propulsion and Power, 16, 923 (2000). 64 J. Rejcek et al., Rad. Phys. Chem. 68, 655 (2003). 65 G. A. Smith, "High Density Storage of Antimatter", Advanced High Energy Storage Conference, MITRE Corp., McLean, VA, Aug. 1 (2005). 66 C. M. Surko and R. G. Greaves, Phys. Plasmas 11, 2333 (2004). 29 UNCLASSIFIED/FOR OFFICIAL USE ONLY
UNCLASSIFIED/FOR OFFICIAL USE ONLY 67 G. A. Smith, "Long-Term Confinement of Dense Positron Plasmas", Final Technical Report, AFRL Contract F08630-02-C-0017, Eglin AFB, FL, January 23 (2007). 68 Ibid. 69 G. A. Smith, "Positron Energy Conversion (PEC)", 53th JANNAF Propulsion Meeting, Subcommittee on Future Technologies for Spacecraft Propulsion (2005 0356 EA), CPIAC JSC CD43 December (2005). 70 G. A. Smith, "Stabilization and Long Term Confinement of Atomic Positronium", Final Technical Report, AFRL Contract F08630-02-C-0018, Eglin AFB, FL, January 23 (2007). 71 S. J. Hoffman and D. I. Kaplan, eds. "Human Exploration of Mars: The Reference Mission of the NASA Mars Exploration Study Team," NASA Special Publication 6107, JSC (1997). 72 G. A. Smith, "Stabilization and Long Term Confinement of Atomic Positronium", Final Technical Report, AFRL Contract F08630-02-C-0018, Eglin AFB, FL, January 23 (2007). 73 J. Lu, "Classical Trajectory Monte Carlo Simulation of Ion-Rydberg Atom Collisions", Ph. D. Dissertation, University of Bielefeld, Germany, May (2003). 74 Ibid. 75 K. Sudarshan et al., J. Phys: Condens. Matter 19, 386 (2007). 76 G. A. Smith, "Stabilization and Long Term Confinement of Atomic Positronium", Final Technical Report, AFRL Contract F08630-02-C-0018, Eglin AFB, FL, January 23 (2007). 30 UNCLASSIFIED/FOR OFFICIAL USE ONLY