UNCLA SSIFIED//FOR OFFICIA L USB ONLY 2 March 2010 IC OD : 1 D ecem ber 2009 D IA -08-1003-002 Defense Intelligence Reference Document A cquisition Threat S upport Positron A erospace Propulsion U N CLA SSI FI E D/ / FOR OFFICIA L USE ONLY UNCLA SSIFIED//FUK OFFICIA L USE UNL! Positron A erospace Propulsion Prepared by: A cquisition Support Division (DWO-3) Defense Warning Office Directorate for A nalysis Defense Intelligence A gency A uthor: A A P Person 69 A dministrative 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, Defense Warning Office's Advanced Aerospace Weapon System Applications (AAWSA) Program. Comments or questions pertaining to this document should be addressed to|AAP Person 1 AAWSA Program Manager, Defense Intelligence Agency, ATTN: CLAR/DWO-3, Bldg 6000, Washington, DC 20340-5100. ii UNCLA SSI FIED //EQR OFFICIAL UCt OliW UNCLA SSIFIED//FOR OFFICIA L U0£ ONW Contents Introduction................................................................................................................v A ntimatter...................................................................................................................1 Positron A ir-Breathing Propulsion............................................... 2 PTRE A pplications.......................................................................................................5 Unmanned A erial Vehicle (UA V)................................................ 5 Ramjet-A ssisted M issile (RA M )....................................... 7 Single-Stage Reusable Vehicle (SSRV).................................. 8 Positron-Powered Rockets.......................................................................................11 The Solid-Core Positron Rocket.............................................................. 12 The Gas-Core Positron Rocket..............................................................................14 The Sanger Photon Positron Rocket....................................................................16 Positron Rocket System Comparison............................................................ 17 Positron Energy Conversion for Onboard Power.................................................18 Positrons for a M anned M ars M ission ............................................................. 19 Positron Production..................................................................................................22 Positron Costs.................................................................. 23 Positron Storage................................................................................................. 24 Formation of Positronium in Porous M edia..........................................................25 Long-Term Storage of Positronium......................................................... 25 Conclusions...................................... 27 Figures Figure 1. Specific Energy for Chemical, Nuclear and A ntimatter M aterials...............1 Figure 2. Tory-IIC Ready for Testing.......................... 3 Figure 3. PTRE Turbojet and Turbo-Ramjet M odes......................... 3 Figure 4. Details of the PTRE Engine.............................................. 4 Figure 5. Combustion Turbo-Ramjet........................................................ 4 Figure 6. UA V Range vs. Positron M ass.....................................................................5 Figure 7. LOCA A S Turbojet Engine ...................................................................... 6 iii UNCLA SSI FIED/ /TOR OFFICIA L USE ONLY UNCLA SSIFIED//FUR UFF1UAL USE ONLY Figure 8. BOM A RC and Talos Ramjet-A ssisted M issiles.................. 8 Figure 9. Positron SSRV Flight Profile............................................... 10 Figure 10. A rtist'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. M odified Sanger Photon Rocket Concept........................................... 17 Figure 14. Closed Brayton Cycle Using Positron A nnihilation................................18 Figure 15. Conceptual 110 Watt Positron Closed Cycle Generator Based on the NA SA Glenn Research Center Stirling Radioisotope Generator (SRG).. 18 Figure 16. M ars Trajectories (X-Coordinates Defined in Direction of A ries)........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 A toms.......................................... 26 Figure 22. TEM of Silica A erogel.............................................. 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 M ass 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 M ars M ission....17 Table 6. Positron and A ntiproton Expected Costs in the Next 10 Years ............... 23 iv U N CLA SSI FI E D/ lEGB^EHGlAIUJfiMNMt UNCLA SSIFIED//FOR OFFICIA L UDE OHM * Positron A erospace Propulsion Introduction A ntimatter 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. A fter 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 A ir Force Research Laboratory, Eglin A ir Force Base, Florida, and the NA SA Institute for A dvanced Concepts, A tlanta, 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 A nderson in 1932.2 A long 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. A ntimatter has a long history of appearing in science fiction literature, dating to a 1942 short story in A stounding 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 A ngels and D em ons. 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 UNCLA SSI FIED//FOP nFFV™» ■■rr?i..u UNCLA SSIFIED//HUH UFF1L1A L USE ONLT A ntimatter A ntim atter appears in the form of fundam ental particles that have their sign of electric charge reversed from their m atter counterpart. For exam ple, the positron, e+ , is the antiparticle to the electron, e-. A ccording to the C PT theorem ,4 properties of m atter and their counterpart antim atter particles are identical. This has been tested in the laboratory to an accuracy of roughly 1 part in 10 m illion. A ntim atter is appealing for aerospace uses because its specific energy by annihilation is 180 MJ/pg, or 10 orders of m agnitude larger than chem ical energy, as show n in Figure 1. 1 00E+03 1.00E+02 1 00E+O1 1 00E+00 1 00E-01 Energy Density I 00E-02 MJ/gg I 00E-03 1.00E-04 1.00E-05 1.00E-06 I 00E-07 1 00E-08 Figure 1. Specific Energy for Chemical, Nuclear, and A ntimatter M aterials In the presence of m atter, the positron binds w ith an electron to form a short-lived atom called positronium (Ps). D epending on the relative spin orientations of the positron and electron, Ps has a m ean lifetim e in a vacuum of 125 picoseconds (para-Ps, spins antiparallel), or 142 nanoseconds (ortho-Ps, spins parallel). From quantum num ber conservation, para-Ps decays into tw o gam m a rays of equal energy, 511 kiloelectronvolts (keV), w hereas ortho-Ps decays into three gam m a rays w hose energies add up to 1022 keV. Hence, w hen Ps self-annihilates, there is 100 percent conversion of m ass into electrom agnetic energy given by Einstein's fam ous equation, E = m e2, w here c is the speed of light. A lthough the energies of positron annihilation gam m a rays are on the nuclear scale, they have none of the undesirable features associated w ith nuclear energy. First, the annihilation evolves rapidly (nanoseconds) and is controllable under predictable electrom agnetic forces. There is no long-term inertia as w ith nuclear reactors. C onsequently, positron-generated thrust can be throttled. 1 UNCLA SSI FIED//FOR OTriCIA L USE OM L¥ UNCLA SSIFIED//FOR. OFFICIA L USE ONLY Second, low -energy gam m a rays from positron annihilation cannot m ake residual radioactivity in surrounding air and containm ent vessels. In contrast, antiprotons annihilate into a host of high-energy particles, including jr-m esons and gam m a rays that can induce residual radioactivity in nearby m aterials. Finally, low -energy gam m a rays from positron annihilation can be readily converted into useful form s of energy, including heat and electricity required for propulsion system s. This contrasts w ith large, com plex system s required for conversion of antiproton annihilation and nuclear fission/fusion energy. There are tw o reasons w hy positrons have yet to be used for aerospace applications. First, it has not been possible to produce them in the num bers required. How ever, recent developm ents in high-energy physics research are resulting in expanding levels of positron production. Second, m ethods for storing positrons for basic research do not hold enough positrons long enough for propulsion applications. R ecent developm ents in storage techniques m ay significantly im prove the situation, w ith lifetim es up to m onths and possibly years. Positron A ir-Breathing Propulsion A eronautical engines burn a m ixture of aviation fuel and oxygen in air to heat a w orking fluid. To keep engines sm all, the com bustion rate in the engine needs to be high.5 A t sea level for a fuel-air m ass ratio of 0.06 8, it is 500,000 kJ/m 3-s. To m aintain speed, the thrust specific fuel consum ption (TSFC ) for turbojets and turbo-ram jets is in the range of 0.075 - 0.11 and 0.17 - 0.26 kilogram /hour-N ew ton (kg/hr-N ), respectively. A ll aeronautical engines are lim ited in range and flight duration by the fuel on board. Because of the aforem entioned perform ance bounds of com bustion engines, the aeronautic industry has w orked diligently to increase the range and payload of aircraft by m axim izing the perform ance of com bustion engines and optim izing aerodynam ic design. Beyond this, the only w ay a com bustion-pow ered aircraft can extend its range and endurance is by in-flight refueling. Tw o projects investigated nuclear pow er as a w ay to increase perform ance. In 19 46 , the U.S. A ir Force established the N uclear Energy for Propulsion of A ircraft program . How ever, this program w as disbanded in 19 51 in favor of the joint A tom ic Energy C om m ission-A ir Force A ircraft N uclear Propulsion program . Im plem enting nuclear fission to pow er an aircraft required tw o approaches. One w as direct cycle, w hereby air w as heated by passing it through a nuclear reactor; the other w as indirect cycle, w hereby the reactor heated a liquid m etal that in turn heated air in a secondary heat exchanger. The program never produced a prototype and w as canceled in 19 6 1. In 19 57, the Pentagon started developm ent of a nuclear ram jet m issile (SLA M, Supersonic Low -A ltitude Missile) to fly below Soviet defenses. The Law rence Liverm ore N ational Laboratory Pluto program successfully tested tw o engines, Tory-IIA and Tory- IIC (Figure 2), at the N evada Test Site. The program w as canceled in 19 6 4.6 -7 2 UNCLA SSI FIED//FOR OmCIA L USE ONLY UNCLA SSIFIED//rOR OrriCIA L USE ONLY Figure 2. Tory-IIC Ready for Testing (courtesy LLNL) In a positron turbojet/ram jet engine (PTR E),8 tungsten shells are heated by gam m a rays, w ith heat transferred to air by convection.9 - 10 Figure 3. PTRE Turbojet (green) and Turbo-Ramjet (red) M odes (courtesy Positronics Research LLC)11 3 UNCLA SSI FIED //FOR OFF^tai iice^mi^ UNCLA SSIFIED//FOR OFFICIA L USE OhL¥ Concentric Annular Tungsten Heating Surfaces Transfer Positron Conversion Heat Energy To Airflow Turbine Powered Electrical Generator is Power Source For Positron Trap and Electric Motor for Compressor Electric Motor Driven , Compressor Chamber Positron Trap Intake Airflow 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 com parison w ith the com bustion turbo-ram jet engine (Figure 5) show s that volum e used for com bustion heating is used by PTR Es for convection heating. turbojet Figure 5. Combustion Turbo-Ramjet (courtesy Positronics Research LLC)13 4 UNCLA SSI FIED //TOR OFFICIA L UDE OHLY UNCLA SSIFIED//F8R OFFICIA L USE ONLY PTRE A pplications Three uses for PTR Es have been investigated: unm anned aerial vehicles (UA Vs), ram jet-assisted m issiles (R A Ms), and single-stage reusable vehicles (SSR Vs). UNM A NNED A ERIA L VEHICLE (UA V) Figure 6 show s range versus positron m ass for a 6 0-kg UA V w ith lift/drag of four that can circum navigate Earth on 150 pg of positrons.14 It is m odeled after the LOC A SS turbojet (Figure 7) at the A ir Force R esearch Laboratory (A FR L), Eglin A ir Force Base, Florida.15 Positron Mass (mg) Figure 6. UA V Range Versus Positron M ass (courtesy Positronics Research LLC)16 5 UNCLA SSI FIED//FOR OFFICIA L USE ONLY UNCLA SSI FI ED//F8R OFFICIA L USE ONLY Figure 7. LOCA A S Turbojet Engine (courtesy of A FRL, Eglin A FB, FL)17 A UA V fueled w ith positrons w ould allow for an ultralong-endurance platform w ith m any "dual-use" applications, including the follow ing: Military • Intelligence, surveillance, and reconnaissance. • R eal-tim e battlefield com m and observation. • Monitoring and early w arning of nuclear, biological, and chem ical w eapons. • Ordnance delivery. • Target laser illum ination/covert target acquisition. • C oastal patrol. • D rug interdiction. • Search and rescue. • Geom agnetic and atm ospheric surveys. C ivil, State, and Local Governm ent • A irborne early w arning (storm s, terrorist attacks, chem ical/biological/nuclear). 6 UNCLA SSI FIED//FOR OFFICIA L USE ONLY UNCLA SSI FI ED//rOR OFFICIA L USE ONLY • Environm ental/pollution m onitoring. • Fire detection and m onitoring. • A erial surveying. • Weather and atm ospheric m onitoring. • Border patrol. • Em ergency com m unications relay. • D rug interdiction. • Law enforcem ent. • Highw ay/road m onitoring. • Search and rescue. C om m ercial • Iceberg patrol/tracking in shipping lanes. • R ailw ay/pipeline/pow er line m onitoring. • C om m ercial fishing reconnaissance/sea-life m onitoring. • Environm ental/pollution m onitoring. • Livestock m onitoring. • Mineral exploration. • Weather sensing. • A griculture. A dditional advantages to the com m ercial airline industry: • Less propellant m eans increased payload/passenger per aircraft. • Global nonstop flights are possible. • Increase in structural m ass allow s m ore electronics/passenger am enities. RA M JET-A SSISTED M ISSILE (RA M ) The BOMA R C 440-kilom eter (km )-range antiaircraft m issile w as developed by the U.S. A ir Force in the 19 50s to counter Soviet bom bers. In the 19 6 0s, the Talos, a long-range surface-to-air m issile, w as developed by the U.S. N avy and later converted to the Vandal m issile. Both ram jets (Figure 8) activated at m ach 1. 7 UNCLA SSI FI ED//ron OrriCIA L UEG QNhY UNCLA SSIFIED//F8R OFFICIA L USE ONLY Figure 8. BOM A RC (left) and Talos (right) Ramjet-A ssisted M issiles (courtesy Boeing and the U.S. Navy) The BOMA R C range could be increased to 2,000 km w ith 1 m illigram (m g) of positrons. In addition, positrons could act as ordnance to destroy electronics on m issiles or aircraft by electrom agnetic pulse (EMP) by detonation of m icrogram s of positrons up to hundreds of m eters from the target.18 SINGLE-STA GE REUSA BLE VEHICLE (SSRV) The D epartm ent of D efense, N A SA , and the aerospace industry are w orking to elim inate m ultistage rockets for access to low Earth orbit (LEO) to low er launch costs. A n SSR V w ould integrate com ponents and allow m ore rapid turnaround on m issions. The com bustion SSR V has a low er payload ratio than conventional rockets. C urrent designs use com bined-cycle or com bination engines for the endoatm ospheric phases of the m ission. Use of air-breathing engines significantly reduces propellant m ass and gross liftoff w eight (GLOW). Im proving payload m ass requires reducing propellant and structural m ass, both of w hich can be accom plished using positrons as fuel. A PTR E elim inates propellant m ass during the endoatm ospheric phase of operation. A ny reduction of propellant m ass has serious positive effects for three reasons. First, it allow s an increased payload/num ber of passengers per aircraft. Second, adding positrons on the m illigram level does not change the lift requirem ents for the aircraft, resulting in longer range flights, perhaps exceeding 100 kilonautical m iles (kN M). Third, the structural m ass m ay be increased. Tables 1 and 2 provide a m ass budget com parison betw een a chem ically fueled SSR V19 and a positron-pow ered SSR V.20 8 UNCLA SSI FIED//FOR OFFICIA L UOE ONLY UNCLA SSIFIED//FUH OFFICIA L USE ONLY Table 1. GLOW for Chemical SSRV21 Vehicle Component Mass Structure 25,7 00 kg Thermal Protection 12,300 kg Propulsion (4 engines) 14,9 00 kg Electronics 7 ,600 kg TOTAL DRY MASS 60,500 kg 15% Margin + Unused Propellants 11,400 kg Payload 11,340 kg (24,9 48 lbs) BURNOUT MASS 83,240 kg TOTAL PROPELLANT 368,300 kg GLOW 451,540 kg (9 9 3,388 lbs) Table 2. GLOW for Positron SSRV22 Vehicle Component Mass Structure 25,7 00 kg Thermal Protection 12,300 kg Propulsion (4 engines) 14,9 00 kg Electronics 7 ,600 kg TOTAL DRY MASS 60,500 kg 15% Margin + Unused Propellants 11.400 kg Payload 11,340 kg (24,9 48 lbs) BURNOUT MASS 83,240 kg TOTAL PROPELLANT 176,000 kg GLOW 259,240 kg (59 0,328 lbs) The GLOW of the positron SSR V is 43 percent less than that of the chem ical SSR V ow ing to reduced propellant m ass. The PTR E w ill dram atically increase the affordability of space transportation by increasing the useful payload. 9 UNCLA SSI FIED //FOR OFFICIA L USE ONLY UNCLA SSIFIED//F8R OFFICIA L USE ONLY Figure 9. Positron SSRV Flight Profile (courtesy Positronics Research LLC)13 The SSR V takes off horizontally (Figure 9 ) from Edw ards A ir Force Base, accelerates to m ach 1.8 as a turbojet, and then goes to the ram jet m ode. A t m ach 8 the rocket is ignited and takes the vehicle into LEO.24 The positron m ass budget for a 6 0,500-kg dry m ass for ascension to LEO is 86 .9 m g (Table 3). Table 3. Total Positron Requirement for SSRV With a Dry M ass 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 A n artist's rendition of the SSR V is provided in Figure 10. 10 UNCLA SSI FIED //FOR OFFICIA L USE OHLY UNCLA SSIFIED//FOR OFFICIA L USE ONLY Figure 10. A rtist'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 N uclear fission reactors contain enorm ous am ounts of highly toxic radioactive m aterial, and antiproton rockets produce radioactivity in surrounding m aterials by interactions of high-energy m esons and gam m a rays from antiproton annihilation. The proposed nuclear fission gas core rocket and antiproton adaptations w ould release radioactive fission fragm ents into the atm osphere. In contrast, positron rockets are totally radioactivity free. Second, in the event of an accidental detonation of the positron fuel, the prom pt (nanoseconds) burst of gam m a rays can be shielded from hum ans on spacecraft. The 1/e absorption length of a 511-keV gam m a ray in lead is 5.6 m illim eters. A 13- centim eter (cm )-thick shield thus reduces the gam m a ray flux by a factor 8.3 x IO-11. To illustrate, a hum an behind such a shield at a distance of 10 m eters from a 100-m g source of annihilating positrons w ould experience a w hole-body radiation dose of 2 rem (roentgen equivalent m an), w hich is w ithin the annual tolerance for radiation w orkers in the United States. The follow ing sections investigate three positron rockets: solid core, gas core, and photon.28 In a solid-core positron rocket, positrons heat a hydrogen w orking fluid through an attenuating solid such as tungsten. In a gas-core positron rocket, gam m a rays directly heat propellant through a one- or tw o-fluid process. In a photon positron rocket, solid propellant is ablated from a surface bom barded by gam m a rays. 11 UNCLA SSI FIED//FOR OFFICIA L UOE ONLY UNCLA SSIFIED//TOW OFFICIA L USE ONLY THE SOLID-CORE POSITRON ROCKET Figure 11 depicts a solid-core positron-pow ered rocket, sim ilar in m any regards to the N ER VA nuclear-therm al concept.29 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 pum p 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 gam m a rays to high tem perature. Hydrogen propellant passes through the attenuating m atrix and is heated and exhausted through a nozzle to generate thrust. A sm all fraction of the hot exit propellant is bled off to a turbine that drives the high- pressure feed pum p. The high-tem perature bleed can either be m ixed w ith cold hydrogen to reduce its tem perature or directly fed to the turbine. If it is directly fed to the turbine, it m ust be m ade of m aterials that can w ithstand high tem peratures. The bleed flow is exhausted from a turbine exit nozzle to space after driving the turbine. A s w ith the N ER VA system , the positron solid-core concept is therm ally lim ited by m aterials in the heating cham ber. The difference is that the fission system requires a reactor and com plex m achinery, w hereas the positron system relies on Ps atom s injected upstream from a storage unit. This has tw o advantages. First, a reduction in the engine m ass for a given thrust is realized; second, there is greater choice in m aterials to be used in the heating cham ber. A therm al-fluids analysis w as conducted to predict perform ance. A specific im pulse of 9 20 seconds is attainable w ith cham ber tem peratures at 3,000 Kelvin. The corresponding thrust and pow er em ulate fission system s. Mars trip burn tim es are on 12 UNCLA SSI FIED//FOR OFFICIA L USE ONLY UNCLA SSI FI ED/7F0A OFFICIA L USE OM L¥ the order of 30 m inutes, indicating that a spacecraft em ploying three 72-kN solid-core engines w ould require 6 -9 m g of positrons per m ission. Solid-core fission and positron system s are com pared in Table 4. Table 4. Comparison of Space Propulsion and Power Systems - Solid Core Fission-Based Positron Pow ered Technology • N ER VA /R over dem onstrated • N ever flight tested • C onceptual • Must dem onstrate positron storage and controlled injection • N ear-term technology dem onstration for positron storage needed Perform ance • lsp 55 9 50 sec • Thrust *72- 123 kN • Pow er * 36 7- 5320 MW (m atched to thrust) • Lifetim e * 2 hours total operation • l5p - as in fission system s • Thrust -variable, sim ilar to fission system s • Pow er - m atched to thrust • Lifetim e - set by m aterial considerations Operation • D esign dictated by neutronics. fuel burn up and fission poisoning • High neutron & gam m a radiation during operation • R equires active, accurate and m assive control • R equires shutdow n cooling to rem ove heat from nuclear w aste • R adiation after shutdow n due to fission products • N o criticality, burn up or poison accum ulation issues • D esign based only upon heat transfer and gam m a attenuation issues • D oes not require shutdow n cooling • Sim ple on-off control, pow er controlled by rate of positron utilization • N ot a radiation source Materials • Material dictated by neutronics • Propellant-heated H 2 • Work! ng flu ids for pow e r system s - inert gas • Uranium in graphite media • Corrosion issues require complex fuel • Material choices dictated by tem perature • Propellant-heated H 2 • Working fluids for pow er system s - inert gas Payload Integration • Requires massive shield from reactor • Requires separation from reactor • Complex design issues due to neutron scattering • Shield required around manned area separated from positron storage • Propulsion and power sources can be integrated into vehicle Post Operation • Not able to return to earth or inhabited surface • Not reusable or refuelable • Able to return to Earth or inhabited surface • Reusable and refuelable 13 UNCLA SSI FIED //BOP DEETCIAL USE QNWE UNCLA SSIFIED//ron OFFICIAL USE ONLY THE GA S-CORE POSITRON ROCKET The gas-core positron concept follow s nuclear gas-core concepts,31'32'33 w hich are different from the solid-core concept in that gam m a rays directly heat a fluid under pressure. The lim it of the solid-core approach is m elting tem peratures of the solid m atrix gam m a ray attenuator. By direct heating, tem peratures can increase significantly as long as the gas does not appreciably heat the w alls. Four versions of the gas-core concept are illustrated in Figure 12. Synchronous w ith pulsed Ps injection are (a-c) pulses of LN 2 or LN e or LH 2 w ith LXe gam m a ray attenuator and (d) pulses of LH2 w here Ps is encapsulated in lead, a gam m a ray converter. For fluid injection, a turbo-pum p (not show n) is located upstream , w ith pow er obtained from a positron Brayton cycle system described later in this paper. R esults from com putational fluid dynam ics codes reveal that high-density regions of the fluid m ove aw ay from the gam m a ray source w hen the pow er in the system exceeds 300 m egaw atts. Under these conditions, the propellant does not efficiently absorb gam m a rays. Furtherm ore, calculations of heat required for continuous operation suggest the vortex configuration of (b) breaks dow n and reverts to tw o-fluid flow . How ever, both the tw o-fluid, flow -through m odel and the Ps lead-cartridge concepts show prom ise if the m ass flow rate of the hydrogen propellant exceeds that of the xenon or lead by a factor of five. By operating in a pulsed m ode, one should be able to control the positron delivery into the cham ber core. With com plete absorption of gam m a rays in the 2-cm lead casing, perform ance of the system m atches that of previously exam ined system s.34'35 Thrusts of 130 kN (1,000 atm ospheres) are predicted for a single-engine system w ith an efficiency of 85 percent. Burn tim es are 30 m inutes for A V = 3.7 km /second (sec) w ith 25 m g of positrons consum ed for a 50,000-kg burnout m ass. The lim it of the gas­ core concept occurs near the thresholds for ionization of hydrogen, corresponding to Isp of ~ 2,500 sec. 14 UNCLA SSI FIED//FOR OFFICIA L USE UNLT UNCLA SSIFIED//FOA OFFICIA L USE ONLY Figure 12. Fluid Systems, (a) One-fluid system of LN ? or N e propellant; (b) Tw o-fluid system of LH? and LXe; (c) Tw o-fluid flow -through system w ith LH? at higher m ass flow rates than LXe; and (d) One-fluid LH? system w ith lead encapsulated Ps (courtesy Positronics R esearch LLC )36 15 UNCLA SSI FIED / /fob nmr™» ..ccnM o/ UNCLA SSIFIED//FOR OFFICIAL MCE QNI Y THE SA NGER PHOTON POSITRON ROCKET In 19 53, Germ an engineer Eugen Sanger proposed the photon rocket. One m eans of providing thrust w as to show er a parabolic m irror w ith positron annihilation gam m a rays.37 Unfortunately, there are no m aterials that reflect gam m a rays at large angles. A schem atic of a m odified Sanger photon rocket is show n in Figure 13. Ps is em itted in 'pellets' from several storage banks located behind the engine. Positrons are program m ed by supporting fields to annihilate in front of a stiffened pressure plate, the shape of w hich w as assum ed to be parabolic for this w ork.38 To m ake the Sanger photon rocket practical, solid propellant that can be ablated from the plate is added. Gam m a rays deposit energy on the surface of the ablation m aterial and jettison high-energy particles w ith large Isp. A s the solid m aterial recedes from the target, the location of the positron annihilation can be correspondingly m oved inw ard to preserve focal properties of the system . A thickness of a few m eters of m aterial is sufficient for a planetary m ission. Figure 13. M odified Sanger Photon Rocket Concept (courtesy Positronics Research LLC)39 The predicted Isp for this system results in fast transit tim es to Mars, w arranting a positron pow er plant. Unlike the solid-core or gas-core concepts, there are no m eans to draw off som e of the ablated m aterial to provide pow er. A separate Brayton-cycle positron energy conversion system provides pow er to the pellet m ass driver and other 16 UNCLA SSI FIED //TOR OFFICIA L USE ONte¥ UNCLA SSIFIED//TOR OmCIA L USE ONLY com ponents. A lternatively, solar collectors or closed-loop nuclear reactors could be em ployed. To im prove ablation efficiency, annihilation gam m a rays m ust be w avelength shifted (WLS) by passing them through a high-Z WLS m aterial. The m aterial of choice, lead, also serves as the shell of the Ps pellet. The pellet vaporizes into high-energy plasm a, and the WLS photons propagate to the pressure plate. Silicon carbide ablation m aterial has been adopted from the antiproton catalyzed m icrofission/fusion concept40' 41 developed at Penn State University by the author and cow orkers. Photon energy distributions are shifted through 2 cm of lead to 1-10 keV from 511 keV w ith 85 percent efficiency. Perform ance depends prim arily on the energy of the WLS photons and the energy per pellet. A t 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 consum ed for a one-w ay trip to Mars over this range of Isp is 15-40 m g w ith 50 percent of gam m a rays striking the plate and a 85 percent WLS efficiency. POSITRON ROCKET SYSTEM COM PA RISON A side-by-side com parison of three positron rocket propulsion concepts is presented in Table 5 for a one-w ay transit to Mars using A V = 3.7 km /sec. Table 5. Comparison of Three Positron Propulsion Concepts for M ars M ission Solid-Core Gas-Core Sanger Ablation Isp 650 - 9 20 sec 1000 - 2500 sec 1200-3000 sec Thrust 7 2 kN, small class 130 kN (1000 atm) 40-145 kN (1 Hz) Limits • Wall and nozzle temperature • Wall and nozzle temperature • H Ionization • Positron density per pellet e+ mass • 6-9 mg (100% efficiency) • <25 mg (85% efficiency) • 15-40 mg (42.5% efficiency) Special Notes • Continuous burn • Multiple engines may be possible • Hot-bleed line possible • Pulsed burn • Multiple engines may be possible • Hot-bleed line possible • Engine can be throttled • Pulsed burn • Multiple engines may be possible • No direct onboard power • Engine can be throttled Future work • Efficiency study • Efficiency study ♦ Lower pressure possible? • Further WLS and radiation transport study 17 UNCLA SSI FIED / ^FOR OmCIA L USE ONLY UN CLA SSI FI ED//TOR. OFFICIA L USE ONLY POSITRON ENERGY CONVERSION FOR ONBOA RD POWER R esearch w as conducted on positron utilization in a standalone, closed-loop, high-pow er system .42 A Brayton cycle engine (Figure 14) w as investigated w ith output pow er of 100 kW, consistent w ith Mars R eference Mission specifications.43'44 R esults show efficiencies of 25-30 percent and positron consum ption of 7 pg/hour. Such a pow er system w ould have practical m eaning for fast transits to Mars w here positron consum ption does not dom inate rocket positron consum ption. Regenerator Radiator Compressor Work (A lternator) Figure 14. Closed Brayton Cycle Using Positron A nnihilation (courtesy Positronics Research LLC)45 Heater­ Attenuator Turbine In addition, a sm all, 110-w att, positron-driven generator (Figure 15) for sm all, onboard tasks w as designed around the N A SA Glenn R esearch C enter Stirling R adioisotope Generator,46 w ith heat provided by Ps gam m a rays. Figure 15. Conceptual 110-Watt Positron Closed-Cycle Generator Based on the NA SA Glenn Research Center Stirling Radioisotope Generator47 (courtesy Positronics Research LLC)48 18 UNCLA SSI FIED / /FOR Om^A L 7» ONI v UNCLASSI FI ED//FOR OFFICIAL WE ONLY Positrons for a M anned M ars M ission Positron propulsion system s im prove engine perform ance, m aking them an attractive substitute for chem ical and nuclear system s for m anned exploration of the planets. One of the boldest challenges is a m anned m ission to Mars. Onboard propellant requires an overall interplanetary system m ass 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 -im pulse interplanetary trajectories to reduce propellant m ass. Missions m ust be established that can transport astronauts to Mars in less than 180 days. D em ands on a positron engine to get from LEO to Mars are based on tw o param eters: m ass of the spacecraft after burnout and the A V provided by orbital m echanics. Efforts to m inim ize burnout m ass for a positron-based rocket spacecraft prom pted exam ination of previously designed system s. The N A SA Mars Exploration Study Team studied such system s in 19 9 7-9 8.49 - 50 C onclusions reached by N A SA and adopted for this study include: • To m ake the Mars m ission econom ically feasible, m ultiple payloads should be launched to Mars instead of a single, "all-in-one" vehicle. This keeps payload m asses w ithin reach of existing chem ical launch system s. • A solid-core nuclear-therm al rocket (N TR ) w as studied. The study adopted existing N ER VA rockets w ith Isp = 9 00 seconds and a core tem perature near 2,800 °C . The 19 9 3 study exam ined 15 kilopound-force klbf and 20 klbf rockets.51 • Each launch had a payload consisting of the N TR w ith its Mars payload. • Unpiloted cargo w as sent on a low -energy ("C 3") Hohm ann-type transfer, generally the slow est m eans of reaching Mars. • The Mars excursion vehicle should be sent on a "fast transit" to Mars from LEO. A fast, 180-day m ission w ould not require artificial gravity on the spacecraft to protect astronauts from w eightlessness. • The Earth return vehicle (ER V) sits in Mars orbit at 250-km periapsis and w aits until astronauts have docked from Mars using a liquid oxygen (LOX)/m ethane propulsion system . The ER V uses a chem ical propulsion system to return hom e to avoid use of a fission-based propulsion plant in the atm osphere. • Minim ization of A V to Mars is perform ed by launching during estim ated planetary conjunctions (every 778 days) and by using aerobraking. • A erobraking uses the chem ical propulsion system of the cargo vessel or lander. Payload is jettisoned from the N TR system (called the trans-Mars insertion system [TMI]) som etim e during the trip to Mars. • To reduce the probability of im pact w ith Earth, an additional A V is given to the TMI stage after the pay load has separated. 19 UNCLASSI FIED//EOH OEHHAb USE ONEY UNCLA SSIFIED//rOR OFFICIA L USE ONLY Benefits of using positrons for a Mars m ission include: • The "disposable A V" used to propel TMI stages into low -probability Earth or Mars intercepts can be elim inated, reducing total propellant m ass. • R eduction in shielding and engine m ass give low er initial m ass low Earth orbit for launch vehicles or faster transits for piloted m issions. • The ER V uses a positron engine instead of LOX/C H4. This gives significant m ass savings or an equivalent reduction in Mars-to-Earth return tim e for astronauts. • The im provem ent in Isp translates to either a reduced launch payload m ass for cargo m issions or reduced transit tim es for piloted m issions to Mars. • More chem ical propellant can be stored on the lander to im prove aerobraking or landing strategies that reduce hazards for astronauts. Launch dates are set for around 2030. A ssum ing m inim um A V for Mars opposition-class m issions, interplanetary scenarios are illustrated in Figure 16 . The A V for an insertion trajectory into Mars for the m anned m ission (Figure 16 b) is A V = 3.7 km /sec. Each m anned trajectory assum es a 180-day transit tim e. Figures 16. M ars Trajectories (X-coordinates defined in direction of A ries): (a) 2029 cargo m ission; (b) 2031 m anned lander to Mars; (c) 2033 m anned return to Earth; (d) 2035 m anned lander to Mars, if necessary (courtesy Positronics R esearch LLC )52 20 UNCLA SSI FIED //FOR OFFICIA L USE ONLY UNCLA SSIFIED//TOR UFF1UA L U3E ONLY The Mars reference m ission53'54 considered payload m asses of 6 0,000 kg for 2015 m issions. This can be reduced to 45,000 kg assum ing technological advances by 2031 w ith a com plete interplanetary spacecraft m ass of 9 0,000 kg. In sum m ary, the m ission scenario for a positron spacecraft is sim ilar to that for existing studies, but w ith the use of less costly launch vehicles. Every 778 days, tw o 45,000-kg payloads are launched from Earth using a Saturn V or equivalent chem ical rocket. One payload is the unm anned system or m anned crew lander sent to Mars; the other contains the positron propulsion system and the propellant tank. They are assem bled as a com plete unit in LEO. Unm anned system s are launched in advance of the crew ed system in order to ensure that the Martian habitat is w ell established. The crew arrives at Mars in late 2033, perform s research for 1 year, and then returns hom e in a sm aller positron spacecraft using a shorter trajectory. A rtists' renditions of tw o possible positron spaceships previously described in this study are show n in Figure 17. (a) (b) Figure 17. Spacecraft Using Positron Engines, (a) Solid-core system enters Mars orbit; (b) Modified Sanger photon rocket system burns for landing on Mars (courtesy Positronics R esearch LLC ).55 A rchitectures for Mars exploration using a positron SSR V are sum m arized below : • Before hum ans leave for Mars on initial flights, cargo ships precede them to Mars on low -energy trajectories to take the com ponents of a Mars space station (MSS) and necessary supplies, including a Mars surface lander (MSL). The MSS w ill be sim ilar to an Earth space station (ESS). The cargo ships w ill utilize positron rocket engines. • Manned positron SSR Vs launched from Earth rendezvous in LEO w ith the ESS. The SSR V is a horizontal-takeoff, horizontal-landing w inged-body, m anned vehicle in w hich the first stages of flight use air-breathing engines w ith positrons heating the air. It sw itches to the rocket engine to com plete the final ascent phase to LEO. • Once ready for interplanetary flights at the ESS, including refueling, the SSR V flies to Mars on a fast, high-energy trajectory, carrying a crew of five or six astronauts and pow ered by positron rocket engines. The SSR V conducts a rendezvous w ith the 21 UNCLA SSI FIED//EQB QFHHAh USE ONLY UNCLA SSIFIED//FQR QrriCIA L UDE ONL^ 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 w orldw ide for basic and applications research. For exam ple, the positron-em itting radioisotope N a22 (2.7 year m ean lifetim e) is m ade by bom barding targets w ith neutrons from a high-energy proton accelerator in the reaction A I27(n,x)N a22. C apture of these positrons is used to form beam s w ith keV (slow ) to MeV (fast) energies. Handling of large radioactive sources results in lim its of 106 slow positrons/sec. For intensities up to 1010/sec, bom bardm ent of m etal targets w ith electron beam s in the 10- to 100-MeV range is used, follow ed by collection and acceleration (deceleration) of positrons to form fast or slow beam s. In addition, it has recently been show n that slow positron beam s of up to lO^ /sec can be realized by converting neutrons in reactors to electron-positron pairs in thin m etal foils. Much higher positron currents are being sought in a variety of proposed solutions. Illustrated below are a few of the m ore prom ising concepts. First, in 19 9 6 , the U.S. N aval R esearch Laboratory56 proposed developing an intense source of fast positrons (1016 /sec) utilizing com pact electron betatron accelerators. Second, tabletop fem tosecond laser-driven positron sources currently under developm ent at the N ational Ignition Facility (Law rence Liverm ore N ational Laboratory), the R utherford-A ppleton Laboratory (United Kingdom ), and the Max Planck Institute (Munich) look prom ising, although m ore m ust be done to dem onstrate efficient collection of positrons into beam s. Finally, a m ost im portant step forw ard is m ulti-gigaelectronvolt (GeV) energy electron storage rings being developed for the high-energy physics International Linear C ollider (ILC ) project that uses undulators in electron beam s to create intense photon beam s that produce intense (1014'16 /sec) positron beam s by pair production.57 A schem atic draw ing from one proposal for the ILC is show n in Figure 18.58 Layout of ILC P ositron S ource Figure 18. Proposed Undulator-Based Positron Source for the International Linear Collider (courtesy KEK, Japan)59 22 UNCLA SSI FIED/ / EQB ClFHHAh USE ONh¥ UNCLA SSIFIED//EQR OFFICIA L USE ONW A ssum ing that 1016 positrons/sec can be realized in the next 10 years, then 150 m icrogram s could be produced in 6 m onths to enable a globe-encircling flight of a sm all air-breathing turbojet UA V as discussed earlier. Positron Costs A n independent study has been done to determ ine future costs of positrons and, for com parison, antiprotons as w ell. The results, show n in Table 6 , are based on data for existing sources and proposals for future sources.6 0'6 1'6 2 Table 6. Positron and A ntiproton Expected Costs in the Next 10 Years Source Trap Injection Energy (MeV) Filling Rate (sec1) When $/JOULE (annihilation) CERN AD (pbar)25 0.01 -0.1 4x 105 Now ? Fermilab (pbar/b <0.002 2.8 x 104 Now 333* e+/14 MeV e linac2//ILC24 0.1 5x1012/1016 2011/19 0.4/0.004** * $100 m illion/year (est. op. cost). ** $5 m illion/year (est. op. cost, adjusted for inflation)/$100 m illion/year (est. op. cost). Tw o clear results of the study should be noted. First, m easured 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 tim es m ore energy per annihilation than a positron, this result appears to defy logic. How ever, the laboratory energy threshold for producing antiprotons is 6 ,000 tim es greater than for positrons, requiring a relatively com plex proton synchrotron that is costly to construct and run. In addition, antiprotons are m ade at m uch 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 system s are com paratively sim ple and less costly to operate and m aintain than proton system s. These factors, com bined w ith the absence of radioactive residue associated w ith positron annihilation, m ake positrons the obvious choice over antiprotons. Second, the cost of positrons is projected to be $0.004/J x 180 MJ/pg = $720K/pg. Hence, the cost of 1 gram is $0.72T, or 5 percent of the 2008 U.S. gross dom estic product (GD P). A 2000 N A SA study6 3 on w hich this author collaborated placed the cost of antiprotons at $6 4T/g, consistent w ith the $333/J figure in the second line of Table 5, and roughly six tim es the 2000 GD P. Unfortunately, this is still being quoted in U.S. scientific and governm ent com m unities. The dram atic reduction in the unit cost of antim atter since 2000 is due to a new em phasis on positrons by the physics com m unity, and hopefully this paper w ill help spread that good new s. Earlier, a nonstop flight around the globe by a sm all positron UA V w as described as equivalent to the 19 27 Spirit of St. Louis transatlantic flight of C harles Lindbergh. From 23 U N CLA SSI FI E D/ / EQLQEEICUUSM NM UNCLA SSIFIED//FOR OFFICIA L USE ONLY Table 5, the required 150 pg for this epic flight could be m anufactured in 6 m onths for $9 6 m illion, or 0.0007 percent of U.S. GD P. Positron Storage C onfinem ent of antim atter has been review ed extensively in the literature.6 4 Historically, the first approach w as the Penning trap.6 5 Stores of 109 positrons for 1 hour have been achieved.6 6 Electric potentials are required to overcom e space charge forces.6 7 To illustrate, confinem ent 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 (IO15). Figure 19. Penning Trap With Trapping Volume of 1,000 Cubic Centimeters (center), Injection A pparatus (left) and Controls (right) (courtesy Positronics Research LLC)68 In addition, w ith a m agnetic field there are m agnetic energy density restrictions on confinem ent of positron plasm as. The Brillouin D ensity Lim it is: 2 n = £ B /2m 8 o e (1) For a practical m agnetic field of 1 Tesla, ne = 9 .7 x 1012/cubic centim eters. In a 10-cm radius sphere, the Brillouin N um ber Lim it is 4 x 1016 (40 picogram s). Therefore, by 24 UNCLA SSI FIED //FOR OFFICIA L USE ONLY UNCLA SSIFIED//rOR OFFICIA L USE ONLY either space charge or m agnetic energy considerations, the storage lim it is tens of picogram s, 7-8 orders of m agnitude short of 100 m icrogram s, w here practical uses of positrons begin to em erge, as illustrated earlier. The second approach is confinem ent of neutral Ps atom s in m anufactured porous m edia of either regular lattices of atom s, such as polym ers, or irregular strands of insulator m aterial encapsulating voids, such as silica aerogel.6 9 '70 R egardless of void size, Ps atom s ultim ately annihilate w ith electrons attached to atom s on the boundaries of voids by the so-called "pickoff" process. Therefore, large, observable lifetim es require m aterials w ith extraordinarily large voids. FORM A TION OF POSITRONIUM IN POROUS M EDIA Positrons are injected into a porous m aterial at low energy (~ 100 keV) to ensure that they stop and form a Ps atom over a distance of a few m illim eters.71 The positron rapidly loses its energy by collisions w ith electrons attached to atom s in the m aterial. A s it nears 6 .8 electronvolts (eV)— the binding energy of the ground state of Ps— it captures a w eakly bound electron and form s Ps. It diffuses through the m aterial, and over about 1 nanosecond, its energy is rendered to the room tem perature of the m aterial, 0.025 eV. This is called therm alization. The quantum m echanical m odel of Ps is rem arkably sim ilar to the hydrogen atom . The m ajor difference is that Ps spontaneously annihilates, w hereas hydrogen is stable. The "self-annihilation" of Ps due to overlap of electron and Figure 20. A Positron Forms Ps on the Edge of a Void, Thermalizes, and Becomes Trapped in a Void Before A nnihilating (courtesy University of M ichigan) positron w ave functions results in extrem ely short lifetim es, as noted earlier. Lifetim es against "self-annihilation" can be dem onstrably increased if the follow ing tw o conditions are m et: (1) a w ay is found to isolate the electron w ave function from the positron w ave function, and (2) m aterials provide voids large enough to allow detection of lifetim es w ell beyond 142 nanoseconds (ns). A high vacuum is required to avoid Ps annihilation on gas m olecules w ithin the voids. The follow ing describes how Positronics R esearch LLC has approached these issues in the laboratory.72 LONG-TERM STORA GE OF POSITRONIUM Under crossed m agnetic and electric fields, Ps assum es a doubly oblate shape (Figure 21), w ith the electron and positron separated by hundreds of nanom eters to tens of m icrom eters, depending on the size of the fields.73 C om putation of lifetim es against quantum m echanical barrier penetration reveals lifetim es in excess of 1 year over a large range of m agnetic and electric fields. 25 UNCLA SSI FIED //FOR OFFICIAL USE ONLY UNCLA SSI FI ED//F8R OFFICIA L USE ONLY Lifetim e shortening ow ing to cyclotron radiation of the electron and positron gyrating in the m agnetic field are not included in this m odel. Because this process is proportional to B 2, m agnetic fields should be sm all— less than 0.1 Tesla based on our com putations of the effect. This, in turn, renders the atom very large, w ith up to 1-m icrom eter elongation. If this can be verified in the laboratory, the first of the tw o conditions laid out in the previous section w ill be satisfied. N ext, it is im portant to identify a storage m edium w ith the largest possible voids. Silica aerogel is a prom ising m aterial that w as developed by N A SA because of its extrem ely low density and excellent therm al insulating properties. It is com posed 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 w ithin an irregular lattice of silica strands (Figure 22). Silica aerogel is X(a.U.> Figure 21. Computer Simulation of Rs A toms in a 5-Tesla M agnetic Field and 100 V/cm Electric Field (1 a.u. = 0.052 nm) (courtesy University of Bielefeld, Germany)74 available com m ercially w ith typically 20-nanom eter average voids. R ecent research has produced silica aerogel w ith up to 1-m icrom eter void sizes. Experim ents in Japan and at Positronics R esearch LLC 33 w ith low -energy positrons in silica aerogel show a high efficiency (~ 35 percent) for m aking Ps through the interaction of the positron w ith silica grains. High radiation exposure from positrons im planted in the m aterial result in it becom ing "param agnetic," perm anently at low tem peratures, w ith a high density of "dangling bonds" containing very loosely bound electrons that explains the high efficiency for Ps form ation. It therefore serves a dual role as source and storage m edium for Ps. What lifetim es m ight be expected w orking w ith this m aterial? Figure 22. TEM of Silica A erogel (courtesy Lawrence Berkeley Laboratory) The Ps decay rate in a porous m aterial is given by:75 A = k'/(R - r') + A T + A q (2) 26 UNCLA SSI FIED//FOR OFFICIAL USE ONLY UNCLA SSI FI ED//rOR OmCIA L UOE ONLY w here k' is characteristic of the electron density for silica strands = 0.016 4 nm -ns1, R is the void radius (nm ), r' is the silica strand thickness (0.6 8 nm ), A t is the self* annihilation rate, and A q is the quenching rate for air (0.00427 ns1 at 0.1 torr vacuum , typical of a standard rotary pum p). To illustrate, w ithout m agnetic and electric fields (A t = 1/142 ns = 0.007 ns’1) and for R = 20 nm , A = 0.0121 ns'1, or r = A ’1 = 82.5 ns. This is consistent w ith our m easurem ents.76 A ssum ing self-annihilation is suppressed (A t ~ 0) by crossed m agnetic and electric fields, a high vacuum (IO-5 torr) is m aintained (A q ~ 0) and R = 20 nm , then A = 8.5 x 10*4 ns'1, or t = 1.2 ps. Proprietary experim ents at Positronics R esearch LLC show that Ps atom s in crossed m agnetic and electric fields in silica aerogel w ith 20-nm voids live up to 10 ps. This is som ew hat longer than the predicted lifetim e ow ing to severe radiation dam age induced in the silica aerogel by positrons that alters k' from the values quoted above. Measured lifetim es tn the present experim ents at Positronics R esearch LLC are w ithin lim its set by diffusion to the trap w alls. This is consistent w ith the expectation that Ps atom s "stabilized" in crossed m agnetic and electric fields w ill be "delocalized" and drift freely across m agnetic field lines. Future experim ents at Positronics R esearch LLC w ill use super-dilute m edia (R = 1,000 nm ), large container volum es (10 cm ), w eak m agnetic fields (< 0.1 Tesla), and electric fields. Substitution of R = 1,000 nm into Equation 2, assum ing again a high vacuum and crossed m agnetic and electric field suppression of self-annihilation, predicts a lifetim e of 6 1 m illiseconds. A ssum ing the follow ing results are consistent w ith this prediction, it w ill be dem onstrated beyond any reasonable doubt that Ps can be stabilized against "self-annihilation" in crossed fields. To ultim ately reach lifetim es of m onths and years required by aerospace propulsion, oscillating electric-gradient-field-confinem ent forces w ill be required to keep Ps atom s off the w alls of a high-vacuum trap. Ps atom s w ill be produced by a beam of low -energy positrons intercepting silica aerogel and be stabilized in the trap using crossed m agnetic and electric fields. Ps in crossed m agnetic and electric fields has an enorm ous electric dipole m om ent, and confinem ent using the classical p«VE force looks very encouraging at this tim e. Conclusions C onceptual designs and m issions for turbojets, turbo-ram jet m issiles, and interplanetary rockets pow ered by positron annihilation have been presented. Positron requirem ents range from 150 m icrogram s for a globe-encircling UA V turbojet flight to 100 m illigram s for a m ission to Mars. A positron-pow ered SSR V could take off from Earth horizontally, go to LEO, launch to Mars for a 1-year exploration of the R ed Planet, and return to LEO and then Earth w ith a horizontal landing w ithout refueling. Within 10 years, the 150 pg of positrons required for a globe-encircling, nonstop turbojet flight could be m ade in 6 m onths at a cost of $6 9 m illion. This first-ever antim atter voyage, approxim ately 9 0 years after Lindbergh's 19 27 Spirit of St. Louis transatlantic flight, w ould stir the public's im agination and eventually lead to positron- pow ered exploration of the solar system in the 21st century. 27 UNCLA SSI FI ED//EQR QFHGIA L USE ONLY UNCLA SSIFIED//FOR OFFICIA L USE ONEY N ew developm ents in stabilizing and storing positronium atom s in a m atrix of dilute m aterials are encouraging. Lifetim es of 10 ps have been achieved, and tens-of- m illisecond lifetim es are expected in the next round of experim ents. Present lim its are due to interactions on the w alls of the trap container. A pplication of oscillating gradient electric fields to the huge electric dipole m om ent of the stable Ps atom should m itigate this problem . With the issue of stabilization in crossed fields now settled, very long lifetim es are but a m atter of engineering! 1 P. A . M. D irac, Proc. R oy. Soc. A .126 , 36 0 (19 3 0). 2 C . D . A nderson, Phys. R ev. 43, 49 1 (19 33). 3 0. C ham berlain, E. Segre, C , Wiegand and T. Ypsilantis, Phys. R ev. 100, 9 47 (19 55). 4 G. Luders, A nn. Phys. 2, 1-15 (19 57). 5 P. Hill and C . Peterson, Mechanics of Therm odynam ics of Propulsion, 2nd ed., A ddison - Wesley Publishing C o. (19 9 2). 6 G. Herken, "The Flying C row bar", A ir and Space Magazine, Vol. 5 (# 1), p. 28, A pril/May (19 9 0) ( uto.htm l).http://w w w .m erkle.com /pluto/pl 7 R . W. Bussard and R . D . D eLauer, Fundam entals of N uclear Flight, McGraw -Hill Book C o. (19 6 5) 8 G. A . Sm ith et al., "R evolutionary Positron C onversion", Final Technical R eport, A FR L C ontract F086 30-00-C - 0010, Eglin A FB, FL, March (2002). 9 O. C ham berlain, E. Segre, C . Wiegand and T. Ypsilantis, Phys. R ev. 100, 9 47 (19 55) 10 G. Luders, A nn. Phys. 2, 1-15 (19 57). 11 G. A . Sm ith et al., "R evolutionary Positron C onversion", Final Technical R eport, A FR L C ontract F086 30-00-C - 0010, Eglin A FB, FL, March (2002). 12 Ibid. 13 Ibid. 14 Ibid. 15 Ibid. 16 Ibid. 17 Ibid. 18 Ibid. 19 N orthrop-Grum m an C orp: R eport to Kaiser-Marquardt for HTHL blended-body SSTO engine, "Vision Vehicle Final R eport," A pril 30 (19 9 8). 20 G. A . Sm ith et al., "A R evolutionary Positron Based SSR V Vehicle for A pplication to Hum an Exploration and D evelopm ent of Space", The A dvanced Space Propulsion Workshop, N A SA Marshall Space Flight C enter, Huntsville, A L, A pril 2-6 (2001). 21 N orthrop-Grum m an C orp: R eport to Kaiser-Marquardt for HTHL blended-body SSTO engine, "Vision Vehicle Final R eport," A pril 30 (19 9 8). 22 G. A . Sm ith et al., "A R evolutionary Positron Based SSR V Vehicle for A pplication to Hum an Exploration and D evelopm ent of Space", The A dvanced Space Propulsion Workshop, N A SA Marshall Space Flight C enter, Huntsville, A L, A pril 2-6 (2001). 23 Ibid. 24 Ibid. 25 Ibid. 26 Ibid. 27 G. A . Sm ith, "Positron Propelled and Pow ered Space Transport Vehicle for Planetary Missions", N IA C Phase I Final R eport, R esearch Subaw ard N o. 076 05-003-048, Septem ber 1, 2005 - March 31, 2006 . 28 Ibid. 29 D . R . Koenig, "Experience Gained from the Space N uclear R ocket Program (R over)," LA -1006 2-H, Los A lam os N ational Laboratory, Los A lam os, N M., May (19 86 ). 30 G. A . Sm ith, "Positron Propelled and Pow ered Space Transport Vehicle for Planetary Missions", N IA C Phase I Final R eport, R esearch Subaw ard N o. 0 76 05-003-048, Septem ber 1, 2 005 - March 31, 2006 . 31 L. E. Thode et al., J. Propulsion and Pow er 14, 4 (19 9 8). 32 D . I. Poston and T. Kam m ash, N uclear Science and Engineering 122, 32 (19 9 6 ). 28 UNCLA SSI FIED//EQB QEEIMAh USE ONLY UNCLA SSIFIED//TOR OFFICIA L USE ONLY 33 S. K. Borow ski et al., "N uclear Therm al R ocket/Vehide D esign Options for Future N A SA Missions to the Moon and Mars/' A IA A -9 3-4170 (N A SA Tech Mem orandum 107 071), (19 9 3). 34 D . I. Poston and T. Kam m ash, N uclear Science and Engineering 122, 32 (19 9 6 ). 35 S. K. Borow ski et al., "N uclear Therm al R ocket/Vehide D esign Options for Future N A SA Missions to the Moon and Mars/' A IA A -9 3-4170 (N A SA Tech Mem orandum 107 071), (19 9 3). 36 G. A . Sm ith, "Positron Propelled and Pow ered Space Transport Vehicle for Planetary Missions", N IA C Phase I Final R eport, R esearch Subaw ard N o. 076 05-003-048, Septem ber 1, 2005 - March 31, 2006 . 37 E. Sanger, Ing. A rch. 21, 213 (19 53). 38 G. A . Sm ith, "Positron Propelled and Pow ered Space Transport Vehicle for Planetary Missions", N IA C Phase I Final R eport, R esearch Subaw ard N o. 076 05-003-048, Septem ber 1, 2005 - March 31, 2006 . 39 Ibid. 40 G. Gaidos et al., "A ntiproton-C atalyzed Microfission/fusion Propulsion System s for Exploration of the Outer Solar System and Beyond," A IA A -9 8-3 589 , Presented at the 3 4th A IA A /A SME/SA E/A SEE Joint Propulsion C onference & Exhibit (19 9 8). 41 W. Lance Werthm an, "A ntiproton-C atalyzed Microfission/fusion Space Propulsion," MS Thesis, D ept, of A erospace Engineering, Penn State University (19 9 5). 42 G. A . Sm ith, "High D ensity Storage of A ntim atter", A dvanced High Energy Storage C onference, MITR E C orp., McLean, VA , A ug. 1 (2005). 43 S. J. Hoffm an and D . I. Kaplan, eds. "Hum an Exploration of Mars: The R eference Mission of the N A SA Mars Exploration Study Team ," N A SA Special Publication 6 107, JSC (19 9 7). 44 B. G. D rake, ed. "R eference Mission Version 3.0: A ddendum to the Hum an Exploration of Mars: The R eference Mission of the N A SA Mars Exploration Study Team /' Hum anExplore/ Exploration/ EXLibrary/docs/ MarsR ef/addendum /index.htm , June (19 9 8). http://ares.jsc.nasa.gov/ 45 G. A . Sm ith, "High D ensity Storage of A ntim atter", A dvanced High Energy Storage C onference, MITR E C orp., McLean, VA , A ug. 1 (2005). 46 J. D ion, "Stirling R adioisotope Generator", N A SA Glenn R esearch C enter, ME 388R .2, Spring (200 5). 47 Ibid. 48 G. A . Sm ith, "High D ensity Storage of A ntim atter", A dvanced High Energy Storage C onference, MITR E C orp., McLean, VA , A ug. 1 (2005). 49 S. J. Hoffm an and D . I. Kaplan, eds. "Hum an Exploration of Mars: The R eference Mission of the N A SA Mars Exploration Study Team ," N A SA Special Publication 6 107, JSC (19 9 7). 50 B. G. D rake, ed. "R eference Mission Version 3.0: A ddendum to the Hum an Exploration of Mars: The R eference Mission of the N A SA Mars Exploration Study Team ," Hum anExplore/ Exploration/ EXLibrary/docs/ MarsR ef/addendum /index.htm , June (19 9 8). http://ares.jsc.nasa.gov/ 51 S. K. Borow ski et al., "N uclear Therm al R ocket/Vehide D esign Options for Future N A SA Missions to the Moon and Mars," A IA A -9 3-4170 (N A SA Tech Mem orandum 107071), (19 9 3). 52 G. A . Sm ith, "Positron Propelled and Pow ered Space Transport Vehicle for Planetary Missions", N IA C Phase I Final R eport, R esearch Subaw ard N o. 076 05-003-048, Septem ber 1, 2005 - March 31, 2006 . 53 S. J. Hoffm an and D . I. Kaplan, eds. "Hum an Exploration of Mars: The R eference Mission of the N A SA Mars Exploration Study Team ," N A SA Special Publication 6 107, JSC (19 9 7). 54 B. G. D rake, ed. "R eference Mission Version 3.0: A ddendum to the Hum an Exploration of Mars: The R eference Mission of the N A SA Mars Exploration Study Team ," Hum anExplore/ Exploration/ EXLibrary/docs/ MarsR ef/addendum /index.htm , June (19 9 8). http://aresjsc.nasa.gov/ 55 G. A . Sm ith, "Positron Propelled and Pow ered Space Transport Vehicle for Planetary Missions", N IA C Phase I Final R eport, R esearch Subaw ard N o. 076 05-003-048, Septem ber 1, 2005 - March 31, 2006 . 56 C .A . Kapatanakos, J. Synchrotron R ad. 3, 26 8-271 (19 9 6 ). 57 ILC undulator-based source, . ILC capitalization is estim ated at $9 B (see ). w w w .ippp.dur.ac.uk/~ gudrid/source/BC D -source w w w .linearcollider.org 58 Ibid. 59 Ibid. 6 0 R . Landua, C ER N , "Precision Experim ents w ith A ntiprotons" Feb. 24 (2003). 6 1 S. How e et al., A IP C onf. Proc. 746 , 520 (200 5). 6 2 14 MeV Electron Linac @ $5M capitalization cost and $5M/yr operating cost (A . Herer et al., "A pplications of High Voltage High Pow ered Electron Beam s", IBA , Belgium , 19 9 7). 6 3 G. R . Schm idt et al., J. Propulsion and Pow er, 16 , 9 23 (2000). 6 4 J. R ejcek etal., R ad. Phys. C hern. 6 8, 6 55 (2003). 6 5 G. A . Sm ith, "High D ensity Storage of A ntim atter", A dvanced High Energy Storage C onference, MITR E C orp., McLean, VA , A ug. 1 (2005). 6 6 C . M. Surko and R . G. Greaves, Phys. Plasm as 11, 2333 (2004). 29 UNCLA SSI FI Fn//yp ^"rm ||CFHM IY UNCLA SSIFIED//FOA OFFICIA L USE ONLY 6 7 G. A . Sm ith, "Long-Term C onfinem ent of D ense Positron Plasm as", Final Technical R eport, A FR L C ontract F086 30-02-C -0017, Eglin A FB, FL, January 23 (2007). 6 8 Ibid. 6 9 G. A . Sm ith, "Positron Energy C onversion (PEC )", 53th JA N N A F Propulsion Meeting, Subcom m ittee on Future Technologies for Spacecraft Propulsion (2005 0356 EA ), C PIA C JSC C D 43 D ecem ber (2005). 70 G. A . Sm ith, "Stabilization and Long Term C onfinem ent of A tom ic Positronium ", Final Technical R eport, A FR L C ontract F086 30-02-C -0018, Eglin A FB, FL, January 23 (2007). 71 S. J. Hoffm an and D . I. Kaplan, eds. "Hum an Exploration of Mars: The R eference Mission of the N A SA Mars Exploration Study Team ," N A SA Special Publication 6 107, JSC (19 9 7). 72 G. A . Sm ith, "Stabilization and Long Term C onfinem ent of A tom ic Positronium ", Final Technical R eport, A FR L C ontract F0 86 30-02-C -0018, Eglin A FB, FL, January 23 (2007). 73 J. Lu, "C lassical Trajectory Monte C arlo Sim ulation of Ion-R ydberg A tom C ollisions", Ph. D . D issertation, University of Bielefeld, Germ any, May (2003). 74 Ibid. 75 K. Sudarshan et aL, J, Phys: C ondens. Matter 19 , 386 (2 007). 76 G. A . Sm ith, "Stabilization and Long Term C onfinem ent of A tom ic Positronium ", Final Technical R eport, A FR L C ontract F086 30-02-C -0018, Eglin A FB, FL, January 23 (2007). 30 UNCLA SSIFIED//FOR OFFICIA L USE ONLY