Previously we looked at a proposal for a laser enabled megawatt class nuclear fusion space propulsion system for about 2020-2025 Now the full eight page paper by John J. Chapman, NASA, Langley Research Center is online.
The preferred fuel for space is the p-boron reaction: p + 11B → 3 4He + (8.7 MeV)
11Boron is an abundant, inexpensive fuel stock and has the major advantage of a clean
fuel for the primary fusion reaction, at the expense of reduced plant gain since currently p-11B energy gain estimates range from ~5 to 15. Previously the parameters for harnessing aneutronic ( p-11B ) fusion had appeared significantly more exigent than for conventional (D-T) fusion fuel cycle. Yet recent scientific studies and engineering lab efforts applicable to micro-scale triggering of advanced fusion fuel solid targets has introduced new concepts with unique aspects that include demonstrations of attainable aneutronic reactions. The successful application of aneutronic methodology to fusion propulsion plants for space deployment will ultimately offer the possibility of enhanced performance from nuclear gain as compared to existing chemical and even ion propulsive engines, while also providing a clean solution to planetary protection, Earth included considerations and requirements. Proton-triggered 11boron fuel (p-11B) offers the potential for abundant ion kinetic energy for in-space vectored thrust applications as well as for direct energy conversion in specialized direct electrical energy conversion plants.
The momentum of the energetic alpha particles provides clean, high ISP ~900,000 as vectored thrust such that p-11B offers a clean fuel with well understood reaction kinematics. Triggering of the favored predictive yields from the p-11B reaction is optimized at two resonant energy levels corresponding to ~163 keV and ~560 KeV (p-11B center of mass).
These ionic energy levels are readily attainable via high contrast ratio pulsed laser triggering (TNSA) of selectively layered targets. Photon impact excitation of ionic species (over 1 MeV) is achieved in the target plane resulting from picosecond duration, high contrast ratio Chirped Pulse Amplified (CPA) laser abrupt wavefront target-normal impingement. The photonically-induced ion acceleration process which occurs at the target material focal point has been demonstrated to spawn fusion reactions in the adjacent composite.
Power efficiency entails the aspects of reaction triggering and associated power loss burdens such as energy spent in lasing medium excitation, ionization and target level internal losses. The quantum efficiency of (Fig.3) CPA laser systems capable of producing high contrast ratio picosecond pulses is below 1%, however for a rocket thrust application (as contrasted with a power production plant) the [symbol] e ratio can be less than 100%. The incremental thrust from a laser triggered p-11B target, assuming ~105 Alphas from a single laser pulse has been estimated to yield a few pico-Newton impulse per laser pulse from a 10 micron square target area. High pulse rate laser systems coupled with multiple square centimeters of active target area could effectively augment the effective thrust level towards Newton magnitude levels, particularly in conjunction with increased alpha yields from optimized target designs. Recent advances in laser technology indicate possibility of higher laser quantum efficiencies (over 25%) and higher femtosecond pulse train rates (~75MhZ). Bremstrahlung radiation and non-productive plasma also result in losses as well as particle collisions with the structure represent additional power losses from the propellant exhaust stream.
Future development and the availability of high efficiency short pulse laser systems may result in overall gains in [symbol] and ISP values that may make the A-LIFT (Aneutronic Laser Induced Fusion Thruster) offering ISP ~900,000 approach an attractive alternative to previous fusion ~1-10 kW/kg or ionic (ISP range from 2000 – 100,000) propulsion for In-Space thrust applications.