Latest Talk by Chapman on Laser pulse fusion propulsion

Last year, John Chapman of NASA proposed a pulsed laser system for megawatt class fusion propulsion.

In Chapman’s aneutronic fusion reactor scheme, a commercially available benchtop laser starts the reaction. A beam with energy on the order of 2 x 10^18 watts per square centimeter, pulse frequencies up to 75 megahertz, and wavelengths between 1 and 10 micrometers is aimed at a two-layer, 20-centimeter-diameter target.

In the present study : The fusion reaction is spawned by a burst of Terawatt laser accelerated protons accelerated via Target Normal Sheath Acceleration (TNSA) from the substrate and hydrogen content to liberate ~180 to 575 KeV Protons which react with the target.

Thrust is realized via Lorentz reaction of electromagnetic forces coupled to the spacecraft frame

Advanced Fusion Reactors for Space Propulsion and Power Systems (8 pages)

In recent years the methodology proposed for conversion of light elements into energy via fusion has made steady progress. Scientific studies and engineering efforts in advanced fusion systems designs have introduced some new concepts with unique aspects including consideration of Aneutronic fuels. The plant parameters for harnessing aneutronic fusion appear more exigent than those required for the conventional fusion fuel cycle. However aneutronic fusion propulsion plants for Space deployment will ultimately offer the possibility of enhanced performance from nuclear gain as compared to existing ionic engines as well as providing a clean solution to Planetary Protection considerations and requirements. Proton triggered 11Boron fuel (p- 11B) will produce abundant ion kinetic energy for In-Space vectored thrust. Thus energetic alpha particles “exhaust “ momentum can be used directly to produce high ISP thrust and also offer possibility of power conversion into electricity. p- 11B is an advanced fusion plant fuel with well understood reaction kinematics but will require some new conceptual thinking as to the most effective implementation.

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. The p-11B reaction is typically triggered via a pumped and pinched plasma process with excited ions using hydrogen-boron fuel. Alternative means of triggering the p-11B reaction may provide a lower overall propulsion plant size and mass.

The Lawson triple product for the p-11B reaction regarding plasma temperature-plasma density-confinement time can be met by Target Normal Sheath Acceleration (TNSA) of “trigger” ions using compact and lightweight hardware. The theoretical principles pertaining to this principle have been recently outlined as the Lawson-Woodward Theorem. 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.

The incremental thrust from a laser triggered p-11B target, assuming ~10^5 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. Power is also lost in transverse momentum resulting in exhaust stream spreading.

Future development and the availability of high efficiency short pulse laser systems may result in overall gains 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.

Direct conversion of Energy:

An important related aspect making aneutronic fusion reactions amenable to space missions is that most of the energy is released in the form of charged particle kinetic energy. This kinetic energy can be converted directly into electricity via conversion techniques based on mature technology derived from other fields, such as microwave technology, resonance-tuned rectennae which entails equipment that is more compact and potentially cheaper than that involved in conventional thermal production of electricity. Various fuels D-T, D-3He, p-11B and 3He-3He have also been investigated and compared for favorable energy yields in the IEC reactor.

Potential Impact

Ultimately advances in clean fusion plant technology using aneutronic fuels offer the potential of far reaching impact upon next generation space missions. Such new technologies could be utilized in satellite station keeping, in the exploration of planets and asteroids and in terrestrial energy applications. Energetic alpha particle “exhaust” momentum can be used directly to produce high ISP thrust and also offers the possibility of direct power conversion into electricity. p-11B is an advanced fusion plant fuel with well understood reaction kinematics but still requires implementation of new conceptual thinking for optimal implementation. Development of aneutronic fusion plants should also be pursued to secure potentially more energetic and abundant sources of clean, sustainable, high density energy, with applications for powering extended space exploration missions. Over recent decades the methodology for effective harnessing of propulsive energy via fusion has made steady progress. But scientific studies and engineering efforts in advanced fusion systems design and development have lately introduced new concepts (i.e. pulsed laser triggering) each with unique aspects and characteristics which offer new oportunity for clean in-space propulsion and power generation.

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