Non-thermal beam-to-propellant energy conversion lacks a satisfactory solution

Develop an effective non-thermal beam–plasma coupling mechanism that transfers the kinetic energy of a relativistic electron beam to propellant exhaust with minimal structural heating, suitable for beam-driven rockets targeting high exhaust velocities and high specific power.

Background

For beam-driven rockets, almost all beam energy is kinetic and should be transferred to propellant as exhaust work rather than momentum reflection. The paper discusses potential approaches that excite strong plasma waves (e.g., wakefield excitation in a waveguide, plasma-filled backward-wave oscillators, relativistic klystrons), but emphasizes that essentially all throughput power must couple to the exhaust or be emitted, with negligible deposition into the structure to avoid intolerable heat loads.

Despite outlining candidate mechanisms, the authors state that they currently lack a satisfactory solution meeting these stringent efficiency and thermal constraints.