Laser Accelerated Heavy-ion Plasma Propulsion System

The Ultrafast “Laser Accelerated Plasma Propulsion System” LAPPS makes use of lasers with very short pulse lengths to accelerate charged particles to relativistic speeds. Recent experimental data reveal that high intensity lasers are capable of producing collimated, charge-neutral proton beams containing more than 6 x 10 particles at a mean energy of 5 MeV with a total beam energy of 500J. If used in propulsion, these systems are capable of generating specific impulses of more than one million seconds albeit at very modest thrusts, and require a nuclear power system to drive them. A major research effort at the University of Michigan is directed at finding ways to enhance the thrust of LAPPS. One approach is to irradiate larger focal spots in solid targets, and the other is to accelerate heavier ions by first removing the moisture from targets then coating them with materials of larger mass numbers such as carbon or fluorine. In this paper we address these issues in light of a recently developed acceleration model. ____________________________ *Associate Fellow AIAA, Professor of Nuclear Engineering and Radiological Sciences NOMENCLATURE Cs = Sound speed d = diameter of focal spot D = linear distance Ei = ion energy F = Thrust g = Gravitational Acceleration h = thickness of electron cloud I = laser intensity Isp = specific impulse λ = wave length Mi = initial mass Mf = final (dry) mass R = radius of focal spot Sf = distance to destination tf = travel time to destination τRT = round trip travel time Ve = exhaust velocity Vf = final vehicle velocity Vmax = maximum ion velocity Z = ion charge η = conversion efficiency 39th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit 20-23 July 2003, Huntsville, Alabama AIAA 2003-4690 Copyright © 2003 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved. INTRODUCTION In a recent publication 1 we examined the propulsion capability of a LAPPS system based on recently generated experimental data 2 in which a kelojoule laser of 1 micron wave length and a 500 fs pulse length was used to irradiate a focal spot of 9 μm in a gold foil. The power of such a laser was 10 W giving rise to an intensity of 3 x 10 w/cm where it was noted that half of the energy (i.e. 500J) appeared in the proton beam that was ejected from the target. Using the often-cited model whereby the electrons from the blow-off plasma are accelerated by the laser, penetrate deeper in the target to set up an electrostatic potential. This positive potential accelerates ions and simultaneously slows down the electrons until the two species drift out of the target at the same (ambipolar) rate. Moreover, simple energy balance dictates that the energy imparted by the laser must appear in these electrons at some efficiency η. Since these electrons create the potential, then the electron energy must be equal that of the potential energy, and that in turn must equal that of the ions acted upon by this potential. When all these facts are put together the energy of the ejected ion can be expressed by 3