Pergamon www.elsevier.com/locate/asr Adv. Space Res. Vol. 29, No. 9, pp. 1385-1390,2002 0 2002 COSPAR. Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain 0273-I 177/02 $22.00 + 0.00 PII: SO273- 1177(02)00185-O 3D ELECTROMAGNETIC PIC SIMULATIONS OF RELATIVISTIC ELECTRON PULSE INJECTIONS FROM SPACECRAFT T. Neubert’, and B.E. Gilchrist ‘Solar-Terrestrial Physics Division, Danish Meteorological Institute, Lyngbyvej 100, 2100 Copenhagen, Denmark *Space Physics Research Laboratory, University of Michigan, 2455 Hayward St., Ann Arbor, MI 48109-2143, USA ABSTRACT Relativistic electron beam accelerators (5 MeV? 0.1 A) can now be flown on spacecraft,. Inject,ion from low- Earth-orbit into the atmosphere makes it possible to perform active perturba.tion esperiments in the 40-60 km altitude range. These include modification of the a.tmospheric electric potential struct,ure over thunderstorm regions and t,he possible stimulation of high-altitude-lightning, as well as st,udies of rela.tivistic elect,ron precipitation effects on chemical rea.ction paths. In this paper, the initial stage of the beam injcct.ion process is simulat,ed by a fully electromagnetic and relativistic Particle-in-Cell (PIG’) code. The self-consist,ent implementation of electric charging of a spacecraft structure in an electroma,gnetic code is demonstra.ted, and bea.m propagation dynamics is explored for a range of beam to ambient plasma. densities. It is shown t.hat the combined effects of ambient plasma and beam self-fields may allow propa,gation in the ion-focused regime and that this regime primarily is expected for rela.tivistic beams. 0 2002 COSPAR. Published by Elsevier Science Ltd. All rights reserved. INTRODUCTION PIC simulations of relativistic electron beam dynamics are undertaken t,o invcstigatc t,hc beam injection process from spa,cecraft in order to give realistic initial conditions of beam para.metcrs for large-scale models of beam propagation in the Earth’s atmosphere-ionosphere-magnetosphere (Neubcrt et al., 1996: liazanov et al., 1999). The beam pulses simulated have the characteristics of the linac being tested for flight by the US .&r Force (Jest,, 1993). The parameters are shown in Table 1. Table 1. Relativistic Beam Pulse Parameters zyxwvutsrqponmlkjihgfedcbaZYXWVUTSR rb F ib rb T<i 0 0 c n; L2,> 0.1 A 5 MeV 4 /Ls 0.001 1.5 inm 1 O.OOOlS m-rad 2.9X10’” n-” ‘L.9X10s rad/s Ib is the beam current, Eb the beam energy, rb the bea.m pulse length: ‘l’(i the duty cycle, a, the beam radius at injection: t the beam emittance, ng the density at injection, and tiEb the beam plasma frequency at injection: w$ = q2ng/60yme, where y is the rela,tivistic factor. The 3-D simulations are performed using the TRISTAN electromagnetic and relativistic particle code (Buneman et al., 1993). The code uses local updates of the fields from particle motions, rather than Poisson’s equation. While this scheme makes the code fast, it is required, that the experin1enta.l conditions are described self-consistently within the simulation domain. It is important,, therefore, t,o incorporate the beam source and the spacecraft in the simulation domain with care. INCORPORATION OF SPACECRAFT CHARGING IN THE TRISTAN CODE zyxwvutsrqponmlkjih The Plasma Response Time The spacecraft potential will initially increase with time, causing beam eleclrons to escape with decreasing energy. The a,mbient plasma will react and att,empt t,o supply an electrically neutralizing return current. In 1385