Decay Time and Instability Measurements of Plasmas Generated by Intense Relativistic Electron Beams

Abstract

We report here on the first measurements of decay times, during the beam pulse, of plasmas generated by intense relativistic electron beams. The pressure scaling of characteristic decay times and detailed temporal evolution are consistent with a simple plasma-electron generation model which includes direct-impact ionization, avalanche, and recombination. The decay times have been correlated with observed frequencies of the beam resistive hose instability, and the inverse relationship between o> and r predicted by linear theory has been experimentally verified.PACS numbers: 52.40.MjCharged-particle beam transport through a background plasma is subject to a number of instabilities. For intense relativistic electron beams (IREB's) in high-pressure gases, the most disruptive of these are the resistive instabilities. 1 The hose instability, which results from the coupling of beam transverse motion and magnetic field diffusion, is the most catastrophic of the resistive modes. 2 Progress in understanding these modes has, in the past, been impeded by a lack of understanding of air-chemistry phenomena which govern beam-channel conductivity.Linear theory 3 " 6 yields dispersion relations which show a peak growth rate for the m = 1, or hose mode, at real laboratory frequencies, w, such that (or d ==0.2-0.6, where the dipole decay time is usually taken to be 5 T d = 7rcr 0 a 2 /2c 2 . cr 0 is the on-axis conductivity, a is the plasma radius, and cis the speed of light in vacuum.The dipole time is clearly the key parameter of the host instability, but it is extremely difficult to measure. It is, however, closely related to the monopole decay time, which we have determined experimentally. The monopole decay time, or fundamental L/R decay time, determines the plasma response time to changes in the axisymmetric inductive fields associated with changes in the beam current.If one assumes an axisymmetric beam-plasma system within a perfectly conducting cylinder of radius R, a simple scalar Ohm's-law plasma, a time-independent net current density profile [j net = I n et(t)f n O), flirr dr xf n (r) =1], and 8/6z « 9/8r, then Maxwell's equations may be integrated radially (ignoring displacement currents) to yield (1) 'plasma : -T",I m I r\Qt where the monopole decay time is 7 (A 4T <* f I ^ cr(r,t) f R dr' fr' ,,.,,,< ,,\ it) = z-I 2irr dr -r--I lirr dr fAr ). ira (2)Thus, simultaneous measurements of 7 net and 7 plasma can be used to determine the time-dependent monopole decay time from Eq. (1). Furthermore, if the conductivity and net current density profiles are known, both r d and o-0 can be calculated throughout the pulse; and, if the momentum-transfer collision frequency can be estimated, the peak plasma electron density can be deduced. We note that others have estimated the monopole decay time from the late-time decay rate of plasma currents, 8 but these estimates do not include the changes in r m during the time of the beam pulse, which is when the hose instability actually occurs.In Fig. 1, we show a schematic of the experime...