Radiation-Induced Instability of Electron Plasma Oscillations

Abstract

It is well known that external radiation of frequency u) 0 incident upon a plasma is absorbed by inverse bremsstrahlung of electrons in the presence of ions. 1 However, when a> 0 is close to the plasma frequency, oop, the radiation energy is preferentially fed into electron plasma oscillations at a rate proportional to I^nckT, where I 0 is the radiation intensity, n the density of electrons, c the speed of light, and kT the electron thermal energy. We wish to point out in this Letter the possibility of electron plasma oscillations becoming unstable and growing in amplitude when they gain energy at a rate faster than they can dissipate it by their dominant damping mechanism; that is, whenwhere y L is the plasma damping rate.Monochromatic coherent external radiation of form lEoexp^ko-r-GOo^J + c.c., where k 0 «E 0 = 0, will modulate the longitudinal polarization, P^(k, u>), in the plasma at the sum and difference frequencies u>±u; 0 : •* NL •*-*-» -+ +(1) E L is the effective longitudinal field in the plasma i=-Pi), XL = Xif *Xj} is the usual equilibrium longitudinal susceptibility with a contribution from the electrons, \L e > and a contribution from the ions, xjj 9 X£ NL is a nonlinear longitudinal susceptibility connecting the polarization to E 0 and E L (W±WQ). A similar equation for -E L (QO-(£Q) follows from (1), which now couples to frequencies a; and io-2co 0 . In fact, a chain of coupled equations is generated. Assume in the following that co is near the plasma resonance frequency, oo L = (up 2 + 3v 2 k 2 ) 1/2 , and co 0 slightly is above it. In the chain of equations we can then neglect all fields E L propagating at frequencies other than oo and co-co 0 (strongly damped low-frequency ion-acoustic waves can still conceivably respond at the difference frequency a;-w 0 ). 2 This leaves only two coupled homogeneous equations for E L $L, O>) and E^(k, OJ-CUQ). 3 The condition for a nontrivial solution is the vanishing of the following function, which is the nonlinear longitudinal dielectric constant e L NL (^f u>), € L (k,co) : €^(k, co)> --NL--

Physics division. Progress report, January 1, 1995--December 31, 1996

“…These neutrons travel with speeds of less than 8 m per second. We are continuing to develop this source with improved cold moderators and better rotor 22 Progress Report 1995-1 996 reflectors ( Fig. 1-7).…”

Raman gain against a background of non-thermal ion fluctuations in a plasma

“…These results naturally focus attention on those plasma-wave interactions that couple Langmuir waves and ion acoustic waves. These are the Langmuir decay instability (LDI), in which a primary Langmuir wave decays into a daughter Langmuir wave and an ion acoustic wave (DuBois and Goldman 1965), and the electromagnetic decay instability (EDI), where the daughter Langmuir wave is replaced by a second scattered electromagnetic wave (Shukla et al 1983;Baker 1996). The link between SRS and the presence or otherwise of ion waves has been firmly established.…”

Analysis of the backscatter spectrum in an ionospheric modification experiment

“….... From experimental results obtained at Arecibo (Carlson, Gordon and Showen, 1972;Kantor, 1974), it is believed that the enhanced heating and anomalous absorption occurring in the ionosphere are mainly due to parametric decay instabilities: the HF ordinary (pump) wave supplied by a high power transmitter (-100 kW), and propagating in the F-layer, decays into electron plasma and ion acoustic waves to enhance the plasma fluctuations (DuBois and Goldman, 1965;Nishikawa, 1968). Several analyses have been carried out of the saturation spectra in homogeneous (DuBois and Goldman, 1972a, b;Valeo, Oberman and Perkins, 1972;Kruer and Valeo, 1973;Harker, 1972;Kuo and Fejer, 1972;Fejer and Kuo, 1973;Perkins, Oberman and Valeo, 1974) and inhomogeneous (Arnush, Fried and Kennel 1974) plasmas, with the assumption that the pump field is well above the threshold for parametric amplification.…”