A. Possibility of Radiation-induced Sound Amplification in a Gas

The possibility of observing acoustic-wave amplification made possible through the heating of neutral particles by electrons in a weakly ionized gas has received some recent attention. -4 The idea is that an existing sound wave, by inducing a space-.time variation in the electron density, would create an acoustic source, thereby generating a wave that interferes constructively with the original 3ound wave. At moderately high sound frequencies (> 104 sec ) the electrons behave isothermally under typical discharge conditions. Then, if acoustic losses resulting from viscosity and heat conduction in the neutral gas are ignored, the resulting exponential growth in the wave intensity in the linear theory is characterized by a time constant T' = T(1+x 2). The parameter T (~1 sec under typical discharge conditions) represents a time of the order of that required for the gas to absorb from the electrons an amount of energy equal to the thermal energy of the gas, while x is the tangent of the phase angle between relative density perturbations in the electrons and neutrals. 4 An analogous calculation for ion waves in a fully ionized gas shows only a very slight wave amplification.5 Here, also, there is no guarantee that the gain would offset the expected losses. In fact, one of the principal difficulties in observing energy-transfer wave amplification is the considerable loss rate associated with the relatively low-pressure gas required to obtain an electrical discharge. This difficulty could be overcome if the energy to be transferred into the translational modes of the gas could be carried internally in the gas molecules themselves. Discharge conditions would not then be required, and one could increase the gas pressure so as to reduce ordinary acoustic losses in the completely un-ionized gas. By carrying the energy internally, moreover, the gas molecules would eliminate the phase difference 4 between relative density perturbations in the gas component that is responsible for heating and the component that is being heated through collisions with the former. The purpose of