A sensitive He-Ne interferometer with passive beam power stabilization for lowdensity pulsedplasma measurements Rev.The characteristics of the He-Ne gas laser used in a new simple interferometric technique have been studied experimentally and theoretically. The interferometer has two novel features: first, the intensity of the laser itself is used to detect the fringes and second, because the intensities of the 0.63-", (red) and 3.39-", (infrared) laser beams are coupled, interference in the infrared can be detected by a simple photomultiplier monitoring the red beam.The system does not respond instantaneously to changes in the optical path length; experimental measurements show that when the red beam is used to follow interference in the infrared, the maximum detectable response is limited to about 3 X 10 6 fringes per second. Discussion of the frequency response and the crosscoupling between the two wavelengths leads to the conclusion that the frequency response is limited by the red channel only.Experimental details of the interferometer are described, including the application of a mUltipass system which, with some loss in spatial resolution, increases the sensitivity of the interferometer by at least a factor of 20.

Nonlinear Landau damping, the nonlinear interaction between two waves in which the beat disturbance is resonant with the particle thermal velocities, has been observed for electron plasma waves. The results confirm the predictions of weak-turbulence theory.

The production of highly energetic ions (up to 10 keV) in, for example, large pinch discharges is well known, and has usually been attributed to acceleration of the ions by electric fields associated with the dynamics of the unstable plasma, i.e., by the fluctuating fields of the large-scale (hydromagnetic) turbulence 1 ; the time scale for this kind of process is necessarily many ion Larmor periods. We wish to report evidence for the noncollisional heating of ions to energies up to 3 keV in a situation in which the heating must be due to fluctuation frequencies CL) »fy, the ion cyclotron frequency, and which we believe results from statistical accelerations in the microfields of the turbulent spectrum of longitudinal plasma waves.The apparatus is shown schematically in Fig. 1. It consists of a silica torus, 65-cm major diam, 10-cm bore, in a quasistatic axial magnetic field Bw~ 3kG. A plasma with density ~10 12 cm"" 3 and electron temperature a few eV is prepared by passing a 1-kA axial current through weakly preionized hydrogen gas at 2-mTorr pressure. A large electric field E

This paper gives further experimental verification of the theory of fast hydromagnetic waves. By a suitable choice of plasma conditions the compressional wave is propagated in a partially ionized cylindrical hydrogen plasma at frequencies below the ion cyclotron frequency. The propagation constants of the wave are measured experimentally by observing the phase velocity and attenuation over a fixed distance. Other plasma parameters, with the exception of the neutral density are measured independently. To obtain good experimental agreement with the theoretical curves computed from the measured data, it is necessary to postulate a considerable neutral particle density in the plasma. This gives rise to loading of the wave, observable as a reduction in the Alfvén speed, and also to wave damping not explicable by resistive effects alone. The enhanced loading produced by neutral particles is unaffected by variations in magnetic field and the fitted value of neutral density derived from wave cut-off data is in agreement with that calculated from attenuation measurements.