Measurement of the plasma radial electric field by the motional Stark effect diagnostic on JET plasmas Rev. Sci. Instrum. 74, 1596 (2003;Strong optical pumping of ions can result after a few microseconds of resonance excitation in a laser beam. However, both Fokker-Planck diffusion and acceleration due to macroscopic electric fields can remove an ion from resonance by changing the ion velocity on a similar timescale. Therefore, the time dependence of laser induced fluorescence can be influenced by particle acceleration and velocity-space diffusion. This effect which has already been used to measured Fokker-Planck diffusion, is extended to include the influence of an electric field and used to measure the electric field associated with an electrostatic shock in a multipolar gas discharge.

Effects induced by the propagation of electrostatic perturbations in a low-density collisionless argon plasma are reported. A space-time-velocity resolved laser-induced fluorescence diagnostic shows clearly that the plasma reacts to a steep density perturbation by the formation of self-organized coherent ion structures. [S0031-9007(98)05742-1] PACS numbers: 52.35.Tc, 52.50.Dg, 52.70.Kz The mutual interaction of nonlinear and dispersive effects can be observed in the propagation of ion perturbations in a collisionless plasma. Since the theoretical work of Moiseev and Sagdeev [1] who predicted the existence of ion acoustic solitons and shock waves, intensive theoretical and experimental studies have been pursued and important results have been obtained in the turbulence and instabilities associated with the propagation of a density perturbation [2][3][4][5][6][7][8][9]. Among the rich variety of nonlinear structures [10][11][12][13][14], there have been observations of envelope solitons and cavitons or density holes dug in a plasma by high frequency waves and able to trap wave packets. These studies, in progress today [15,16], provide an explanation for the satellite observations of electrostatic ion shock waves [17].Besides computer simulations, the experimental work that has been done to test the various assumptions of the nonlinear theory has employed mainly Langmuir probes and retarding field energy analyzers. With the rapid progress of optical plasma diagnostics, new tools are available to study nonlinear propagating phenomena. Among laser aided plasma diagnostics [18], the laser-induced fluorescence (LIF) technique [19,20] provides a means to nonintrusively probe plasmas and to measure the ion velocity distribution function (IVDF) with unprecedented resolution in velocity and space. The resolution in velocity is obtained because the Doppler effect is the dominant linebroadening mechanism for a wide range of plasma conditions. Spatial resolution is obtained by collecting the fluorescence light through a viewing volume which intersects the laser beam in a localized region. LIF depends on the fact that laser pumping of a transition will modify the ionic state distributions and, thus, modify the plasma line radiation. Fundamentally, the time resolution of LIF is limited by the lifetime of the upper quantum state of the pumped transition (usually around 10 nsec). In practice, the number of scattered photons is low enough that some form of time averaging is necessary to obtain a distribution function.To experimentally study propagation, relaxation processes, or instabilities in weakly collisional (low density) plasma, it is necessary to optimize the time resolution. The signal to noise ratio of the fluorescence can be enhanced considerably by averaging over a large number of periods if the observed phenomenon is repeatable. Time resolution can be obtained by using a boxcar averager [21] or a multichannel analyzer synchronized with the observed phenomenon [22,23].In this paper, after briefly describing the instru...