Strong Langmuir turbulence driven by a relativistic electron beam has been investigated by the laser scattering technique. The special features of the experiments are as follows: (i) in the regime under study, Langmuir turbulence is well developed, and the temporal and spatial scales of the turbulent region far exceed those of a single caviton; (ii) the dispersion of the electron plasma waves is governed by the magnetic field, although ωpe≫ωBe; and (iii) the temperature of plasma electrons is much higher than that of ions, i.e., the damping of ion-acoustic waves is small. k-Spectra of electron plasma waves are measured in a broad spectral region by means of CO2-laser scattering. Criterion of modulational instability of observed spectra is estimated. The role of conversion and scattering of Langmuir waves by ion-acoustic waves in nonlinear energy transfer is discussed.
The ultra-soft x-ray duochromator based on two multilayer mirrors installed on the reversed field pinch experiment (RFX) reversed field pinch is described. Built at the Budker Institute of Nuclear Physics of Novosibirsk, it has been designed to monitor the time behavior of the hydrogen and helium-like resonant transitions of carbon and oxygen, the main impurities in the RFX emission spectrum, with the possibility of scanning the wavelengths between 16 and 45 Å. The wavelengths can be independently selected for both the mirrors (made of Ni/C and Cr/C layers, respectively) via a Θ–2Θ mechanism, without breaking the vacuum. The detector is based on a scintillator layer deposited on the head of a photomultiplier. Vacuum ultraviolet and visible radiation is cut by means of thin metal films (Fe, Al, Cr) deposited on a mylar substrate and on the scintillator. The wavelength resolution (λ/Δλ) of 40 to 50 in the region of interest is provided by a Soller collimator and a precollimator stage placed in front of the mirror. The relatively large etendue allows one to perform observation in a wide range of experimental conditions. Sample results of the chordal observations are compared in absolute units with the predictions of a collisional–radiative one-dimensional impurity transport code.
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