Cooperative Light Scattering fromθ-Pinch Plasmas

Results are reported of an experiment to measure the cooperative scattering from density fluctuations in the plasma of a 150-kJ, high-voltage $\theta$ pinch with a low-pressure (20 mTorr) filling of deuterium gas. The beam from a giant-pulse ruby laser is focused at the center of the plasma, and the light emerging at angles between 5.5° and 7° to the beam and plasma axis is collected and spectrally resolved by a spectrometer consisting of an interference filter and two identical Fabry-Perot interferometers in series. The $\theta$-pinch plasma parameters, separately measured in another experiment, are plasma density $n=2.8\mathrm{}\times {10}^{16}$ ${\mathrm{cm}}^{-3\mathrm{}}$, $k{T}_e=345\mathrm{}$ eV, and $k{T}_i=2.0\mathrm{}$ keV, corresponding to Salpeter's parameters $\alpha =1.2\mathrm{}$, $\beta =0.33\mathrm{}$. Scattered spectra are reduced to absolute values of $\frac{n{d}^2\sigma }{d\Omega d\lambda }$ by calibration against Rayleigh scattering from dry ${\mathrm{N}}_2$ gas. The results show a narrow central peak superimposed on a broader tail of lower magnitude. In the case of no bias field ($B_0=0\mathrm{}$), the tail has a width and magnitude equal within experimental error to those of the theoretical thermal-ion feature, corresponding to Doppler broadening of the scattered light by an amount characteristic of the thermal-ion motion at $k{T}_i=2\mathrm{}$ keV. The central peak has a maximum magnitude 15 times greater than that of the theoretical ion feature, and is 0.44 times as wide. For $B_0=-750\mathrm{}$ G, the central peak increases in intensity by another factor of 3, while retaining approximately the same width. The central peak is attributed to superthermal-plasma density fluctuations, superimposed on the background represented by the broader thermal-ion feature.