Spatially resolved light scattering diagnostic on plasma focus devices

Abstract

Three Mather-type focus devices with capacitor bank energies of 280, 250 and 12 kJ, respectively, have been studied by collective scattering of ruby laser light. In order to determine the thermal plasma parameters and detect the occurrence of suprathermal scattering from plasma turbulences, the frequency and wavevector spectra of the ion feature of scattered light have been measured. All scattering vectors were oriented in an azimuthal plane perpendicular to the focus pinch axis. The measured plasma temperature and densities are below 2 keV and 1019 cm-3, respectively. Therefore, only a few per cent of the observed neutron emission from the studied focus plasmas can be of thermonuclear origin. During the compression phase, and after maximum compression during the rise of the neutron emission, suprathermal forward scattering has been observed. This is interpreted as scattering from enhanced density fluctuations due to plasma turbulences, which may take part in the heating of the focus plasma and the acceleration of the high-energy deuterons which are responsible for the dominant neutron generation in the focus plasma by beam-target reactions. The results are in agreement with those of former light scattering measurements at lower energy focus devices.