Measurement of the free electron line density in a spherical theta-pinch plasma target by single wavelength interferometry

A single wavelength heterodyne interferometer has been set up to investigate the free electron density integrated axially along the line of sight (line density) in a theta-pinch plasma to determine its applicability as a plasma target for ion beam stripping. The maximal line density reached in this experiment was (3.57±0.28)1018cm-2 at 80 Pa and 20 kV. The findings demonstrate the pulsed character of the line density and its increase by raising the load voltage and the working gas pressure. Additionally, the results were compared with spectroscopic free electron density estimations, which were carried out by Hbeta-line broadening and peaks separation. The time behavior of the line density indicates that its peak value is delayed by about 10 µs compared to the spectroscopic results. This effect is due to the formation of an extended, magnetically compressed plasma column in the vicinity of the current maximum, although the highest volumetric free electron density is reached near the current zero crossing. Since the line density is an essential parameter in describing the stripping capabilities of the plasma target, the interferometric diagnostic is superior to a spectroscopic diagnostic, because it directly provides integrated values along the line of side. Furthermore, the measurements of the line density in this experiment partially show nonphysical negative values, which is due to gaseous effects and residual shot vibration.