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.

https://iopscience.iop.org/article/10.1088/1361-6463/abf956/pdf

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

A hydrogen theta-pinch plasma is diagnosed by two-color interferometry to determine the line density of the free electrons and the hydrogen gas. From the ratio of these line densities, the effective ionization can be calculated. The free electron line density and the effective ionization degree are the most essential quantities for the evaluation of the plasma regarding its applicability as a target in context of plasma ion beam interaction to increase the charge state of the ion beam (plasma stripper). The two-color interferometric diagnostic shows that both line densities exhibit a periodic behavior predefined by the periodic current and at the time the free electron line density reaches a local maximum, the hydrogen line density falls to a local minimum. This occurs, because the plasma axially expands from the coil center by evading the radial compression force of the magnetic field, creating an ionizing wave in the residual gas. The theta-pinch was set up in two versions, with one of them using a cylindrical coil and the other using a spherical coil. For the cylindrical version, the best working point regarding the free electron line density and the effective ionization degree is 1.45±0.04×1018 cm−2 and 0.826±0.022 at 20 Pa and 16 kV. In contrast, for the spherical version, lower values of 1.23±0.03×1018 cm−2 and 0.699±0.019 at 20 Pa and 18 kV were found. Additionally, a new set-up is proposed for optimizing the plasma target regarding the free electron line density and the effective ionization degree, as well as how to maintain them on a sufficient level for several 10 μ s.

https://iopscience.iop.org/article/10.1088/1361-6463/ac4f90/pdf

Time-resolved measurement of the free electron and neutral gas line density in a hydrogen theta-pinch plasma target by two-color interferometry

Optical refractivity in Ar–CO2, plasmas at λ=632.8 nm

The influence of excited atoms on the plasma refractivity is studied for hydrogen and argon plasmas. Using tabulated f-values [1] in the case of hydrogen and calculated f-values [2], [3] for argon, the index of refraction is obtained for typical are conditions. The results indicate that the accuracy of electron density determination in plasmas from laser interferometry is not severely influenced by the excited species. This is clearly a contradiction to the interpretation of the results of a similar study of hydrogen given by GLASSER et al. [4].

Influence of Excited Atoms on the Plasma Refractivity

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Influence of Excited Atoms on the Plasma Refractivity