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

Abstract

Abstract 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 × 10 18 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 × 10 18   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.