Multiframe interferometry as a diagnostic to measure the free electron density in laser generated plasma

The investigation of phenomena concerning the interaction of heavy ions with plasmas is a key research activity of the plasma physics group at the Gesellschaft für Schwerionenforschung (GSI). The laboratory at GSI offers the unique opportunity to measure the charge state distribution and the energy loss of swift heavy ions after they have penetrated a hot and dense plasma [1]. The laser induced plasma expands considerably on a nanosecond timescale, which leads to significant changes of the plasma parameters along the projectile-plasma interaction path. This requires plasma diagnostics sophisticated enough to measure key plasma parameters such as temperature and free electron density with both space and time resolution during the interaction time. The density of free electrons is an important plasma parameter for the theoretical description of energy loss in ionised matter. An interferometer was developed and constructed to determine the electron density with spatial resolution, and for the first time simultaneously at different times [2]. This diagnostic provides images of the interference pattern with a time difference of 1.5 ns and allows determination of the free electron density up to a maximum density of 3 × 1020cm−3. Figure 1 shows the experimental