Space charge saturation of Faraday cup detectors used in laser-produced plasmas studies

Abstract

Summary form only given, as follows. Faraday cups are commonly used to measure plasma expansion characteristics in laser-produced plasmas. In its simplest design a Faraday cup consists of a grounded entrance grid and a biased collector plate. As an expanding laser plasma flows into the Faraday cup, the entrance grid, with a mesh size smaller than the local Debye screening length, separates electrons from the ions to be measured and the transmitted ion flux is subsequently collected by the collector plate. When large signals are measured, space charge effects can significantly distort the signals. Such distortion may still lead to apparently reasonable looking ion signal waveforms which are, however, limited in amplitude or truncated from the full correct signal. Thus, the design of Faraday cups should include a consideration of the space charge saturation effects. An analytic approach in one dimension (1D) for calculation of the self consistent fields and effect on the streaming ions in the space charge limited regime is developed to model this process. Based on this approach the ion current modification due to a longitudinal space charge field is investigated theoretically and compared experimentally to measurements for a simple two-electrode ion detector system. The measurements were carried out with carbon plasmas produced by 25 ns KrF laser pulses. In order to maintain the 1D geometry, the size of the gap between the two electrodes in the ion detector is kept small compared to the size of the entrance aperture. The experimental results and comparison to theory will be presented.