Space charge effects in Faraday cup ion detectors

Faraday cup ion detectors are widely used in the investigation of laser-produced plasmas. However, it is well known that these detectors are susceptible to space charge distortion of the ion signal. Although an analytical model describing the threshold condition for distortion was developed in 1970 by Green, this model has never been tested experimentally. In the present investigation, it is found that the analytic model derived by Green appears to be inadequate when compared in detail to experiments performed using KrF (248 nm, 15 ns) laser pulses incident on a graphite target at an intensity of 1.7/spl times/10/sup 10/ W/cm/sup 2/. By comparing the results of particle-in-cell (PIC) simulations with these experiments, it has been discovered that electrons play an important role in the space charge distortion phenomenon.

I. Introduction

Faraday cup ion detectors continue to be widely used in the investigation of laser-produced plasmas [1]–[7]. In particular, Faraday cup detectors are used extensively for the characterization of ion emission in pulsed laser deposition studies. However, the limitation of the Faraday cup detector has been known for some time. The potential for distortion of the cup signal due to space charge effects was recognized and characterized in 1970 by Green [8]. Due to the presence of a significant number of ions, a very large positive space charge accumulates inside the cup. If the space charge field is sufficiently strong, additional arriving ions will be repelled and the cup signal is said to be distorted by space charge effects. Green constructed an analytic model describing the behavior of the ions inside the cup, and derived a condition to ensure that space charge distortion is avoided. In spite of the widespread use of the Faraday cup ion detector, this model has never been tested experimentally. In the present investigation, it is found that this analytic model of Faraday cup operation is incomplete and does not account for the possible accumulation of electrons in the space charge region. Fig. 1. Schematic of the geometry of the Faraday cup employed in this work.