System for voltage control and for data acquisition of retarding field energy analyzer

Abstract

One of the main concerns when working with negative ion beams is collimation during propagation. The beam is composed of like charged particles; therefore they tend to repel each other causing the beam to widen after acceleration. In environments where the beam needs to travel long distances before reaching its target, the compensation of its space charge is necessary; the most straightforward way of achieving this is the creation of a plasma around the beam, created by ionization of the background gas by the beam itself. This effect, called space charge compensation, plays therefore a key role in the beam propagation and a deep understanding of this phenomenon is fundamental when operating negative ion sources. In order to perform measurements of the particles forming such beam-generated plasma, a retarding field energy analyzer (RFEA) can be adopted, which is an indirect non perturbing instrument, analysing the particles emitted radially from the beam plasma, and allowing to estimate the plasma parameters from the energy distribution function of ions and electrons. The paper presents the design and the layout of the system for voltage control and data acquisition of the RFEA, developed at Consorzio RFX and used for the experimental campaigns in NIO1 to study the phenomena that cause space charge compensation.