Linac4 H$^-$ source R&D;: Cusp free ICP and magnetron discharge

The 2MHz radio-frequency inductively coupled plasma heating (ICP RF) of Linac4’s IS03 H- source is more efficient without its octupole cusp in offset hallbach configuration. This was shown by Particle in cell Monte-Carlo (PICMC) simulation using the NINJA software [1] and confirmed by plasma characterization via optical emission spectroscopy [2,3], an easier plasma ignition is also anticipated. In this paper, we present preliminary results of an Alumina plasma chamber IS03 H- source [4] operated without magnetic cusp. Operation under monthly cesiation induces a slow evolution of the molybdenum cesiated surface correlated with an increase of the co-extracted electron yield. An improved stability of the extracted H- beam is achieved by compensating the Cs-losses. The high intensity option for Linac4 features an adaptation of BNL’s Magnetron. Simulation of this complex H2-Cs arc discharge plasma, where electrons are emitted from a cesiated molybdenum cathode, requires characterization of the plasma impedance and knowledge of hydrogen and cesium densities. We present a measurement of plasma impedance over the range of discharge current, hydrogen and cesium-densities.