Plasma meniscus and extraction electrode studies of the ISIS H− ion source

Abstract

In order to reduce the emittance and increase the transported beam current from the ISIS Penning-type H(-) ion source, improvements to the extraction system are required. This ion source is currently being commissioned on the front end test stand at the Rutherford Appleton Laboratory, which demands higher extraction energies, higher beam currents, and smaller emittances. To facilitate this, the present geometry requires optimization. This paper details the experimental and simulation studies performed of the plasma meniscus and the possible electrode geometry modifications needed to extract the highest quality beam.

The front end test stand high performance H− ion source at Rutherford Appleton Laboratory

“…Increased aperture width and extraction voltage are yet to be implemented. Work is underway to improve extraction, 13 these additional modifications should further increase beam current and decrease emittance. When the limitations of the power supplies are overcome it should be possible to meet the beam requirements for FETS.…”

“…Extraction from the source has been investigated previously 11,12 and this work is continuing with a detailed study into the plasma meniscus and electrode geometry. 13 It has been demonstrated 14 that beam currents of 78 mA can be achieved by increasing the width of the aperture slit from 0.6 to 0.8 mm.…”

“…Discharge current and hydrogen flow rate are the two main factors affecting plasma density in the source. Recent experimental work 13 has shown that there is an optimum discharge current for a specific extraction voltage and geometry. For the ISIS source the discharge current should be in excess of 65 A to produce a beam with minimum divergence.…”

Overview of recent studies and design changes for the FNAL magnetron ion source

“…3a. Changes in source pressure affect the arc and beam currents, as well as the shape of the plasma meniscus [5] which affect the losses and transmission efficiency of the linac. Other labs such as ISIS use a collar around the valve for cooling [6], however this would be difficult to implement due to our current extraction scheme.…”

Diagnostic experiments at a 3 MeV test stand at Rutherford Appleton Laboratory (United Kingdom)

“…Further implications concerning the extraction are a recent matter of investigation. 12 To use the detector as an energy analyzer the cup signal must be measured at different "jacket potentials" to provide the particle energy E RGI . The pulses I FDC ͑t , U RGI ͒ are then converted to energy depending signals Ĩ͑E RGI , t͒ at different time steps which then has to be differentiated dI RGI / dU ret ͑see Fig.…”