Rise/fall time enhancement of the spallation neutron source Linac LEBT chopper system

The Spallation Neutron Source (SNS) Linac Low Energy Beam Transport (LEBT) chopper system provides fast chopping of the H^- ion beam in the LEBT structure. Four identical pulsed power supplies (pulsers) create a series of ± 2.5 kV pulses to the four deflection electrodes floating on the focusing voltage of -50 kV. Each pulser is connected to the electrode through the network which consists of high voltage (HV) cables, a blocking capacitor, HV feed-through connectors, current-limiting resistors and transient voltage suppressors. Effective beam chopping requires minimal rise/fall time of the rectangular HV pulses on the load. In the present configuration these values are approximately 100 ns. Methods of reducing rise/fall time on the LEBT electrodes are discussed. Results of simulation and comparative measurements of the original and upgraded system on the test stand are presented. Furthermore, the effect of these changes on reliability degradation caused by arcing in the LEBT structure is discussed.