Simulation of the low energy H− injector into the National Spallation Neutron Source radio frequency quadrupole accelerator

The low energy beam transport system (LEBT) for the National Spallation Neutron Source is required to transport 35 mA of a 65 keV H− ion beam from the ion source to the radio frequency quadrupole accelerator (RFQ) entrance with a normalized root-mean-square emittance < 0.15 pi mm mrad. At a later time, it is anticipated that this beam current will be increased to 70 mA. For this purpose a radio-frequency driven, magnetically filtered multicusp ion source is under development at LBNL. The H− beam extraction from the ion source is being simulated utilizing the three-dimensional (3D) computer code ARGUS. H− ion beam extraction from the ion source requires proper simulation of the extraction of both plasma electrons and the desired H− ions. The goal is to optimize the LEBT system so that the electrons are completely deflected between the first two electrodes and the desired H− beam is properly matched into the RFQ accelerator. The magnetic field in the extraction region is being optimized by using the 3D magnetic code OPERA3D. The performance of the all electrostatic LEBT for the transport of higher currents (>70 mA) is discussed.