Evolution of the Lebt Layout for Sns*

As the startup Low-Energy Beam Transport (LEBT) for the Spallation Neutron Source (SNS) project is undergoing commissioning aimed at transporting a 35-mA Hbeam, a new LEBT layout has been developed that is designed to transport a beam current of 65 mA, consistent with the revised nominal SNS beampower goal of 2 MW. The SNS LEBT employs two electrostatic einzel lenses and provides static beam steering as well as fast chopping, implemented by splitting the second lens into four quadrants. The development was carried out using the code IGUN in a novel way that allows working with finite ion temperatures without encountering any problems. The changes with respect to the Startup LEBT layout affect electrode shapes in the upstream part only; the downstream part is kept as previously designed, to preserve the chopping action. In addition, the overall length was kept constant, and only minor changes had to be made to the design of some of the support structures; the vacuum tank design is identical to the previous one. The expected performance of this new LEBT, based on the simulations, is described for a large range of output Twiss-parameters and compared to simulation results for the startup configuration.