High Fidelity Simulation of Low-Energy Ion Beam Chopping for the Spallation Neutron Source

The Oak Ridge National Laboratory ion source group will construct and operate a two-solenoid low-energy beam transport (LEBT) system for 60 mA H- beams, including rf chopping. This could increase the beam current in the Spallation Neutron Source (SNS), while eliminating the sparking problems of the existing electrostatic LEBT. Previous solenoid-based LEBT designs with beam chopping showed problems, due to background ion plasma around the chopper. The parallel simulation framework VORPAL will be used to simulate H- ion beam transport experiments being planned as part of the long-term SNS power upgrade project at Oak Ridge National Lab (ORNL). Tech-X personnel have established a collaboration with the ion source group at ORNL during the Phase I effort and this collaboration will be continued and strengthened during Phase II, enabling us to provide key computational support during design and commissioning of the LEBT and to benefit from the insight and guidance of ORNL physicists. It was demonstrated that the parallel VORPAL framework can model all key aspects of chopping in the ORNL LEBT design: 3D electrostatic particle-in-cell (PIC), complex 4-quadrant chopper geometry and radio frequency quadrupole (RFQ) entrance, gradual build-up of background ion plasma, converging ion beam with initial phase space taken from ORNL specifications, and external solenoidal magnetic field. The design potentials were applied to the chopper quadrants with specified rise and fall times and the beam steering angle was verified, while strong plasma dynamics was observed. All possible avenues for beam-plasma instabilities will be explored over multiple time scales: electron plasma frequency (very fast), ion plasma frequency (fast) and rf chopper frequency (slow). Previous experiments and present intuition suggest that problems are occurring on the slow time scale, for which a fast Boltzmann electron model will be used. Code validation and experimental support during LEBT design and commissioning are planned. VORPAL training will be offered to the ion source community and ease of use will be improved, especially through improvements to the graphical user interface (GUI), known as VORPAL Composer. Commercial applications and other benefits: The commercial VORPAL application will be further developed, making it more directly relevant to important commercial applications like compact deuterium on tritium neutron sources for homeland security and oil exploration, as well as ion implantation for the semiconductor industry. The work will also help to reduce risk and potentially lower costs for the planned SNS power upgrade