3D WARP simulation of a novel merging-beamlet ion beam injector. On the order of 100 beamlets are independently accelerated to 1.2 MeV and then merged to form a single beam with a current of 0.5 A. The transverse phase space, (x, vx/vz) projection at 1.6 m after the end of the accelerating Pierce column, shows the beamlets as they begin to mix. The particles are color coded based on the beamlet they began in.

Alex Friedman, William M. Sharp, and David P. Grote, Lawrence Berkeley National Laboratory

Research Objectives
To model beam dynamics for heavy-ion fusion (HIF) in accelerators and in fusion chambers.

Computational Approach
A hierarchy of codes is used for numerical simulation of beam dynamics in heavy-ion accelerators. IBEAM is first used for global optimization. A truncated-moment beam module in WARP is then used for refining the physics design and for generating the fields for beam acceleration, compression, and longitudinal control. Detailed accelerator simulations are done mainly with the electrostatic particle-in-cell (PIC) modules of WARP. A 2D semi-Lagrangian Vlasov code is being developed to study halo questions and to corroborate the PIC modeling. Beam transport modeling in the fusion chamber has been done with the axisymmetric PIC code BICrz and with the more modern 2D/3D code BPIC. Mission Research Corporation will use their parallel 2D/3D electromagnetic PIC code LSP to investigate chamber-transport scenarios using higher plasma densities.

Accomplishments
Much of the simulation work has focused on developing the physics design of the Integrated Research Experiment (IRE). WARP simulations of several representative layouts were made to quantify such phenomena as emittance growth, beam halo formation, and transverse-longitudinal coupling. Simulations examining the effects of quadrupole offsets and idealized steering were also carried out. Initial steps have been taken to design and simulate a drift compression system.

Simulations were also used to design and support other experiments. For beam injectors, extensive simulation was carried out to determine the sensitivity of the 2 MeV injector to changes in geometry, voltage, and source uniformity. A multiple-beamlet injector has been designed that promises a higher average current density than a single large-aperture beam injector. Simulations were also done for the High-Current Experiment (HCX), the Scaled Final-Focus Experiment, the University of Maryland electron ring experiment, and the European approach to heavy-ion fusion.

Finally, simulations were used to study several basic questions of accelerator physics, including beam splitting and halos. A preliminary investigation of impedance effects on longitudinal instability found that a coasting beam under some conditions remains stable through hundreds of accelerating modules.

Significance
The U.S. HIF program is charged with developing an accelerator and focusing system capable of igniting inertial-fusion targets at a cost that is competitive with other long-term energy sources. Achieving this objective requires significant advances in beam physics and in accelerator science and technology. Numerical simulations are essential for all aspects of the HIF program, including interpreting the results of ongoing experiments and developing and optimizing the designs of future experiments.

Publications
A. Friedman, D. P. Grote, E. P. Lee, and E. Sonnendrucker, "Beam simulations for IRE and driver: Status and strategy," 13th Inter-national Heavy-Ion Fusion Symposium, 12—17 March 2000, San Diego, CA; Nuclear Instruments and Methods in Physics Research (in press).

D. P. Grote, A. Friedman, G. D. Craig, W. M. Sharp, and I. Haber, "Progress toward source-to-target simulations," 13th International Heavy-Ion Fusion Symposium, 12—17 March 2000, San Diego, CA; Nuclear Instruments and Methods in Physics Research (in press).

E. Sonnendrucker, A. Friedman, J. J. Barnard, D. P. Grote, and

S. M. Lund, "Simulation of heavy ion beams with a semi-Lagrangian Vlasov solver," 13th International Heavy-Ion Fusion Symposium, 12—17 March 2000, San Diego, CA; Nuclear Instruments and Methods in Physics Research (in press).