High-Current Beam Studies for Heavy-Ion Fusion
A. Friedman, D. P. Grote, S. M. Lund, J. J. Barnard, W. M. Sharp, and G. D. Craig, Lawrence Livermore National Laboratory
I. Haber, Naval Research Laboratory
Research Objectives
Simulate heavy-ion induction acceleration to study basic beam physics, analyze beam behavior in experiments, and predict beam behavior in future facilities.
Computational Approach
The NERSC computers are used for challenging calculations based on two principal models: an envelope equation/fluid model embodied in the code CIRCE, and a particle-in-cell (PIC) model embodied in the code WARP. CIRCE offers rapid synthesis and optimization, but neglects higher-order effects that can lead to beam degradation. WARP provides a fully kinetic description of beam behavior in 2D and 3D. The codes are sometimes linked.
Accomplishments
We extended our understanding of high-current beam transport and acceleration and increased the power of our computational tools.
Earlier simulations showed evidence of an instability that drives thermal energy exchange in intense beams initially colder longitudinally than transversely. Recent simulations, coupled with theoretical analysis, have led to a more complete understanding of the mode, which constrains the parameter space available to a fusion driver.
An experiment that accelerates a space-charge dominated beam through a 90-degree bend is under way at Lawrence Livermore National Laboratory. Extensive simulations have examined the optimization of the envelope match, emittance growth from longitudinal dispersion, injector transients, and the effects of beam nonuniformities (see figure). A complete ring, proposed for the experimental program, has served as a simulation testbed for novel feed-forward beam steering and adaptive control algorithm development.
The HIBALL II final-focus system, including an axially compressing beam, was modeled for the first time self-consistently in 3D. This work validated earlier 2D simulations of the design.
We began simulating the long-term behavior of a beam in a next-step inertial fusion energy facility incorporating hundreds of fundamental lattice periods and using both electric and magnetic focusing (confinement). With ideal linear applied fields and realistic tolerances on transverse misalignments, the beam is seen to suffer little degradation in focusability.
A new interface to WARP was built using the Python interpreter/scripting language, which was readily adapted to the parallel environment. This added a fully interactive code-steering capability to the existing parallel code.
Significance
The principal approach to developing an inertial fusion driver in the DOE Office of Fusion Energy Science’s Inertial Fusion Energy program is the heavy-ion induction accelerator.
Contours (in the transverse plane) of the density of a high-current beam at four stations along the LLNL bend experiment. The first frame shows the beam as it exits the diode, reconstructed from experimental measurements at that location. The following frames show a space-charge wave that spreads out and bounces from the edge of the beam, ultimately to reform a high-density spot similar to that at the start. Red denotes highest density, blue lowest.
A key challenge, essential to a cost-effective driver, is maintaining the focusability of the intense ion beams (non-neutral plasmas) through numerous manipulations in the presence of the strong, nonlinear self-fields.
Publications
D. P. Grote, A. Friedman, I. Haber, W. M. Fawley, and J. L. Vay, "New developments in WARP3d: Progress toward end-to-end simulation," Proc. Int. Sympos. on HIIF, (Heidelberg, September 1997); Nuclear Instruments and Methods A, in press.
S. M. Lund, J. J. Barnard, G. D. Craig, A. Friedman, D. P. Grote, H. S. Hopkins, T. C. Sangster, W. M. Sharp, S. Eylon, T. J. Fessenden, E. Henestroza, S. S. Yu, and I. Haber, "Numerical simulation of intense-beam experiments at LLNL and LBNL," Proc. Int. Sympos. on HIIF, (Heidelberg, September 1997); Nuclear Instruments and Methods A, in press.
W. M. Sharp, D. P. Grote, M. A. Hernandez, and G. W. Kamin, "Steering algorithms for a small recirculating heavy-ion accelerator," Proc. Int. Sympos. on HIIF, (Heidelberg, September 1997); Nuclear Instruments and Methods A, in press.