San Diego, California, 17-19 June 2002

Threat assessment from charged particles to the SOMBRERO target chamber wall and mitigation scenarios

Jeff Latkowski
latkowski1llnl.gov,
D. T. Blackfield
dtbllnl.gov
Lawrence Livermore National Laboratory

W. Meier
meier5llnl.gov,
L.J. Perkins
perkins3llnl.gov
Lawrence Livermore National Laboratory

D. Welch
drwelchmrc.com,
Mission Research;

With the latest NRL 400 MJ target design and detailed target physics calculations using LASNEX, a greater threat to the SOMBRERO target chamber wall survivability from high-energy charged particles now exists. Although the original SOMBRERO chamber design included 0.5 Torr of xenon gas to prevent damage to the chamber wall from charged particles and x-rays, recent calculations indicate that target heating during injection will limit the xenon gas pressure to less than 0.05 Torr. TRIM [1] calculations predict that, with this reduced pressure, most ions will now reach the chamber wall, hence significant radiation damage from ion bombardment will occur.

To increase the wall lifetime, we present a scenario using coils outside of the target chamber to create a magnetic field that prevents ions from reaching the chamber walls. Instead, ions are directed out holes in the chamber wall to be collected in beam dumps. We first assess the engineering and physics feasibility of using several coils to create a uniform magnetic field. We use the 3D finite element code TOSCA [2] to design a coil configuration that will produce a uniform vacuum magnetic field. Seven loop coils with radii from 350 cm to 850 cm and currents from 500 to 1200 A/cm² will create a uniform vacuum magnetic field of 1.6 T. In this field, the plasma expands in the radial direction a distance well short of the chamber wall and exits axially. We use 2D and 3D PIC simulations [3] to determine the radius of plasma expansion and exit hole diameters. We use TART [4] to determine coil shield and heat removal requirements.

With the presence of a neutral gas, there is a possibility that charge transfer processes may allow a significant fraction of ions to be neutralized, move across field lines, and strike the chamber wall. We examine various charge transfer cross sections to make an initial assessment of the impact of these atomic collisions.

Finally, having assembled this suite of tools, we begin to examine other candidate coil configurations such as those that will produce magnetic mirror and cusp external fields.

[1] J.P. Biersack and L. Haggmark, "A Monte Carlo Computer Program for the Transport of Energetic Ions in Amorphous Targets", Nucl. Instr. and Meth. 174 (1980) 257 J. F. Ziegler, "The Stopping and Range of Ions in Matter," vol. 2-6, Pergamon Press (1977-1985)

"OPERA-3d User Guide", Vector Fields Limited, 24 Bankside, Kidlington, Oxford, OX5 1JE, England

[3] Dale R. Welch, "Hybrid Implicit Algorithms for IPROP and Lsp", Mission Research Report MRC/ABQ-R-1942, Mission Research Corp. Alberquerque, NM (Oct. 1999)

[4] D. E. Cullen, "TART98: A Coupled Neutron Photon, 3-D, Combinatorial Geometry, Time Dependent, Monte Carlo Transport Code", LLNL Report UCRL-ID-126455, Rev. 2 (1998)