5C06: 5C07:

Electron Beam Distrnption Due to Ion Release From Targets-Comparison Between Computer Calculations And

Observations’ A particle simulation study of self-hias potential for plasma

ion confinement in multiple-pulse radiography

T. P. Hughes*, H. A. Davis, C. Vermare, D. C. Moir, R. T. Olson, and W. M. Wood

Los Alamos National Laborarory, Los Alamos, NM 87544

Experiments have been performed on the first axis of the DARHT electron beam accelerator to study the effects on the beam of ion emission from surfaces struck by electrons (H. A. Davis et. al., this meeting). Calculations using the two-and three-dimensional Lsp panicle-in-cell code were performed to compare with experimental results. Two-dimensional calculations, which allow the release of ions when the impacted surface temperature increases by 400K accurately replicate experimental results showing beam blow-up downstream of the surface just after ion emission. Three-dimensional calculations show the onset of a hose instability after beam blow up in agreement with observations. The frequency of this instability indicates a large component of €f ions, which may come from the “cracking” of desorbed water. Calculations in support of experiments to identify the desorbed species are presented.

‘Work sponsored by US DOE contract W-405-ENG-36. *Mission Research Corporation, Albuquerque, NM.

Lin Yen and Thomas J. T. Kwan Los Alamos National Laboratory

Applied P hysics Division L o s Alamos, NM 87545

The intense deposition of energy on to a high-Z target from a high current relativistic electron beam produces bremsstrahlung photons which can be used to generate radiographs of a dynamically evolving object. The interaction of the intense electron beam with the target can generate plasmas from the target surface. Consequently, a column of plasma ions expands away from the target due to the space charge potential of the electron beam. This expansion can lead to a larger beam spot size resulting in degradation of spatial resolution of radiographic images. Previous studies have shown that a self-biased electric potential between the target and the collimator can confine the ion column and maintain the electron beam spot size.[l] In this work a two-dimensional particle-in-cell code is used to study a novel target configuration in which the converter target and the beam dump are connect electrically to maintain the self-biased potential in multiple-pulse radiography such as in the second axis of DARHT. Our simulation study showed that the self-bias electric potential between the target and the collimator was effective in confining the ions emitted from the target during multiple-pulse operation. The effects of the inducrance of the target chamber on the decay time of the self-biased potential are also examined.

[I1 Thomas J. T. Kwan, et al., Phys. Plasmas Vol. 7, No. 5 , 2215 (2000).

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