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Analysis of a Small-Scale Magnetic Deflection Experiment *

Analysis of a Small-Scale Magnetic Deflection Experiment *. D. V. Rose,** T. C. Genoni ATK Mission Research E. Robson, J. D. Sethian Naval Research Laboratory HAPL Meeting, June 2005, LLNL. *R. E. Pechacek, et al ., Phys. Rev. Lett. 45 , 256 (1980). **David.Rose@atk.com.

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Analysis of a Small-Scale Magnetic Deflection Experiment *

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  1. Analysis of a Small-Scale Magnetic Deflection Experiment* D. V. Rose,** T. C. Genoni ATK Mission Research E. Robson, J. D. SethianNaval Research LaboratoryHAPL Meeting, June 2005, LLNL *R. E. Pechacek, et al., Phys. Rev. Lett. 45, 256 (1980). **David.Rose@atk.com

  2. Experiment Description • A two-stage laser system drives a 1-mm scale, solid D2 pellet forming a plasma. • The plasma is created inside the void of a cusp magnetic field. • The adiabatically expanding plasma compresses the cusp field lines. • Plasma ions escape from the “point” and “ring” cusps in the field geometry. • Plasma ions are “deflected” away from the chamber walls

  3. Experimental Parameters • Chamber wall radius is 30 cm (not shown) • External field coils, 67 or 70 cm diam, 70 cm separation. • |B| = 2.0 kG at ring cusp. • 2x1019 D2 ions produced from cylindrical target of 1-mm diam., 1-mm length.

  4. We have examined three different initialize plasma conditions: • A uniform density, thermal distribution of D2+ ions with an initial radius of 4 cm. • A non-Maxwellian energy distribution, estimated from experimental measurements, with a 2 cm initial radius and uniform density. • A non-Maxwellian energy distribution, with a 2.5 cm initial radius with radial energy and density distribution obtained from time-of-flight ballistic expansion from a 2-mm radius “pellet”.

  5. 1. Initialize with thermal ion cloud, uniform density, 4 cm radius

  6. T=0 ns T=1000 ns T=2000 ns

  7. At r=22 cm inside ring cusp, electron density was measured at 5 different times.

  8. Compare experimental data with an “orbit” calculation (no field pushing, no field-particle energy exchange) and EMHD simulation: peak density unaffected, but FWHM of escaping ions closer to measured value.

  9. Plasma/Field boundary along 27 degree radial line from the cusp center:

  10. 2A. Estimate of Initial Energy Distribution in the Laser-Created D2+ Plasma

  11. Hand-digitize data from 1980 PRL to estimate initial ion energy distribution: Measured J, no appliedB-field Model calculation (Haught & Polk, 1970).

  12. Use experimentally measured J from shot without applied B-field to determine approximate source distribution function:

  13. Sample from this calculated distribution in an LSP orbit calculation, assuming a 4-cm radius, uniform density ion cloud with radially directed velocities.

  14. 2B. EMHD simulations with the non-Maxwellian distribution function, 2-cm initial radius and uniform density

  15. LSP run: bob9b.lsp Sigma = 1e14 (1/s) Ne-background=2e14 (cm-3) Non-Maxwellian ion distribution Initial magnetic field magnitude 0.00015 ms 2-cm initial plasma radius (uniform density)

  16. 1.5 ms 0.5 ms 1.0 ms

  17. 2.0 ms 2.5 ms 3.0 ms

  18. 4.5 ms 3.5 ms 4.0 ms

  19. 5.0 ms 5.5 ms 6.0 ms

  20. Magnetic field swept back too quickly, and takes too long to snap back into place.

  21. 3. Replace 2-cm radius, uniform density plasma blob with density distribution that results from time-of-flight expansion from 2-mm radius, uniform density blob. Use same non-Maxwellian speed distribution from previous simulation.

  22. Field-free expansion of 2-mm radius ion cloud due to non-Maxwellian speed distribution at 75 ns. Particle Positions Density Contours Note reduced chamber dimensions for this “special” calculation.

  23. No significant change to the answer with a non-uniform, pre-loaded plasma

  24. Comparison between EMHD model and simple orbit calculation Total ion energy inside 30 cmradius chamber vs. time Total ion charge inside 30 cmradius chamber vs. time EMHD orbit orbit EMHD Ion charge slower to leave chamber, due in part to energy exchange with magnetic field.

  25. Status: • Present simulation results suggest numerical (artificial) current & field diffusion into plasma volume. • Recent tests suggest that this numerical diffusion can be significantly reduced with more conservative calculation of computational constraints. • Reasonable agreement obtained between the Robson “shell” model and 1D LSP tests using the more conservative constraints.

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