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Update on Plasma Reconstructions in Vacuum

Update on Plasma Reconstructions in Vacuum. NCSX SWG Meeting Reiersen, Georgiyevskiy, et al PPPL June 17, 1999. Topics. Review last week’s results d 2 B z /dr 2 scan B t scan Symmetrization at 100% and 110% toroidal field Harmonic analysis Conclusions. Review of last week’s results.

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Update on Plasma Reconstructions in Vacuum

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  1. Update on Plasma Reconstructions in Vacuum NCSX SWG Meeting Reiersen, Georgiyevskiy, et al PPPL June 17, 1999

  2. Topics • Review last week’s results • d2Bz/dr2 scan • Bt scan • Symmetrization at 100% and 110% toroidal field • Harmonic analysis • Conclusions

  3. Review of last week’s results • Fixed boundary VMEC had iotas from 0.033-0.288 • Fixed boundary PIES results showed few good surfaces outside 3/19 island chain • Free boundary PIES results were similar • Iotas range from 0.05-0.158 • Large stochastic region apparent outside of 3/19 islands • Approximately half the plasma volume is lost • Outer region packed with n=3 resonances • Nine primary resonances in this region (m= 18 -10) • Spacing between resonances ranges from 7-12 mm • Initial efforts to beat down resonances unsuccessful • J in large saddles up by almost 60%

  4. Fixed/free boundary PIES results Fixed Boundary Free Boundary

  5. Bz varied from 0 - 1x reference • Plasma shifts outward with more positive Bz • Minor radius at midplane increases, less indentation • Little impact on iota or number of good surfaces prior to scraping off plasma Reference 0.5 Reference Zero Bz

  6. dBz/dr varied from 0 - 2x reference • Increasing dBz/dr did not move axis or change height • Lowered elongation, increasing volume with good surfaces • Doubled central iota from .042 to .100 for 2x ref, lesser impact on edge iota Zero dBz/dr Reference 2x Reference

  7. d2Bz/dr2 varied from -1 to +4x reference

  8. More concave -> more crescent-shaped plasma • More convex - > more square-shaped More concave (-1x reference) Reference d2Bz/dr2 More convex (+4x reference)

  9. Bt varied from 80-110% • All other coil currents fixed • Lowering Bt raises iota, decreases plasma volume by moving edge limiting resonances into the plasma region • Raising Bt lowers iota, increasing the plasma volume by moving edge limiting resonances beyond the first wall

  10. 80% TF 100% TF 110% TF Iota(a)=0.208 Iota(a)=0.158 Iota(a)=0.157

  11. Modest improvements can be achieved by symmetrization with PF • Bz used to control radial position • dBz/dr used to control elongation • d2Bz/dr2 used to control “squareness”

  12. Edge on symmetrized reference vacuum configuration defined by onset of overlapping islands • Highlights need to suppress islands to grow plasma

  13. Edge on symmetrized 110% Bt configuration defined by first wall • Highlights need to expand first wall boundary to grow plasma • Particularly in square x-section (top, bottom, and outboard) and bean-shaped cross section (inboard)

  14. Harmonic analyses • Poor correlation found between island size and Bmn - lower Bmn’s had larger islands than higher Bmn’s closer to the plasma edge • Discrepancy may be due to not decomposing Bn in straight line coordinates (Reiman, Hirshman) • Valanju has developed a code to do this - awaiting input on fixed boundary equilibrium from Ku for reference vacuum case • Correlation study will be repeated

  15. TF ripple appears to have no impact on magnetic surfaces • TF linear dimensions roughly doubled, lowering TF ripple by orders of magnitude • No discernible impact apparent in Poincare plots • Bmn’s impacted in fourth significant digit

  16. Conclusions • The PF system provides useful knobs for controlling radial position, elongation, and triangularity/squareness • The TF system provides a useful knob for raising/lowering iota, moving problematic resonances into or out of the plasma in the process • TF ripple does not have a significant impact on magnetic surfaces • To increase plasma volume and iota, need to: • Expand first wall boundary in tight spots • Have capability to surgically suppress problematic resonances • Reduce current density in reference configuration • Need headroom for coil currents to increase at other operating points • Important for flexibility!

  17. Next steps • Explore expanded first wall boundary • Explore plasma shape and iota profile control with saddle coils • Validate harmonic analysis methodology as basis for suppressing islands • Explore suppression of islands with existing coil systems • Develop functional requirements for trim coils • Continue to explore options for reduced J in reference configuration • Study magnetic surfaces in the reference high beta, full current configuration

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