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C. H. Skinner, D. Mueller, R. Maingi Boundary Physics ET Meeting, Wednesday 12/17/03

Density Control through plasma termination control. C. H. Skinner, D. Mueller, R. Maingi Boundary Physics ET Meeting, Wednesday 12/17/03. Motivation:. The IPPA 5 year goal for end FY 2005 includes:

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C. H. Skinner, D. Mueller, R. Maingi Boundary Physics ET Meeting, Wednesday 12/17/03

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  1. Density Control through plasma termination control. C. H. Skinner, D. Mueller, R. Maingi Boundary Physics ET Meeting, Wednesday 12/17/03

  2. Motivation: The IPPA 5 year goal for end FY 2005 includes: “Determine the ability for managing intense energy and particle fluxes in the edge geometry and for increasing pulse durations...” Uncontrolled (non-disruptive) density rise in long pulse H-modes Initial work done in FY2003 XP304: Goal 1: Assess role of divertor temperature in density rise and impurity generation by strike point jog. Result: roll over in density observed after jog, but motion away from CS caused major part. Goal 2: Reduce density rise by depleting D in divertor with He conditioning with NBI Result: Modest change - after 17 discharges D-alpha decreased by 40% cf x10 TFTR. Potential factors : • Heat flux profiles on divertor not matched • Inefficient gas fueling during discharge (unlike TFTR) • H-mode physics (SOL transparency, transport barrier) • Tail end disruptions. #108728 Ip [MA] PNBI/10 [MW] ne [1019 m-3] Da [au] Sgas[torr-l/s] SNBI dNe/dt WMHD*10 [MJ] H98pby2 From Maingi ‘BP_NSTX_5yrpln_061703.ppt’

  3. RF helium discharges did better RF current drive XP on next day (Feb 11th 2003) showed dramatic reduction in density. • Much better job of degassing tiles than NBI heated He discharges. • Every He conditioning discharge ended with a disruption whereas Dennis took care that RF discharges did not. • This may be key step to controlling H-mode density rise. Comparison of 110118 (toward end of He conditioning XP304 11 Feb 2003) and 110162 (toward end of HHFW CD XP 312 12 Feb 2003)

  4. Propose to explore effect of plasma termination on density rise in dedicated XP in upcoming run. Draft Conceptual Proposal for XP: • Establish long pulse H-mode fiducial • Run conditioning RF heated He discharges with smooth termination (XP 312 Feb 12th 2003) to deplete limiter • Repeat attempts at long deuterium H-modes - check for reduced density rise. • Continue H-modes to restore original conditions • Repeat conditioning discharges varying plasma termination, RF power, He/D... • Identify density rise dependence on: • plasma termination (disruptions are know to be deleterious for wall conditions) • RF power (heats edge plasma and increases penetration of ions) • He gas feed (displaces D in limiter) • .....

  5. Detailed shot list and decision points: Key: F = Fiducial: LSN H-mode 4 MW NBI with center stack gas puff e.g. # 108728 C = conditioning discharge, from XP-312 He+RF, e.g. 110152 Sm = smooth rampdown of Ip with reduced reconnection event. Ro = rough rampdown of Ip with regular reconnection event If Fiducial is better, then repeat to restore original bad conditions for next test. 'Better' means slower density rise upward arrow means time of day a factor also. He can displace D from wall; RF increases edge temperature, interaction depth and increase D depletion; Sm can avoid deconditioning the wall - aim is to identify which is most important in logical sequence of discharges 19 - 46 discharges conclude that if 6 conditioning discharges are needed then logic will be incomplete.

  6. Prerequisites: • (from XP304) Fiducial: 1-2 shots high performance long pulse shot e.g. 108741 LSN, H-mode in D, similar to 109063 record 0.8 MA, 1 s, 0.7 s flattop 4.8 MW NBI H-mode in D, but with lower voltage beams and longer flattop. Two NB sources required. Minimum duration of discharge after L-H transition is about 0.3 s. 108741 was a 5 kG discharge but we will run it at 4.5 kG. Beams @ 80 kV to improve reliability. Diagnostics to include VIPS2 looking for CD molecular bands at 431 nm, IR divertor camera viewing divertor, realtime EFIT. Backup fiducial is 107796. This is a 4.5kG discharge, but the startup used more flux than 108741. • (from XP312) Ip ≈500kA, Bt ≈ 0.45 T, He for long pulsesRF needs all six transmitters operating for long pulses at 6a00 - 750 kW each phase control feedback and I-V cube signals (phase and magnitude).Need plasma TV for antenna hot spot surveillance. • If RF unavailable, then proceed checking effect of plasma termination and He/D

  7. Model H-mode Fiducial

  8. Model Conditioning discharge

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