1. Dec. 12, 2008Takahashi MHD SFG Mtg1 Scrape-Off-Layer Current (SOLC) Study in NSTX Hiro Takahashi MHD Science Focus Group Meeting Princeton Plasma Physics Laboratory December 12, 2008 Thanks: Eric Fredrickson Stefan Gerhardt Robert Kaita Related Papers: PRL100(2008)205001 EPS(2005)4.018 NF44(2004)1075-1096 NF42(2002)448-485

2. Dec. 12, 2008Takahashi MHD SFG Mtg2 Issues in SOLC Study in NSTX 1.Elucidate Origin of SOLC a)Thermo-electric potential (sheath-lithium interplay?) b)Coupled MHD-SOLC (mutually driven) c)Flux linkage with time-varying plasma (“halo current”) 2.Determine Time-varying Spatial Structure of SOLC Including Filaments a)ELM (onset period) b)RWM-event (onset period) c)NTM-event d)Locked Mode/momentum transfer e)Blobs (current-carrying flux tubes?) f)High-performance-discharge termination (n index change?) g)CHI (after closed surface formation) h)Disruption halo current (initial stage) 3.Examine Effect of SOLC a)Masquerade as MHD modes b)Modify appearance of MHD (spoil MHD feedback by confusing error signals) c)Help destabilize MHD modes 4.Control SOLC (Future) a)Drive SOLC actively by external supply to generate ELMs on demand b)Cut off intrinsic SOLC (“insulated” tiles) to eliminate ELMs

3. Dec. 12, 2008Takahashi MHD SFG Mtg3 SOLC-Lithium Synergy in ELM Study 1.SOLC-ELM Causality Hypothesis. a)Sheath-breakdown ELM Trigger Model. b)Thermo-electric potential impressed across sheath exceeds breakdown limit. c)Use lithium to alter sheath conditions for systematic study. 2.ELMs in NSTX Suggest Different Model? a)Lithium can create ELM-free discharges. b)RMPs can generate, not suppress, ELMs. 3.Lithium Effect in NSTX in Need of Physical Explanations? a)Lithium raising ion saturation current density or lowering sheath Te ratio? b)Lithium raising sheath breakdown limit (perhaps through work function)? 4.Sheath Density and Temperature Control by Lithium Aids Systematic Test of SOLC-ELM Causality Hypothesis.

4. Dec. 12, 2008Takahashi MHD SFG Mtg4 Sheath-Breakdown ELM Trigger Model  = T hot /T cold  =  Cold /  SOL Thermo-electric Potential Contours Test results, though limited in scope, are suggestive that sheath breakdown plays a role ( EPS(2005)4.018). Flux-tube SOLC Circuit Model (Two-parameter System:  ) Abrupt and rapid (~ 20  s) change observed in SOLC at ELM trigger before edge thermal collapse (ECE). 3 ms SOLC during ELM (A/tile) ELMing No ELM

5. Dec. 12, 2008Takahashi MHD SFG Mtg5 Forward-Problem Solving Successful at ELM Peak Poloidal VariationToroidal Variation (Outboard Mid-plane) Observed Modeled PRL100(2008)205001 Forward-problem solving, SOLC B-field successfully modeled measured field at ELM peak (but used interpolated toroidal profile and event-averaged radial profile for SOLC). Forward-problem solving is inadequate at ELM onset because of filamentary (not amenable to interpolation) and irreproducible (not amenable to event-averaging) SOLC profiles. Inverse-problem solving, SOLC B-field more suitable for causality investigation at ELM onset.

6. Dec. 12, 2008Takahashi MHD SFG Mtg6 Technical Approaches to SOLC Study in NSTX 1.Utilize New and Existing Diagnostics. a)Magnetic New tri-axial magnetic probe toroidal and poloidal arrays Existing Mirnov coils (only those not shielded) b)Tile Current New array(s) in top divertor Existing 4-sensor array (Gerhardt) in bottom divertor c)Langmuir Probe New closely-spaced radial 1D array(s) in top divertor Existing probes around machine New 2D array (UI) in bottom divertor 2.Develop Analysis Methods. a)Inverse-problem (   -optimization fitting) for SOLC distribution b)Langmuir I-V curve analysis for magnetized plasma