1. Nov. 3-5, 2003Takahashi - Active Control of MHD1 Using Actively Driven SOL Current for Controlling Vertical Instability and Other MHD Modes in Tokamaks H. Takahashi and E.D. Fredrickson Princeton University Workshop on Active Control of MHD Stability: Extension to the Burning Plasma Regime November 3 - 5, 2003 University of Texas Austin, TX - Bringing back Old Ideas into a New Environment -

2. Nov. 3-5, 2003Takahashi - Active Control of MHD2 Can ITER/Reactor Design Be Improved? ITER and reactors will have large control coils far from plasma. Control coils far from plasma are inefficient. –Multipole fields decay fast with distance. –Coil power supplies need high current, large bandwidth. –Inductive heating of cryogenic assembly requires additional cooling. There is, perhaps, room for innovation here… –Closer feedback circuit would be more efficient.

3. Nov. 3-5, 2003Takahashi - Active Control of MHD3 Using Scrape-Off-Layer Current (SOLC) for: (1)MHD stability (2)Confinement improvement (3)H-mode power threshold reduction (4)Other worthy causes is an old (and good) idea*. *See, e.g., “Workshop for Feedback Stabilization of MHD Instabilities (1996)” (K. M. McGuire, et al., NF 37(1997)1647-1655): But it has rarely been carried out in major facilities.

4. Nov. 3-5, 2003Takahashi - Active Control of MHD4 Electrodes Previously Proposed to Drive SOLC (1) S.C. Jardin and J.A. Schmidt, “Numerical Simulation of Feedback Stabilization of Axisymmetric Modes in Tokamaks Using Driven Halo Current,” NF 38(1998)1105-1112. (2) R. Goldston, “Toroidally Segmented Divertor Biasing and Current Injection,” Plasma Phys. and Controlled Fusion. (3) H.W. Kugel, et al., “Feedback Stabilitzation Experiment for MHD Control with Edge Current,” SOFE 1997. Vertical Control Toroidally Segmented Divertor Biasing Ref. (1)Ref. (2-3) n = 0 n > 0

5. Nov. 3-5, 2003Takahashi - Active Control of MHD5 What New Environment? (1)Increased knowledge of SOLC (2)More urgent need for MHD control: future has drawn closer. (3)Opportunities for carrying out active SOLC control experiment in high betaN tokamaks: DIII-D, NSTX, MAST, AUG, … (4)Extensive and expanding MHD feedback programs exist or planned. (5)Opportunities to make contributions to ITER.

6. Nov. 3-5, 2003Takahashi - Active Control of MHD6 What Increased Knowledge of SOLC? Measurement SOLC in DIII-D, TCA - presence of large intrinsic current during MHD Experience with Driven “SOLC” in NSTX (helicity injection) Measurement of SOL properties in DIII-D, MAST, AUG, TCA, … Some example measurements in DIII-D follow…

7. Nov. 3-5, 2003Takahashi - Active Control of MHD7 B-field Signal Pollution Feedback Control Tokamak Operation Equilibrium Reconstruction ? Potential Effects of Error Field Generated by SOLC SOLC Intrinsic or Driven Resonant B-field Normal to Flux Surfaces Flux Surface Distortion MHD Stability ? ? MHD Control

8. Nov. 3-5, 2003Takahashi - Active Control of MHD8 SOLC Flows Just Outside Separatrix The origin of the SOLC* is not yet fully understood - not a subject of this talk. *See, e.g., discussion by M. Schaffer and B. Leikind, NF 31(1991)1750. Topology of SOLC path can change for small shift in location (compare red and blue curves on the right). Line Current Model The simplest model SOLC flows along an open field line and closes its circuit through the tokamak structure.

9. Nov. 3-5, 2003Takahashi - Active Control of MHD9 SOLC Generates Helical Field Pattern B-field Normal to q=3 Surface Produced by SOLC

10. Nov. 3-5, 2003Takahashi - Active Control of MHD10 RWM Produces Helical Field Pattern *From M. Okabayashi, et al. Poloidal angle 180 0 -180 External coils try to emulate RWM field pattern. Why not match helical with helical using SOLC? B-field Normal to Plasma Surface Produced by RWM*

11. Nov. 3-5, 2003Takahashi - Active Control of MHD11 Control with Different Current Path Topologies Secondary feedback loop keeps SOLC in a desired toroidal distribution by applying control through toroidally segmented electrodes. Vertical Control (n = 0)MHD Control (n > 0)

12. Nov. 3-5, 2003Takahashi - Active Control of MHD12 DIII-D Has Sensor Arrays for Measuring Current through Divertor Tiles Bottom Divertor Top Divertor A narrow SOL current channel may escape detection, because less than 10 % of tiles in only selected tile-rings have sensors. Each of shaded divertor tiles is instrumented with a resistive-element current sensor (tile representation merely schematic). *Schaffer, et al., Poster 3Q21, APS-DPP, 1996, Denver, CO, Nov. 11-15.

13. Nov. 3-5, 2003Takahashi - Active Control of MHD13 SOLC Spikes Accompany ELMs… Inner and outer divertor tile rings are connected via open field lines without obstruction in-between. Notion that SOLC flows along open field lines is generally borne out, though not always in quantitative details. SOLC spikes can be an indicator of ELMs. outer strike point inner strike point Discharge Summary Positive signal means current flowing from plasma into tile. Tile Current D  Light

14. Nov. 3-5, 2003Takahashi - Active Control of MHD14 SOL Current Can Be Oscillating… SOLC (bi-polar) Mirnov (B-dot) 4.9kHz 6.5mTp-p Discharge Summary SOLC Mirnov

15. Nov. 3-5, 2003Takahashi - Active Control of MHD15 SOL Current Can be Large, Non-axisymmetric… Over 800 A thru one tile Peak current does not always occur at the same toroidal location. Large SOL current may be non- linearly coupled with Ip evolution caused by thermal collapse. tile at 0 deg tile at 150 deg Discharge Summary

16. Nov. 3-5, 2003Takahashi - Active Control of MHD16 SOLC Spreads Radially Far during ELM Re-circulating current flows from (probably) top (ring #11A) to bottom (#11B) in near SOL and from bottom (#12B) to top ( #12A) in far SOL. Some current in very far SOL also. Nearly 400 A flowed through a single tile during a large ELM. Discharge Summary N/A SOLC over Wide Radial Region During an ELM SOLC was spread over at least 21 cm, possibly 36 cm, beyond bottom outboard strike point (at least 5 cm when measured in outboard mid-plane). 1 cm spacing Wall at 6 cm SOLC fills space between plasma and wall during ELM, and reverses its direction, possibly twice. Top Divertor Mid-plane Bot Divertor

17. Nov. 3-5, 2003Takahashi - Active Control of MHD17 SOLC Has Complex Radial Structure during ELM Discharge Summary Waveforms are different on adjacent rings. Temporal and spatial structures of SOLC are complex during an ELM. SOLC in adjacent tile rings during a single ELM in expanded time scale Radial Sensor Array Tor/Rad Width=7.5deg/7.1cm Tor/Rad Width=7.5deg/13.9cm Tor/Rad Width=5.0deg/14.3cm Ring #11B Ring #12B Ring #13B

18. Nov. 3-5, 2003Takahashi - Active Control of MHD18 Staged Experiment Stage-I: Install toroidally segmented electrodes with leads having “on/off” switching capability for grounding. (a)Effect of cutting-off SOLC on MHD activity, including RWM, ELM, NTM, and LM - use on/off switching capability to establish causality. Stage-IIa: Add power supplies. (a)How much current can be driven? (b)Can SOLC-generated error field affect MHD? (c)Can SOLC rotate plasma through “entraining?” (d)Do driven and intrinsic SOLC interact? Configure a multi-staged experiment whose ultimate goals are to actively exploit SOLC for controlling vertical instability and other non-axisymmetric MHD modes.

19. Nov. 3-5, 2003Takahashi - Active Control of MHD19 Staged Experiment-Cont. Stage-III: Install primary feedback based on magnetic (or other position sensor) signals for vertical position control. (a)Demonstrate feedback control of vertical positional instability. Stage-VI: Install primary feedback based on magnetic sensor signals for non-axisymmetric MHD modes. (a)Demonstrate feedback control of non-axisymmetric MHD modes. Stage-IIb: Add secondary feedback based on current sensor signals. (a)Develop technique to maintain desired toroidal SOLC distribution. (b)Examine effect of symmetrized SOLC on MHD activity. (c)Examine effect of non-axisymmetric SOLC on MHD activity.

20. Nov. 3-5, 2003Takahashi - Active Control of MHD20 Summary The use of actively driven SOL current (SOLC) was considered with the following goals in mind: I.To develop efficient techniques for controlling vertical instability and other low-frequency MHD modes in ITER. II.To offer, through a staged experiment, opportunities to answer a number of physics questions about SOLC: (a)Effect of cutting-off SOLC on MHD activity, including RWM, ELM, NTM, and LM. (b)Interaction of intrinsic and driven SOLC. (c)Effect of symmetrized SOLC on MHD activity. (d)Effect of non-axisymmetric SOLC on MHD activity.