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H. Takahashi and E.D. Fredrickson Princeton University

Using Actively Driven SOL Current for Controlling Vertical Instability and Other MHD Modes in Tokamaks. - Bringing back Old Ideas into a New Environment -. H. Takahashi and E.D. Fredrickson Princeton University. Workshop on

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H. Takahashi and E.D. Fredrickson Princeton University

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  1. Using Actively Driven SOL Current for Controlling Vertical Instability and Other MHD Modes in Tokamaks - Bringing back Old Ideas into a New Environment - 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 Takahashi - Active Control of MHD

  2. 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. Takahashi - Active Control of MHD

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

  4. Vertical Control Toroidally Segmented Divertor Biasing n > 0 n = 0 Ref. (1) Ref. (2-3) 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. Takahashi - Active Control of MHD

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

  6. 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… Takahashi - Active Control of MHD

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

  8. SOLC Flows Just Outside Separatrix Line Current Model The simplest model SOLC flows along an open field line and closes its circuit through the tokamak structure. Topology of SOLC path can change for small shift in location (compare red and blue curves on the right). 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. Takahashi - Active Control of MHD

  9. SOLC Generates Helical Field Pattern B-field Normal to q=3 Surface Produced by SOLC Takahashi - Active Control of MHD

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

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

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

  13. inner strike point outer strike point Discharge Summary SOLC Spikes Accompany ELMs… Positive signal means current flowing from plasma into tile. 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. Tile Current Da Light Takahashi - Active Control of MHD

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

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

  16. Discharge Summary SOLC Spreads Radially Far during ELM Top Divertor Mid-plane 1 cm spacing Wall at 6 cm SOLC over Wide Radial Region N/A N/A 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. Bot Divertor 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). SOLC fills space between plasma and wall during ELM, and reverses its direction, possibly twice. Takahashi - Active Control of MHD

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

  18. Staged Experiment Configure a multi-staged experiment whose ultimate goals are to actively exploit SOLC for controlling vertical instability and other non-axisymmetric MHD modes. Stage-I: Install toroidally segmented electrodes with leads having “on/off” switching capability for grounding. 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. How much current can be driven? Can SOLC-generated error field affect MHD? Can SOLC rotate plasma through “entraining?” Do driven and intrinsic SOLC interact? Takahashi - Active Control of MHD

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

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

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