h. takahashi and e.d. fredrickson princeton university

Nov. 3-5, 2003 Takahashi - Active Control of MHD 1

PRINCETON PLASMA PHYSICS LABORATORY

PPPL

Using Actively Driven SOL Current for Controlling Vertical Instability and Other MHD Modes

in Tokamaks

H. Takahashi and E.D. FredricksonPrinceton University

Workshop on Active Control of MHD Stability: Extension to the Burning Plasma Regime

November 3 - 5, 2003University of Texas

Austin, TX

- Bringing back Old Ideas into a New Environment -



PPPL

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.



PPPL

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.



PPPL

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



PPPL

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.



PPPL

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…



PPPL

B-field Signal Pollution

Feedback ControlTokamak OperationEquilibrium Reconstruction

?

Potential Effects of Error Field Generated by SOLC

SOLCIntrinsic or Driven

Resonant B-field Normal to Flux Surfaces

Flux Surface Distortion MHD Stability

?

?

MHD Control



PPPL

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.



PPPL

SOLC Generates Helical Field Pattern

B-field Normal to q=3 Surface Produced by SOLC

NATIONAL FUSION FACILITYS A N D I E G O

DIII-D



PPPL

RWM Produces Helical Field Pattern

*From M. Okabayashi, et al.

Pol

oid

al a

ngl

e

180

0

-180

External coils try to emulate RWM field pattern.Why not match helical with helical using SOLC?


DIII-D

B-field Normal to Plasma Surface Produced by RWM*



PPPL

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)


DIII-D



PPPL

DIII-D Has Sensor Arrays for Measuring Current through Divertor Tiles

BottomDivertor

TopDivertor

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.



PPPL

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


DIII-D



PPPL

SOL Current Can Be Oscillating…

SOLC(bi-polar)

Mirnov(B-dot)

4.9kHz6.5mTp-p


DIII-D

Discharge Summary

SOLC

Mirnov



PPPL

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


DIII-D

Discharge Summary



PPPL

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 N/ASOLC 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 spacingWall at 6 cm

SOLC fills space between plasma and wall during ELM, and reverses its direction, possibly twice.

Top Divertor

Mid-plane

Bot Divertor


DIII-D



PPPL

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 ArrayTor/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


DIII-D



PPPL

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.



PPPL

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.



PPPL

SummaryThe use of actively driven SOL current (SOLC) wasconsidered 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.

h. takahashi and e.d. fredrickson princeton university

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