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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 - PowerPoint PPT Presentation
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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 -
Nov. 3-5, 2003 Takahashi - Active Control of MHD 2
PRINCETON PLASMA PHYSICS LABORATORY
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.
Nov. 3-5, 2003 Takahashi - Active Control of MHD 3
PRINCETON PLASMA PHYSICS LABORATORY
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.
Nov. 3-5, 2003 Takahashi - Active Control of MHD 4
PRINCETON PLASMA PHYSICS LABORATORY
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
Nov. 3-5, 2003 Takahashi - Active Control of MHD 5
PRINCETON PLASMA PHYSICS LABORATORY
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.
Nov. 3-5, 2003 Takahashi - Active Control of MHD 6
PRINCETON PLASMA PHYSICS LABORATORY
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…
Nov. 3-5, 2003 Takahashi - Active Control of MHD 7
PRINCETON PLASMA PHYSICS LABORATORY
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
Nov. 3-5, 2003 Takahashi - Active Control of MHD 8
PRINCETON PLASMA PHYSICS LABORATORY
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.
Nov. 3-5, 2003 Takahashi - Active Control of MHD 9
PRINCETON PLASMA PHYSICS LABORATORY
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
Nov. 3-5, 2003 Takahashi - Active Control of MHD 10
PRINCETON PLASMA PHYSICS LABORATORY
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?
NATIONAL FUSION FACILITYS A N D I E G O
DIII-D
B-field Normal to Plasma Surface Produced by RWM*
Nov. 3-5, 2003 Takahashi - Active Control of MHD 11
PRINCETON PLASMA PHYSICS LABORATORY
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)
NATIONAL FUSION FACILITYS A N D I E G O
DIII-D
Nov. 3-5, 2003 Takahashi - Active Control of MHD 12
PRINCETON PLASMA PHYSICS LABORATORY
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.
Nov. 3-5, 2003 Takahashi - Active Control of MHD 13
PRINCETON PLASMA PHYSICS LABORATORY
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
NATIONAL FUSION FACILITYS A N D I E G O
DIII-D
Nov. 3-5, 2003 Takahashi - Active Control of MHD 14
PRINCETON PLASMA PHYSICS LABORATORY
PPPL
SOL Current Can Be Oscillating…
SOLC(bi-polar)
Mirnov(B-dot)
4.9kHz6.5mTp-p
NATIONAL FUSION FACILITYS A N D I E G O
DIII-D
Discharge Summary
SOLC
Mirnov
Nov. 3-5, 2003 Takahashi - Active Control of MHD 15
PRINCETON PLASMA PHYSICS LABORATORY
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
NATIONAL FUSION FACILITYS A N D I E G O
DIII-D
Discharge Summary
Nov. 3-5, 2003 Takahashi - Active Control of MHD 16
PRINCETON PLASMA PHYSICS LABORATORY
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
NATIONAL FUSION FACILITYS A N D I E G O
DIII-D
Nov. 3-5, 2003 Takahashi - Active Control of MHD 17
PRINCETON PLASMA PHYSICS LABORATORY
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
NATIONAL FUSION FACILITYS A N D I E G O
DIII-D
Nov. 3-5, 2003 Takahashi - Active Control of MHD 18
PRINCETON PLASMA PHYSICS LABORATORY
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.
Nov. 3-5, 2003 Takahashi - Active Control of MHD 19
PRINCETON PLASMA PHYSICS LABORATORY
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.
Nov. 3-5, 2003 Takahashi - Active Control of MHD 20
PRINCETON PLASMA PHYSICS LABORATORY
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.