First Demonstration of Smart-Shell Suppression of Wall Modes in - - PowerPoint PPT Presentation

First Demonstration of “Smart-Shell” Suppression of Wall Modes in HBT-EP

Mike Mauel for the HBT-EP Group Columbia University

– Identification of the Wall Mode by retracting the thick, aluminum shells. – Demonstration of passive stabilization (again) by inserting the aluminum shells near the plasma’s edge. – Demonstration of active mode control using a 30-element “smart-shell”.

1

Please come to the HBT-EP poster session!

Tuesday, Afternoon (at the same time as the DIII-D oral session!) GP1.76 Overview of HBT-EP Experimental Program and Plans, HBT-EP Group Active mode control research using the HBT-EP is now entering it’s third phase:

• 1. Understanding & Passive Control of External Kink Modes.
• 2. Understanding & Active Control of Internal Tearing Modes.

et al.

resonant magnetic perturbations on the HBT-EP tokamak D. A. Maurer, et al.

Taylor, et al.

• 3. Understanding & Active Control of Wall Modes and

stable Tokamak Plasmas M. Shilov, et al.

2

Initial Active Feedback Experiments to Control the Resistive Wall Mode (RWM) in HBT-EP

HBT-EP investigates active RWM control with (1) a segmented adjustable resistive wall, and (2) a distributed "smart-shell" active feedback system. 10 Independently Adjustable "Thick" Aluminum Shells 10 Independently Adjustable "Thin" SS Shells (each with 3 flux sensors and 3 active control create 30 independent feedback circuits.)

Experimental Procedure:

1. Generate discharges with strong edge current using a plasma- current ramp (~ 1.6 MA/s). 2. With the steel shells located near the plasma edge, move the alumi- num shells from the plasma to excite m = 3, n = 1 resistive wall modes. (These modes are similar to the external kinks reported by Ivers, et al., Phy. Plasmas, 1996.) 3. With the Al shells withdrawn, switch-on the 30 independent active feedback coils (located on the steel shells) to observe RWM suppression. HBT-EP

10 1 10 0 10 -1 10 -2 10 -3 10 -4 10 -5 10 -6 10 -2 10 -1 10 0 10 1 10 2 10 3 10 4 10 5 10 6

No Feedback Gain = 102 Gain = 103 Gain = 104

growth rate passive system slowed growth rate with feedback

S ∝ −

free fixed

β

− β

β β

MHD Drive: Growth Rate (s-1)

VALEN Model Calculations Show RWM Control Can Be Achieved with HBT-EP's Sensor and Control Coil Locations

Control Coils Flux Sensors 5 toroidal locations: 6 poloidal locations:

Stainless Steel Shell

Resistive Wall Model Confirmed with Simple Tests

10-5 10-4 103 104

Measured VALEN One Circuit Model Two Circuit Model

freq(Hz)

Single Coil Transform Function

Wall Coils

0.0 0.2 0.4 0.6 0.8 1.0 0 100 5 103 10 103 15 103

Bandwidth of Initial Feedback System Detected Voltage (with FB/without FB) Frequency (Hz)

Measured Calculated

Frequency (Hz)

τ1 ~ 500 µs τ2 ~ 70 µs Vs / Ic

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1 2 3 4 time (ms) Loop Voltage (V) Plasma Current (kA) Safety Factor (q) 20 10 4 8 12 2 3 4 2.5 MA/s

Discharge Parameters for Initial “Smart-Shell” Tests

B ~ 0.31 T a ~ 0.12 m R ~ 0.95 m R/a ~ 7.5 <n> ~ 0.8 x 10^19 m^(-3) <T> ~ 25 eV tauE ~ 0.4 ms S ~ 500-1000

C L

b/a ~ 1.7 Vessel Plasma b/a ~ 1.08

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1 2 3 4 time (ms) Loop Voltage (V) Plasma Current (kA) Safety Factor (q) 20 10 4 8 12 2 3 4 2.5 MA/s

Discharge Parameters for Initial “Smart-Shell” Tests

B ~ 0.31 T a ~ 0.12 m R ~ 0.95 m R/a ~ 7.5 <n> ~ 0.8 x 10^19 m^(-3) <T> ~ 25 eV tauE ~ 0.4 ms S ~ 500-1000

q(a) q(0)

4 3 2 1 5 10 radius (cm) 5 10 radius (cm) 0.6 0.4 0.2 0.0 J (MA/m2) Safety Factor (q)

Wesson Diagram

Hydromagnetic Stability of Tokamaks, Nuc. Fusion (1978)

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1 2 3 4 time (ms) Safety Factor (q) 8 4 30 60 90 2 3 4

Large-Aspect Ratio Stability Analysis for Initial “Smart-Shell” Tests

B ~ 0.31 T a ~ 0.12 m R ~ 0.95 m R/a ~ 7.5 <n> ~ 0.8 x 10^19 m^(-3) <T> ~ 25 eV tauE ~ 0.4 ms S ~ 500-1000 Kink Growth (1/ms)

b/a = 1.08 b/a = 1.70 m = 3 m = 4 m = 5

6 2 Rutherford Rate (1/ms)

m = 3 m = 2

Tearing Mode Ideal

q(a) q(0)

Approximate Safety Factor

q

n = 1 Mode Amplitude Volt / m2 Al "Thick" Shells Retracted Al Shells Inserted 22780 22763 Flux Rate

m = 3

With the Al "Thick" Shells Retracted, Slowly Rotating RWMs Appear as the Edge Safety Factor Passes Below 3

Tearing m = 2 Wall Mode

1.5 1.0 0.5 0.0

• 0.5
• 1.0
• 1.5

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Poloidal Angle Al "Thick" Shells Retracted

1.5 1.0 0.5 0.0

• 0.5
• 1.0
• 1.5

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Poloidal Angle Al Shells Inserted

m = 3 Wall Mode m = 2 Tearing m = 3 Tearing

1.5 2.0 2.5 3.0 time (ms)

• 10
• 5

5 10 Poloidal Field Fluctuations (G)

Approximate Safety Factor

q

n = 1 Mode Amplitude Volt / m2

m = 3

No Shells / No Feedback With Shells No Shells / RWM Feedback 22890 22763 22781

With Active RWM Feedback ON, RWM Suppression is Similiar to that seen with "Thick" Aluminum Shells

Flux Rate

m = 2

1.5 1.0 0.5 0.0

• 0.5
• 1.0
• 1.5

22781

1.5 1.0 0.5 0.0

• 0.5
• 1.0
• 1.5

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m = 2 Tearing m = 3 Tearing

Poloidal Angle Poloidal Angle 1.5 2.0 2.5 3.0 time (ms) Feedback ON Al Shells Inserted With RWM Feedback

• 10
• 5

5 10 Poloidal Field Fluctuations (G)

RWM Suppression with Feedback ON

Columbia University

Princeton Plasma Physics Laboratory

HBT-EP Research In Progress

• Investigate and optimize feedback circuit parame-

ters for RWM control.

• Investigate alternate feedback algorithms, includ-

ing phase-shifting “rotating shells”.

• Compare measured feedback performance to ana-

lytical (Boozer, 1999) and numerical (VALEN) mod- els.

• Investigate the coupling of external kink modes to

the external coil system by applying resonant per- turbations and observing the plasma's response.

• Install 200 kW ICRF system built by PPPL and

LANL to enable investigation of beta-driven RWM instabilities.

• Document the maximum beta limits achievable

with active RWM control.

HBT-EP's ICRF Antenna has been Tested to > 400 kW for 100 µsec.