First Demonstration of “Smart-Shell” Suppression of Wall Modes in HBT-EP Mike Mauel for the HBT-EP Group Columbia University � Description of the experiment � Description of the discharge � Results: – 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. � GP1.82 Magnetic Feedback Experiments on the 2 = 1 Tearing Mode in HBT-EP D. L. Nadle, n = et al. � GP1.83 Suppression, growth, and frequency locking of magnetic islands induced by rotating resonant magnetic perturbations on the HBT-EP tokamak D. A. Maurer, et al. � GP1.81 Effect of Magnetic Islands on the Local Plasma Behavior in the HBT-EP Tokamak E. D. Taylor, et al. 3. Understanding & Active Control of Wall Modes and � Enhancement. � GP1.77 Active Feedback and Wall Stabilization of MHD Instabilities on HBT-EP C. Cates, et al. � GP1.79 Measurement of the Mutual Inductances of Active Control Coils in the Presence of Un- stable Tokamak Plasmas M. Shilov, et al. � GP1.78 Beta Enhancement Program in HBT-EP H. Dahi, et al. � GP1.80 New Results from the HBT-EP Thomson Scattering System S. Mukherjee, et al. 2
Initial Active Feedback Experiments to Control the Resistive Wall Mode (RWM) in HBT-EP HBT-EP investigates active RWM control with Experimental Procedure: (1) a segmented adjustable resistive wall, and (2) a distributed "smart-shell" active feedback system. 1. Generate discharges with strong edge current using a plasma- 10 Independently Adjustable current ramp (~ 1.6 MA/s). "Thick" Aluminum Shells HBT-EP 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 With the Al shells withdrawn, switch-on the 30 independent active feedback coils (located on the steel shells) to observe RWM 10 Independently Adjustable "Thin" SS Shells suppression. (each with 3 flux sensors and 3 active control create 30 independent feedback circuits.)
VALEN Model Calculations Show RWM Control Can Be Achieved with HBT-EP's Sensor and Control Coil Locations 5 toroidal locations: 6 poloidal locations: 10 6 No Feedback Gain = 102 10 5 Control Coils Gain = 103 10 4 Gain = 104 Growth Rate (s-1) 10 3 Flux Sensors growth rate passive system 10 2 10 1 slowed 10 0 growth rate with feedback 10 -1 10 -2 10 -6 10 -5 10 -4 10 -3 10 -2 10 -1 10 0 10 1 β β − S ∝ MHD Drive: free β − β Stainless Steel Shell fixed
Resistive Wall Model Confirmed with Simple Tests Single Coil Transform Function 10 -4 Measured VALEN One Circuit Model Two Circuit Model V s / I c Wall 10 -5 τ 1 ~ 500 µ s τ 2 ~ 70 µ s Coils 10 3 10 4 Frequency (Hz) freq(Hz) Bandwidth of Initial Feedback System Detected Voltage (with FB/without FB) 1.0 0.8 0.6 0.4 0.2 Measured Calculated 0.0 0 10 0 5 10 3 10 10 3 15 10 3 Frequency (Hz)
Discharge Parameters for Initial “Smart-Shell” Tests 4 22763 B ~ 0.31 T Safety Factor (q) a ~ 0.12 m R ~ 0.95 m 3 R/a ~ 7.5 <n> ~ 0.8 x 10^19 m^(-3) <T> ~ 25 eV tauE ~ 0.4 ms 2 S ~ 500-1000 12 Plasma Current (kA) 2.5 MA/s 8 Vessel 4 Plasma C L 0 20 Loop Voltage (V) b/a ~ 1.08 b/a ~ 1.7 10 0 0 1 2 3 4 time (ms)
Discharge Parameters for Initial “Smart-Shell” Tests 4 22763 B ~ 0.31 T Safety Factor (q) q(a) a ~ 0.12 m R ~ 0.95 m 3 R/a ~ 7.5 <n> ~ 0.8 x 10^19 m^(-3) q(0) <T> ~ 25 eV tauE ~ 0.4 ms 2 S ~ 500-1000 12 Plasma Current (kA) 2.5 MA/s 8 0.6 4 4 Safety Factor (q) 3 0.4 J (MA/m2) 2 0 0.2 1 20 Loop Voltage (V) 0.0 0 0 5 10 0 5 10 radius (cm) radius (cm) 10 0 0 1 2 3 4 time (ms)
Wesson Diagram Hydromagnetic Stability of Tokamaks, Nuc. Fusion (1978)
Large-Aspect Ratio Stability Analysis for Initial “Smart-Shell” Tests 4 22763 B ~ 0.31 T Safety Factor (q) q(a) a ~ 0.12 m R ~ 0.95 m 3 R/a ~ 7.5 <n> ~ 0.8 x 10^19 m^(-3) q(0) <T> ~ 25 eV tauE ~ 0.4 ms 2 S ~ 500-1000 90 Kink Growth (1/ms) m = 5 b/a = 1.08 b/a = 1.70 60 m = 4 m = 3 Ideal 30 0 Rutherford Rate (1/ms) 8 Tearing Mode m = 2 6 4 m = 3 2 0 0 1 2 3 4 time (ms)
With the Al "Thick" Shells Retracted, Slowly Rotating RWMs Appear as the Edge Safety Factor Passes Below 3 Al "Thick" Shells Retracted 22780 Al "Thick" Shells Retracted 22780 1.5 Al Shells Inserted 22763 Poloidal Angle 1.0 0.5 Approximate Safety Factor 0.0 -0.5 -1.0 q -1.5 m = 3 Wall Mode m = 3 m = 2 Tearing m = 3 m = 2 Tearing Al Shells Inserted 22763 n = 1 Mode Amplitude Volt / m2 Wall Mode 1.5 Poloidal Angle 1.0 Flux Rate Tearing 0.5 0.0 -0.5 -1.0 -1.5 1.5 2.0 2.5 3.0 time (ms) -10 -5 0 5 10 Poloidal Field Fluctuations (G)
With Active RWM Feedback ON, RWM Suppression is Similiar to that seen with "Thick" Aluminum Shells No Shells / No Feedback 22890 With Shells 22763 Al Shells Inserted 22781 22763 No Shells / RWM Feedback 1.5 Approximate Safety Factor Poloidal Angle 1.0 0.5 0.0 q -0.5 -1.0 -1.5 m = 3 m = 2 m = 3 m = 2 Tearing Tearing n = 1 Mode Amplitude Feedback ON Volt / m2 22781 With RWM Flux Rate 1.5 Feedback Poloidal Angle 1.0 0.5 0.0 -0.5 -1.0 -1.5 1.5 2.0 2.5 3.0 time (ms) RWM Suppression -10 -5 0 5 10 with Feedback ON Poloidal Field Fluctuations (G)
HBT-EP Research In Progress HBT-EP's ICRF Antenna has been • Investigate and optimize feedback circuit parame- Tested to > 400 kW for 100 µsec. 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 Princeton Plasma instabilities. Physics Laboratory • Document the maximum beta limits achievable Columbia University with active RWM control.
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