# Presentation: 818053

## XP728: RWM active stabilization and optimization – ITER scenario

- _XP728: RWM active stabilization and optimization – ITER scenario_

***Columbia U***
***Comp-X***
***General Atomics***
***INEL***
***Johns Hopkins U***
***LANL***
***LLNL***
***Lodestar***
***MIT***
***Nova Photonics***
***NYU***
***ORNL***
***PPPL***
***PSI***
***SNL***
***UC Davis***
***UC Irvine***
***UCLA***
***UCSD***
***U Maryland***
***U New Mexico***
***U Rochester***
***U Washington***
***U Wisconsin***
***Culham Sci Ctr***
***Hiroshima U***
***HIST***
***Kyushu Tokai U***
***Niigata U***
***Tsukuba U***
***U Tokyo***
***JAERI***
***Ioffe Inst***
***TRINITI***
***KBSI***
***KAIST***
***ENEA, Frascati***
***CEA, Cadarache***
***IPP, J******ü******lich***
***IPP, Garching***
***U Quebec***

**NSTX Team XP Review Meeting**
- May 8th, 2007
- Princeton Plasma Physics Laboratory

- V1.1

*1**Department of Applied Physics, Columbia University, New York, NY*
*2**Plasma Physics Laboratory, Princeton University, Princeton, NJ, USA*
*	** **3**Nova Photonics, Inc., Princeton, NJ, USA*
*	**4**Johns Hopkins University, Baltimore, MD, USA*

## XP728: RWM active stabilization and optimization

- Goals
- Investigate variations of control sensor combinations to optimize RWM stabilization at low plasma rotation,  (more robust, reach higher N)
- Use upper/lower RWM Br, Bp sensors for feedback (ran out of time in 2006)
- Examine possible poloidal deformation of RWM during feedback
- Investigate active stabilization of recent plasmas that exhibit unstable RWM activity leading to discharge termination at _*high*_ .
- Explore possible stable region at  < i with feedback is turned off
- Investigate RWM active stabilization of low  plasma with superposed time-averaged n = 1 error field correction + n = 3 magnetic braking
- (Fredrickson, Garofalo suggestion from 2006, but no run time)
- Measure n =2-3 RFA, attempt to destabilize n = 2 RWM with n = 1 stable
- Introduce and study effect of applied time delay on feedback (ITER support)
- Depends on control system time delay capability in 2007
- Addresses
_**NSTX milestone R(07-2)**_**,** NSTX PAC request
- ITPA experiment MDC-2, ITER issue card RWM-1, USBPO MHD task

## RWM actively stabilized at low, ITER-relevant rotation

- N

- IA (kA)

- Bpun=1 (G)

- Bpun=2 (G)

- /2 (kHz)

- 120047

- 120712

-  < crit

- 92 x (1/RWM )

- 6

- 4

- 2

- 0

- 8

- 4

- 0

- 1.5

- 1.0

- 0.5

- 0.0

- 20

- 10

- 0

- 20

- 10

- 0

- Control OFF

- Control ON

- (Sabbagh, et al., PRL **97** (2006) 045004.)

- Logical next-step of XP615 addresses several key issues
- Optimal RWM sensor configuration
- Dependence of active stabilization on 
- Possible stable region at sufficiently _low_  without active stabilization
- If stable region with low  is found, scan magnitude to determine range of stable .
- Approach
- Follow established Xp615 procedure to generate RWM stabilized, low rotation target
- Make control system parameters scans, rotation scans to fulfill stated goals

## Poloidal deformation of RWM sometimes observed
Poloidal n = 1 RWM field decreases to near zero; radial field increasing
Subsequent growth of poloidal RWM field
Asymmetric above/below midplane
Radial sensors show RWM bulging at midplane
midplane signal increases, upper/lower signals decrease 
Theory: may be due to other stable ideal n = 1 modes becoming less stable
Approach
Include full set of RWM sensors (upper and lower, Bp and Br) in feedback circuit – new PCS capability (tested piggyback 2006)

- Poloidal deformation of RWM sometimes observed
- Poloidal *n* = 1 RWM field decreases to near zero; radial field increasing
- Subsequent growth of poloidal RWM field
*Asymmetric* above/below midplane
- Radial sensors show RWM bulging at midplane
- midplane signal increases, upper/lower signals decrease
- Theory: may be due to other stable ideal *n* = 1 modes becoming less stable
- Approach
- Include full set of RWM sensors (upper and lower, Bp and Br) in feedback circuit – new PCS capability (tested piggyback 2006)

- _Test improved control with addition of new sensors_

-  ~ crit

- upper

- lower

## Does stable high N, low  region exist without feedback in NSTX?

- /A

- passively stabilized by rotation

- (120038, t = 0.535s)

- experimental
- critical rotation profile

- (120712, t = 0.575s)

- actively
- stabilized

- (120047
- t = 0.825s)

- R (m)

- crit/A

- (axis)

- (q=2)

- _Does stable high ____N__, low ______ region exist __*without*__ feedback in NSTX?_

- Non-resonant n = 3 magnetic braking used to slow profile
- The /A < 0.01|q = 2
- The /crit = 0.2|q = 2
- The /crit = 0.3|axis
- Less than 1⁄2 of ITER Advanced Scenario 4 /crit (Liu, et al., NF **45** (2005) 1131.)
- Possible energy dissipation at low rotation speeds
- trapped particle precession drift stabilization at  < i (Betti and Hu, PRL 93 (2004) 105002.)
- Approach
- Generate N > Nno-wall (n = 1) actively stabilized plasma at low  magnitude
- Gate feedback off/on to probe low rotation stable operating regime (seen in DIII-D – Reimerdes, et al. PRL **98** (2007) 055001.)

- (120717, t = 0.915s)

- }

## n = 2 RWM does not become unstable during n = 1 stabilization

- Internal mode ~ 25 kHz (n=2)

- ...but, can the n = 2 RWM be driven unstable at higher N?
- Unstable *n* = 1 – 3 RWMs already observed in NSTX (Sabbagh, et al., NF **46** (2006) 635.)
- Generate controlled, measured n = 2 RFA during n = 1 stabilization, and drive unstable
- Approach
- RFA measurements for n = 2 and 3 can be made for most conditions
- To destabilize n = 2 RWM, use “optimized” control system configuration and plasma configuration to maximize N

## XP728: Active RWM Stabilization - Run plan (Part 1)

- _	__Task__	__	__	__	__	__	__                    Number of Shots_
- 1) _Create target plasma_
- 	A) Run active feedback in piggyback mode in prior experiments to verify operation	-
- 	B) 3 NBI,  > 2.2,  N > Nno-wall (control shot - 123529 as setup shot)		1
- 	C) Drop Ip to 0.9 MA from 1.0 MA					1
- 2) _Reproduce active RWM stabilization at low plasma rotation_
- 	A) n = 3 braking, n = 1 feedback w/Bpu sensors, adjust n = 3 braking if  > 0.5 crit 	2
- 	B) Reproduce (2A) with n = 1 feedback off - demonstrate unstable RWM at low  	2
- 3) _Optimize n = 1 feedback sensors at low ____ _
- 	A) Adjust relative phase between sensors / RWM coil current if (2A) <> shot 120717	3
- 	B) Add Bpl sensors to feedback circuit					1
- 	C) Use Bpu + Bpl average (150 degree spatial offset)				1
- 	F) Vary relative phase between sensors / RWM coil				4
- 	D) Add upper/lower Br sensors to feedback circuit				1
- 	E) Add Bru + Brl average (260 degree spatial offset)				2
- 	G) Vary relative phase / feedback parameters to further optimize performance	6
- _	__	__	_____________________________________________________________
- 								       Total: 24

## XP728: Active RWM Stabilization - Run plan (Part 2)

- _XP728: Active RWM Stabilization - Run plan (Part 2)_

- _	__Task__	__	__	__	__	__	__                    Number of Shots_
- 4) _n = 1 RWM stabilization with various rotation profiles < ___crit
-            (best feedback settings from step (3))
- 	A) Vary n = 3 braking current to create scan of profiles 0 <  << crit 		8
- 	       Gate off active feedback for many wall times (100 ms) to determine which, if
- 	        any profiles are stable at low rotation without n = 1 feedback
- 	B) If any  profiles are stable without n = 1 feedback in (5A), re-run shot with	2
- 	        feedback turned off
- 5) _Check pre-programmed average of n = 1 feedback current for stabilization_
- 	A) Attempt stabilization using avg. n = 1 feedback current for best case of (3) above	2
- 	B) If successful, vary plasma parameter(s) (e.g. ) to test robustness of stabilization	2
- 6) _Measure n > 1 RFA at maximum _N_; attempt n = 2 RWM destabilization with n = 1 stable_
- 	A) Take highest N stabilized plasma at low and run at maximum N/Nno-wall
- 	      (options: increase NBI power, optimize DRSEP, use lithium, drop Ip by 100A)  	2
- 7) _Examine feedback performance vs. feedback system latency_
- 	A) Increase feedback system latency from optimized settings to find critical latency
-               for mode stabilization						6
- _	__	__	__	_____________________________________________________
- 					   Total: _**16**_ w/o latency scan; 22 with latency scan

## XP728: Active RWM stabilization - Diagnostics

- Required diagnostics
- Internal RWM sensors
- CHERS toroidal rotation measurement
- Thomson scattering (30 point)
- USXR
- MSE
- Toroidal Mirnov array / between-shots spectrogram with toroidal mode number analysis
- Diamagnetic loop
- Desired diagnostics
- FIReTip
- Fast camera