NSTX S. A. Sabbagh

XP407: Passive Stabilization Physics of the RWM in High N ST Plasmas – 4/13/04

Goals Define RWM stability boundary in (V, N) space

• Use past XP experience and new theories to determine boundary Present theory and NSTX data suggests: crit/A ~ 1/4q2

• Define the operating space and criteria for active stabilization Measure n > 1 resistive wall modes

• Present high N plasmas are computed to be n = 2 unstable

• Compare to theoretical computation of n > 1 stability (structure?)

• Look for finite mode frequency Set up conditions to examine physics of RWM passive

stabilization in high N ST plasmas to compare to theory

• Critical rotation frequency: XP by A. Sontag

• Rotation damping physics: XP408 by W. Zhu

NSTX S. A. Sabbagh

Stability boundary probed and modes observed q and pressure peaking variation were main techniques

q variation through slow Bt reduction

Bt reduction placed discharge in stabilized high /A region

Bt reduction yielded variation of pressure peaking

Low frequency 370Hz mode observed before mode lock Global collapse in rotation profile observed as in past RWM XPs

High targets reached at high toroidal rotation speed /A = 0.48; N = 6.7 (highest value at constant Ip); t = 38%

New diagnostic capabilities used to measure RWM Internal locked mode sensor array shows n=1 ~ 10-20G; n=2 ~ 5G 51 channel, 10 ms resolution CHERS yields unprecedented detail

• detail of global rotation collapse

USXR data taken at two toroidal positions (not yet examined)

NSTX S. A. Sabbagh

Past experiments show crit depends on q

Chu/Bondeson critical rotation crit/A = 1/(4q2) applies across the profile in CY02 plasmas

/

A

q1 2 3 4 5

0.0

0.1

0.2

0.3

0.41084201076361/(4q2)

(a)

stabilized region

1/(4q2)

0.27 s0.29 s0.31 s0.33 s0.35 s

108197

/

A

0.1

0.2

0.3

0.4

q1 2 3 4 5

0.0

(b)

NSTX S. A. Sabbagh

Approaches to probing (V, N) stability boundary

Rapidly drive across increasing growth rate contours This is the standard approach

Destabilize RWM by increasing crit

Decrease Bt slowly in stable discharge

Stabilize RWM once triggered Decrease crit by increasing Bt slowly in unstable discharge

NBI step-down after destabilization

Drop out of H mode with N >> NNo-wall

Vary timing of NBI and Bt variation to change position in (V, N ) space

NSTX S. A. Sabbagh

Schematic approach to probing stability boundary

Technique 3 was most extensively used Technique 4 occurred as pressure peaking increased at lower Bt

N

/A

Unstable

UnstableN wall

N no-wall

decreased qincreased q

1

2

3

4 drop N no-wall

NSTX S. A. Sabbagh

XP407 Passive RWM - Waveforms: 4/13/04

Aspect ratio in 110184 increased in time – vary A in similar way

6

0 0.1 0.2 0.3 0.4 0.50

0.5

1.0

X10

Am

ps

2

4

6

(arb

)

Time (Seconds)

Source B

Source C (start with this source delayed)

Default NBI timing

Plasma current

0 0.1 0.2 0.3 0.4 0.50

Source A

4.5

Run NBI at max power

0.6 0.7

0.6 0.7

Use setup shot 111957 LSN

4.0

3.5

Toroidal field

Optional delayed Ip ramp

NSTX S. A. Sabbagh

XP407 Passive RWM - Run plan: 4/13/04 Scan boundary of passive stabilization space (F, N)

Task Number of Shots

A) Restore shot 111957, ~ 2.1 LSN (or newer similar target)

(Ip ramped to 0.9 MA Bt = 4.4 kG (or above), =2.1, li ~ 0.65)

(i) 3 NBI sources (rerun 111957 with 3rd source at t = 0.4s) 1

(ii) 2 NBI + delayed 1 NBI if collapse occurs before 0.5 s 2

B) Use toroidal field ramp to influence rotation evolution

(i) Bt = 4.4 kG, reduced to Bt = 3.5 kG, vary start time of ramp 3

(ii) Bt = 4.4 kG, reduced to Bt ~ 4.0 kG, vary start time of ramp 3

(iii) choose best Bt ramp, vary start time (2 or 3 unique times)4

(iv) Bt ~ 4.0 kG, reproduce collapse time, ramp Bt up to delay crash 4

C) Vary NBI timing to best cover (F, N) space

(i) 3 NBI sources with source step-down / vary TS laser timing 5

D) Take plasma out of H-mode (ramp PF2) for high N / NNo-wall 4

Total shots: 26

NSTX S. A. Sabbagh

RWM mode locking and slow phase rotation observed

Low frequency ~ 440Hz precursor to mode lock

Mode locks at t=0.49s Rotating modes (typically

islands) are clearly rotating at this time

Very slow rotation of n=1 phase observed after lock Apparent f ~ 50 Hz

NSTX S. A. Sabbagh

Global rotation collapse when core n=1 is triggered

Global rotation collapse when core n=1 is triggered n=1 RWM appears to trigger core n=1 mode RWM locks at least 30 ms before islands eventually lock

No apparent momentum transfer, which would be observed if braking were due to electromagnetic torque at island

rotation profile evolution

Radius (cm)

rapid globalcollapse

440 Hz odd-nobserved

core n=1 triggered,RWM locked

V = 1kHz (> 440 Hz)

PRELIMINARY

NSTX S. A. Sabbagh

Slower collapse – RWM near marginal stability?

RWM rotation frequency ~ 370 Hz in this case Mode locks/unlocks before plasma rotation terminates

Momentum transfer across rational surface observed before insland lock at 0.51s RWM “marginally stable” near 0.5s, collapses plasma again and locks at 0.52s

rotation profile evolution

Radius (cm)

PRELIMINARY

370 Hz odd-nobserved

core n=1 triggered,RWM locks

NSTX S. A. Sabbagh

RWM sensor data taken during XP407

Detail of RWM slow rotation, mode locking underway

J. MenardA. Sontag

Bp = 4G

Bp = 10Gupper array

lower array