# Presentation: 922674

## Macroscopic Stability TSG XP statusmid-run 2008

_**Macroscopic Stability TSG XP status**__**mid-run 2008**_

- S.A. Sabbagh and S. Gerhardt
- for the
- NSTX Macroscopic Stability Topical Science Group
**NSTX mid-run assessment**
**Princeton Plasma Physics Laboratory**
- April 16th, 2008

***Culham Sci Ctr***
***U St. Andrews***
***York U***
***Chubu U***
***Fukui U***
***Hiroshima U***
***Hyogo U***
***Kyoto U***
***Kyushu U***
***Kyushu Tokai U***
***NIFS***
***Niigata U***
***U Tokyo***
***JAEA***
***Hebrew U***
***Ioffe Inst***
***RRC Kurchatov Inst***
***TRINITI***
***KBSI***
***KAIST***
***ENEA, Frascati***
***CEA, Cadarache***
***IPP, J******ü******lich***
***IPP, Garching***
***ASCR, Czech Rep***
***U Quebec***

***College W&M***
***Colorado Sch Mines***
***Columbia U***
***Comp-X***
***General Atomics***
***INEL***
***Johns Hopkins U***
***LANL***
***LLNL***
***Lodestar***
***MIT***
***Nova Photonics***
***New York U***
***Old Dominion U***
***ORNL***
***PPPL***
***PSI***
***Princeton U***
***SNL***
***Think Tank, Inc.***
***UC Davis***
***UC Irvine***
***UCLA***
***UCSD***
***U Colorado***
***U Maryland***
***U Rochester***
***U Washington***
***U Wisconsin***

- v1.1

## MHD XPs for 2008 guided by Milestones, ITPA needs

- General mission statement (2008 Forum)
*Develop physics understanding applicable to the ST development path and to tokamaks in general, leveraged by the unique low-A and high-**** operational regime of NSTX*
- Priorities (summarized in two lines – 2008 Forum)
- Assess active and passive RWM stabilization physics for improved mode control (NSTX Milestone R09-1)
- Evaluate MHD sources of plasma viscosity and assess the impact of plasma rotation on plasma stability, including NTM (Joule milestone)
- XPs serve NSTX and DOE (Joule) Milestones, ITPA joint XPs, ITER support, several joint experiments / comparisons
- 80% support Joule milestone
- 80% support ITPA / ITER

## Macroscopic MHD TSG XPs – Forum 2008 Priority

- XP Idea presentations requesting run time
- Beta ramp down 2/1 tearing mode: study of self-stabilization (LaHaye)	1.0 days
- Rotation dependence of 2/1 NTM thresholds (Buttery)
- Active RWM stabilization optimization and ITER support (Sabbagh)	1.0 days
- SXR tomography of neon-seeded RWM stabilized plasmas (Tritz) 0.5 days
- Comparison of NTV among tokamaks (n = 2 fields, i scaling) (Sabbagh)	1.0 days
- Testing NTV theory of error field penetration (Buttery)
- Studies of the 3/2 NTM: Rotation and Beta Rampdown (Gerhardt/Gates)	1.0 – 1.5 days
- RWM stabilization physics – comparison to theory (Berkery)		1.5 days
- DIII-D/NSTX RWM joint XP – stability vs. A, rotation profile, i (Berkery)
- Parametric dependence RWM damping in rotating plasmas (Reimerdes)
- n=2 intrinsic error fields and RWM critical rotation (Gerhardt/Menard)	1.0 days
- Island-induced neoclassical toroidal viscosity (NTV) (Sabbagh/Shaing)	1.0 days
- Deformation of RWM and multi-mode characteristics (Sabbagh)	1.0 days
- Assessment of non-rigidity in RWM feedback (Okabayashi)
- Measurement of Halo Currents in the Lower Divertor (Gerhardt)	piggyback

- Run days: 15.5 - 16.5

- Run time guidance: 6.5 – 8.0 run days

- V1.1

- 6.5 - 7
- days

- 9
- days

## Macroscopic MHD TSG 2008 XPs – Status 4/14/08

- _Macroscopic MHD TSG 2008 XPs – Status 4/14/08_

- XP Idea presentations requesting run time
- 801: Beta ramp down 2/1 tearing mode: self-stabilization study (LaHaye)	1.0 days
- 810: n = 1 error field, V influence on 2/1 NTM thresholds (Buttery)	0.5 days
- 802: Active RWM stabilization optimization and ITER support (Sabbagh)	1.0 days
- 803: SXR tomography of neon-seeded RWM stabilized plasmas (Tritz)	piggyback
- 804: Comparison of NTV among tokamaks (n = 2 fields, i) (Sabbagh)	1.0 days
- Testing NTV theory of error field penetration (Buttery)
- ###: Studies of the 3/2 NTM: Rotation and Beta Rampdown (Gerhardt)	1.0 – 1.5 days
- 830: RWM stabilization physics – comparison to theory (Berkery)	1.5 days
- DIII-D/NSTX RWM joint XP – stability vs. A, rotation profile, i (Berkery)
- Parametric dependence RWM damping in rotating plasmas (Reimerdes)
- 805: n=2 intrinsic error fields and RWM critical rotation (Gerhardt/JEM)	1.0 days
- 743: Island-induced neoclassical toroidal viscosity (NTV) (SAS/Shaing)	1.0 days
- ###: Deformation of RWM and multi-mode characteristics (Sabbagh)	1.0 days
- Assessment of non-rigidity in RWM feedback (Okabayashi)
- Measurement of Halo Currents in the Lower Divertor (Gerhardt)	piggyback
- 818: ELM mitigation with midplane control coils (SAS/JKP/RM/SG)	3.0 days

- V1.1

- base

- extended

## XP801 examines the small island physics of 2/1 NTM

- Goal / Approach
- Trigger the 2/1 mode, then step down NB power to restabilize mode.
- This was accomplished in 1 post-lithium shot in CY2007 run
- Data to be compared with DIII-D 2/1 rampdown data.
- Key to i) Stay in H-mode and & ii) Avoid mode locking before restabilizing.
- Vary the rotation with n = 3 braking, and vary “drive” with an IP scan.
- Status
- This was essentially the first XP of the year, and was plagued with technical problems (NBI, ground loops,...).
- A number of  ramp-down attempts were completed, all of which ended with a mode-lock before island was restabilized.
- Plan
- Return to XP - use EF correction in order to allow a lower locking threshold
- These techniques are now being optimized in the MHD (XP 802) and ASC groups (XP 823) with improved feedback control.
- Share 1 run day with continuation of XP810 – run post-lithium to reduce MHD

## Data collected was useful for demonstrating the neoclassical nature of the mode

- NTM should have island width proportional to ***P*

- 123873: Survived into ** ramp-down phase, re-stabilized in CY2007

- 126965, 127009: Locked in CY2008

- Arrows Indicate Direction of Time

## XP810 Examining role of n=1 error fields at high and low rotation

- Goal / Approach
- Trigger the 2/1 mode at high and low rotation, with and without n =1 error field
- Maintain H-mode and avoid mode-locking
- Determine if the threshold in N changes...important for assesing role of EF in plasmas without momentum input (read: ITER)
- Status
- Limited data collected, lack of error field correction made interpretation difficult
- This was essentially the second run day of the year, plagued with technical problems
- Beta ramp-down techniques to stabilize mode implemented, mode locking problems
- Possibly related to machine conditions and intrinsic error fields
- 4 point 2/1 NTM onset scan obtained vs. n=1 field
- Error fields act to lower rotation and decrease NTM  threshold
- 2 point scan of n=1 field obtained with modest n=3 braking
***Scope very limited by available time - higher n=3 & n=1 levels desired to explore key question – is error field sensitivity worse at low rotation*********
- Plan
- Return and complete XP, with well-conditioned machine and n = 1 feedback capability to allow error field correction if desired

## XP810 Initial results – lower N threshold for 2/1 mode with n = 1 field applied

**R. Buttery**

- Preliminary onset scan obtained with n=1 fields & 2 beam recipe...
- ...but very limited data withn=1 applied when lowering rotation from n=3 braking...
***(this was main objective)***
*Nevertheless, useful **extension of present NSTX **database to get at rotation vs. **rotation shear issue...*

## XP802 RWM feedback Optimization - good initial run

- Goal / Approach
- Alter active control configuration to achieve _highly reliable_ RWM stabilization at various plasma rotation, 
- Upper/lower RWM Br, Bp sensors, follow from best CY2007 feedback settings
- Determine if stable, low  < i operation exists with feedback gated off
- If achieved, control system open as a tool for all NSTX XPs as desired
- Specific ITER support requests (not yet run)
- Determine impact of a large toroidal gap on active RWM stabilization to simulate ITER port plug coil geometry (take out one of six control coils)
- Study effect of applied time delay on feedback
- Status
- Rotating (~ V) n = 1 mode activity stronger than post-lithium run last year
- Bpu,l sensor feedback successful, good statistics, relatively broad V created
- Neon puff taken on one shot to better diagnose RWM (K. Tritz XP803 piggyback)
- Bru,l sensor feedback showed n = 1 amplitude successfully reduced
- Necessary but not sufficient for mode stabilization - _need more understanding here_
- Plan
- Complete shot list in plasma target with reduced n = 1 rotating mode activity

## Bru,l sensor feedback- n = 1 amplitude successfully reduced

- Analyze role of n = 1 Br amplitude in stabilization
- Necessary to keep < 10G, not sufficient for RWM stability
- Influence on n = 1 rotating mode unclear
- Changing Br re-zero time didn’t make large difference in pulse length

- 128479 - no feedback
- 128484 - 270o phase
- 128487 – 270o phase
-                0.2s baseline
-                re-zero
- 128486 – 45o phase

- Ip (MA)

- N

- Brun=1
-     (T)

- Bpun=1
-     (T)

- Br-midn=1
-      (G)

- n = 1 Br feedback
- (Gain = 1.5)

- Feedback off

## XP804: Comparison of neoclassical toroidal viscosity (NTV) among tokamaks (n = 2 fields, i scaling)

- Goals
- Compare NTV results/analysis on NSTX to other devices (MAST, JET, etc.)
- Test NTV theory for n = 2 applied field configuration
- n = 2 may be best for comparison to other devices (n = 1 strongest resonant rotation damping, n = 3 weak in some devices, many machines run n = 2)
- Examine possible RFA effects by varying proximity to no-wall limit
- Investigate damping over widest possible range of ion collisionality
- _Key for ITER,_ determine affect on rotation damping and compare to theory
- Compare to braking due to using n = 1, 3 fields
- Status
- Inferior plasma conditions led to many lost shots during startup
- Regardless, good data taken for ~ 7 shots!
- n = 2 braking clearly demonstrated, braking profile established
- Plans
- Complete shot list by varying collisionality – operation after lithium deposition

- V1.1

## XP804: Clear braking observed due to n = 2 field

- n = 2 has broader braking profile than n = 3 field (field spectrum?)
- Next step: analyze non-resonant NTV profile, examine resonant effects
- Joint XP proposed to MAST (didn’t see strong n = 2 braking, while JET has)

- _Rotation evolution during n = 3 braking_

- broad braking region,
- Peak change
- (1.3 < R(m) < 1.35

- (kHz)

- R(m)

- 124010

## XP805: Designed to Isolate the Presence of an n=2 Error Field

- Goal / Approach
- Apply n=2 error field of varying phases and magntitude
- Look for asymmetric response in (and increase of) pulse length and plasma rotation.
- Mimics procedure that has found the n=3 EF in XP701, XP823
- Status
- Day was plagued by irreproducible startup and early MHD.
- Two phase scans were completed:
- 150A: No strong effect on performance
- 300 A: Some indication of a favorable applied field phase
- Plan
- Run remainder of XP in tandem with the other EF proposal (XP823) to benefit from similar discharges and methods.
- 1/2 day on schedule in last week of April

## Evidence of n = 2 correcting phase found, needs confirmation

-  Longest fiducial up to that date
-  Rotation increases across the profile when field is applied
-  Tolerates the rotating mode for longer
*Need to confirm this trend!*

- Rotation at 1.2m

- Rotation at 1.3m

- IP

- RWM Coil Current

- _Evidence of n = 2 correcting phase found, needs confirmation_

## XP818: Exploratory approach to finding ELM mitigation solution with midplane non-axisymmetric coils 

- Goal / Approach
- Demonstration of ELM mitigation with NSTX midplane RWM coil set
- Target development: (i) low *q**95* < 6; (ii) swept *q**95*
- Application of DC fields (broader *n* spectrum, new 2008 capabilities)
- New combined odd/even parity field (theoretical favorite *n* = 2 + 3 field)
- New even parity field (dominant *n* = 2) created with new RWM coil patch panel
- Application of AC fields
- Status
- ELMs not fully mitigated; PHAT ELMs created in some cases
- n = 2 + 3 configuration was not particularly favorable
- PHAT ELMs produced in other field configurations
- (aside) Good non-resonant and resonant magnetic braking detail shown
- Plans
- Re-run most favorable cases in lower recycling conditions – post-lithium run

## ELMs not fully mitigated by n = 2 + 3 field; frequency decreased

- Decrease in ELM frequency at maximum applied field
- Continue to investigate physical cause for changes in ELM behavior
- Results consistent with Chirikov parameter > 1 being necessary, not sufficient condition for ELM mitigation; but could be due to different physics

- _n = 2+3 field, 2.0 – 3.0kA peak RWM current_

- Ip (kA)

- IRWM (A)

- D(arb)

- t (s)

- t (s)

- _ELM target control shot (no n > 1 field, )_

- q95

## Upgraded Halo Current Diagnostics Significantly Improve Measurements

- Method
- Array of 12 BT sensors
- “Inner Ring” Inside of OBD
- “Outer Ring” Outside of OBD
- Treat as sections of a partial rogowski coil
- Use in combination with existing measurements
- Status
- Collecting data on every shot
- New Sensors Increase the HCD estimate by factor of two
- HCF much larger when current path doesn’t include the CHI buswork
- Plan: group suggests dedicated XP - examine
- halo current vs. plasma current
- halo current fraction vs. diamag. flux (**)

## New Sensors Increase the Halo Current Fraction by a Factor of Two

*Interpretation: HCF much larger when current path doesn’t include the CHI buswork*

## Suggestions for remainder of run – Macro Stability XPs

- Run XPs presently on schedule
- XP830 RWM stabilization physics: April 24th
- Key input for R09-1 milestone
- kinetic W analysis (J. Berkery) starting to show trends in data
- XP805 n = 2 Error field reduction: April 29th (0.5 day)
- Continue XPs / re-run shots from XPs under lithium
- Supports call by Jon for extrapolation of ST to low collisionality – a significant goal of future NSTX research
- New XPs suggested by present results
- Dedicated halo current XP (Gerhardt)
- RWM feedback examining poloidal deformation of mode
- Role of islands in resonant / non-resonant rotation braking (XP743)
- Estimated run time to complete XPs
- Active XPs: 4.5 run days (+ cross-cutting XP818 ELM Mitigation run)
- Scheduled XPs: 1.5 run days
- Run planned / new XPs: 4 run days