# Presentation: 743058

## Edge Electrode Biasing 

_**Edge Electrode Biasing**_** **
_**Experiments on NSTX**_

- S. Zweben, C. Bush, R. Maqueda, L. Roquemore, R. Marasla
- M. Bell, J. Boedo, R. Kaita, Y. Ratises, B. Stratton
- R. Cohen, D. Ryutov, M. Umansky (LLNL)
-  NSTX Meeting 5/21/07

- •  Basic idea for SOL control
- •  Theory and previous experiments
- •  Biased Electrodes and Probes (BEaP) in NSTX
- •  Initial results from checkout of hardware (XMP-051)
- •  Additional tests which can be done in this run

## SOL Control by Edge Biasing

- -    Vr(cm/sec) = 108 Epol(V/cm)/B(G)
- turbulent ‘blob’ speed ≤ 1 km/sec
- need only Vr ~ 5 V/cm to broaden
- SOL in NSTX (because of low B)

- •   Create localized poloidal electric fields in SOL to make
- 	local radial Vr=EpolxB drift to drive plasma outward
- •   If Vr is larger than the outward turbulent transport speed,
- 	local SOL width will be increased (particles and heat)

## Theory of Active SOL Broadening 

- •   Apply toroidally asymmetric perturbations in SOL near divertor
- 	plate to create ExB flows to induce turbulent broadening
- 	[R.H. Cohen et al, Nucl. Fusion (1997); D.D. Ryutov et al,
- 	    PPCF (2001), R.H. Cohen et al, PPCF (2007)]
- 	-   perturbations can be biasing, ‘wavy’ plates, gas puffing
- 	-   effects confined to divertor region by X-point shear
- 	-   potential distribution along B-field line was calculated
- •    Open questions:
- 	-   how far do perturbations extend along and across B ?
- 	-   do these perturbations locally induce turbulence ?
- 	-   can the effects be simulated with BOUT (or XGC) ?

## Previous Experiments

- •   Most tokamak biasing experiments aimed to modify Er and 	poloidal flow [e.g. PBX-M, DIII-D, TdeV, TEXTOR...]
- •   MAST experiment done to test idea of Cohen and Ryutov,
- 	resulting in partial confirmation of theory

_**Previous Experiments**_

- G.F. Counsell, EPS Conference (2003)

- biased (and grounded) “ribs” in divertor (~80 V, 3 kA total)

- peak
- of D

## NSTX Electrode Biasing Experiment

- •    Locate electrodes on B field line of GPI to see effect
- 	of bias on local turbulence and D profile (e.g.
- 	see ‘blobs’ move faster outward, or inward)
- •    Include Langmuir probes between the electrodes to
- 	measure potential, ne and Te, and turbulence
- •    Correlate fluctuations in probe and GPI to determine
- 	location of electrodes in GPI field of view
*   ****Did not expect to make “global” modification of SOL or plasma with this initial set-up !***

## Biased Electrodes and Probes in NSTX

- •  installed ~ Dec. ‘06
- •  leading edge ~ 1 cm
-     behind RF limiter
- •  4 electrodes and 5
-     Langmuir probes
- •  look for effect of bias
-     in GPI and probes

**Bay B**

**B**

## Closer View of BEaP

- Present capabilities:
- •  ± 100 V, 10 A on
-    any two electrodes
- •  5 swept probes
- •  change outer gap
-    to control density
-    at electrodes

## Initial Results (from 3 shots) 

- •  Electrode and probe currents and voltages
- •  Effect of biasing on probe signals
- •  Mapping of fluctuations from probes to GPI
- •  Effect of biasing on GPI signals

## Currents and Voltages vs. Time

_**Currents and Voltages vs. Time**_

- I=0.8 MA
- B=4.5 kG

-      outer gap
- 15 cm => 1 cm

-     electrode V
-   -70 V@ 50Hz

-     electrode Isat,i
-     ≤ 2 amps ave.

- probe Isat,e
- ≤ 0.3 amps ave.

## Electrode Configurations Tried

- •  Should have made Epol ~ ± 35 Volts/4 cm ~ ±10 V/cm
- 	between Electrodes #1 and #2
- •  Should have made Epol ~ 70 Volts/4 cm ~ 20 Volts
- 	between Electrodes #2 and #3

## Effect of Biasing on Probe Currents

- •  ***No visible effect***
-    of biasing on mean
-    or rms/mean of any
-    probes signals
- •  Might have expected
-    density to change if
-    large Vr were made
- •  Would have expected
-     rms/mean to change
-     if turbulence was
-     affected by Vr

## Effect of Bias on Probe Correlations

- •  **No visible effect**
-    of biasing on probe
-    cross-correlation
-    or autocorrelation
-    time
- •  Might have expected
-    these to change if
-    large Vr was created

## Mapping from Probes to GPI 

- Time ->

- #1

- #2

- #3

- #4

- #5

- probe

- •  Good correlation of
-     fluctuations across
-     ~ 1 m along B field
-     (C12 ~ 50-80%)
- •   Agrees well with
-     EFIT02 field lines
- •   Size of correlation
-      volume ~ blob size
-      as expected

## Effect of Bias on GPI Turbulence

- •  No visible effect on turbulent structures or motion

- Electrode #1

- Electrode #2

- Electrode #3

## Effect of Bias on Local D Profiles

- D
- intensity
- profiles

- Poloidal distance

- red = on
- black = off

- •   No visible effect of bias on local D profiles

- 10 msec
- averages

- Poloidal distance
- from Electrode #1
- to Electrode #3

## Summary and Plans

- •   So far, no visible effect of electrode biasing on local edge
- 	plasma (still can do more data analysis)
- •   Would like to try ~ 6-8 more test shots:
- 	-  increase electrode voltage from -70  to -100 Volts
- 	-  increase electrode current from ~ 2 to ~ 10 amps
- 	-  decrease B field from 4.5 to 3 kG to increase Vr
- •   Collaborate with LLNL and CPES to understand results
- •   Possibly upgrade hardware for next run (divertor bias ?)

## Slide 17