# Presentation: 446533

## Biased Electrodes for SOL 

_**Control in NSTX**_

- S.J. Zweben, R.J. Maqueda*, L. Roquemore,
- C.E. Bush**,  R. Kaita, R.J. Marsala, Y. Raitses,
- R.H. Cohen***, D.D. Ryutov***
- PPPL, Princeton NJ USA
- *  Nova Photonics, Princeton NJ USA
- **ORNL, Oak Ridge TN USA
- ***LLNL, Livermore, CA USA

*Poster P3-O3 @ PSI Meeting May 2008*

## Abstract

- 	Small electrodes were installed near the outer midplane of NSTX to control the width of the scrape-off layer (SOL) by creating a strong local poloidal electric field.
- 	Clear increases were seen in the plasma density and potential in between these biased electrodes when the applied Epol x B drift was directed radially outward.
- 	However, little or no change was seen in the Da emission profile ~1 meter downstream from the electrodes along the magnetic field, implying that the poloidal electric field did not propagate this far along B.

## Theory of SOL Control by Epol

- •   Create localized Epol in SOL near divertor plate to cause
- 	radial EpolxB flows to move SOL strike point or induce
- 	turbulent broadening of SOL
- 	R.H. Cohen et al, NF 37, 621 (1997)
- 	D.D. Ryutov et al, PPCF 43, 1399 (2001)
- 	R.H. Cohen et al, PPCF 49, 1 (2007)]
- •    Main questions:
- 	-   do these perturbations affect the local SOL ?
- 	-   do these perturbations locally induce turbulence ?
- 	-   how far do perturbations extend II and  B ?

## Previous Experiments

- •   Some experiments have shown creation of local Epol in SOL
- 	JFT-2M  [Hara et al, J. Nucl. Mat. 241-243, 338 (1997)]
- 	MAST [Counsell et al, J. Nucl. Mat. 313-316, 804 (2003)]
- 	CASTOR [Stockel et al, PPCF 47, 635 (2005)]
- •   MAST experiment was done to test idea of Cohen/Ryutov,
- 	resulting in partial confirmation of theory, e.g. movement
- 	of D strike point at biased divertor “ribs”
- •  Other experiments have seen potential propagate along B
- 	DITE [Pitts and Stangeby, Plasma Phys. Cont. Fusion  32, 1237 (1990)]
- 	TEXT [Winslow et al, Phys. Plasmas 5, 752 (1998)]
- 	W7-AS [Thomsen et al, Plasma Phys. Cont. Fusion 47, 1401 (2005)]

_**Previous Experiments**_

## NSTX Electrode Biasing Experiment

- •   Uses electrodes near outer midplane in far-SOL
- •   Uses probes to measure local potential, ne and Te
- •   Uses GPI to see effects on D ~ 1 m along B
- =>      Applied Epol ≤ 200 V/cm (vacuum) could create a
- 	EpolxB of Vr ≤ 5x106 cm/sec, ~50 times larger than 	the typical radial velocity in the SOL (~ blob velocity)
*     ****bias ***_***should***_*** have strong effect on local SOL (it did)***
***       ******bias ***_***should not***_*** affect global plasma (it did not)***

## Biased Electrodes and Probes in NSTX

- •  4 electrodes of size
-     3 cm x 3 cm
- •  8 Langmuir probes
-     next to electrodes
- •  leading edge ~ 1 cm
-     behind RF antenna
- •  biased field lines seen
-    by GPI diagnostic

**Biased electrodes **
**and probes**

**GPI**
**puffer**

**HHFW **
**antenna**

**B field**

## Biased Electrodes and Probes

- •  Electrodes ≤100 V@30 A (or -100V@10 A), mod. @ 50 Hz
- •  Nearby Langmuir probes biased DC or swept ± 50 volts

- electrodes

- probes

- RF
- antenna

- radial
- probe
- array

- Epol x B
- radially
- outward

## NSTX Discharge Conditions

_**NSTX Discharge Conditions**_

- •  Typically I=0.8 MA, B=4.5 kG, PNBI = 2-4 MW
- •  Edge density in SOL increases with smaller outer gap

- Ip (kA)

- NBI (MW)

- outer gap (m)

- E3 voltage

- E3 current

## Electrode and Probe Signals vs. Time

_**Electrode and Probe Signals vs. Time**_

- •  Here E2 @ - 90 volts, E3 at + 90 volts, P3b @ +45 volts
- •  See clear increase in probe current with each biasing

- H-mode

- L-mode

- #127054

## Typical (I,V) Characteristics

_**Typical (I,V) Characteristics**_

- •  Electrode current  Ie/Ii ~ 7 at ±40 V
- •  Probe current  Ie/Ii ~ 24 at  ±40 V
- 	ne ~ few x1011 cm-3
- 	Te ~ 6±3 eV
- •  Ii(electrode) / Ii(probe) ~ 100,
-        about area ratio (as expected)
- •  Electrode electron current more
-        than ‘double-probe’ limit, but
-        less than ‘single-probe’ limit

## Probe Floating Potential Response

_**Probe Floating Potential Response**_

- •  floating potential of
-    probes near - bias
-    electrode doesn’t
-    change significantly
- •  floating potential of
-    probes near + bias
-    electrode go up ~20%
-    of voltage on electrode
- =>  positive electrode
-         affects local Vf
-          negative electrode
- 	does not

## Local SOL Density Profile Effects

- •  Epol x B directed outward between electrodes E2 and E3
- •  Radial profiles of Ie ( ne) measured with probes P3a-P3d
- •  Local density increases x3 to x10 with ±90 volts on E2-E3

## Effects on Local Turbulence

- •   Relative density fluctuations decrease x2 with biasing
- 	(floating potential fluctuations increase with biasing)
- •   Autocorrelation times and radial correlation lengths in
- 	L-mode are ~ unchanged by biasing
- •   During H-mode biasing, small ELMs are seen at probes,
- 	which increases correlation times and lengths

## Ohmic and RF Heated Cases

- •  Similar density profile changes were seen in Ohmic
- 	plasmas (±90 volts) and RF heated plasmas
- 	(± 50 volts), with biasing between E2 and E3

## Correlation of Probe & GPI Fluctuations 

- •  Good correlation of
-     fluctuations along
-     ~ 1 m along B field
-     (C ~ 50-80%)
- •   Mapping of probes
-     to GPI agrees with
-     EFIT02 field lines
- •   Size of correlation
-      volume ~ 4 cm, as
-      expected (~ blob)

- GPI imaging region overlayed with
-   probe correlation strength (red)

## Radial Profile of D Emission

- •  Demission from a GPI gas puff measured ~ 1 m along B
- •  No significant change seen with biasing

- •  radial profile as measured
-     between E2 and E3, using
-     alignment from correlations
- green dots = electrode centers
-   white line = range of this plot
- red circle = overlap with probes

## Summary of Experimental Results

- •	Poloidal electric field had a significant effect on the SOL
- 		profiles as measured the local Langmuir probe array:
- 	SOL density increased by ~ factor of x3-10 over ~ 4 cm
- •    Poloidal electric field did not have a significant effect on
- 		the local turbulence (except during small ELMs)
- •    Positive bias had a small effect on the local floating 		potential, but negative bias did not have any effect
- •    No significant change was seen in the D profile and 		turbulence in the GPI diagnostic ~1 m away along B

## Tentative Theoretical Interpretations

- •    Outward Epol x B caused the increase in SOL density, but
- 	a quantitative comparison with theory can not be done
- 	without more information on penetration II and  to B
- •    Changes in local potential seen with (+) bias and not (-)
- 	bias is consistent with sheath model of Ryutov et al
- •    Ratio of electron/ion current to electrodes I(+) ~ 7 x I(-)
- 	suggests a significant cross-field current, which may
- 	explaining the absence of effect on D ~ 1 m along B
- •    No clear evidence of increased turbulence due to biasing,
- 	as might be driven by K-H instabilities

## Sorry I could not come to this meeting

- If you have questions or comments,
- please ask Ricky Maqueda at Poster
- P3-23,  or email me at:
- _[szweben@pppl.gov](mailto:szweben@pppl.gov)_
- Stewart