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| Title: **Edge      |                      |       |                 |
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| for SOL Control**  |                      |       |                 |
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| **Responsible      |                      |       | > Date 531/07   |
| Author: S.         |                      |       |                 |
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NSTX EXPERIMENTAL PROPOSAL

TITLE: **Edge Electrode Biasing for SOL Control** No. **OP-XP-744**

AUTHOR: **S. Zweben, C. Bush, R. Maqueda, L. Roquemore** DATE:
**5/25/07**

# 1. Overview of planned experiment 

This XP will use the newly installed edge electrodes to test the theory
of Cohen and Ryutov concerning the effect of a local poloidal electric
field on the SOL transport. The electrode hardware has been successfully
tested with XMP-051 (completed May 10, 2007). This XP will extend these
test results to higher electrode voltages and currents, will vary the
plasma conditions from the single case tried in XMP-051, will try a
positive biased electrode, and will (if there is any extra time
available) try electrode biasing in an Ohmic H-mode.

# 2. Theoretical/ empirical justification

> The main justification for this XP is to test one of the very few
> existing theoretical ideas for modifying the SOL in magnetically
> confined plasmas. If successful, this could lead to methods to control
> and reduce the peak particle and heat flux in any magnetic fusion
> device.

The theory and its previous application to the MAST experiment has been
described in: R.H. Cohen et al, "Plasma Convection Induced by Toroidal
Asymmetries of the Divertor Plates and Gas Puffing", Nucl. Fusion 37
(1997) 621 ; D.D. Ryutov et al, "On the Possibility of Inducing Strong
Plasma Convection in the Divertor of MAST", Plasma Phys. Cont. Fusion 43
(2001) 1388 ; G.F. Counsell et al, "Reduction of Divertor Power Loading
in MAST", 30th EPS Conference (2003) paper P-3.202 ; and R.H. Cohen et
al, "Current and Potential Distribtion in a Divertor with
Toroidally-Asymmetric Biasing of the Divertor Plate", Plasma Phys. Cont.
Fusion 49 (2007) 1. The basic idea is very simple: the application of a
local poloidal (DC) electric field will cause a local Vr = Epol x B
drift in the radial direction, which can in principle move and/or
broaden the SOL. The electric field needed to do this in NSTX should be
quite low (\~ 10 V/cm) since the B field is quite low. This field level
can be created by the small electrode set installed in the shadow of the
RF antenna for this run. The main physics question is how far the
potential induced by the electrodes penetrates along and across the B
field. Some success was obtained by using a (somewhat differently
designed) electrode set in MAST. A secondary physics question is to
determine whether the application of this local bias affects the edge
turbulence locally, which could cause additional changes in the SOL
width.

# 3. Experimental run plan

> The run plan is based on repeating the plasma condition used in
> XMP-051 (Checkout of Biased Probe and Electrodes), e.g. shot #124062
> (B= 4.5 kG, I=0.8 MA, Ohmic, outer gap slowly reduced from \~ 10 cm to
> \~ 1 cm over the duration of the discharge). This condition had the
> proper B field line angle for connecting the biased electrodes to the
> GPI diagnostic.
>
> **shot 1:** set electrode #1 and #2 voltages to -100 V / -50 volts
> (compared with -70 volts/ -35 volts used previously in XMP-051), and
> increase the density with extra gas puffing to \<ne\>\~ 4x10^13^
> cm^-3^. Electrodes #3 is at zero volts and electrode #4 is hard
> grounded.
>
> **shot 2:** repeat shot #1 with electrodes #1 and 2 to -50 V/ -100 V
> to change sign of local B. Increase density further if possible. Keep
> electrodes #3 and at zero volts and #4 hard grounded.
>
> **shot 3:** if a clear effect of electrode bias is seen in shot #1 or
> #2, as determined by the GPI or probe measurements, then repeat that
> shot.
>
> **shot 4:** using the same shot above, keeping electrodes #1 and 2 at
> -50 V / -100 V, now set electrode #3 at +10 volts (if hardware
> allows), to see effect of electron collection in electrode.
>
> **shot 5:** same as shot 4, but increase electrode #3 voltage to +70
> volts in electron saturation.
>
> **shot** **6**: if either shots #5 or #6 make a clear change in the
> local SOL, as determined by the GPI or probe measurements, then repeat
> that shot.
>
> **shot 7:** change to B=3.5 kG, I=620 kA to see effect of increased Vr
> due to lower B; Ohmic plasma, moderate density, same outer gap as part
> (1). Set electrode #1 and #2 voltages to -100 V / -50 volts. Set
> electrode #3 as guided by shots 5,6, and electrode #4 is hard
> grounded.
>
> **shot 8:** using the same B=3.5 kG shot above, switch electrodes #1
> and 2 to -50 V/ -100 V to change sign of local B. Keep electrodes #3
> and 4 at zero volts and hard grounded.
>
> *only if extra time is available:*
>
> **shots 9-12:** repeat shot #117260 to create Ohmic H-mode, but with
> outer gap slowly reduced from \~ 5 cm to \~ 1 cm, and try various
> electrode configurations (as above) to modify the SOL.

# 4. Required machine, NBI, RF, CHI and diagnostic capabilities

Ohmic plasmas, no NBI or RF or CHI

Necessary Diagnostics: Langmuir probes mounted in the biased electrode
holder (Bush) GPI with gas puff (Maqueda), fast camera to view electrode
region from the other side of the machine (Roquemore).

**5. Planned analysis**

The effect of the electrodes on the local SOL can be determined by the
GPI and by the Langmuir probes mounted in the biased electrode holder.
If there is a fast radial convection induced by the electrode bias, the
GPI gas cloud should move outward radially (as seen in the MAST
experiment). The turbulence radial speed can also be measured by the GPI
images. The edge turbulence fluctuation levels, correlation lengths, and
autocorrelation times between the electrodes can be measured by the
local probes.

The results will be directly compared with the analytical models
mentioned above. Comparisons with the XGC code of CS Chang will be
attempted. Comparison with the MAST results will also be made.

# 6. Planned publication of results

The results could be published as a full-length paper in Phys. Plasmas,
NF, or PPCF.

PHYSICS OPERATIONS REQUEST

Title: Edge Electrode Biasing for SOL Control OP-XP-744

Machine conditions (specify ranges as appropriate)

> I~TF~ (kA): **-53, -42** Flattop start/stop (s):

I~P~ (MA): **0.8, 0.6** Flattop start/stop (s):

Configuration: **Shots #1-6 exactly same as #124062 , except for higher
density**

**Shots #7-8 same as #1-6 but with I~TF~=-42kA, I~p~=620 kA**

**Shots #9-12: exactly like #117260, but with outer gap to 1 cm** Outer
gap (m): **down to \~ 1 cm** Inner gap (m):

Elongation : Triangularity :

Z position (m):

> Gas Species: **D** Injector(s):
>
> NBI -- off

ICRF -- off

CHI -- off

Shot numbers for setup: **124062 (shots #1-6); scale down I~p~, I~TF~ or
use 121356 with modified outer gap (shots #7, 8); 117260 (#9-12)**

DIAGNOSTIC CHECKLIST

OP-XP-744

  -------------------------------------------------------------------------------
  **Diagnostic**             **Need**   **Desire**   **Instructions**
  -------------------------- ---------- ------------ ----------------------------
  Bolometer -- tangential               **X**        
  array                                              

  Bolometer -- divertor                 **X**        

  CHERS -- toroidal                                  

  CHERS -- poloidal                                  

  Divertor fast camera                  **X**        

  Dust detector                                      

  EBW radiometers                                    

  Edge deposition monitors                           

  Edge pressure gauges                               

  Edge rotation diagnostic              **X**        

  Fast ion D_alpha - FIDA               **X**        

  Fast lost ion probes -                             
  IFLIP                                              

  Fast lost ion probes -                             
  SFLIP                                              

  Filterscopes                          **X**        

  FIReTIP                               **X**        

  Gas puff imaging           **X**                   

  Hα camera - 1D                        **X**        

  High-k scattering                                  

  Infrared cameras                      **X**        

  Interferometer - 1 mm                 **X**        

  Langmuir probes - divertor            **X**        

  Langmuir probes -- RF                 **X**        
  antenna                                            

  Magnetics -- Diamagnetism             **X**        

  Magnetics - Flux loops                **X**        

  Magnetics - Locked modes              **X**        

  Magnetics - Pickup coils              **X**        

  Magnetics - Rogowski coils            **X**        

  Magnetics - RWM sensors               **X**        

  Mirnov coils -- high                  **X**        
  frequency                                          

  Mirnov coils -- poloidal              **X**        
  array                                              

  Mirnov coils -- toroidal              **X**        
  array                                              

  MSE                                                

  NPA -- ExB scanning                                

  NPA -- solid state                                 

  Neutron measurements                               

  Plasma TV                             **X**        

  Reciprocating probe                   **X**        

  Reflectometer -- 65GHz                **X**        

  Reflectometer --                      **X**        
  correlation                                        

  Reflectometer -- FM/CW                **X**        

  Reflectometer -- fixed f              **X**        

  Reflectometer -- SOL                  **X**        

  RF edge probes                        **X**        

  Spectrometer -- SPRED                 **X**        

  Spectrometer -- VIPS                  **X**        

  SWIFT -- 2D flow                      **X**        

  Thomson scattering         **X**                   

  Ultrasoft X-ray arrays                **X**        

  Ultrasoft X-ray arrays --             **X**        
  bicolor                                            

  Ultrasoft X-rays -- TG                **X**        
  spectr.                                            

  Visible bremsstrahlung                **X**        
  det.                                               

  X-ray crystal                                      
  spectrometer - H                                   

  X-ray crystal                                      
  spectrometer - V                                   

  X-ray fast pinhole camera                          
  -------------------------------------------------------------------------------