Divertor Characteristics from Probe Measurements during NBI

NSTX Results Review 26-28 July 2006

PPPL Princeton, NJ

C.E. Busha, R. Kaitab, P. Roneyb, R. Maingia,

E.. Fredricksonb, S. Sabbaghc, V. Soukhanovskiid, J.G. Watkinse

aOak Ridge National Laboratory, Oak Ridge, TN USA bPrinceton Plasma Physics Laboratory, Princeton, NJ USA

cColumbia University, NY USAdLawrence Livermore National Laboratory, Livermore, CA USAeSandia National Laboratories, Albuquerque, New Mexico, USA

ResearchSupported by

C. Bush, ResultsRev_Jul2006 Page 2

Outline

•Initial NSTX Langmuir Probe System is a coarse array–Sample IV Characteristic data

•Langmuir Probe data obtained for vast majority of shots–Especially the last 3 campaigns–Data for many configurations–Data for many conditions

•Report Emphasizes a Beam Power Scan–Also a series of Gas Puff experiments

•Isat, Te reported for a limited set of experiments

•Summary

Backup Slides

END

The EFIT equilibrium shows the location of the inner and outer strike-points on the divertor plates of NSTX . In general the strike-points and measured probe positions are in good agreement.

Outer Strike-pointInnerStrike-point

LowerSingle-nullDivertor

C. Bush, ResultsRev_Jul2006 Page 4

Divertor Langmuir Probes

1234

1 R(probe 1) = 1.02 m2 R(probe 2) =

0.91 m3 R(probe 3) = 0.80 m4 R(probe 4) = 0.49 m

C. Bush, ResultsRev_Jul2006 Page 5

Fit to IV Characteristic for Flush Probe

Sweep voltage ± 50 volts

Only points V < VLimit included in fit.

Here VLimit = +15 V

The relatively flat upper part of curve due to 10 volt limit on digitizer not to electron current saturation.

* Te fit corrected for nonsaturationof ion current.Te = 6.72 eV(12 eV uncorrected).

Fit to IV data

Solid lines, data for

2 different sweeps

Isat(ion)

Probe sweep voltage

I

V

0

~Isat

I (A)

IV Characteristic

0.04

-0.04

0.08

0.12

-40 0 40

V (Volts)

14

C. Bush, ResultsRev_Jul2006 Page 6

Outer Divertor Probe Te vs Time, Pb = 4 MW

Shot 112498Pb = 4 MWRprobe = 0.91 m

C. Bush, ResultsRev_Jul2006 Page 7

Heat flux profile becomes more peaked with increasing NBI

power

0

1

2

3

4

5

0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1

2 MW (#112508)3MW (#112507)4 MW (#112498)6 MW (#112499)

Heat Flux [MW/m

2]

Radius [m]

0.306 sec

R. Maingi

• Peak in Isat occurs near peak heat flux

• In general peak Isat is independent of Pb, heating power

•Isat Probe 3

IsatProbe 2

40

30

20

10

0

50

Isat (mA)

IsatProbe 1

C. Bush, ResultsRev_Jul2006 Page 8

Isat Does not Depend Strongly on NBI Power112508112498112499Shots:

112508112498112499Shots:

Isat(A)

Time (s)0 0.2 0.60.4 0.8

Probe 1

Pb(MW)

0

3

6

Isat(A)Probe 2Isat(A)Probe 3

0.05

0.05

0.05

0.10

0

0

0

Shots112508112498112499

C. Bush, ResultsRev_Jul2006 Page 9

Isat for Probes - PDD with LDGIS GAS Puff 700 Torr

0

20

40

60

Current

(mA)

2711 2812 2913

116486

0

20

40

60

80

1307 1408 1509

0.4

0.6

0.8

1.0

Strike point

radius (m)

Current

(mA)

Time (s)

0.0 0.1 0.2 0.3 0.4 0.5

C. Bush, ResultsRev_Jul2006 Page 10

Summary

•NSTX has a coarse probe array

–Radial variation of Isat Outer Divertor similar to that of the heat flux

–Te along divertor surface in range 4 to 20 eV over the beam power scan

•Response to Gas Puff Experiments

–Probes in SOL show correlation with Dα measured at divertor plate (possible ~10 ms lag)

–Signals large in SOL, smaller in private flux region

–Other features

C. Bush, ResultsRev_Jul2006 Page 11

END

C. Bush, ResultsRev_Jul2006 Page 12

Corrected Fit

Normal IV Characteristic Fit for 3 parameters Isat, Vf, Te

I = Is*[1-exp(e(V-Vf)/kTe)]

For nonsaturation of I(V) we need a 4th

I=[Is+ I(Vf-V)][1-exp(e(V-Vf)/kTe)]

C. Bush, ResultsRev_Jul2006 Page 13

Isat Does not Depend on NBI Power112508112498112499Shots:

112508112498112499Shots:

Isat(A)

Time (s)0 0.2 0.60.4 0.3

Probe 3

D(arb)

Pb(MW)

0

3

60

2

4

0

0.05

0.10

C. Bush, ResultsRev_Jul2006 Page 14

Isat for Probes - Neon GAS Puff 190 Torr, 4Tls

0

5

10

15

Current

(mA)

2711 2812 2913

116489

0 5

10

15

20

25

Current

(mA)

1307 1408 1509

0.4

0.6

0.8

1.0

Strike point

radius (m)

Time (s)

0.0 0.1 0.2 0.3 0.4

Isat

Isat

C. Bush, ResultsRev_Jul2006 Page 15

Histograms of Isat for Outer Divertor Probes

0.08

0.040

0.04

00.020.01

0

0

4

0

400

0.9

0.70.0 0.2 0.4

Time (s)

Isat(13) (A)

Isat(14) (A)

Isat(15) (A)

Ip(kA)

Dα(arb.)

R(m)