R. Maingi, NSTX Research Forum 9/04Page 1

Rajesh MaingiOak Ridge National Laboratory

Hendrik MeyerUKAEA Fusion

And the NSTX and MAST Teams

NSTX Results ReviewSept. 20-21, 2004

Princeton, NJ

Results from the Power Threshold Fueling Location and NSTX/MAST Identity Experiments

ResearchSupported by

R. Maingi, NSTX Research Forum 9/04Page 2

NSTX and MAST save similar plasma size but MAST vacuum vessel larger and outer walls further away

Passive stabilizing plates

Vpl/VVV ~ 35-40%MAST

NSTX Vpl/VVV ~ 6-10%

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Good progress made on first detailed MAST/NSTX identity experiment

Goals:

compare L-H power threshold in same CDND (double-null with drsep < 3mm) shape with same Ip, Bt, density, inner-wall midplane fueling location, etc.

determine if PLH lowest with drsep=0 (go to +/- 10 mm)

Results:

CDND NBI power very similar near LH transition (~300-350 kW); need to compute PLOSS

LDND (DN with drsep ~ 10 mm, downward) had higher PLH > 1.7 MW

NSTX shape under rtEFIT was a little bigger

R. Maingi, NSTX Research Forum 9/04Page 4

drsep or sep defined as distance between separatrices mapped to outer midplane

Courtesy of H. Meyer, EPS 2002

R. Maingi, NSTX Research Forum 9/04Page 50.0 0.1 Time[s] 0.2 0.3

Dithery H-mode in CDND near PLH in MAST

Ip [MA]

PNBI [MW]

ne [1019 m-3]

D [au]

PLH

0.5

0.01.0

0.00.4

0.0

0.0

---- 8842 ---- 8851---- 8848 ---- 8852

Type III ELMs

dithers

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NSTX dithers not as ‘periodic’ as MAST (CDND near PLH)

“dithers”

“firm” L-H

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CDND Shapes were reasonably well matched, although NSTX shape (under rtEFIT) was a little larger

NSTX #111378 277msec (NBI 700 kW)MAST #8851 200msec (NBI 580 kW)

R (m)

Z (m)

NSTX #111378 343msecMAST #8851 300msec

Z (m)

R (m)

Double-nullr

sep<1mm(“CDND”)NSTX vac. ves.

Double-nullr

sep<1mm(“CDND”)NSTX vac. ves.

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NSTX ne profile more peaked and Te profile broader early

R [m] R [m] R [m]

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NSTX ne (Te) lower(higher) later in H-mode phase

R [m]R [m]R [m]

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Clear H-mode Transition in LDND near PLH in MAST

Ip [MA]

PNBI [MW]

ne [1019 m-3]

D [au]

PLH

0.4

0.02.0

0.00.6

0.0

0.0

---- 8856---- 8857---- 8859

L-H (ELM-free)L-H (ELMy)

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No transition observed in LDND in NSTX (too low density or MHD at NBI turn-on?)

(Reconstructed)

Sawteeth

(no H-mode)

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NSTX LDND shape a little larger than MASTMagnetic balance (drsep) scan possible with rtEFIT

NSTX #111381 301msecNSTX #111380 301msec

R(m)

Z(m)

NSTX #111383 391msec (NBI 1.1 MW)MAST #8859 280 msec (NBI 1.4 MW)

R(m)

Z(m)

Single-nullr

sep~10mm(“LDND”)NSTX vac. ves.

Single-nullr

sep~+/- 3mm(“LDND”,”UDND”)NSTX vac. ves.

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NSTX never achieved H-mode in LDND, but maybe needed a little more power or higher density?

R [m]R [m]R [m]

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Summary and Future Work

First joint NSTX/MAST experiment was a good first step

CDND power threshold was remarkably similar in terms of NBI power -> wall proximity does not play a big role in these conditions?!

ne profiles in these low Ip H-modes peaked like ohmic H-modes

Higher power CDND discharge had type IIII ELMs in MAST but largely ELM-free (?) in NSTX

Need more LDND data from NSTX (w/edge rotation data…)

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Summary of Power Threshold Dependence on Fueling Location

Motivation: Some gas fueling always required for H-mode access

Test a neoclassical theory formulation which predicted that high-field side (i.e. center stack) gas fueling could lead to a lower power threshold than low-field side (i.e. outer) fueling

Results:

Power threshold and Er not markedly different

Complications:

Early NBI heating needed to avoid sawteeth but affects discharge evolution and power threshold

Power threshold was slowly increasing during run day (do long L-mode phases de-condition the wall w/small outer gap?)

Early H-mode experiment removed H-mode gas fueling requirement

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