Energy Confinement Scaling in the Low Aspect Ratio National Spherical Torus Experiment (NSTX)

S. M. Kaye, M.G. Bell, R.E. Bell, E.D. Fredrickson, B.P. LeBlanc, K.C. Lee,

S. Lynch and S.A. Sabbagh47th APS – DPP Meeting

Oct 24-28, 2005Denver, Co.

Supported byOffice ofScience

Culham Sci CtrU St. Andrews

York UChubu UFukui U

Hiroshima UHyogo UKyoto U

Kyushu UKyushu Tokai U

NIFSNiigata UU Tokyo

JAERIHebrew UIoffe Inst

RRC Kurchatov InstTRINITI

KBSIKAIST

ENEA, FrascatiCEA, Cadarache

IPP, JülichIPP, Garching

ASCR, Czech RepU Quebec

College W&MColorado Sch MinesColumbia UComp-XGeneral AtomicsINELJohns Hopkins ULANLLLNLLodestarMITNova PhotonicsNew York UOld Dominion UORNLPPPLPSIPrinceton USNLThink Tank, Inc.UC DavisUC IrvineUCLAUCSDU ColoradoU MarylandU RochesterU WashingtonU Wisconsin

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Scaling Experiments Performed to Assess Effect of Aspect Ratio on Energy Confinement

• Data from 2004 experiments (ELM-free and ELMy H-mode)

Ip=0.6-1.2MA, BT=0.3-0.45 T, ne=1.5-7e19 m-3, PNBI=1.5-7 MW, =1.6-2.4

Data from 2005 experiments being analyzed

• NSTX extends parameter space– Allows for exploration of confinement trends at

high-, low-

– Connect to conventional aspect ratio database for assessment of R/a

International H-modeConfinement Database

NSTX

correlation

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Results of Systematic Parametric Scans Indicate Some Scalings Similar to Those at Conventional R/a

• ~ Linear current scaling (correlated with density)• Slightly weaker power degradation than at conventional R/a

0

5

0.0 0.1 0.2 0.3 0.4 0.50

100

Time (s)

0

5

0

1

0.5 1.0 1.5Radius (m)

Ip [MA]

PNB [MW]

WMHD [kJ]

We [kJ]

100200

300

0

1

0

ne [1019m-3]

Te [keV]

BT=0.45 TE,thermal calculated in TRANSP

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Data Exhibits a BT Dependence and Significant Shot – Shot Variability

H-factor depends on ELM severity and inversely on plasma shaping

98y,2 ~ 0.8

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Statistical Analyses Indicate a Stronger BT but Weaker Ip Dependence Than Exhibited at Conventional Aspect Ratio

B ~ -5.81 -0.65

-0.43 B ~ -3.75 -0.01

-0.45

• Variation in power scaling exponents among methods leads to range of -scaling dependences (from null to unfavorable) • BT and scaling to be studied in future experiments through dedicated scans (strong – correlation in present NSTX dataset)

Principal Component – Error in Variables Ordinary Least Squares Regression

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NSTX Confinement Data In the International H-mode Database Has Been Used to Determine Aspect Ratio Scaling

E,thIPB98(y,2) ~ Ip

0.93 BT0.15 ne

0.41 PL,th-0.69

0.78 M0.19 R1.97 0.58

B ~ 2.7 -0.9

0.01

E,thfit ~ Ip

0.73 BT0.36 ne

0.39 PL,th-0.62

R2.14 1.03

B ~ 2.78 -0.48

0.12

ITER98PB(y,2) and other scalingsdeveloped with data from <0.5 donot describe low R/a data well

Scalings developed with low R/a dataexhibit stronger and less unfavorable dependences

– correlated in database: leads to difficulty inestablishing these dependences uniquely

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NSTX Confinement Studies Have Been Carried Out

• Systematic scans indicate current, power scaling similar to those at higher aspect ratio– Density increases with plasma current– Weaker current scaling using statistical methods

– Scalings show B ~ 0 to -0.65 ( – correlation)

• Data shows scaling with BT and significant shot – shot variability– Some variability can be attributed to the effects of shaping,

ELM amplitude, density fluctuation amplitude (weak)

• More studies need to be done– Assess BT dependence through dedicated scans

– Perform dedicated and shape () scans– Complete analysis of more recent data

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Shot – Shot Confinement Variability Studied in Context of Other Plasma Variations

No dependence on rotation or MHD activity(at any frequency)

Weak dependence on density fluctuationamplitude