Exploration of High Harmonic Fast Wave Heating on NSTX

J. R. Wilson

2002 APS Division of Plasma Physics Meeting

November 11-15, 2002

Orlando, Florida

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R.E, Bell, S. Bernabei, M. Bitter, R. Dumont, D. Gates, J. Hosea, B. LeBlanc, S. Medley, J. Menard, D. Mueller, M. Ono, C. K. Phillips,

A. RosenbergPrinceton Plasma Physics Lab

P. Ryan, D. Swain, J. Wilgen, ORNLP. Bonoli, MIT

T.K. Mau, UCSD R. I. Pinsker, GA

R. Raman, U. of Washington S. Sabbagh, Columbia U.

D. Stutman, Johns Hopkins U.

Y. Takase, U of Tokyo and the NSTX Team

Supported by

With contributions from

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OUTLINE

• Role and Characteristics of High Harmonic Fast Wave (HHFW) heating

• Technical Application on NSTX• Electron Heating and Confinement Results• Interaction with Fast Ions• Current Drive Experiments

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HHFW HEATING HAS BEEN IMPLEMENTED ON NSTX TO PROVIDE A TOOL FOR ELECTRON

HEATING AND CURRENT DRIVE

• ST’s need auxiliary current drive• High beta plasma makes Lower Hybrid and

Conventional Electron cyclotron CD impossible

• HHFW in high beta plasmas has strong single pass absorption on electronsCan allow off-axis deposition

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NSTX UTILIZES THE TFTR ICRF SYSTEM

• 30 MHz Frequency corresponds to D = 9-13

• 6 MW from 6 Transmitters for up to 5 s• 12 Element antenna with active phase control

allows wide variety of wave spectra kT = ± 3-14 m-1

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• Antenna takes up almost 90° toroidally • Provides high power capability with good spectral selectivity

HHFW 12 ELEMENT ANTENNA ARRAY

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1 2 3 4 5 6 7 8 9 10 11 12

Antennas

D1 D2 D3 D4 D5

D6

P1 P2 P3 P4 P5 P6

V1 V3 V4 V5 V6V2

RF Power Sources

5 Port Cubes

Cube Voltages

DecouplerElements

I1 I7

V21

• Digital based phase feedback control is used to set the phase between the voltages of antenna elements 1 through 6• Decouplers compensate for large mutual coupling between elements and facilitate phase control

PHASE FEEDBACK CONTROL CONFIGURATION

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ELECTRON HEATING AND CONFINEMENT

• For typical NSTX plasmas HHFW deposits all its power into electrons

• Energy Confinement on NSTX follows L-mode and H-mode scaling when heat is applied via the electron channel

• Improved electron energy confinement discharges have been observed

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HHHFW PROVIDES STRONG ELECTRON HEATING

Te > Ti

He Discharge

NOTE LOW INTIAL Te

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Both Ions and Electrons Exceed Neoclassical Transport in HHFW L-mode Plasmas

iNC < i

, e

A. Rosenberg

Increasing e with radius

RF power deposition fromHPRT ray tracing code

t = 0.24 s

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HEATING WITH HHFW IS IN QUANTITATIVE AGREEMENT WITH CONVENTIONAL SCALING

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Deuterium, 0.8 MA, 4.5 kG

LARGE Te INCREASE

SMALL ne DECREASE

SMALL ne DECREASE

SOME DISCHARGES DISPLAY BEHAVIOR OF INTERNAL TRANSPORT BARRIER

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Increase in Te Corresponds to Decrease in e

• Power deposition from ray tracing

• Tio(t) obtained from X-ray crystal spectrometer

• e progressively decreases in the central region

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EXPERIMENTS WITH DIRECTED SPECTRA HAVE EXHIBITED CHARACETIRISTICS OF CURRENT

DRIVE

• Compare discharges with wave phased ±3-7 m-1

• Adjust power levels and fueling to match density and temperature profiles

• Loop voltage differences seen in the absence of central MHD (sawteeth, m=1)

• Amount of driven current inferred from circuit analysis of magnetic signals is comparable to theoretical predictions

IP = 500 kA, BT = 4.5 kG

ELECTRON PARAMETERS MADE COMPARABLE BY ADJUSTING POWER AND FUELING

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0.01.61.41.21.00.80.60.40.2

Radius (m)

Electron Temperature for co-, counter-CD Phasing(107899, 107907)

co-CD (t = 393 ms) co-CD (t = 510 ms) counter-CD (t = 393 ms) counter-CD (t = 510 ms)

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• Less loop voltage is required to maintain IP constant when driving HHFW current in the co direction• Internal inductance is similar for the two cases and V is not caused by dli/dt

SIGNIFICANT DIFFERENCE IN LOOP VOLTAGE OBSERVED BETWEEN CO AND CTR PHASING

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CURRENT DRIVE MODELING ANALYSIS AND CODE PREDICTIONS IN ROUGH AGREEMENT

Circuit analysis (0D): IP = (V- 0.5*IP*dLi/dt)/RP + IBS + ICD

(Assumes steady state, RP and IBS (pressure profiles) independent of array phasing, ICD PRF/ne) ICO ≈ 110 kA (0.053 A/W)

Codes - Calculated electron power absorption profiles are coupled to Ehst-Karney adjoint solution for current drive efficiency to obtain current density profiles

• TORIC: Full wave ICRF field solver [(ki)2 << 1, B = 0 for electric field polarization] ICO ≈ 96 kA (0.046 A/W)

• CURRAY: Ray tracing code (damping is linear on Maxwellian species, all orders in ki, k determined locally) ICO ≈ 162 kA (0.077 A/W)

Calculated Driven Current Density Profiles

• The “no trapping” profile is indicative of the power deposition profile• Trapped electrons are predicted to reduce fw from 0.19 to 0.046

TRAPPING PLAYS SIGNIFICANT ROLE IN REDUCING DRIVEN CURRENT

TORIC CALCULATION FOR NSTX CO CD SHOT

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• Current drive figure of merit, fw, falls in range of DIII-D data at lower Te(0) • Dimensionless current drive efficiency, fw = fw *3.27/Te(0) (keV), is comparable to that for DIII-D at lower temperatures

HHFW CURRENT DRIVE CONSISTENT WITH D-IIID AND TFTR CD EXPERIMENTS

OPERATION AT INCREASED Te WILL BE NEEDED TO ACHIEVE NSTX GOALS

TFTR MCCD

TFTR FWCD

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HHFW INTERACTION WITH FAST IONS

• Even at high harmonic numbers (n ≥ 9) ion damping can be important due to large kperpi effectsOn NSTX kperpi ~ x for 80 keV beam ion

Damping important if kperpi)2/2 ≥ n

• Damping on beam ions may reduce current drive efficiency

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• D+ tail extends to 130 keV• Tail saturates in time during HHFW

beam injectionenergy

at RF end+10 ms+20 ms

noise level

108251

neL(0)

Te(0)

PNB

PRF

Ip

• Tail decays on collisional time scale

NPA SHOWS FAST ION TAIL BUILD-UP AND DECAY

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• HPRT computes hot plasma absorption over cold ion/hot electron ray path

• 25 rays used

• TRANSP output used as input for fast ion temp and density distribution

• Total power evenly split

• Fast ions dominate central absorption

• Electrons dominate further off-axis

ne0 = 3.2 1013 cm-3

nb0 = 2.5 1012 cm-3

Te0 = 638 eVTb0 = 17.3 keV

Pe=54.5%Pb=45.5%

Dbeam

e-

Shot 105908 Time 195 ms

RAY TRACING PREDICTS SIGNIFICANT FAST ION ABSORPTION

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B0=4.5 kGB0=4.0 kGB0=3.5 kG

B0=4.5 kGB0=4.0 kGB0=3.5 kGNo RF (B0=4.5 kG)

RF turn on

RF full power

RF turnoff

• As B0 increases, the NPA sees a larger tail

• Enhanced neutron rate with higher B0 until MHD ~240 ms

• Likely due to larger with lower B, which promotes greater off-axis absorption where fast ion population is small

STRONGER TAIL AT HIGHER BT

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NBI Source A, 80kV 1.6 MWNBI Sources B & C, 60 kV,.8 MW each (1.6 MW total)

Ip= 1 MAIp= 750 kANo RF(Ip=750 kA)

• Larger tail observed with greater total beam energy, fixing power

• Tail of low vs. high Ip similar, but neutron rate greater enhanced

• Greater ion absorption predicted with lower k||, but surprisingly little variation in tail, small neutron enhancement with higher k||

SCANS IN BEAM ENERGY, PLASMA CURRENT AND WAVE PHASE VELOCITY PERFORMED

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NPA scan, k||= 7 m-1, B0=4.5 kG

NPA at 90 cmNPA at 70 cmNPA at 50 cm

• Depletion in particle flux

with NPA Rtan further

off-axis

• Tail extends to same energy range

MEASUREMENTS SHOULD ALLOW DETAILED COMPARISON TO DEPOSITION MODELING

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• HHFW PROVIDES MEANS OF ELECTRON HEATING ON NSTX CONFINEMENT CONSISTENT WITH STANDARD SCALINGS IMPROVED CONFINEMENT REGIME OBSERVED

• INITIAL EXPERIMENTS WITH DIRECTED PHASING PROVIDE EVIDENCE FOR CURRENT DRIVE DRIVEN CURRENT APPEARS CONSISTENT WITH

MODELING AND PREVIOUS FWCD EXPERIEMENTS TO ACHIEVE NSTX GOALS INCREASED Te NEEDED

• INTERACTION BETWEEN HHFW AND NBI IONS OBESERVED

SUMMARY