Advances In High Harmonic Fast Wave Heating of NSTX H-mode Plasmas

P. M. Ryan, J-W Ahn, G. Chen, D. L. Green, E. F. Jaeger, R. Maingi, J. B. Wilgen - Oak Ridge National Laboratory, Oak Ridge, TN, USAR. E. Bell, J. C. Hosea, S. M. Kaye, B. P. LeBlanc, C. K. Phillips, M. Podestá, G. Taylor, J. R. Wilson - Princeton Plasma Physics Laboratory, Princeton, NJ, USAP. T. Bonoli - MIT Plasma Science and Fusion Center, Cambridge, MA, USAR. W. Harvey - CompX, Del Mar, CA, USA

• Goal was to bring system voltage limit with plasma (~15 kV) up to its vacuum limit (~25 kV), thus increasing the power limit by a factor of ~2.8.• For a given strap current

– Peak strap voltage would be halved.– Peak voltage on vacuum side of feedthrough would be reduced by ~30%.– Peak system voltage in transmission line would remain unchanged.

• Tests whether electric fields in strap/Faraday shield region or the strap/antenna frame currents set the limit for plasma operation

12 Antenna Straps

30 MHz RF Power Sources5 PortCubes

DecouplerElements

HHFW antenna extends toroidally 90º

• 12 straps are connected to form two adjacent 6-element arrays, 180º out of phase with each other.• For phase shifts of 30º, 90º, and 150º, it operates as a 12-element array with a single highly directional

peak.

3λ/4 (7.5 m)

λ/2 (5 m)

λ/2 (5 m)

Transmitter power

5λ/4 (12.5 m)

Original

Upgrade

Transmitter power

Original RF Feeds Original RF Feeds Original RF Feeds Original RF Feeds

Previous GroundPrevious Ground Previous GroundPrevious Ground

New Ground New Ground New Ground New Ground

New FeedsNew FeedsNew FeedsNew Feeds

Antenna Upgraded To Reduce Peak Voltages on the Straps

Extensive Antenna Conditioning Needed For Li Operation

Dual Liquid Lithium

Evaporators

~

Dual Lithium Powder

Droppers

Liquid Lithium Divertor

Observation of Arcs During Plasma Conditioning• Li deposited on antenna

contributed to arcing and had to be cleaned by plasma conditioning

• Three visible light cameras (1000, 2000, and >30,000 fps) observing antenna during plasma condtioning in He, L-mode plasma.

• Shot 135242 had three transmitter trips during a 200 ms, 2.6 MW pulse at -90º phasing.

0.188s 0.418s

RF

Po

wer

(M

W)

Transmitter 1 (straps 1 & 7) trips off at 188.1 ms

8 910

1112

1 2 3 4 5 6 7 8 9

189.755 ms 191.755 ms 193.755 ms 195.755 ms

189.749 ms 191.751 ms 193.753 ms 195.755 ms

189.742 ms 191.742 ms 193.742 ms195.742 ms

2000 fps

38,460 fps

1000 fps

Standard camera sees straps 6-12

Fast camera sees straps 8-12

Additional camera sees straps 1-9

Transmitter 5 (straps 5 & 11) trips off at 418.1 ms414.655 ms 416.655 ms 419.655 ms

414.649 ms 416.651 ms 419.615 ms

Ohmically-Heated Helium Target Plasma Transitions to H-Mode During 2.6 MW HHFW Pulse

k = -8 m-1

•GENRAY ray tracing simulation predicts HHFW power deposited on electrons inside ~ 0.5.

•Broader power deposition during H-mode.

•Negligible damping on He predicted, some H minority heating (1 MW, k = -8 m-1, He with 3% H minority)

H-mode Initiated & Maintained Through ELMs with PRF ~ 2.7 MW During ~ 2 MW D2 NBI (Shot 135340)

• Transition to H-mode occurs after RF turn on and without RF arc.

• With reflection coefficient trip level set to 0.7, RF stays on during ELMing plasma.

k = - 13 m-1

Li Operation Improves Power Coupling to the Core

RECORD Te

• CQL3D simulation predicts ~ 40% of RF antenna power coupled to plasma for k = -13 m-1 (-150º) Heating

• Prf used in CQL3D modeling reduced to match simulated and measured neutron rate

In NBI-driven H-mode, Power Coupling to Core Decreases ~20% Compared to L-mode

Electron Pressure Coupling Characteristics

We ~ 13 kJ

Wtot ~ 25 kJ

We ~ 7 kJ

Wtot ~ 14 kJ

(0.353 sec) (PNB = 2 MW, IP = 1 MA,BT = 0.55 T)

R. Harvey

CompX

Shot

130608

k = -13 m-1

• TRANSP/TORIC modeling predicts RF absorption by NBI fast-ions lasts well after NBI turn off

• All rf power absorbed by electrons prior to NBI pulse• After NBI turn-on, the fast-ion population absorbs HHFW power at

the expense of the electrons• Trend confirmed by single time point calculations with AORSA,

GENRAY and TORIC

TRANSP/TORIC

• RF Power Absorption by Fast-Ions Decreases as Fast-Ions Thermalize During RF-Heated NBI H-Mode

• Electron increases with time as density rises, increasing RF heating on electrons

k = -13 m-1

BT(0) = 5.5 kG

Results for HHFW Heating of NBI-driven H-modes

k = -8 m-1

RF-acceleration of fast ions from NBI

Fast IR camera for studying ELM-resolved heat flux

ORNL

Three bolometer views for studying divertor radiation

Fast IR camera view

• "Hot" region in outboard divertor more pronounced at k = –8 m-1 than –13 m-1

• Linked along field lines to scrape-off plasma in front of antenna

• 3 MW/m2 measured by IR cameraduring 2.6 MW of k = –8 m-1 RF heating

• Time for “hot” spot to decay away is ~ 20 ms at –8 m-1 and ~ 8 ms at –13 m-1

Visible & IR Images Show Significant RF Power Flows to Divertor, Particularly for Lower k Heating

Pnbi = 2 MW

Prf = 1.8 MWk = - 8 m-1

Pnbi = 2 MW IR View IR View

Prf = 1.8 MWk = -13 m-1

Pnbi = 2 MW

Visible Camera

(with NBI-only light subtracted)

IR Camera•

HeatFlux

(MW/m2)

3

Bay I

IR view

Bay G

IR view

439.64 – 439.31 ms

RF

RF

-90° -90°

452 ms 455 ms

RF heated pattern on lower divertor plate follows the change in magnetic field pitch

• Large increase in neutron rate when HHFW is applied to NBI plasmas (as predicted originally by CQL3D/GENRAY)

• Measured significant enhancement & broadening of NBI fast-ions profile over a range of ion cyclotron harmonics and

Wtot ~ 20 ms gives eff ~ 66%, 40% for -150°, -90° antenna phasings

• PRF losses coupled to edge are ~ 0.7MW, 1.1 MW for -150°, -90°, out of 1.8 MW launched by antenna

Summary

• Antenna grounds were moved and power feeds added to reduce voltage on the straps.

• Li wall conditioning, needed for improved power coupling through SOL, imposes additional antenna conditioning requirements.

• Achieved record Te (>6 keV) in He.• H-modes accessed with HHFW alone.• Heating of core electrons in NBI-driven

H-modes for k of 3, 8, 13 m-1.• Significant damping on the fast ions

from neutral beams.• Increased edge plasma power losses in

NBI-driven H-modes.• Increased heating of outer divertor with

HHFW.

Summary

• Antenna grounds were moved and power feeds added to reduce voltage on the straps.

• Li wall conditioning, needed for improved power coupling through SOL, imposes additional antenna conditioning requirements.

• Achieved record Te (>6 keV) in He.• H-modes accessed with HHFW alone.• Heating of core electrons in NBI-driven

H-modes for k of 3, 8, 13 m-1.• Significant damping on the fast ions

from neutral beams.• Increased edge plasma power losses in

NBI-driven H-modes.• Increased heating of outer divertor with

HHFW.