Initial Exploration of HHFW Current Drive on NSTX

J. Hosea, M. Bell, S. Bernabei, S. Kaye, B. LeBlanc, J. Menard, M. OnoC.K. Phillips, A. Rosenberg, J.R. Wilson

Princeton Plasma Physics Laboratory

M. Carter, P. Ryan, D. Swain Oak Ridge National Laboratory

R. Pinsker General Atomic

P. Bonoli Massachusetts Institute of Technology

T.K. Mau University of California at San Diego

NSTX Team

APS DPP Meeting11-15 November 2002

Orlando, Florida

Goal: Develop HHFW to support non-inductive operation of the ST concept

Outline:

• HHFW antenna arrangement

• Phase feedback control configuration for selecting antenna spectra

• Selected case for evaluating co/counter current drive effects– -/+ 90° phasing for k = 7.6 m-1

– Closely match plasma parameters [Te(r), ne(r)]

– Measure change in loop voltage

• Modeling of current drive effects

• Conclusions and future directions

HHFW 12-strap antenna array on NSTX

NSTX antennas installed in the vacuum vessel

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

B

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

Phase Feedback Control Configuration

• 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

Spectra Launched for Co and Counter Current Drive with k = 7.6 m-1

• Large pitch angle of the magnetic field results in asymmetric spectra• Loading is larger for co-CD and heating efficiency is larger for counter-CD• To compare VLoop between co and counter cases we use different RF powers to

produce very similar electron parameters

k (m-1)

Co = - 90∞Counter

= + 90∞

GLOSI/RANT3D calculations of power spectra

DipoleS

pect

ral P

ower

(au

)

Electron Parameters Made Similar for Co and Counter CD By adjusting PRF and Gas Feed

107899: Co-CD PHHFW = 2.1MW ( solid lines)

108907: Counter-CD PHHFW = 1.2MW ( dotted lines)

n eL

(cm

-2) Te 0 (keV

)

time (s)

IP = 500 kA, BT = 4.5 kG, D2

Electron Temperature and Density Profiles are Very Similar for Co and Counter Cases

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0.0

Te (k

eV

)

1.61.41.21.00.80.60.40.2

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)

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0.0

ne (x

10-1

9 m

-3)

1.61.41.21.00.80.60.40.2

Radius (m)

Plasma Density 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)

Radius (m) Radius (m)

Te ne

CoCounter

Te

(keV

)

n e

(10

19 m

-3)

A Significant Difference in Loop Voltage is Observed Between Co and Counter Current Drive

• 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

RF on

Counter-CD

Co-CD

V ≈ .23V

Loo p

Vo l

tage

(V

)

time (sec)

ICD from Circuit Analysis is Bracketed by Current Drive Modeling Predictions

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)

See posters - P.M. Ryan et al., GP1.121; T.K. Mau et al., GP1.124; and C.K. Phillips et al., GP1.123 - Tuesday afternoon

C. Petty et al., Plasma Physics and Controlled Fusion 43 (2001) 1747

DIII-D (With NBI)

HHFW Current Drive on NSTX is Consistent with DIII-D Results

• Current drive figure of merit, fw, falls in range of DIII-D data at lower Te(0) • Dimensionless CD efficiency, fw = fw *3.27/Te(0) (keV), is comparable to that for DIII-D at lower temperatures• RF power losses are important in reducing the HHFW current - - trapped electrons are predicted to reduce fw significantly for NSTX

(HHFW only)

Calculated Driven Current Density Profiles

JR

F (

A/m

2 /W

inc)

TORIC Code Predicts a Large Reduction in Current DriveDue to Trapped Electrons

• The “no trapping” profile is indicative of the power deposition profile

Summary and Future Directions

• Digital phase feedback control has been used successfully to compare co and counter current drive with HHFW on NSTX

• A significant reduction in VLOOP is observed for co-CD relative to counter-CD for comparable discharge parameters

– Heating effects on VLOOP are mitigated by closely matching Te(r) and ne(r)

– Ohmic Poynting flux reduction of ~ 30% is observed (VLOOP ~ 0.23V)

• Modeling gives values of dimensionless current drive efficiency comparable with high harmonic DIII-D results

• Future directions for extending the study of HHFW CD on NSTX include:– Increasing HHFW CD by pushing RF power toward 6 MW and increasing Te

– Bringing the MSE system on-line to afford direct measurement of the HHFW CD effects