A. L. Rosenberg, for the NSTX Team

With contributions from:

J. E. Menard, J. R. Wilson, S. S. Medley, R. Andre, M. Bitter, D. S. Darrow, J. Egedal, R. W. Harvey, E. F. Jaeger, B. P. LeBlanc, T. K. Mau, C. K. Phillips, M. H. Redi, A. L.

Roquemore, P. M. Ryan, S. A. Sabbagh, and D. W. Swain

45th Annual Meeting of Division of Plasma PhysicsAmerican Physical Society

October 27 – 31, 2003Albuquerque, New Mexico

Fast Ion Absorption of the High Harmonic Fast Wave in the National

Spherical Torus Experiment

Columbia UComp-X

General AtomicsINEL

Johns Hopkins ULANLLLNL

LodestarMIT

Nova PhotonicsNYU

ORNLPPPL

PSISNL

UC DavisUC Irvine

UCLAUCSD

U MarylandU New Mexico

U RochesterU Washington

U WisconsinCulham Sci Ctr

Hiroshima UHIST

Kyushu Tokai UNiigata U

Tsukuba UU Tokyo

JAERIIoffe Inst

TRINITIKBSI

KAISTENEA, Frascati

CEA, CadaracheIPP, Jülich

IPP, GarchingU Quebec

Supported by

Introduction and Motivation

• Ion absorption critically important to assessing viability of HHFW to heat and drive current in STs– ECH cut off, LHCD & ICRF have low e- CD efficiency – HHFW predicted to be completely absorbed in one pass

• Experimental evidence of HHFW interaction with NBI– Neutral Particle Analyzer (NPA) scannable at midplane

– neutron rates, Fast Lost Ion probes

• Modeling supports observations– HPRT, CQL3D, CURRAY, AORSA

NSTX top-down layout

HHFW antenna array delivers controllable phase

12 element antenna for 6 MW @ 30 MHz

Antenna phasing controls heating or current drive

-20kz (m-1)

0 20

co-CD

HeatingB

Antenna extends almost 90° toroidally

D = 9-13

counter-CD

• Neutral beam at 80 keV• D+ tail extends to 130 keV• Tail saturates in time during HHFW

HHFW generates a fast ion tail with NBI

• Tail decays on collisional time scale

• Typical density, temperature, Ip

HHFW enhances neutron rate

105906

No RF vs. TRANSP

105908

RF vs. TRANSP

RF

no RF

Measured RF vs. no RF

TRANSP

Measurement

After RF turnoff, rate decays close to

measured and predicted no RF value

Measured neutron rate for RF shot significantly exceeds TRANSP prediction without RF

TRANSP

Measurement

Enhancements in tail and neutron rate simultaneously observed

• NPA horizontally scanned from 108249-51, shots otherwise

set up identically

r/a

Ray tracing predicts fast ion absorption competitive with electrons

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

• 25-50 rays used

• Fast ions modeled as effective Maxwellians– Tfast & nfast from TRANSP

• Fast ions dominate central absorption, electrons further off-axis

• Ti,th = 2 Te, no thermal ion absorption

ne0 = 3.2 1013 cm-3

nf0 = 2.5 1012 cm-3

Te0 = .6 keVTf0 = 17.3 keV

Pe = 55%Pfi = 45%

Dfast

e-

Shot 105908 Time 195 ms

p

• HPRT, CURRAY, AORSA find 30-35% fast absorption – CQL3D finds similar ion deposition profile using model f(v), 3

harmonics

Agreement between HPRT,

CURRAY, AORSA, and CQL3D for RF + NBI

• N = 24, t = 5%, fast ion shot

Ray-tracing and full wave codes see similar significant 2D effect in propagation

HPRT50 rays

(99% absorbed)

AORSA

• TRANSP– Monte Carlo, FLR, finite banana width, time-dependent,

asymmetric equilibria– No package to calculate effect of HHFW on fast ion dist. fct.

• CQL3D – Harvey (CompX)– Only code w/Fokker-Planck treatment of HHFW-NBI– No neutrals, FLR, banana width (other than loss), time-dependent

profile, or asymmetric equilibria effects – Only 3 consecutive harmonic numbers currently possible

• TRANSP-based NPA Simulator – Andre (PPPL)– Standalone now reads in CQL3D fast ion dist. function– Function averaged in pitch-angle space to counter unphysical

particle localization in ST without FLR effects; unnormalized

Quantitative comparison tools for neutron rates, NPA

Significant RF-induced neutron rate enhancement predicted and observed

• Equilibria at t = 235 ms used– Time-weighted solution also using t = 180 ms equilibria

• Error in absolute neutron rate at least +/- 10 %• Time dependence, total RF in bulk plasma significant effects

NPA simulator in good agreement with measurement

• Agreement above 80 keV within error bars• Discrepancy below ~30 keV likely due to large uncertainty in 1D

wall neutral model• Half-to-full RF power spans error bars; edge absorption observed

full power

half power

Tail reduced with lower B-field, higher t

• Larger t expected to promote greater off-axis electron absorption

• Reduces fraction of power available to core fast ion population

• Two codes used to check ion loss at B0=4.5 kG vs. 3.5 kG for 80-120 keV ions– CONBEAM – Egedal (MIT-PSFC)

• Loss fraction at 120 keV B0 = 4.5 kG: 21%

– EIGOL – Darrow (PPPL)• Loss fraction at 120 keV B0 = 4.5 kG: 17%

• Tail at highest B0 stronger by factor ~8– Small change in loss fraction insignificant compared to major tail

reduction

• Changes in tail population due to RF, not fast ion loss

Can ion loss account for tail reduction with lower B0?

B0 = 3.5 kG: 25%

B0 = 3.5 kG: 23%

r/a r/a

Observation of less fast ion absorption at higher t consistent with ray-tracing theory

t = 9%, B0=3.5 kG

Pe = 73%Pfi = 24%

e-

Dfast

• Lower on-axis absorption for lower B, higher t predicted

t = 5%, B0=4.5 kG

Pe = 62%Pfi = 37%

Dfast

e-

Shot 108250 Shot 108252HPRT HPRT

Smaller neutron rate enhancement at higher t predicted and observed

• Measured neutron rate ratio is very similar to simulated

ratio

Absolute neutron rate Ratio btw high/low B0

Smaller RF-induced fast ion tail at higher t predicted and observed

• Good agreement between experiment and theory for t scan

k|| has little observed effect on fast ions

• Theory predicts greater ion absorption with lower k||

• Surprisingly little variation in tail, small neutron enhancement with higher k||

Greater fast ion absorption predicted at lower k||, in contrast to measurement

• CURRAY, AORSA, and CQL3D also show this trend

HPRT

Larger neutron rate enhancement at lower k|| predicted, smaller rate observed

Neu

tron

rat

e (1

013 n

/s)

Much larger fast ion tail at higher k|| predicted, but smaller tail observed

• No k|| evolution measurement available• Edge-coupling effects, theory breakdown at low k||?

– To be further investigated

Summary

• Clear RF-induced fast ion tail observed with NBI

• Neutron rate and modeling support this observation

• Tail formation suppressed with higher t, consistent with theory

• Observed effect of k|| scan opposite to theory– Edge coupling or theory breakdown at low k|| an issue?

• Good agreement between HPRT, AORSA, CURRAY, CQL3D for HHFW + NBI

Effective Maxwellian matches TRANSP f(E) well

NPA scan indicates induced tail well off-axis

• Depletion in particle flux with NPA Rtan further off-axis

• Tail extends to same energy range

• Future: scan over wider range of r/a