# Presentation: 894102

## Initial Exploration of HHFW Current Drive on NSTX

- J. Hosea, M. Bell, S. Bernabei, S. Kaye, B. LeBlanc, J. Menard, M. Ono
- C.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 Meeting
- 11-15 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**

## Phase Feedback Control Configuration 

- 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

## Electron Parameters Made Similar for Co and Counter CD By

-     adjusting PRF and Gas Feed

- neL (cm-2)

## Electron Temperature and Density Profiles are Very Similar 

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

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

-   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

## ICD from Circuit Analysis is Bracketed by Current Drive Modeling Predictions

- 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 [(ki)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 ki, 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

## 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)

## TORIC Code Predicts a Large Reduction in Current Drive

- Calculated Driven Current Density Profiles

-  J**RF** (A/m**2**/W**inc**)

- Due 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