# Presentation: 266548

## 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

## 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. RosenbergPrinceton Plasma Physics Lab  P. Ryan,  D. Swain, J. Wilgen, ORNLP. 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

- With contributions from

## 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

## 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 electrons
- Can allow off-axis deposition

## 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

## HHFW 12 ELEMENT ANTENNA ARRAY

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

## PHASE FEEDBACK CONTROL CONFIGURATION

** **

**1**

**2**

**3**

**4**

**5**

**6**

**7**

**8**

**9**

**10**

**11**

**12**

**Antennas**

- 

- 

**D1           D2          D3            D4           D5**

**D6**

**P****1**

**P****2**

**P****3**

**P****4**

**P****5**

**P****6**

**V****1**

**V****3**

**V****4**

**V****5**

**V****6**

**V****2**

**RF Power Sources**

**5 Port Cubes**

**Cube Voltages**

**Decoupler**
**Elements**

**I****1**

**I****7**

******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

## 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

## HHHFW PROVIDES STRONG ELECTRON HEATING

*T**e **> T**i*

- He Discharge

- NOTE LOW INTIAL Te

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

-  ***i**NC* < ***i** **, ****e*

*A. Rosenberg*

- _Increasing_ e with radius

- RF power deposition from
- HPRT ray tracing code

*t = 0.24 s*

## HEATING WITH HHFW IS IN QUANTITATIVE AGREEMENT WITH CONVENTIONAL SCALING

## SOME DISCHARGES DISPLAY BEHAVIOR OF INTERNAL TRANSPORT BARRIER

***LARGE T******e****** INCREASE***

*SMALL n**e** DECREASE*

***SMALL n******e****** DECREASE***

## 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

## 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

## ELECTRON PARAMETERS MADE COMPARABLE BY ADJUSTING POWER AND FUELING

- IP = 500 kA, BT = 4.5 kG

## SIGNIFICANT DIFFERENCE IN LOOP VOLTAGE OBSERVED BETWEEN CO AND CTR PHASING

-   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

## 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)
**	********I****CO**** ≈ 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]
**  ****    ****I****CO ****≈ 96 kA  (0.046 A/W)**
**CURRAY**:  Ray tracing code (damping is linear on Maxwellian species, all orders in ki, k determined locally)
**  ****    ****I****CO**** ≈ 162 kA   (0.077 A/W)**

## TRAPPING PLAYS SIGNIFICANT ROLE IN REDUCING DRIVEN CURRENT

- 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

- TORIC
- CALCULATION FOR NSTX CO CD SHOT

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

-    Dimensionless current drive efficiency, fw = fw *3.27/Te(0) (keV), is comparable to that for DIII-D at lower temperatures

- OPERATION AT INCREASED Te WILL BE NEEDED TO ACHIEVE NSTX GOALS

## HHFW INTERACTION WITH FAST IONS

- Even at high harmonic numbers (n ≥ 9) ion damping can be important due to large kperpi effects
- On NSTX kperpi ~ x for 80 keV beam ion
- Damping important if kperpi)2/2 ≥ n
- Damping on beam ions may reduce current drive efficiency

## NPA SHOWS FAST ION TAIL BUILD-UP AND DECAY

-  Tail saturates in time during HHFW

- beam injection
- energy

- at RF end
- +10 ms
- +20 ms

- noise level

- 108251

- neL(0)

- Te(0)

- PNB

- PRF

- Ip

-  Tail decays on collisional time scale

## RAY TRACING PREDICTS SIGNIFICANT FAST ION ABSORPTION

- 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 eV
- Tb0 = 17.3 keV
- Pe=54.5%
- Pb=45.5%

- Dbeam

- e-

**Shot 105908    Time 195 ms**

## STRONGER TAIL AT HIGHER BT

- B0=4.5 kG
- B0=4.0 kG
- B0=3.5 kG

- B0=4.5 kG
- B0=4.0 kG
- B0=3.5 kG
- No 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

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

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

- Ip= 1 MA
- Ip= 750 kA
- No 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||

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

- NPA at 90 cm
- NPA at 70 cm
- NPA 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

## SUMMARY

- 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