# Presentation: 474762

## Characterization of Fast Ion Power Absorption of HHFW in NSTXA. L. Rosenberg, J. E. Menard , J.R. Wilson, S. Medley, R. Andre, D. Darrow, R. Dumont, B. P. LeBlanc, C. K. Phillips, M. Redi, Princeton Plasma Physics Laboratory, T. K. Mau, University of California – San Diego, R. W. Harvey, CompX, E. F Jaeger, P. M. Ryan, D. W. Swain, Oak Ridge National Laboratory, J. Egedal – MIT, and the NSTX Team15th Topical Conference on RF in PlasmasGrand Teton National Park, WyomingMay 19-21, 2003

## Introduction and Motivation

- Ion absorption critically important to assessing viability of HHFW to heat and drive current in STs
- Experimental evidence of HHFW interaction with NBI
- Neutral Particle Analyzer (NPA) scannable at midplane
- neutron rates, Fast Lost Ion probes
- Thompson scattering measures Te, ne profiles
- X-ray Crystal Spectroscopy measures peak Ti
- Computational evidence
- HPRT, TRANSP, CURRAY, AORSA, METS

## 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
-  k|| = ± 3-14 m-1

## HHFW 12 element antenna array

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

**B**

## 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

## HHFW can generate a fast ion tail with NBI 

-  D+ tail extends to 130 keV
-  Tail saturates in time during HHFW

- HHFW can generate a fast ion tail with NBI

- beam injection
- energy

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

- noise level

- 108251

-  Tail decays on collisional time scale

-  Typical shot

**NBI 1.6 MW**

**RF 2.4**

- ne

- Te(0)

- Ip

## HHFW enhances neutron rate

- After RF turnoff, rate decays close to measured and predicted no RF value
- TRANSP neutron rate predictions without RF input fall shorter than measured rate for RF shot

**105906**

- No RF

**105908**

- RF

- RF

- no RF

- Measurement

- TRANSP

- Measured RF vs. no RF

- TRANSP

- Measurement

**NBI**

**HHFW**

## Tail reduced with lower B-field, higher t 

- Tail reduced with lower B-field, higher t

- Larger t promotes greater off-axis electron absorption
- Reduces fraction of power available to core fast ion population

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

- beam injection
- energy

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

## 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%

Small change in loss fraction insignificant compared to major tail reduction
More likely an RF effect

- 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%
- Small change in loss fraction insignificant compared to major tail reduction
- More likely an RF effect

- Ion loss with lower B-field can’t account for reduction in tail

- B0 = 3.5 kG: 25%

- B0 = 3.5 kG: 23%

## k|| has little observed effect on fast ions

**10**

**12**

**6**

**4**

**2**

**140**

**120**

**100**

**80**

**60**

**40**

**20**

**Energy (keV)**

**ln (flux / Energy****1/2****)**

- k|| has little observed effect on fast ions

- Greater ion absorption predicted with lower k||, but surprisingly little variation in tail, small neutron enhancement with higher k||

- k||=14 m-1
- k||=10.5 m-1
- k||=7 m-1
- No RF

- k||=14 m-1
- k||=10.5 m-1
- k||=7 m-1
- No RF

**NBI**

**HHFW**

- NPA

- beam injection
- energy

## 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
- TRANSP output used as input for fast ion temp and density distribution
- Fast ions dominate central absorption, electrons further off-axis
- Ti,th = 2 Te (XCS), no thermal ion absorption

- ne0 = 3.2  1013 cm-3
- nb0 = 2.5  1012 cm-3
- Te0 = .6 keV
- Tb0 = 17.3 keV
**P****e****=55%**
**P****b****=45%**

- Dbeam

- e-

**Shot 105908    Time 195 ms**

*p*

## Effective Maxwellian a good approximation for fast ions

- TRANSP fast ion dist. fcn.
- One effective Maxwellian
- 8 Maxwellians

- One effective Maxellian, exactly matching TRANSP energy density and temp., fits f(E) well, though neutron rate off +20%
- More Maxwellians (including negative) can match all moments

**Shot 108251    Time 235 ms   r/a=3%**

## Observation of less fast ion absorption at higher t consistent with theory

**r/a**

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

- t=9%, B0=3.5 kG

**P****e****=73%**
**P****b****=24%**

- e-

- Dbeam

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

- t=5%, B0=4.5 kG

**P****e****=62%**
**P****b****=37%**

- Dbeam

- e-

## Energy moment of Vlasov eq. + vector identities leads to [Menard, RF 1999]: 

- HPRT Power Absorption

- The first non-vanishing terms of this are:

- Kinetic flux term is necessary if one uses the full hot, complex dielectric tensor

## In Stix (4-15)

- Clarifying Kinetic Flux

- In Stix (4-17,19)

- 1st important note – Stix, p. 75: “In expression for **T**, the dot products are between **E********* **and *h*, and *h* and **E**.”

- So, *unambiguous ***T*** *is:

- Does (4-15) = (4-17)? Yes! However...

## Only ∙T is unique

- also equals:

- So expression for **T** is *not unique*; only ∙**T** well-defined
- Common loss-free plasma approximation of group velocity:		      is invalid, as is any expression which uses **T **in this form.
- 2nd important note:
- RHS gets wrong cross derivatives wrt **k**

## NPA scan indicates induced tail well off-axis

- NPA at r/a = .02
- NPA at r/a = .14
- NPA at r/a = .36

- Depletion in particle flux with NPA Rtan further off-axis
- Tail extends to same energy range
- Future: scan over wider range of r/a

- beam injection
- energy

## Summary

- Clear RF-induced fast ion tail observed with NBI
- Neutron rate and modeling support interaction
- Good agreement between HPRT, AORSA, CURRAY
- Tail formation suppressed with higher t
- Little effect with k|| observed
- Effective Maxwellians can represent fast ion f(E)
- Kinetic flux clarified