# Presentation: 871241

## 
Suppression of frequency chirping in 
NSTX by HHFW heating of Beam Ions

E.Ruskov, W.Heidbrink, UC Irvine
E.Fredrickson, D.Darrow, S.Medley, R.Wilson
Princeton Plasma Physics Laboratory

NSTX Results Review 
September 20-21, 2004

**Suppression of frequency chirping in **
**NSTX by HHFW heating of Beam Ions**
- E.Ruskov, W.Heidbrink, *UC Irvine*
- E.Fredrickson, D.Darrow, S.Medley, R.Wilson
*Princeton Plasma Physics Laboratory*
- NSTX Results Review
*September 20-21, 2004*

## Motivation

**Motivation**

**1.  Nonlinear saturation of fast-ion instabilities:**
**determines their ultimate impact on fast-ion transport**
**must be understood in order to predict the effect of alpha driven instabilities in ITER and other burning plasmas**
**2.  Despite many similarities between existing devices,  frequency chirping in the sub TAE frequency range is not universal              ***( common in NSTX,MAST and START, but rare in DIII-D)****.***
**3.  The Berk-Brezman theory successfully explained the suppression of ****fast electron chirping modes**** with RF power.   Does it apply to beam-ions in NSTX?**

***2***

## Pictorial view of the Berk-Brezman theory

**Pictorial view of the Berk-Brezman theory**

- D.Maslovsky *et al*., Phys.Plasmas **10** (2003) 1549

***3***

## The goal of our experiment

**The goal of our experiment**

**Q: How do we get the HHFW to detrap the resonant ions?**
**A: Select an operating regime where:**
** **
**Chirping causes drops in neutron rate**
**HHFW  increases  the neutron rate**

**We used HHFW in ****helium L-mode plasma**** to accelerate beam**
**ions and produced a rich set of fast-ion driven instabilities.**

***4***

## HHFW increases the neutron rate.  Chirping causes rapid 5-25% drops

** ****Successfully developed our target helium L-mode plasma **
** ****Early chirping ***(during current ramp-up)*** seen only for the most tangential full energy beam injection (source A, 2MW / 90 keV).**
** ****Late chirping seen in all shots.**

***5***

## NPA data proves that HHFW accelerates beam ions

**NPA data proves that HHFW accelerates beam ions**

**• ****Comparable RF acceleration of neutral beam ions observed at E****b ****~ 65 keV and E****b ****~ 90 keV for all NB sources. **
**•**** ****The energetic ion tails form in < 15 ms for P****HHFW**** ~ 2 MW.**
**•**** ****Tail decay time ~ 12 ms****. **

***6***

## HHFW suppresses MHD modes: early TAEs  and ...

**HHFW suppresses MHD modes: early TAEs  and ...**

**... ****chirping, but weakly, most often barely**

**Mode number**

_***Note:***_** These two shots use beams ****B ****and ****C ****with ****1MW /  60KeV****, and have nearly identical plasma parameters****.**

**Coherence**

***7***

## Experimental summary of suppression of beam-ion driven instabilities by HHFW heating

**Experimental summary of suppression of beam-ion driven instabilities by HHFW heating**

***8***

## Conclusions

**Conclusions**

**We successfully achieved regimes with strong instabilities and effective HHFW acceleration of beam ions. **
**Early, “steady” TAE-like modes are most strongly suppressed by low energy (60keV)  beams with large perpendicular component. **
**Hardly any suppression of chirping instabilities takes place with applied HHFW heating.**
**Preliminary hypothesis about this difference is that the early instabilities are not that strong  and thus modest changes in the beam distribution function by HHFW alter their nonlinear saturation. This is not the case with the later, strong chirping instabilities which are violent and therefore harder to suppress.    **

***9***

## To do list

**To do list**

**Prepare TRANSP runs **_***with CHERS data***_** for all shots of interest and get the beam-distribution  (without HHFW) from TRANSP.**
**Analyze NPA  and model the distribution function with applied HHFW heating.**
** **
**Analyze soft X-ray and reflectometer data to identify the modes.**
**Calculate the linear TAE growth rate for the beam-distribution function with and without HHFW and check whether their difference can explain the early suppression of TAE-like modes.**
**Develop quantitative estimate of the instability growth rates, and the collisionality, and relate them to the  theoretical models.   **

***10***

## Increased Collisions Suppress Chirping in a Dipole Experiment

**Increased Collisions Suppress Chirping in a Dipole Experiment**

** ****Interchange instabilities driven by *****energetic electrons***** trapped by a magnetic dipole produced frequency sweeping modes. **
** ****This chirping was suppressed with low-power RF fields.**
**  ****A self-consistent nonlinear simulation reproduced the chirping and identified phase-space holes as predicted by the Berk-Brezman theory.**