Measurement of fast ion losses due to MHD modes driven by fast ions in the Large Helical Device

2009/03

Kunihiro OGAWAA, Mitsutaka ISOBE, Kazuo TOI, LHD experiment group

A Nagoya University

National Institute for Fusion Science (NIFS)

At ASIPP

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Contents

Background & Purpose

Experimental setup

– Scintillator based lost fast-ion probe

Result

– Typical discharge & Scintillation image

– Loss flux correlate with TAE/EPM burst

– Relation of fluctuation level and increase of loss flux

Summary & Future plan

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Background & Purpose

Understanding of fast ion loss process due to fast ion driven MHD mode such as Alfvèn eigenmode (AE) is needed

– fast ion losses may cause a damage of the first wall in a fusion device

– the effect of AEs on fast ion loss must be clarified

example : in NSTX

– fast ions which have wide range of pitch angle are lost due to AE

understanding of the effect of AE on co-going fast-ions in LHD

contribution to understanding of fast ion loss process induced by AE in tokamaks

pitchD. Darrow NF (2008) pitch angle

Fast ion loss due to AE in NSTX

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Scintillator based lost fast-ion probe (SLIP) scintillator probe

– it works as a magnetic spectrometer

– it has a set of apertures (front/rear)

• it allow to enter ions having certain velocities

– scintillation points

• it have information of velocity and pitch angle of ions

this SLIP has two set of apertures

– it can be applicable to the case of CW or CCW direction of Bt

observation of co-going lost fast-ions at relatively low field (Bt < 0.75 T)

model of scintillator head

LHD & location of SLIP

R0 / a = 3.9 m / 0.6 mVNBacc = 180 keV

pitchangle arctan( ) / /v v

picture of SLIP

orbit of co-going ion @ CW-Bt

orbit of co-going ion @ CCW-Bt

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Typical discharge & Scintillation image

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Observation of lost fast-ion flux using Photomultiplier (PMT) array

observation of loss flux with PMT

– loss flux correlates with TAE/EPM burst

PMT array (consist of 16-PMT)

– it has high-time resolution (~ 2 s)

– we mainly monitor the signal of Ch. 14

• it sees high luminescent point

– we haven’t done calibration of image pattern

• At present, this image doesn’t give information of velocity and pitch angle of lost fast ions.

SLIP Ch. 14

Scintillation image (t = 2.78 s)and position of PMT

TAE(f~45 kHz)

EPM(chirping)

scintillator PMT

AMP

PCI

CAMERA

PC

half mirror

block diagram of data acquisition system

MP spectra

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Loss flux correlate with TAE/EPM burst

Bt = 0.75 T, Rax = 3.75 m, =1.254

when NB#2 and #3 inject

a) : filtered magnetic fluctuations

b) : PMT signal (Ch. 14)

increase in ion loss flux induced by TAE

– #90048 case

– mode structure : m~1, n=1 (from MP array)

– frequency : 55 ~ 75 kHz

increase in ion loss flux induced by EPM

– #90044 case

– mode structure : m=2 , n=2 (from MP array)

– frequency : 10 ~ 40 kHz

– frequency sharply chirping down(~ 2 ms)

TAE fluc. and SLIP sig.

EPM fluc. and SLIP sig.

a)

b)

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Relation of fluctuation level and increase in loss flux evaluation of fluctuation level

– : magnetic fluctuation @ MP position

evaluation of increase in loss flux

– ΔSLIP : ISLIP (mode exist) – ISLIP (no mode)

TAE case (m ~ 1, n = 1)

–

EPM case (m=2, n=2)

– ΔSLIP has threshold in , then constant

– #90043 :

• it is due to difference of density?

• m/n=4/3 mode affects transport?#90043 MP spectra

m/n = 4/3

EPM

TAE

peakb

Fluctuation level and increase of loss flux

#90045 MP spectra

EPM

TAE

2~ peakSLIP b

2~ peakSLIP b

peakb

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Summary

co-going lost fast-ions are measured with a scintillator-based lost fast-ion probe– recurrent increase of lost fast-ion flux induced by TAE or EPM burst

is observed

– increase in loss flux due to TAE/EPM (toward Ch.14)

• TAE : loss flux is expressed as

• EPM : loss flux have a certain threshold(?), then constant

but in #90043

Future plan– calibration of scintillation image

• scintillation pattern gives us information of velocity and pitch angle of lost ions

– Are there effects of radial structure of TAE/EPM in lost fast-ion flux?

2~ peakb

2~ peakb