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

2009/03Kunihiro OGAWAA, Mitsutaka ISOBE, Kazuo TOI, LHD experiment groupA Nagoya UniversityNational Institute for Fusion Science (NIFS)

At ASIPP

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Contents

Background & PurposeExperimental setup

– Scintillator based lost fast-ion probeResult

– 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 mirrorblock 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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Summaryco-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