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Comparison of the onset of pellet triggered and spontaneous ELMs

G. KocsisG. Kocsis1)1)

A. AlonsoA. Alonso2)2) , B. Alper , B. Alper3)3), G. Arnoux, G. Arnoux3)3), J. Figueiredo, J. Figueiredo4)4) , D. Frigione , D. Frigione5)5) , , L. GarzottiL. Garzotti3)3) , M. Lampert , M. Lampert1)1), P.T. Lang, P.T. Lang6)6),, G. Petravich1), R. Wenninger6)

1) KFKI RMKI, EURATOM Association, P.O.Box 49, H-1525 Budapest-114, Hungary1) KFKI RMKI, EURATOM Association, P.O.Box 49, H-1525 Budapest-114, Hungary2) 2) Laboratorio Nacional de Fusion, Euratom-CIEMAT, 28040 Madrid, SpainLaboratorio Nacional de Fusion, Euratom-CIEMAT, 28040 Madrid, Spain3) Euratom/CCFE Association, Culham Science Centre, Abingdon, Oxon, OX14 3DB, UK4) Assocoation EURATOM-IST, Av. Rovisco Pais, 1049-001 Lisbon, Portugal5) Associazione EURATOM-ENEA sulla Fusione, CP 65, Frascati, Rome, Italy 6) MPI für Plasmaphysik, EURATOM Association, Boltzmannstrasse 2., D-85748 Garching,

Germany

G. Kocsis, p1

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Outline

Introduction: ♠ pellet pacemaking

♠ radial localisation of pellets at ELM trigger

♠ possible seed perturbation of ELM trigger

Summary and outlook

Pellet ELM triggering investigation on JET:

♠ experimental set-up

♠ data processing

♠ comparison of the onset of filamentary structures for pellet triggered and spontaneous ELMs

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Pellet ELM pacemaking

Injection of frequent small and shallow penetrating cryogenic pellets has been found a promising technique to mitigate ELM effect.

The technique works but the underlying physical processes of the ELM triggering are not well understood. Therefore the aim is to study

where and how a pellet triggers an ELM

to be able to make predictions for future machines and to optimise the pellet pacing tool.

MHDEELMELM WfW 2,000

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ELM-delay

B31-14

500 s

separatrix crossing time

PELLET

Ablation monitor

Pick-up coil

+

pedestal

Consistently with peeling-ballooning model of the ELM predicting instability onset localized to the pedestal steep gradient region

Only 5-15% of the expected pellet mass is ablated until the position of the seed perturb.

Radial localization of the pellet at onset of the triggered ELM for ASDEX Upgrade

Data from DIII-D also indicates that the pellet triggers an ELM before the pellet reaches half way up the pedestal

ELM onset time

ELM (type-I)

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Possible seed perturbation of pellet ELM triggerringPossible seed perturbation of pellet ELM triggerring

According to the observation on ASDEX Upgrade

♠ the localized high pressure pellet cloud elongated along the field line

♠ the axisymmetric plasma cooling

have been recognized as candidate perturbation for trigger mechanism.

Pellet injection

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The pellet ELM triggering was studied on JET using fuelling size pellets

The onset of spontaneous and triggered ELM was investigated by comparing the onset measured by

♠ magnetic coils: MHD ELM onset

♠ fast framing camera diagnostics: bright spots on plasma facing components caused by the interaction of ELM filament with limiters

Pellet ELM triggering investigation on JET:Pellet ELM triggering investigation on JET:comparison of the onset of filamentary comparison of the onset of filamentary

structures for pellet triggered and structures for pellet triggered and spontaneous ELMs spontaneous ELMs

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Several Mirnov coils were used

ELM: high frequency quasi periodic oscillation on Mirnov coil signals Bandpower of 100-300kHz was calculated which was taken as magnitude of the ELM related MHD activity

An appropriate threshold was used to determine the onset on each signal

The earliest of the onset times of the individual signals were selected as MHD ELM onset time.

ELM onset determination ELM onset determination using magnetic diagnosticusing magnetic diagnostic

MHD ELMonset Div Hα

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JET fast framing camera observation geometry

View: full poloidal cross section covering 90 degrees toidally and seeing the plasma facing components (limiters) on the outboard wall.

The cross section of the LFS pellet injection falls into the middle of the view. Time resolution: 10-15μs

LFS pellet injection

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Fast framing camera: typical ROIs

Max. 18kframe/s, 56μs

320 x 384 pixelsdivertor + pellet + ELM

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Inter limiter

box

2 53 4 6 7

Limiter box

pellet box

Max. 30 kframe/s, 33μs 240 x 256 pixels

Part of the divertor + pellet + ELM

Fast framing image over plotted

on slow framing image

EFIT, #76930, t=58.919s

1 7532 4 6

Max. 70kframe/s, 14μs 128x 128 pixels

pellet + ELM fast dynamics

Model image of some plasma facing elements

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Hot spot observation during spontaneous ELM

1 2 3 4 5 6 7 1 2 3 4 5 6 7 1 2 3 4 5 6 7

1 2 3 4 5 6 7 1 2 3 4 5 6 7 1 2 3 4 5 6 7

I1 I2 I3

I4 I5 I6

~100μs

First bright spot

Filament

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Filament observation during ELM trigger for LFS pellet injection

1 2 3 4 5 6 7

1 2 3 4 5 6 7

1 2 3 4 5 6 71 2 3 4 5 6 7

1 2 3 4 5 6 71 2 3 4 5 6 7

I1 I2 I3

I4 I5 I6

pellet

filament

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Images were spatially calibrated:

The bright spots are on wall elements

calibration possible from one view

for each pixel the poloidal and toroidal angle of the wall element seen is calculated

Image processing to reveal features of the bright spots

1

Inter limiter

box

2 53 4 6 7

Limiter box

pellet box

1 7532 4 6

Limiterbox

Inter limiter box

Pellet box

1st step 2nd step

1

2

7

6

5

4

3

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Image processing to reveal features of the bright spots

3rd step2nd step

1

2

7

6

5

4

3

1

2

3

5 5

3

2

1

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Comparison of spontaneous and triggered ELM footprints

#76930#76997Box 1 Box 3 Box 1 Box 3

Bright spot moves up

spot stops

spot stops

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1

3

5

2

Comparison of first appearance of bright spotsfor triggered and spontaneous ELM

From the ‘box radiations’ the poloidal location of the maximum radiation was calculated as a function of time.

The poloidal location of the first (early) appearance (onset) of the bright spots are plotted as a function of time elapsed after the MHD ELM onset for

triggered (◊)and spontaneous (+) ELMs

The triggered ones are around the MHD ELM onset

The spontaneous ones are [0,100]μs after the MHD ELM onset

The location for spontaneous ELMs has a larger scatter, but both cases are around the mid plane

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Localisation of the seed perturbation for LFS pellets

Plasma wall interaction footprint evolution is more regular

No delay relative to magnetic ELM onset

Pellet triggered type-I ELM (100-200kJ)

Up to 100 μs delay relative to magnetic ELM onset

Plasma wall interaction footprint evolution is less regular

Natural type-I ELM (100-200kJ)

The seed perturbation for the ELM triggering may be the pellet cloudThe ELM is probably born at the location of the pellet ablation

This conclusion is confirmed at DIII-D ( exp. in January, 2010), too. (L. Baylor)

Accordingly for LFS pellet ELM triggering the interaction of the filaments are immediately observable.

For spontaneous ELMs filaments may born at arbitrary toroidal location and need time to reach the camera observation volume.

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Summary and outlook

Pellet ELM triggering investigation on JET:comparison of the onset of filamentary structures for pellet triggered and spontaneous ELMs was performed

For LFS fuelling size pellets the high pressure pellet cloud formed around the pellet seems to be the seed perturbation

Initial nonlinear MHD simulation for pellet ELM triggering shows destabilization of medium-n ballooning modes. High pressure in pellet plasmoid drives MHD instability forming a single helical perturbation at the pellet position for LFS injected pellets, which agrees with the observations on JET Huysmans, EPS 2009

Future plans:

The present number of investigated triggered events is low, therefore evaluation of more pellet triggered ELMs is foreseen

Investigation of VHFS pellet ELM triggering (only in new campaigns, 2011-)

Reduction of the pellet size (only in new campaigns, 2011-)

V

LH

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Backup transparencies

19slowing down, n=4

Type-I ELMy H-mode

el. diam. Drift, n>0ion diam. Drift n<0

washboard-type precursor mode No precursor

15 pellet induced ELMs have been analysed for their basic poloidal/toroidal structure.

The single, upward rotating structure represents a rather specific case. Typically, two or more such pronounced structures occur simultaneously.

Despite the fixed pellet launch position, they appear initially at a more or less random toroidal position relative to the pellet, i.e. they are not directly growing out of the pellet plasmoid.

Prompttriggered

delayedtriggered

Neuhauser et al, NF, 48, 045005, 2008

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