elm triggering by deuterium pellets

1

ELM triggering by deuterium pellets

G. KocsisG. Kocsis1)1)

Acknowledgement:Acknowledgement:

A. AlonsoA. Alonso2)2) , L.R. Baylor , L.R. Baylor3)3) , , G. Huysmans4), S. Kálvin S. Kálvin1)1), K. Lackner, K. Lackner5)5) , P.T. Lang , P.T. Lang5)5),,

J. NeuhauserJ. Neuhauser5)5), M. Maraschek, M. Maraschek5)5), B. Pegourie, B. Pegourie6)6) , G. Pokol , G. Pokol7)7) , T. Szepesi , T. Szepesi1)1),,

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) Oak Ridge National Laboratory, Oak Ridge, TN, USA4) Association Euratom-CEA Cadarache5) MPI für Plasmaphysik, EURATOM Association, Boltzmannstrasse 2., D-85748 Garching, Germany6) CEA, IRFM, F-13108 Saint-Paul-lez-Durance, France7) BME NTI, EURATOM Association, P.O.Box 91, H-1521 Budapest, Hungary

2

Outline

Introduction: ELM problem, pellet pacemaking

Summary and outlook

Poloidal scan of pellet triggering capability

Radial localisation of pellets at ELM trigger

Pellet caused MHD perturbation in type-I, type-III and OH discharges

Pellet caused plasma cooling in type-I ELMy H-mode

Pellet plasma interaction:

Nonlinear MHD modelling of pellet ELM triggering

3

Introduction

4

ELMs can cause critically high power load

Type-I ELMs can cause critically high transient power load on plasma facing components.

Full performance ITER plasma W pedestal can be as much as 100 MJ leading to ~20 MJ ELM loss if ν*ped is the controlling parameter.

Loarte et al., Nuclear Fusion, ITER Physics Basis,Chapter 4

«Edge Localised Modes» (ELMs) are

MHD instabilities

destabilised by the pressure gradient in the H-mode edge pedestal:

Losses up to 10% plasma energy in several 100 micro-seconds

Filaments evolving during ELM on MAST.

Kirk, et al. PRL 2004.

5A. Kallenbach, Ringberg seminar,2006

ELM pacing is mandatory in presence of medium-Z radiators

ELMs are not only harmful, but also remove impurities from the plasmapreventing impurity accumulation and radiation collapse. In presence of medium-Z radiators the ELM pacing is mandatory.

Failure of two consecutive ELMs (pellets) are already problematic in scenarios with medium-Z radiators

6

Solving the ELM problem: mitigation by frequency enhancement

It was recognized that ELM energy loss scales inversely with ELM frequency ►►► envisaged solution for the ELM problem:

Mitigate ELMs below the damage threshold by enhancing their frequencyby pellet injection

Observation:

ΔWELM ~ PHEAT f ELM

for many scenarios with natural ELMs

Approach:

Split large ELMs into many small ones

A. Herrmann, PPCF 44 (2002) 883

7

Pellet ELM pacemaking

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

The technique works but the underlying physical processes of the ELM triggering are not well understood. Therefore our 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

8

Poloidal scan of pellet triggering capability

9

P.T. Lang et al., NF 47 (2007) 754

Pellet size: 4mm cube velocity: 150-300m/s

Fueling pellets injected from all locations on JET trigger ELMs

V

LH

Fueling pellets injected from all locations (V,H,L) on JET trigger prompt ELMs detected by Mirnov coils and on divertor Hα radiation

10

Fueling pellets injected from all locations on DIII-D trigger ELMs

Pellets injected from the 5 different injection locations on DIII-D are observed to trigger immediate ELMs.

L. Baylor et al., POP 7 (2000) 1878

Pellet size: 2.7mm diamPellet velocity:150-800m/s

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Pellets injected from all locations on ASDEX Upgrade trigger ELMs

HFS: pellet size: 1.4-2.1mm velocity: 240-1000m/sLFS: pellet size: 1-2mm velocity: 100-200m/s

Prompt pellet ELM trigger for HFS injection.

ELM event released 20–50μs after ablation onset with a minor part of the pellet mass ablated.

P.T. Lang et al., NF 47 (2004) 665

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Radial localisation of pellets at ELM trigger

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Radial localisation of HFS pellets at ELM trigger

Assumption: to trigger an ELM pellet has to reach a certain magnetic surface (lseed)independently of the pellet mass and velocity

dtELM ONSET = (lseed – lseparatrix ) / VP + t0

Perturbation spreads: finally an instability starts to grow which develops into an ELM.

ELM (type-I)

ELM-delay

B31-14

500 s

separatrix crossing time

PELLET

Ablation monitor

Pick-up coil

+

+

Pellet velocity scan: t0 , lseed can be determined

Every pellet injected into type-I ELMy H-mode plasma of ASDEX Upgrade triggered an ELM

This ELM was observed only if the pellet entered into the confined plasma crossing the separatrix:

dtELM ONSET = tELM ONSET – tpellet @ separatrix >0

dtELM ONSET depends on pellet velocities

lsep lped

lseed

instabilitygrowth rate

pert1

pert2

Pellet path (l)

ELM onset time

14


t0 = 50 ± 7 μs lseed = 2.7 ± 0.4 cm

dtELM ONSET = (lseed – lseparatrix ) / VP + t0

G. Kocsis et al., NF 47 (2007) 1166

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 perturbation

Still question: smaller pellets still reaching the same location of pedestal gradient region can trigger an ELM or not (according perturbation reduces with pellet size)

pedestal

15

0.00

0.02

0.04

0.06

0.08

0.10

1.5

1.6

1.7

1.8

1.9

-40

-20

0

20

40

60

80

2772.2 2772.4 2772.6 2772.8 2773.0-50

0

50

100

150

200

PelletEntersPlasma

ELMTriggered

ELMEnd

neL(1014m-2)

dBr/dt (a.u.)Inner wall

dBr/dt (a.u.)Outer shelf

Divertor D

HFS Pellet Induced ELM Details from DIII-D

Pellet D

The ELM is triggered 0.05 ms after the pellet enters the plasma or 147 m/s* 0.05 ms = 7.4 mm penetration depth.

DIII-D 1291951.8mm pellet injectedfrom innerwall is ~10%density perturbation

No MHD precursorto pellet induced ELM

Pellet Drepresentsablation of pellet in the plasma with assumed constant radial speed.

Time (ms)

L.R. Baylor et al, Fuelling WS, Crete, 2008

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Te ped (keV)

0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0

% P

ed

0.0

0.2

0.4

0.6

0.8

1.0

Non-RMP HFSRMP HFS Non-RMP LFS

Non-RMP HFS RMP HFS Non-RMP LFS RMP LFS

0

5

10

15

20

25

30

0.6 0.7 0.8 0.9 1 1.1

Pe

(kP

a)

Electron Pressure Pedestal

DIII-D 129195 2.772sPre-pellet and ELM

Pellet Locationwhen ELMTriggered

Localisation of pellet when ELM is triggered in DIII-DLocalisation of pellet when ELM is triggered in DIII-D

L.R. Baylor et al, Fuelling WS, Crete, 2008

Data from DIII-D indicates that the pellet triggers an ELM before the pellet reaches half way up the pedestal (% Ped is % of Te pedestal height).

Note: Pellets must penetrate deeper to trigger an ELM when RMP is applied.

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Radial location of pellets at trigger on JET

P.T. Lang et al., NF 47 (2007) 754

For all H track pellets an ELM onset delay of 200 ± 30 μs is derived, related to position of s = 30 ± 4.5mm along the pellet trajectory or r = 23 ± 3.4 mm radilly on the outer mid-plane, respectively.

Until the ELM trigger only 1% (or less) of the initial pellet mass is ablated.

Pellet size: 4mm cube velocity: 150-300m/s

Intrinsic ELMTriggered ELM

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Is the ELM triggered at the location of the pellet?Is the perturbation localised polidally/toroidally?

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JET fast framing camera observation geometry

1

Inter limiter

box

2 53 4 6 7

Limiter box

pellet box

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

Kocsis et al, EPS 2009

20

1

Inter limiter

box

2 53 4 6 7

Limiter box

pellet box

LFS Pellet triggered ELM: ♠ field line elongated filament is growing out from the pellet cloud ♠ bright spots on limiter elements: interaction of this filament with the plasma facing components ♠ appearance of this field aligned structure coincides with the MHD ELM onset. ♠ after detachment from the pellet cloud - bright spots in limiter elements move upward - slowed down after 50-100μs: rotates toroidally/poloidally

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

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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 80 μs delay relative to magnetic ELM onset

Plasma wall interaction footprint evolution is irregular

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 but this picture should be further refined/confirmed investigating HFS pellet ELM triggering

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Pellet – plasma interaction

23

→ neutral cloud formed on µs timescale

→ diamagnetic pellet cloud launches broadband Alfvén waves (vA~5 ·106m/s) that communicated on the whole magnetic surface on a few 10µs -100μs

→ ionised cloud elongated along field lines expansion vel.: ion sound speed < 105m/s localised in non-axisymetric filament < 10m high pressure

Pellet-plasma interaction

Geometry at ASDEX Upgrade:

Type-I ELMy H-mode pedestal along the pellet trajectory ~ 6cm Pellet cloud radius perpendicular to the magnetic field ~ 1cm

Pellet velocity: 240-1000m/s, pellet mass: 3-9(20) 1019 deuterium

Incoming incoming heat flux

pellet

spherical neutral cloud

ionised cloud

Pellet ablation in the first 50-100µs:

→ pellet cloud absorbs the incoming energy flux, cooling wave travels with electron thermal speed ~ 107m/s → homogenised on the whole magnetic surface in a few 10µs-100μs

→ further complicated by grad B caused cloud drift to LFS

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Axisymmetric plasma cooling

25

Axisymmetric plasma cooling I

ELM onset

vP=1000m/s

Pellet trajectory

vP=600m/s

ELM onset

Pellet trajectory

vP=240m/s

ELM onset

Pellet trajectory

Pellet caused local cooling is homogeneously distributed on the magnetic surface in a few 10µs-100μs (fast electron cooling wave travels with electron thermal speed ~107m/s).

The local cooling appears on fast ECE electron temperature measurement located toroidally 90o from the location of the pellet injection in a few 10µs. 200µs


26

Pellet caused cooling:

● appears almost immediately after the pellet reached the according magnetic surface

● causing remarkable temperature drop on a short timescale ● the cooling front moves together with the pellet for all pellet velocities.

● pellet plasma cooling lasts until the pellet is completely ablated and the plasma starts to recover but on a ms timescale.

● relative temperature drop is in the range of few 10% seems to depend on the pellet velocity.

● temperature decrease caused by the triggered ELM is slower than direct pellet one, therefore they can be discriminated.

Axisymmetric plasma cooling II

vP=600m/s

ELM onset

Pellet trajectory

vP=240m/s

ELM onset

Pellet trajectory

200µs


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Pellet caused magnetic perturbation

28

Pellet-driven MHD masked by type-I ELM footprint:

in the early phase of the ablation

(100- 200 μs) the ELM is triggered

the pellet-driven perturbation is small,

masked by the ELM

after the ELM the pellet-driven mode is visible

if the pellet lifetime is long enough

→ but only for long-life pellets

(type-I) ELM pellet

ELM-delay

B31-14

500 s

separatrix crossing

tpels v t

distance from separatrix

Therefore: the pellet driven magnetic perturbation was analyzed in different scenarios: OH, HD type-III and type-I H-mode

Pick-up coil signals were analyzed: high frequency component: bandpower in 100-300 kHz called ‘envelope’: magnetic perturbation strength

spectra and toroidal mode number spectra were studied

Pellet caused magnetic perturbation in OH, HD type-III and type-I H-mode

Pick-upcoil

Ablation monitor

Szepesi, EPhF 2009

29

ELM

pellet

type-I

type-III

OH

→ no dependence on pellet mass

→ slight/no dependence on pellet speed

→ depends on pellet penetration

BUT only in one scenario

implies dependence on plasma parameters instead of penetration

ELM- and pellet-related MHD activity can be separated

OH

type-I

type-III

Magnetic perturbation strength

Electron pressure [Pa]

Mag

net

ic p

ert

urb

atio

n s

tren

gth

[a.

u.]

Szepesi, EPhF 2009

30

Spectral properties of the magnetic perturbation

b)

a)

magnetic spectrum coherent mode spectrum

OH

type-III

type-I

→ in type-I the ELM makes the plasma prone to the n = -6 oscillation, and this is enhanced by the pellet (cooling of plasma edge, emergence of turbulence)

→ type-III case: more similar to OH

→ the pellet produces no new phenomena, only enhances what is already present

Pellet-induced perturbation:

→ mode frequency: 100 - 300 kHz

→ tor. mode number: n = -6

(ion diamagnetic drift direction)

• TAE: the same parameters

• but: ELMs are different

n = 3, 4 (Neuhauser, NF 2008)

TAE oscillation Maraschek, PRL79

pel

pel

pel

Washboardn = 3, 4

Szepesi, EPhF 2009

Onliving mode n=-6

31

Nonlinear MHD simulation of pellet ELM triggering

32

flux-aligned grid(reduced resolution)

Non linear MHD code JOREK

JOREK has been developed with the specific aim to simulate ELMs

– domain with closed and open field lines

– non-linear reduced MHD in toroidal geometry

• Density, temperature, electric potential (perp. flow),parallel velocity, poloidal flux

• Ideal wall conditions on walls

• Mach one, free outflow at divertor target

Aim of the pellet related simulations is to answer the following questions:

ELM occurs when plasma crosses linear MHD limit after which plasma returns to stable state

How can a pellet trigger an ELM in the stable inter-ELM phase?

Is pellet a trigger or a cause for an ELM?

Huysmans, EPS 2009

33

Pellets are approximated as an initial perturbation

to the density profile:

– poloidally and toroidally localised

• Amplitude typically 25 times central density

• Total amount of added particles 3-6%

– constant pressure (i.e. low temperature)

10-10

10-9

10-8

10-7

10-6

10-5

ener

gies

340320300280260240220time

n=1 (M)

n=2 (M)

n=3 (M)n=4 (M)

n=0 (K)

n=5 (M)

n=10 (M)

Pellet triggered MHD in H-mode pedestal:

– Initial non-linear MHD simulations of pellets injected in H-mode pedestal show:

• Destabilisation of medium-n ballooning modes

• High pressure in pellet plasmoid drives MHD instability forming a single helical perturbation at the pellet position

– suggests pellets can cause ELMs (instead of being a trigger)

Non linear MHD code JOREK: modelling of pellet ELM triggering

Huysmans, EPS 2009x ~0.5μs

34

Summary

Millimeter sized fuelling pellets trigger ELMs independently of the poloidal injection angle (LFS, HFS)

Pellets are located in the mid-pedestal at ELM trigger, and only a small fraction of their mass is ablated until the trigger, but it is still not known that smaller pellets just reaching the same radial location are still trigger ELMs

For LFS pellets the high pressure pellet cloud formed around the pellet seems to be the seed perturbation but for HFS injection the situation is not yet clear

The high pressure pellet cloud, the axisymmetric plasma cooling and the magnetic perturbation of the pellet cloud have been recognized as candidate perturbation for trigger mechanism, and were investigated in details

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

35

Outlook

HFS-LFS asymmetry of the ELM triggering will be further investigated ►► on ASDEX Upgrade with LFS injection (measurement of the internal delay for LFS) ►► on JET with HFS injection (fast visible imaging of filaments)

The investigation of pellet caused magnetic perturbation and plasma cooling is an ongoing project on JET, results will be published soon.

The simulation should be run to clarify the determining mechanism of the pellet Elm triggering (HFS/LFS)

Further investigations would be necessary with reduced pellet size or with small impurity pellets.

37

Backup transparencies

38

38

• Pellet cloud releases from pellet and expands along a flux tube.

• Density from the cloud expands along flux tube at the sound speed cs.

• Temperature ‘cold wave’ travels along the flux tube at the thermal speed. Heat is absorbed in the cloud resulting in a temperature deficit far from the cloud.

• Pressure decays and expands along the flux tube with a lower pressure far from the cloud.

• Strong local cross field pressure gradients result along the flux tube that form on s time scales.

What Causes an ELM Trigger from a Pellet?

L

necs

Teqe

L

Pe

L

LRB Crete Jun2008

39

long exposure

Cam. exposures

Ablation monitor signal

time [s]

Trajectoryrecons-truction

separatrix

tpellet @ separatrix

Vide angle viewphotodiode

Experimental set-up I

short multiple exposures

on time: 10µs off time: 90µs

exposures

Pellet localization (space, time):

- fast CCD cameras + spatial calibration

→ def: penetration = distance from separatrix

Injection geometry

40

Processing of the pickup coil signals

-eliminate LF component by moving box average

- calculate the envelope of the remaining HF component (25 s box)

- assume: envelope ~ MHD perturbation magnitude

Experimental set-up II

Toroidal pick-up coil setabout 1800, 5 coils

Mirnov coil set3600, 30 coils

Poloidal pick-upcoil set, 7 coilsabout 600

ELM (type-I)

ELM-delay

B31-14

500 s

separatrix crossing

Calibrated Electron Cyclotron Emission (ECE) profiles are measured in secondharmonic X-mode with a fast 60-channel heterodyneradiometer

PELLET

Ablation monitor

Pick-up coil signal

41

Experimental set-up III

short-time Fourier transform (STFT): time shift and frequency shift invariant

continuous analytical wavelet transform: time shift and scaling invariant

Mode numbers as least squares fit to

cross-phase

as function of relative probe position :

),(arg),( ,, uCSu gfyx

yx

yxyxyxm muuwuQ,

2,,, ),(),(),(

yx,

Processing of pick-up coil signals: spectral properties

42


Low pellet injection frequency : trigger events occur at different times in the ELM cycle that is we perform the analysis as a function of the time elapsed after the previous ELM

Pellets can trigger ELMs at any time in the ELM cycle: plasma edge is not stable against HFS pellet induced seed perturbation on ASDEX Upgrade

dtELM_ONSET saturates with increasing dtelapsed

dtelapsed > 8ms: dt ELM_ONSET ~ constant

G. Kocsis et al., NF 47 (2007) 1166

43

→ axisymmetric decrease of the plasma pressure causing a pressure gradient increase in front of the pellet

rough estimate: 10-30% for AUG

→ the incoming energy flux is absorbed and concentrated in the HFS localised helical non axisymmetric cloud causing a cloud pressure higher then that of the target plasma rough estimate: >10 Pe

Pellet-plasma interaction

Pellet affects the plasma at a magnetic surface as long as

t=DiamCLOUD /vP =20-100µs

Estimation of the pellet cloud pressure and plasma pressure drop

44

Injection scenarios

Measurement of the ELM onset delay as a function of the pellet velocity → VP=240,600,880,1000 m/sHFS looping system: pellet erosion is velocity dependent → rP=0.71, 0.67, 0.58, 0.51 mm NP=9 , 7, 5, 3 x 1019

Characterization of the injection scenarios → Neutral Gas Shielding model calculation

dNP /dt = - Const. rP4/3 n e

1/3 Te1.64

dNP /dl VP = - Const. rP4/3 n e

1/3 Te1.64

Designated pellet path

Integrating this diff.equation along thedesignated pellet paththe ablation rate(ablated particles /s)is calculated

P.B. Parks et al., PoP 21 (1978) 1735

45

240 m/s, r=0.71mm, N=9 ·1019

600 m/s, r=0.67mm, N=7.4880 m/s, r=0.58mm, N=5.e191000m/s,r=0.51mm, N=3.3e19

Injection scenarios

Pellets penetrate deep into the plasma crossing the pedestal region completely

In the pedestal region the ablation rate is nearly similar for all injection scenarios

Particle deposition scales with 1/VP

46slowing 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

elm triggering by deuterium pellets

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