Elm triggering by deuterium pellets
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G. Kocsis 1) Acknowledgement: A. Alonso 2) , L.R. Baylor 3) , G. Huysmans 4) , S. Kálvin 1) , K. Lackner 5) , P.T. Lang 5) , J. Neuhauser 5) , M. Maraschek 5) , B. Pegourie 6) , G. Pokol 7) , T. Szepesi 1) ,

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ELM triggering by deuterium pellets

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Elm triggering by deuterium pellets

G. Kocsis1)

Acknowledgement:

A. Alonso2) , L.R. Baylor3) , G. Huysmans4), S. Kálvin1), K. Lackner5) , P.T. Lang5),

J. Neuhauser5), M. Maraschek5), B. Pegourie6) , G. Pokol7) , T. Szepesi1),

1) KFKI RMKI, EURATOM Association, P.O.Box 49, H-1525 Budapest-114, Hungary

2) Laboratorio Nacional de Fusion, Euratom-CIEMAT, 28040 Madrid, Spain

3) Oak Ridge National Laboratory, Oak Ridge, TN, USA

4) Association Euratom-CEA Cadarache

5) MPI für Plasmaphysik, EURATOM Association, Boltzmannstrasse 2., D-85748 Garching, Germany

6) CEA, IRFM, F-13108 Saint-Paul-lez-Durance, France

7) BME NTI, EURATOM Association, P.O.Box 91, H-1521 Budapest, Hungary

ELM triggering by deuterium pellets


Elm triggering by deuterium pellets

Outline

Introduction: ELM problem, pellet pacemaking

Radial localisation of pellets at ELM trigger

Pellet plasma interaction:

Summary and outlook

Pellet caused MHD perturbation in type-I, type-III

and OH discharges

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

Nonlinear MHD modelling of pellet ELM triggering

Poloidal scan of pellet triggering capability


Elm triggering by deuterium pellets

Introduction


Elm triggering by deuterium pellets

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.


Elm triggering by deuterium pellets

ELM pacing is mandatory in

presence of medium-Z radiators

ELMs are not only harmful, but also remove impurities from the plasma

preventing 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

A. Kallenbach, Ringberg seminar,2006


Elm triggering by deuterium pellets

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 frequency

by 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


Elm triggering by deuterium pellets

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 ableto make predictions for future machines and to optimise the

pellet pacing tool.


Elm triggering by deuterium pellets

Poloidal scan of pellet triggering capability


Elm triggering by deuterium pellets

V

H

L

Fueling pellets injected from all locations

on JET trigger ELMs

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

Pellet size: 4mm cube

velocity: 150-300m/s

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


Elm triggering by deuterium pellets

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.

Pellet size: 2.7mm diam

Pellet velocity:150-800m/s

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


Elm triggering by deuterium pellets

Pellets injected from all locations

on ASDEX Upgrade trigger ELMs

Prompt pellet ELM trigger for HFS injection.

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

HFS: pellet size: 1.4-2.1mm

velocity: 240-1000m/s

LFS: pellet size: 1-2mm

velocity: 100-200m/s

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


Elm triggering by deuterium pellets

Radial localisation of pellets at ELM trigger


Elm triggering by deuterium pellets

Assumption: to trigger an ELM pellet has to reach a certain

magnetic surface (lseed)

independently of the pellet mass and velocity

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

Pellet velocity scan: t0 , lseed can be determined

Radial localisation of HFS pellets

at ELM trigger

instability

growth

rate

pert1

pert2

Pellet

path (l)

lseed

lsep lped

PELLET

500 ms

separatrix crossing time

Ablation

monitor

Pick-up coil

+

ELM-delay

ELM onset

time

Perturbation spreads: finally an instability

starts to grow which develops into an ELM.

B31-14

+

ELM (type-I)

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


Elm triggering by deuterium pellets

Radial localisation of HFS pellets at ELM trigger

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

t0 = 50 ± 7 μs

lseed = 2.7 ± 0.4 cm

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

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


Elm triggering by deuterium pellets

HFS Pellet Induced ELM Details from DIII-D

ELM

Triggered

ELM

End

Pellet Da

represents

ablation of pellet in the plasma with assumed constant radial speed.

DIII-D 129195

1.8mm pellet

injected

from inner

wall is ~10%

density

perturbation

0.10

Pellet

Enters

Plasma

0.08

Pellet Da

0.06

0.04

0.02

0.00

1.9

neL(1014m-2)

1.8

Divertor Da

1.7

1.6

1.5

80

60

dBr/dt (a.u.)

Inner wall

40

No MHD precursor

to pellet induced ELM

20

0

-20

-40

200

150

dBr/dt (a.u.)

Outer shelf

100

50

0

-50

2773.0

2772.2

2772.4

2772.6

2772.8

Time (ms)

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.

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


Localisation of pellet when elm is triggered in diii d

Localisation of pellet when ELM is triggered in DIII-D

Electron Pressure Pedestal

30

DIII-D 129195 2.772s

Pre-pellet and ELM

25

Non-RMP HFS

RMP HFS

Non-RMP LFS

RMP LFS

20

Pe (kPa)

15

10

5

0

0.6

0.7

0.8

0.9

1

1.1

r

Pellet Location

when ELM

Triggered

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.

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


Elm triggering by deuterium pellets

Radial location of pellets at trigger on JET

Pellet size: 4mm cube

velocity: 150-300m/s

Intrinsic ELM

Triggered ELM

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.

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


Elm triggering by deuterium pellets

Is the ELM triggered at the location of the pellet?

Is the perturbation localised polidally/toroidally?


Elm triggering by deuterium pellets

JET fast framing camera observation geometry

Inter

limiter

box

Limiter

box

1

2

3

4

5

6

7

pellet

box

LFS pellet injection

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

Kocsis et al, EPS 2009


Elm triggering by deuterium pellets

Filament observation during ELM trigger

for LFS pellet injection

Inter

limiter

box

Limiter

box

1

2

3

4

5

6

7

pellet

box

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

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

Kocsis et al, EPS 2009


Elm triggering by deuterium pellets

Localisation of the seed perturbation

for LFS pellets

Natural type-I ELM

(100-200kJ)

Pellet triggered type-I ELM

(100-200kJ)

Plasma wall interaction

footprint evolution is

more regular

Plasma wall interaction

footprint evolution is

irregular

No delay relative to

magnetic ELM onset

Up to 80 μs delay relative

to magnetic ELM onset

The seed perturbation for the ELM triggering may be the pellet cloud

The ELM is probably born at the location of the pellet ablation

but

this picture should be further refined/confirmed investigating HFS

pellet ELM triggering


Elm triggering by deuterium pellets

Pellet – plasma interaction


Elm triggering by deuterium pellets

Pellet-plasma interaction

→ 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 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

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

→ neutral cloud formed on µs timescale

→ further complicated by grad B caused

cloud drift to LFS

Pellet ablation in the first 50-100µs:

pellet

ionised cloud

incoming heat flux

Incoming

spherical neutral

cloud


Elm triggering by deuterium pellets

Axisymmetric plasma cooling


Elm triggering by deuterium pellets

Axisymmetric plasma cooling I

Pellet trajectory

ELM onset

vP=240m/s

Pellet trajectory

ELM onset

vP=600m/s

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

Pellet trajectory

ELM onset

vP=1000m/s

Kocsis et al, EPS 2008


Elm triggering by deuterium pellets

Axisymmetric plasma cooling II

Pellet trajectory

  • 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.

ELM onset

vP=240m/s

200µs

● 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.

Pellet trajectory

ELM onset

vP=600m/s

Kocsis et al, EPS 2008


Elm triggering by deuterium pellets

Pellet caused magnetic perturbation


Elm triggering by deuterium pellets

Pellet caused magnetic perturbation in

OH, HD type-III and type-I H-mode

500 ms

distance from separatrix

separatrix crossing

Dt

ELM-delay

B31-14

(type-I) ELM pellet

→ but only for long-life pellets

  • 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

Ablation

monitor

Pick-up

coil

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

Szepesi, EPhF 2009


Elm triggering by deuterium pellets

type-I

type-III

OH

ELM

pellet

Magnetic perturbation strength

  • → 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

OH

type-III

Magnetic perturbation strength [a.u.]

type-I

Electron pressure [Pa]

 ELM- and pellet-related MHD activity can be separated

Szepesi, EPhF 2009


Elm triggering by deuterium pellets

Spectral properties of the magnetic perturbation

magnetic spectrum

coherent mode spectrum

Pellet-induced perturbation:

pel

OH

  • → 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

type-III

→ 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

pel

type-I

Onliving mode n=-6

Washboard

n = 3, 4

Szepesi, EPhF 2009


Elm triggering by deuterium pellets

Nonlinear MHD simulation of pellet ELM triggering


Elm triggering by deuterium pellets

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?

flux-aligned grid

(reduced resolution)

Huysmans, EPS 2009


Elm triggering by deuterium pellets

Non linear MHD code JOREK: modelling of pellet ELM triggering

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)

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)

Huysmans, EPS 2009

~0.5μs

x


Elm triggering by deuterium pellets

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


Elm triggering by deuterium pellets

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.


Elm triggering by deuterium pellets

Backup transparencies


Elm triggering by deuterium pellets

ne

cs

L

Te

qe

L

Pe

L

q

f

What Causes an ELM Trigger from a Pellet?

  • 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 ms time scales.

LRB Crete Jun2008

38


Elm triggering by deuterium pellets

Experimental set-up I

on time: 10µs

off time: 90µs

Ablation monitor

signal

Cam. exposures

time [s]

Trajectory

recons-

truction

separatrix

Vide angle

view

photodiode

long exposure

short multiple exposures

exposures

Injection geometry

tpellet @ separatrix

Pellet localization (space, time):

- fast CCD cameras + spatial calibration

→ def: penetration = distance from separatrix


Elm triggering by deuterium pellets

Experimental set-up II

Poloidal pick-up

coil set, 7 coils

about 600

Calibrated Electron

Cyclotron Emission

(ECE) profiles are

measured in second

harmonic X-mode with

a fast 60-channel

heterodyne

radiometer

Toroidal pick-up coil set

about 1800, 5 coils

Mirnov coil set

3600, 30 coils

PELLET

500 ms

Ablation monitor

separatrix crossing

Pick-up coil signal

ELM-delay

  • Processing of the pickup coil signals

  • eliminate LF component by moving box average

  • calculate the envelope of the remaining HF component (25 ms box)

  • assume: envelope ~ MHD perturbation magnitude

B31-14

ELM (type-I)


Elm triggering by deuterium pellets

Experimental set-up III

Processing of pick-up coil signals: spectral properties

continuous analytical wavelet transform: time shift and scaling invariant

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

Mode numbers as least squares fit to

cross-phase

as function of relative probe position :


Elm triggering by deuterium pellets

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

Radial localisation of pellets at ELM trigger

Radial localisation of HFS pellets at ELM trigger

dtELM_ONSET saturates with increasing dtelapsed

dtelapsed > 8ms: dt ELM_ONSET ~ constant

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


Elm triggering by deuterium pellets

Estimation of the pellet cloud pressure

and plasma pressure drop

Pellet-plasma interaction

Pellet affects the plasma at a magnetic surface as long as

t=DiamCLOUD /vP =20-100µs

→ 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

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

front of the pellet

rough estimate: 10-30% for AUG


Elm triggering by deuterium pellets

Injection scenarios

Measurement of the ELM onset delay as a function of the pellet velocity →

VP=240,600,880,1000 m/s

HFS 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

Designated pellet path

dNP /dt = - Const. rP4/3 n e1/3 Te1.64

dNP /dl VP = - Const. rP4/3 n e1/3 Te1.64

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

Integrating this diff.

equation along the

designated pellet path

the ablation rate

(ablated particles /s)

is calculated


Elm triggering by deuterium pellets

Injection scenarios

240 m/s, r=0.71mm, N=9 ·1019

600 m/s, r=0.67mm, N=7.4

880 m/s, r=0.58mm, N=5.e19

1000m/s,r=0.51mm, N=3.3e19

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

Pellets penetrate deep into

the plasma crossing the

pedestal region completely

Particle deposition scales with

1/VP


Elm triggering by deuterium pellets

Type-I ELMy H-mode

el. diam.

Drift, n>0

ion diam.

Drift n<0

delayed

triggered

Prompt

triggered

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.

washboard-type

precursor mode

slowing

down, n=4

No precursor

Neuhauser et al, NF, 48, 045005, 2008


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