ELM%20triggering%20by%20deuterium%20pellets

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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

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Introduction
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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.
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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
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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 ?WELM PHEAT f ELM for many
scenarios with natural ELMs Approach Split
large ELMs into many small ones
A. Herrmann, PPCF 44 (2002) 883
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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 Ha radiation
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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 velocity150-800m/s
L. Baylor et al., POP 7 (2000) 1878
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Pellets injected from all locations on ASDEX
Upgrade trigger ELMs
Prompt pellet ELM trigger for HFS injection. ELM
event released 2050µs 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
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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
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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
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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
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No MHD precursor to pellet induced ELM
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0
-20
-40
200
150
dBr/dt (a.u.) Outer shelf
100
50
0
-50
2772.2
2772.4
2772.6
2772.8
2773.0
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
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Localisation of pellet when ELM is triggered in
DIII-D
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
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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
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Is the ELM triggered at the location of the
pellet? Is the perturbation localised
polidally/toroidally?
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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
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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
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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
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Pellet plasma interaction
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pellet
ionised cloud
incoming heat flux
Incoming
spherical neutral cloud
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Axisymmetric plasma cooling
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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
vP1000m/s
Kocsis et al, EPS 2008
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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
vP240m/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
vP600m/s
Kocsis et al, EPS 2008
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Pellet caused magnetic perturbation
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Pellet caused magnetic perturbation in OH, HD
type-III and type-I H-mode
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
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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
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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
type-III
pel
? 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
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Nonlinear MHD simulation of pellet ELM triggering
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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
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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
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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
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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.
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Backup transparencies
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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
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Pellet localization (space, time) - fast CCD
cameras spatial calibration ? def penetration
distance from separatrix
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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)
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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
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G. Kocsis et al., NF 47 (2007) 1166
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Pellet affects the plasma at a magnetic surface
as long as
tDiamCLOUD /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

gt10 Pe
? axisymmetric decrease of the plasma pressure
causing a pressure gradient increase in
front of the pellet rough
estimate
10-30 for AUG
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Measurement of the ELM onset delay as a function
of the pellet velocity ?

VP240,600,880,1000 m/s HFS looping
system pellet erosion is velocity dependent ?

rP0.71,
0.67, 0.58, 0.51 mm

NP9 , 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
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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, n4
No precursor
Neuhauser et al, NF, 48, 045005, 2008