* See The Appendix Of

F Romanelli

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Comparison of divertor power loads of spontaneous and  
pellet triggered ELMs at JET 
 
R. Wenninger1, T. Eich1, W. Fundamenski2, P. T. Lang1, S. Devaux1, K. Gál3, A. Geraud4, D. 
Harting5, G. Kocsis3, H.W. Müller1, H. Thomsen1 and JET-EFDA contributors* 
JET-EFDA, Culham Science Centre,OX14 3DB, Abingdon, OXON, United Kingdom 
1 Max-Planck-Institut für Plasmaphysik, EURATOM Association, Boltzmannstr. 2, 85748 Garching, Germany 
2 EURATOM-UKAEA Fusion Association, Culham Science Centre, Abingdon, UK 
3 MTA-KFKI-RMKI, EURATOM Association, P.O. Box 49, H-1525 Budapest-114, Hungary 
4CEA, IRFM, F-13108 Saint-Paul-lez-Durance, France 
5Institute for Energy Research - Plasma Physics, Forschungszentrum Juelich, Association EURATOM-FZJ, 
Trilateral Euregio Cluster, 52425 Juelich, Germany 
* See the Appendix of F. Romanelli et al., Fusion Energy 2008 (Proc. 22nd IAEA Conf., Geneva, 2008) 
 
1. Introduction 
It is currently assumed that a reliable technique for the mitigation of Edge Located Modes 
(ELMs) is mandatory for the success of ITER. The severity of the ELM impact on the 
divertor plasma-facing components is determined by ELM rise time, wetted area and energy 
deposited on the divertor targets during the ELM rise time. For any candidate control 
technique to mitigate ELMs to 1MJ detailed analysis of divertor power load characteristics 
has to be carried out. Experiments at ASDEX Upgrade have shown that by Pellet ELM pacing 
[1] ELMs can be triggered reliably up to frequencies twice the natural ELM frequency. We 
present a comparison of several divertor power load aspects of spontaneous and pellet 
triggered ELMs employing infrared thermography [2]. 
 
2. Experimental Setup 
Currently a new pellet injection system for JET is in the phase of commissioning. The High 
Frequency Pellet Injector (HFPI) was designed to deliver small (0.7 – 1.3 × 1020 D) pellets at 
a rate of up to 60 Hz and in the speed range 50 – 200 m/s for Pellet ELM Pacing and large (24 
– 40 × 1020 D) pellets up to 15 Hz and 100 – 500 m/s for Deuterium Fuelling [3]. For the 
experiments described here large pellets with a nominal particle content of 2.4  × 1021 D have 
been used. Pellets produced by the HFPI were guided in our experiments via the flight line, 
which is associated with a normal injection from the magnetic low field side (LFS). Due to 
the High-β Plasmoid Drift [4], which for LFS injection leads to a low fuelling efficiency 
compared to injection from the high field side, this injection direction is regarded as 
favourable for pellet ELM pacing with minimal fuelling. The LFS injection line is equipped 
with a microwave cavity for pellet detection 3.4m before the vessel. 
A new divertor infrared camera has been extensively exploited in these studies. It has a slim 
rectangular view on the divertor elongated in the poloidal direction and a temporal resolution 
from ~30µs (reduced poloidal extension) to ~90µs (full view). Its spatial resolution is 1.63mm 
respectively 5.0mm for the observable area on the outer respectively inner divertor. 
 
3. Comparison of general ELM power load aspects 
For the analysis described below a high triangularity H-mode discharge (JPN 76697) with 
IP=2.5MA, Bt=2.7T, q95=3.4, PNBI≈13MW / 16.5MW has been performed. The discharge is 
optimised for divertor power load analysis employing the divertor IR camera with a time 
resolution of 86µs. Therefore, the strike points were positioned in a way that the full outer 
power deposition but less than one decay length on the inner divertor have been observed . 
Fig. 1 shows time traces describing main discharge parameter between 18s and 22s. Before 
the start of pellet injection type I ELMs with an average frequency of ~10 Hz are observed. 
From 17.5s to 22s pellets are fired at 10Hz frequency and a velocity of about 150m/s.  
36th EPS Conference on Plasma Phys. Sofia, June 29 - July 3, 2009 ECA Vol.33E, D-4.001  (2009)
 
14 pellet triggered and 21 spontaneous type I 
ELMs, which are interleaved in the period 
between 18s and 22s (Fig. 1e), have been 
identified on the basis of the following ELM 
selection criteria. Generally only ELMs 
appearing more then 30ms after their 
predecessor have been considered to ensure a 
minimum in comparability of pedestal 
conditions. ELMs are classified as pellet 
triggered, if the flight time between pellet 
detection by microwave cavity in the flight tube 
and ELM detection in the plasma correspond to 
the measured pellet velocity plus if, a significant 
density jump is observed on the edge channel of 
the interferometer. ELMs, for which just one of 
these criteria is met, have been excluded from 
the comparison. All other ELMs have been 
classified as spontaneous. A standard algorithm 
using power and the maximum power flux 
density on the outer strike plate evaluated by IR calculates for all selected ELMs the three 
times tELM,start, tELM,max and tELM,end. In Fig. 3 average values of these times are indicated. 
The selected sets of ELMs have similar average time intervals to the ELM before (spont. 
44ms, trig. 39ms) and similar average plasma energies at tELM,start (spont. 5.4MJ, trig. 5.2MJ). 
 
3.1. Thermal energy losses and radiated energy 
For the evaluation of the ELM 
associated loss of thermal energy for 
both diamagnetic energy (∆WDIA) and 
thermal energy acquired via the 
equilibrium code EFIT (∆WMHD) the 
mean values at tELM,end are subtracted 
from the mean values at tELM,start. This 
approach tends to underestimate the loss 
of thermal energy due input power 
constantly raising the thermal energy. 
However for both spontaneous and 
triggered ELMs comparable amounts of 
input energy (WINP) have entered the 
plasma between tELM,start and tELM,end 
(78±10kJ resp. 72±6kJ). Spontaneous 
ELMs (∆WDIA=151±46kJ, ∆WMHD=154 
±50kJ) show significantly lower loss of 
thermal energy compared to losses 
during pellet triggered ELMs (∆WDIA= 
259±40kJ, ∆WMHD=249±42kJ). 
The amount of energy radiated between 
tELM,start and tELM,end (∆WRAD) has been integrated from bolometry data (temporal resolution: 
0.2ms). Again clearly lower values have been found for spontaneous ELMs 
(∆WRAD=113±20kJ) than for triggered ELMs (∆WRAD=218±33kJ). However especially for 
the ELM rise phase it can not be assumed that the higher amount of released thermal energy 
for pellet triggered ELMs is balanced by radiation [5]. 
From the edge channel of the interferometer the pellet associated fast density increment has 
been acquired, which is assumed to be roughly proportional to the pellet mass. As Fig. 2a 
0
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t [s]
Fig. 2: Scaling with pellet size (edge density increment): 
a) ∆WMHD and ∆WRAD b) WDIV,OUT,RISE and WDIV,OUT,TOT  c)  
tELM,max – tELM,start  d) average wetted area for entire ELM 
phase. Dashed lines indicate linear regressions, squares 
represent respective mean values of pontaneous ELMs 
a) 
b) 
c) 
d) 
Fig.1: JPN 76697 (18-22s) – main discharge 
parameter: a) input power, b) thermal energy, c) 
and d) pedestal values of Te and ne e) power to 
outer divertor  - squares: analysed spontaneous 
(blue) and triggerd (red) ELMs 
a) 
b) 
c) 
d) 
e) 
36th EPS 2009; R.Wenninger et al. : Comparison of divertor power loads of spontaneous and pellet triggered EL... 2 of 4
 
shows, for a variation of the density jump by a factor of 5 only a weak scaling of ∆WMHD and 
∆WRAD has been found. For ∆WMHD the extrapolation of the linear fit to zero density 
increment is clearly above the values for spontaneous ELMs. 
 
3.2. Deposition on the outer divertor target 
The deteriorating effect of ELMs on the divertor targets roughly can be expressed via the 
temperature increase ∆TELM due to a single ELM, which in approximation is a function of 
thermal material properties Cthermal,mater, timescales of the ELM rise phase τELM,RISE, wetted 
area AELM and the energy ETAR,RISE deposited on the target during the ELM rise phase [6]:  
∆TELM ≈ Cthermal,mater. × ETAR,RISE / (AELM × sqrt(τELM,RISE)) 
Making use of the high spatial  resolution of 
the IR system at the outer target the non 
material related quantities for this area have 
been analysed in further detail. 
Fig. 3 shows a comparison of the power 
deposition dynamics on the outer target of the 
investigated ELMs. Time traces for each ELM 
have been aligned by tELM,max. Subsequently 
averages over both sorts of ELMs have been 
taken.The ELM rise time (tELM,max – tELM,start) 
for pellet triggered ELMs (0.47±0.18ms) is 
higher by about 30% in relation to 
spontaneous ELMs (0.36±0.11ms). At the 
same time  triggered ELMs show a slower 
decay, which is temporarily almost linear and 
not exponential in time as for the spontaneous 
ELMs. Fig. 2c shows that both ELM rise time 
and duration of the full ELM do not scale 
significantly with the pellet size.  
Assuming toroidal homogeneity the wetted 
area AELM can be calculated as the relation 
between the power and the maximum power 
flux density of the deposition on the divertor 
target. Accounting for the ELM rise phase 
(tELM,max-0.34ms to tELM,max) resp. a longer 
phase including the early part of the decay 
phase (tELM,max-0.34ms to tELM,max+ 0.69ms) 
yields for spontaneous ELMs a wetted area on 
the outer divertor target of 0.50m2 resp. 
0.76m2 and for pellet triggered ELMs 0.60m2 
resp. 0.65m2. Thus during the crucial ELM 
rise phase the investigated pellet triggered 
ELMs have a slightly lower wetted area. This 
difference in AELM is stronger for the early rise phase and vanishes towards tELM,max. Power 
flux deposition profiles averaged over the described phases are illustrated in Fig. 4. 
For the investigated pellet triggered ELMs the maximum power deposited on the outer target 
is lower (spont.: 29±4MW, trig.: 26±5MW) than for their spontaneous counterparts, although 
the earlier ones are associated with an about 60% higher release of thermal energy. 
Nevertheless due to their slower dynamics pellet triggered ELMs are associated with the 
higher power deposition in the earlier part of the ELM rise phase (and in the later part of the 
decay phase). Due to this pellet triggered ELMs are associated with a higher energy 
WDIV,OUT,RISE deposited during the rise phase (tELM,start to tELM,max) (6.9±2.9kJ) compared to 
spontaneous ELMs (5.8±1.4kJ). As well for the energy deposited during the entire ELM 
−2 0 2 4 6
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Fig. 4: Average power flux densities for 
spontaneous (blue) and pellet triggered ELMs (red): 
a) tELM,max-0.34ms to tELM,max b) tELM,max-0.34ms to 
tELM,max+0.69ms 
a) b) 
Fig. 3: Comparison of power deposition dynamics 
(Coherent ELM averaging): a) Complete ELM b) 
ELM rise phase. Vertical lines indicate averages of 
tELM,start, tELM,max and tELM,end for both sorts of ELMs 
36th EPS 2009; R.Wenninger et al. : Comparison of divertor power loads of spontaneous and pellet triggered EL... 3 of 4
 
phase WDIV,OUT,TOT triggered ELMs (60±7kJ) exhibit higher values compared to spontaneous 
ones (47±7kJ). Fig. 2b shows that WDIV,OUT,RISE and WDIV,OUT,TOT are not scaling with the 
pellet size. 
 
3.3. Estimation of energy deposited on the inner divertor target 
For JET H-mode plasmas with normal field direction it has been reported [7], that the 
deposited energy per type-I ELM on the inner target is higher than that on the outer target. 
Due to the position of the inner strike point in the analysed discharge the energy deposited in 
this area between tELM,start and tELM,end (WIN) can only be estimated. For this it has been 
assumed that WIN equals the energy deposited in the area observed by IR (WIN,IR) multiplied 
by a constant factor c accounting for the very partial view of the deposition area. The ratio 
between WOUT and WIN,IR describing the in-out-balance of the ELM energy deposition up to 
the factor c is 2.4±0.4 for spontaneous ELMs and 3.6±0.5 for triggered ELMs. This indicates 
that the latter ones have a similar or a more balanced in-out-symmetry compared to 
spontaneous ELMs.  
 
4. Nonaxisymmetric Energy Deposition Patterns 
The spatial and temporal resolution of the IR 
measurements especially on the outer target 
allows observation of deposition peaks, which 
can be linked to toroidal asymmetric structures 
in the SOL [8]. As observed before for 
spontaneous ELMs, for a number of pellet 
triggered ELMs several statistically distributed 
and laterally displaced stripes, many of which 
are well separated from each other and from the 
main strike zone, have been observed especially 
during the ELM rise phase. These structures can 
be interpreted as footprints of helical 
perturbations at the low field side of the plasma 
edge, which are approximately aligned with the 
magnetic field structure. Fig. 5 shows the 
evolution of the deposition pattern during the rise phase of a pellet triggered ELM from a high 
triangularity type-I ELMy H-mode discharge. 
 
5. Summary 
The investigated pellet triggered ELMs in comparison to the spontaneous ELMs show the 
following features: They are associated with 60% higher losses of thermal energy (not clearly 
scaling with the pellet size), as well as a higher radiated energy. The energy deposited on the 
outer target during the ELM rise phase is higher by ~20%, while the wetted area during this 
phase is slightly lower. The ELM rise time is higher by ~30%. 
There is an indication that pellet triggered ELMs have a similar or more balanced divertor in-
out-symmetry compared to spontaneous ELMs. As for spontaneous ELMs deposition 
structures have been observed for pellet triggered ELMs, which suggest a toroidally 
asymmetric component of the SOL transport. 
 
References 
[1] P. T. Lang et al., Nucl. Fusion 43 No 10 (2003) 1110-1120. 
[2] T. Eich et al., P. P. C. F. 49 (2007) 573–604 
[3] A. Geraud et al., Fus. Eng. and Des. 82 (2007) 2183-2188. 
[4] H. W. Müller et al., Phys. Rev. Lett. 83, 2199 (1999). 
[5] P. Monier-Garbet et al., Nucl. Fusion 45(2005) 1404-1410.  
[6] T. Eich et al., J. Nucl. Mater. 337-339 (2006) 669. 
[7] T. Eich et al., J. Nucl. Mater. 363-365 (2007) 989. 
[8] T. Eich et al., Phys. Rev. Lett. 91, 195003 (2003). 
 
Acknowledgement: 
This work, supported by the European Communities under the contract of Association between EURATOM/IPP, was carried out within the 
framework of the European Fusion Development Agreement. The views and opinions expressed herein do not necessarily reflect those of the 
European Commission 
2.7 2.72 2.74 2.76 2.78 2.8 2.82
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Inter ELM
Fig. 5: Development of deposition pattern for a 
pellet triggered ELM: The times are relative to 
tELM,sta 
36th EPS 2009; R.Wenninger et al. : Comparison of divertor power loads of spontaneous and pellet triggered EL... 4 of 4


Comparison of divertor power loads of spontaneous and pellet triggered ELMs at JET

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Comparison of divertor power loads of spontaneous and pellet triggered ELMs at JET

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