23P082-P
Integrated simulation study of ELM pacing by pellet injection with TOPICS-IB
ペレット入射によるELMペーシングに関するTOPICS-IBコードを用いた統合シミュレーション研究
Nobuhiko Hayashi, Nobuyuki Aiba, Tomonori Takizuka and Naoyuki Oyama
林 伸彦, 相羽信行，滝塚知典，大山直幸
Japan Atomic Energy Agency
801-1, Mukouyama, Naka, Ibaraki 311-0193, Japan
日本原子力研究開発機構 〒311-0193 茨城県那珂市向山801-1
Simulations of pellet triggered ELM (edge-localized mode) have been done by the integrated code
TOPICS-IB with various pellet parameters (location, timing, size and speed) to clarify optimum parameters
for ELM pacing by the pellet. The pellet energy absorption effect is considered as an ELM triggering
mechanism. It is found that an ELM is triggered when a pellet enters more deeply in the pedestal region forearlier pellet injection in the natural ELM cycle, smaller size or faster speed, independently of pellet
injection location. The energy loss by the pellet triggered ELM just before next natural ELM onset isalmost the same as the natural ELM in wide ranges of parameters, but earlier injection can reduce theenergy loss depending on parameters. The energy loss by the pellet triggered ELM just after previous
natural ELM becomes smaller for smaller or slower pellet. At the middle timing of pellet injection in thenatural ELM cycle, the energy loss becomes smaller for larger (smaller) or slower (faster) pellet fromhigh-field side [HFS] (low-field side [LFS]). The reduced energy loss is mainly attributed to the onset ofhigh-n ballooning mode, while the natural ELM is triggered by intermediate-n peeling-ballooning mode.

1. Introduction
The large energy loss caused by ELM is crucialfor the erosion of divertor plates in tokamak
reactors. Pellet injection is considered as one ofpromising methods to increase the ELM
frequency and to reduce the ELM energy loss.
ELM pacing and mitigation by the pellet injection
have been demonstrated in several experiments[1]. A few ideas have been explored in attempts
to explain the ELM triggering by the pellet, but
the physical mechanisms, however, are not fully
understood yet. In order to study the pellet
triggered ELM, we have developed an integrated
core transport code TOPICS-IB [2] (1.5D coretransport code TOPICS extended to the integrated
simulation for burning plasmas) coupled with alinear MHD stability code MARG2D, a dynamic
five-point model for SOL-divertor transport and apellet model APLEX [3]. TOPICS-IB simulations
showed that the energy absorption by a pellet in
the pedestal steepens locally the pressure gradientand the ELM is destabilized. It is also found thatthe transport enhancement during the pellet
ablation is able to steepen the local pressure
gradient and to trigger the ELM as well.
For the ELM pacing by the pellet, ELMs should
be reliably triggered by small pellets to minimize
undesirable fuelling and impact on the target
pedestal pressure/temperature. Experiments indicate
the existence of trigger threshold of the pellet

parameters such as the pellet size [1]. In the above
TOPICS-IB simulation for a JT-60U H-mode
plasma, a single pellet with a fixed radius rp=0.6
mm (particle content ~ 5×1019 D) and speed vp=120m/s triggers an ELM at about 10 % lower pedestalpressure than that triggering a "natural" ELM.
Further TOPICS-IB simulations with different
parameters help to study the threshold.
In this paper, we carry out TOPICS-IB
simulations with various pallet parameters of pellet
location, injection timing in the natural ELM cycle,
size and speed, and search for the trigger thresholdand conditions satisfying the demands for ELM
pacing by the pellet.
2. Simulation results
We use JT-60U parameters for simulations withTOPICS-IB whose detailed models are shown in [3].
In a simulation without the pellet injection, a seriesof natural ELMs by unstable modes with thetoroidal mode number n~20 occurs with the periodof τcyc=26 ms and their energy losses ΔWELM areabout 6 % of pedestal stored energy Wped~1 MJ.
Three timings in the natural ELM cycle are chosen
for the pellet injection only with the pellet energy
absorption effect as a first step. Figure 1 shows thetime evolution of profiles for those injection
timings of a pellet with the same size and speed as
described above. The unstable mode triggered by
the pellet just before the next natural ELM onset

(Δt/τcyc=0.96) is almost the same as the natural
ELM and the early injection triggers higher-nmodes. As shown in Fig.2, ELM is triggered when
the pellet enters more deeply in the pedestal region
for earlier injection, independently of pellet
injection location. The energy loss becomes smaller
for earlier injection of a pellet with one set of sizeand speed, because of higher-n modes become
unstable.

slower pellet from HFS or LFS. On the other hand,at the late injection timing (Δt/τcyc=0.96), the trigger
threshold of size and speed is lower than earlier
timing. The energy loss does not much change fromthe level of natural ELM in wide ranges of size andspeed.

Fig.2. (a) Pellet position at ELM onset, minimum
values of pellet position (i.e. penetration depth) and (b)normalized ELM energy loss as functions of Δt/τcyc
where triangles in (b) denote energy losses by naturalELMs.

Fig.1. Time evolution of profiles of total pressure (top),normalized pressure gradient (middle) and electron
diffusivity when a HFS pellet is injected at timings ofΔt/τcyc=0.19 (left), 0.58 (center) and 0.96 (right) where Δtdenote time from previous natural ELM, dotted lines
profiles just before pellet injection, solid ones just atELM onset, broken ones just after ELM and verticaldotted ones pellet position at ELM onset.

We next study the dependence on the pellet sizeand speed at the middle injection timing(Δt/τcyc=0.96). ELM is triggered when a pellet withsize rp>0.3 mm enters the pedestal region andsmallerpelletpenetrates
moredeeply,
independently of injection location. In Fig.3, theenergy loss becomes smaller for smaller pellet
injected from LFS, while larger loss for smallerpellet from HFS. On the other hand, ELM istriggered when a pellet with speed vp>5 m/s fromHFS (vp>0.5 m/s from LFS) enters the pedestal
region and faster pellet penetrates more deeply,
independently of injection location. The energy loss
becomes smaller for slower pellet from HFS, whilelarger loss for slower pellet from LFS except forvery low speed region in Fig.3.
At the early injection timing (Δt/τcyc=0.19), thetrigger threshold of size and speed is almost thesame as the middle timing case for the HFS pellet,
while larger size rp>0.4 mm or faster speed vp>2m/s is necessary for triggering by the LFS pellet.
The energy loss becomes smaller for smaller orFig.3. Normalized ELM energy loss as functions of rpand vp.

3. Discussions
The simulation results have potential to explain
experimental observations such as variations ofpellet position within the pedestal at the ELM
onset and energy loss, smaller pellet enough fortriggering at later injection in the natural ELM
cycle [1]. The simulation results suggest that theearly injection in the natural ELM cycle isdesirable for the energy loss reduction if thecontrol of pellet size and speed is difficult, but thisleads to the reduction of pedestal pressure (14 % atΔt/τcyc=0.19).
Acknowledgments
This work was partly supported by JSPS,Grant-in-Aid for Scientific Research.
References[1] ex. P.T. Lang et al: Nucl. Fusion 51 (2011) 033010
and G. Kocsis et al: Nucl. Fusion 47 (2007) 1166.
[2] N. Hayashi, et al: Nucl. Fusion 49 (2009) 095015.
[3] N. Hayashi, et al: Nucl. Fusion 51 (2011) 103030.