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# 28th IAEA Fusion Energy Conference (FEC 2020)

10-15 May 2021
Nice, France
Europe/Vienna timezone
The Conference will be held virtually from 10-15 May 2021

## Pellet gas comparisons with matched ELM frequency of pedestal, SOL, ELMs and core confinement in JET-ILW

12 May 2021, 08:30
4h
Nice, France

#### Nice, France

Regular Poster Magnetic Fusion Experiments

### Speaker

Dr Christian Perez von Thun (IPPLM)

### Description

Cryogenic pellet injection will almost certainly become indispensable in ITER during H-mode, for which the fuelling efficiency of gas fuelling and recycled neutrals is predicted to be very low. In addition, ELM pacemaking with pellets is foreseen as a back-up technique to ELM control with RMPs. With this in mind, the HFS pellet injector at JET has been recently refurbished to increase the pellet reliability, in particular for the smallest pellets. In addition, the fast visible camera diagnostic has benefited from a new optics layout and the addition of a second fast camera, enabling tracking of the pellet trajectory in the plasma with higher spatial resolution and with improved toroidal coverage (figure 1).
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Making full use of these enhancements, dedicated experiments have been executed in JET-ILW with the aim to assess more systematically the effect of the pellets on type I ELMy H-modes. These experiments encompassed pellet-gas replacement scans for documenting differences in pedestal, SOL and divertor conditions, ELMs, impurity transport and core confinement. The scans were repeated with several target plasmas from low to high plasma current ($I_p=1.4$-3MA) to vary the pedestal height over an as wide range as possible in terms of density ($n_{e,ped} \sim 2.4-6.5\times 10^{19}$ m$^{-3}$), temperature ($T_{e,ped} \sim 0.4-0.7$ keV) and pressure ($p_{e,ped} \sim 2.1-7.8$ kPa). In addition, the influence of the pellet mass was also assessed, through the use of either small or large pellets (factor $\sim$9 difference in mass). As a result, the average value of the maximum radial pellet penetration depth could be varied from moderately deep to very shallow, $<$$\psi_N$$>_{max}=$ 0.70 - 0.95.
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One of the aims of the experiment was to examine how the ELM triggering efficiency depends on the pedestal parameters. Within the range of pellet sizes available on JET and for the range of pedestal pressures explored, it was found that all delivered pellets are capable of triggering type-I ELMs fairly reliably. Even for the shallowest pellet deposition obtained, i.e. $<$$\psi_N$$>_{max}=$ 0.95 for small pellets at high $I_p$, no decrease in pellet triggering efficiency was found. Lower triggering efficiency was only encountered when the pedestal pressure is far away from the stability limit for type-I ELMs, in particular in the presence of type-III ELMs.
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While the high ELM triggering efficiency turns pellets into an efficient tool to increase $f_{ELM}$, locking the ELM frequency to the pellet frequency (ELM pacing) is found to be much harder on JET. ELM pacing is hampered by two effects: (a) the tendency to obtain additional (spontaneous) type-I ELMs due to the finite pellet fuelling effect, and -more importantly- (b) the tendency to trigger compound ELMs (backtransition to type-III ELMs after an initial type-I ELM),even with small pellets. Empirically, the latter effect is more prevalent in plasmas with shallow pellet deposition and low edge safety factor ($q_{95}\sim 3$). The key observation is that the drop in edge $T_e$ is significantly larger for pellet triggered ELMs than for natural ELMs (as shown in figure 2), mainly due to the additional plasma edge cooling effect by the pellet. If $T_{e,ped}$ is pushed below a critical value $T_{e,ped}^{crit}$ (see reference 1), and for as long as $T_{e,ped}$ remains there, the plasma produces transiently type-III ELMs. The consequence is that with pellets the plasma can remain in compound ELM regime at levels of auxiliary power that would normally comfortably yield clean type-I ELMs. At low plasma current ($I_p=2.1$MA) and low $q_{95}$, it has been possible to recover clean and regular type-I ELMs only through a substantial increase in the auxiliary power ($P_{tot}/P_{LH,Martin08}> \sim 3$). In such case, the temperature drop $\Delta T_e$ for pellet triggered ELMs retains its large amplitude, but, because the starting value of $T_{e,ped}$ is higher, it no longer falls below $T_{e,ped}^{crit}$. On the other hand, for $I_p\geq3$MA and $q_{95}=3$ it has not been possible so far to recover regular (non-compound) type-I ELMs with pellets even with the maximum levels of auxiliary heating available at JET. This could have implications also for ITER, where ELM pacemaking will rely on shallow pellet deposition.
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Pellet-gas comparisons of global confinement behaviour reveal a complex picture. On one hand, there are dedicated experiments which have produced essentially unchanged global confinement when gas is partially or fully replaced by pellets. In contrast to this, there are also well documented cases where access to improved core (not pedestal) confinement in the baseline scenario has been obtained with a combination of pellets and low gas dosing (references 2,3). Comparison of the two datasets reveals that the mere gas substitution by pellets does not suffice to trigger this core confinement improvement, and that three additional ingredients need to be fulfilled: the presence of compound/type-III ELMs, high enough neutral beam power (or torque), and a low enough level of gas dosing. Understanding the physics mechanism behind this difference will be one of the aims of this work.
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While with clean regular type-I ELMs some differences have been observed in terms of pre-ELM pedestal pressure height and width in individual gas-pellet pairs, no consistent improvement of pedestal confinement when pellets replace gas has been found. Overall, the impact of pellets on the pedestal pressure is found to be modest. On the other hand, the relative balance between pedestal density and pedestal temperature is affected and varies depending on the pellet size. For fixed ELM frequency we obtain with small pellets $n_{e,ped}$(pellets) $<$ $n_{e,sep}$(gas), whereas with large pellets $n_{e,ped}$(pellets) $>$ $n_{e,ped}$(gas). Further downstream, the replacement of gas with pellets always results in a significant reduction in inner divertor radiation, irrespective of the pellet size, which is indicative of less dense, hotter target plasmas. This renders pellets into an efficient tool to decouple upstream plasma boundary and pedestal from downstream conditions on JET.

Acknowledgment
This work has been carried out within the framework of the EUROfusion Consortium and has received funding from the Euratom research and training programme 2014-2018 and 2019-2020 under grant agreement No 633053. The views and opinions expressed herein do not necessarily reflect those of the European Commission. This scientific work was supported by Polish Ministry of Science and Higher Education within the framework of the scientific financial resources in the years 2019 and 2020 allocated for the realization of the international co-financed project.

References
1 Sartori, R. et al Plasma Phys. Control. Fusion 46 (2004) 723-750.
2 Kim H.-T. et al Nucl. Fusion {\bf 58} (2018) 036020.
3 Garcia J. et al, Integrated scenario development at JET for DT operation and ITER risk mitigation, at this conference

Figures

Affiliation Institute for Plasmaphysics and Laser Microfusion (IPPLM), Hery Str 23, 01-497 Warsaw, Poland Poland

### Co-authors

Dr Elena De La Luna (Laboratorio Nacional de Fusión, Asociación EURATOM-CIEMAT) Lorenzo Frassinetti (KTH, Royal Institute of Technology) Dr Scott Silburn (UKAEA) Spyridon Aleiferis (National Centre for Scientific Research ‘Demokritos’, Athens, Greece) Itziar Balboa (CCFE, Culham Science Centre, Abingdon, Oxon, OX14 3DB, UK) Dr Mathias Brix (UKAEA) Dr Ivo Carvalho (Instituto de Plasmas e Fusão Nuclear, Instituto Superior Técnico, Universidade de Lisboa, Portugal) Dr Pedro Carvalho (IST/IPFN, Lisbon, Portugal) Dr Agata Chomiczewska (Institute of Plasma Physics and Laser Microfusion) Anthony Field (UKAEA) Luca Garzotti (United Kingdom Atomic Energy Agency - Culham Centre for Fusion Energy) Dr Carine Giroud (UKAEA) Dr Rafael Henriques (IST/IPFN, Lisbon, Portugal) Dr Laszlo Horvath (ukaea) Hyun-Tae Kim (EUROfusion Consortium JET) Ms Natalia Krawczyk (IPPLM, Warsaw, Poland) Morten Lennholm (European Commission) Dr Bartosz Lomanowski (Oak Ridge National Laboratory) Dr Peter Lomas (Culham Centre for Fusion Energy, Abingdon, OX143DB) Ulises Losada (Laboratorio Nacional de Fusión CIEMAT) Dr Ana Manzanares (Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas, Madrid, Spain) Sara Moradi (Ecole polytechnique) Dr Andrew Meigs (UKAEA) Dr Duarte Nina (IST/IPFN, Lisbon, Portugal) Dr Nerea Panadero (Laboratorio Nacional de Fusion (CIEMAT)) Dr Tiago Pereira (Instituto de Plasmas e Fusão Nuclear, Instituto Superior Técnico, Universidade de Lisboa, Portugal) Mr Daniel Réfy (Centre for Emergy Research) Dr Fernanda Rimini (UKAEA) Dr Marco Sertoli (Max-Planck-Institut für Plasmaphysik) Emilia R. Solano (EsCiemat) giuseppe telesca (IPPLM) Dr Jari Varje (VTT)

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