Speaker
Martin Valovic
Description
See the full Abstract at http://ocs.ciemat.es/EPS2018ABS/pdf/P2.1075.pdf
Shallow pellet fuelling under conditions of RMP ELM mitigation or
divertor detachment in ASDEX Upgrade
M Valovič1, P T Lang2, A Kirk1, W Suttrop2, M Bernert2, M Cavedon2, M Dunne2,
R Fischer2, L Garzotti1, L Guimarais2, F Janky2, N Leuthold2, PJ Mc Carthy3, A Mlynek2,
B Plőckl2, G Tardini2, E Viezzer2, E Wolfrum2, the ASDEX Upgrade team2 and the
EUROfusion MST1 team4
1
CCFE, Culham Science Centre, Abingdon, OX14 3DB, UK; 2Max-Planck-Institut fűr
Plasmaphysik, Boltzmannstrasse 2, D-85748 Garching, Germany; 3Department of Physics,
University College Cork, Cork, Ireland; 4See H. Meyer et al., Nucl. Fusion 57 (2017) 102014
Pellets are used in ASDEX Upgrade [1] to control plasma density under conditions of ELM
control or divertor detachment. In experiments presented here direct fuelling by gas is
negligible. Relative pellet size and pellet deposition are aimed to approach those in ITER but
differences still remain. ELMs are controlled by n=2 RMPs in feed forward mode [2]. Divertor
detachment is controlled by nitrogen gas in feedback mode.
In low upper triangularity plasmas with ELM control by RMPs, pellets can refuel the
RMP pump out using both gradual [3] and prompt [4] application of pellet trains. With
application of pellets promptly after activation of the RMP fields the duration of the density
transient can be reduced to 3 energy confinement times. The required pellet particle throughput
to restore pre-RMP density is about pel~5.6×1021at/s~0.07Paux/Tped (Paux is the auxiliary
heating power and Tped is the pedestal temperature) which is comparable to the RMP pump out
rate RMP~1.7×1021at/s determined from the time derivative of the plasma density after the
RMP is switched on. The density increase by pellets approximately preserves ion pedestal
pressure in the RMP phase. An unwanted side effect of pellet refuelling is the transition from
ELM suppression to an ELMy regime, triggered by the first pellet. A favourable observation
is that ELMs with pellet fuelling are still smaller than those without RMPs, and the ELM
frequency is not modulated by pellets. At elevated upper triangularity the ELM suppression is
restored after an ELM-like events triggered by the pellets [4]. With increasing density plasma
eventually transitions to ELMy regime, similarly as in low triangularity case.
In plasmas with divertor detachment, density control by pellets without gas is
demonstrated. The required particle throughput is about pel~19×1021at/s. When normalised to
heat flux the throughput is pelTped /Paux~0.1 which is broadly agreeing with ITER prediction
[5]. With detachment, an unwanted side effect is the pellet induced modulation of the plasma
temperature (~2x) at the outer strike point and a consequential modulation of the nitrogen gas
due to feedback control. This is caused by the modulation of the ELM frequency by pellets and
not by a pellet induced cooling wave as one might expect.
[1] Lang P T et al 2012 Nucl. Fusion 52 024002; [2] Suttrop W et al 2017 Plasma Phys. Control. Fusion 59
014050, [3] Valovič M et al 2015 Nucl. Fusion 55 013011, [4] Valovič M et al 2018 Plasma Physics Contr.
Fusion to be submitted, [5] Polevoi A R et al 2017 Nucl. Fusion 57 022014
