Speaker
Pascal Devynck
Description
See the full Abstract at http://ocs.ciemat.es/EPS2018ABS/pdf/P4.1035.pdf
JET ITER-like and JET Carbon wall discharges:
2 different scaling for the frequency of type I ELMs.
Consequences for the pedestal structure.
P. Devynck1 and JET Contributors*
EUROfusion Consortium, JET, Culham Science Centre, Abingdon, OX14 3DB, UK
1
CEA, IRFM, F-13108 St. Paul-lez-Durance cedex, France
We compare the results of the scaling previously obtained for the frequency of type I ELMs in
JET ITER-like wall ( ILW) with new results in JET carbon wall obtained over a database of
270 shots. The frequency scaling in JET ILW has been found to be a linear function of
Np/dNp [1] where Np is the pedestal density and dNp is the density drop of the pedestal after
the ELM crash. We find that in JET carbon wall discharges; this is no more the case. The
frequency scaling is a linear function of Psep/dWp, where Psep is the power through the
separatrix and dWp is the energy drop after the ELM crash. Such a result is in agreement with
what has been found in the literature [2].
We propose a simple interpretation for these two different scaling. After the ELM
crash, energy and density are expelled from the plasma and must be recovered before the next
ELM crash can be triggered. In both cases, the recovery is the result of an imbalance between
source terms and outward transport. For the recovery of the energy, the source term is the
additional heating (Psep) while for the density recovery the source term is provided by the
neutrals fueling the plasma. These two processes occur on different time scales and the
slowest one will set the frequency scaling of the ELMs.
In the case of JET carbon wall, it is speculated that the wall releases during the
discharge a very high flux of trapped particles towards the plasma providing a strong fueling.
The density is faster than the energy recovery and one must wait for the additional heating to
rise the temperature to the threshold pressure gradient. In the case of JET ILW, the situation is
reversed. The wall does not outgas anymore and only the recycling and gas fueling are
available to feed the plasma. The top pedestal temperature saturates and one must wait for the
density to reach the peeling ballooning threshold pressure gradient.
We expect the competition between these two processes to have some consequences
for the pedestal structure. In carbon wall, the pedestal density will reach first the maximum
allowed by the instabilities and a lower temperature will be sufficient to reach the threshold
pressure gradient for the ELM crash. In ILW, it will be the opposite. As a result for the same
pedestal pressure and pressure gradient, density and temperature are expected to be different
in the two machines. A very simplified simulation of the competition between these two
processes, allows recovering this behavior. Additionally we show that the coupling between
these two nonlinear processes through the pressure can generate in some cases the appearance
of several mixed frequencies for the ELMs.
[1]P. Devynck et al. Plasma Phys. Control. Fusion 58 (2016) 125014 (9pp)
[2] H. Zhom, Plasma Phys. Control. Fusion 38 (1996) 105–128
• See the author list of X. Litaudon et al., Nuclear Fusion 57, 10 (2017)
