58th Annual Meeting of the APS Division of Plasma Physics
Volume 61, Number 18
Monday–Friday, October 31–November 4 2016;
San Jose, California
Session GI2: MFE: Pedestal & Impurities
9:30 AM–12:30 PM,
Tuesday, November 1, 2016
Room: 210 CDGH
Chair: Howard Wilson, University of York
Abstract ID: BAPS.2016.DPP.GI2.3
Abstract: GI2.00003 : On the recovery of pedestal temperature of JET-ILW plasmas with injection of low-Z impurities
10:30 AM–11:00 AM
Preview Abstract
Abstract
Author:
Carine Giroud
(CCFE)
The pedestal confinement has significantly decreased in JET with its
metallic ITER-like wall with reference to the carbon wall phase of JET
(JET-C). A reduction in pedestal temperature is observed in all scenarios
regardless of the level of D-gas injection or value of $\beta
_{\mathrm{N}}$. In particular, the JET-ILW 2.5MA/2.7T high-$\delta $
($\delta =$0.4) plasmas at n$_{\mathrm{ped}}$/n$_{\mathrm{GW}}\ge $0.7,
discharges most comparable with JET-C, the pedestal pressure has reduced by
40{\%} with a decrease in pedestal temperature from 0.9keV to 0.5keV with
the change of wall. The pedestal stability has been modified with the new
wall: the reference JET-C plasmas pedestals had an operational point in the
corner of the Peeling-Ballooning (PB) diagram, with pressure limited by
intermediate n-numbers (n$=$5-20), whereas the JET-ILW unseeded plasmas have
a lower pressure gradient limited by high n-numbers $\ge $70 (ballooning
modes). Seeding N, a low-Z impurity, almost recovers the thermal stored
energy, pedestal pressure and pedestal temperature to JET-C levels and with
an operation point in the corner of the PB diagram. The mechanisms linked to
the pedestal recovery with N are likely related to the mechanisms leading to
a decrease in pedestal temperature in the absence of C in the plasma
composition. The improved pedestal stability with N is not solely linked to
the ideal linear PB stability since N-seeded plasmas in JET-ILW can be in
type-III ELM regime and have a higher pedestal pressure than unseeded type-I
ELMy H-mode. An increased pedestal pressure via an inward movement of the
pedestal pressure from the separatrix is not observed with N seeding.
However, we have identified two mechanisms responsible. A first initial
mechanism linked to the change in ELM energy losses which raises modestly
the average global beta by 10{\%} but allows in return a second mechanism to
take place. The considered high-$\delta $ plasmas can then benefit, if in
type-I ELM regime, from the virtuous cycle (2nd mechanisms) of an increased
Shafranov shift, higher pedestal pressure allowing increased core pressure.
The operational point can climb towards the corner of the PB diagram. The
1$^{\mathrm{st\thinspace }}$mechanism which reduces the average ELM energy
losses has to be identified but seems to be linked to the SOL/separatrix
conditions. The effects of ion diamagnetic drift and plasma rotation on the
stability of high-n ballooning modes are being investigated.
To cite this abstract, use the following reference: http://meetings.aps.org/link/BAPS.2016.DPP.GI2.3