RF Sheath-Enhanced Plasma Surface Interaction Studies using Beryllium Optical Emission Spectroscopy in JET ITER-Like Wall

^a Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6169, USA
^b IRFM CEA Cadarache 13108 Saint Paul lez Durance France
^c LPP-ERM/KMS, Royal Military Academy, 30 Avenue de la Renaissance B-1000, Brussels, Belgium
^d Max-Planck-Institut für Plasmaphysik, D-85748 Garching, Germany
^e CCFE, Culham Science Centre, Abingdon, Oxfordshire OX14 3DB, UK

^* Corresponding author: kleppercc@ornl.gov

Published online: 23 October 2017

Abstract

A dedicated study on JET-ILW, deploying two types of ICRH antennas and spectroscopic observation spots at two outboard, beryllium limiters, has provided insight on long-range (up to 6m) RFenhanced plasma-surface interactions (RF-PSI) due to near-antenna electric fields. To aid in the interpretation of optical emission measurements of these effects, the antenna near-fields are computed using the TOPICA code, specifically run for the ITER-like antenna (ILA); similar modelling already existed for the standard JET antennas (A2). In the experiment, both antennas were operated in current drive mode, as RF-PSI tends to be higher in this phasing and at similar power (∼0.5 MW). When sweeping the edge magnetic field pitch angle, peaked RF-PSI effects, in the form of 2-4 fold increase in the local Be source,are consistently measured with the observation spots magnetically connect to regions of TOPICAL-calculated high near-fields, particularly at the near-antenna limiters. It is also found that similar RF-PSI effects are produced by the two types of antenna on similarly distant limiters. Although this mapping of calculated near-fields to enhanced RF-PSI gives only qualitative interpretion of the data, the present dataset is expected to provide a sound experimental basis for emerging RF sheath simulation model validation.