Abstract
During high-power, ion cyclotron resonance heating (ICRH), RF sheath rectification
and RF induced plasma-wall interactions (RF-PWI) can potentially limit long-pulse
operation. With toroidally-spaced ICRH antennas, in an ITER-like wall (ILW)
environment, JET provides an ideal environment for ITER-relevant, RF-PWI studies.
JET pulses combining sequential toggling of the antennas with q95 (edge safety
factor) sweeping were recently used to localize RF-enhanced Be I and Be II spectral
line emission at outboard poloidal (beryllium) limiters. These measurements were
carried out in the early stages of JET-ILW and in ICRF-only, L-mode discharges. The
appearance of enhanced emission spots was explained by their magnetic connection
to regions of ICRH antennas associated with higher RF-sheath rectification [1]. The
measured emission lines were the same as those already qualified in ERO modelling
of inboard limiter beryllium erosion in JET limiter plasmas [2].
In the present work, we revisit this spectroscopic study with the focus on obtaining
estimates of the impact of these RF-PWI on sputtering and on net erosion of the
affected limiter regions. To do this, the ERO erosion and re-deposition code [2] is
deployed with the detailed geometry of a JET outboard limiter. The effect of RF-PWI
on sputtering is represented by varying the surface negative biasing, which affects
the incidence energy and the resulting sputtering yield. The observed variations in
line emission, from [1], for JET pulse 81173 of about factor 3 can be reproduced with
~ 100 – 200 V bias. ERO simulations show that the influence of the respective E-field
on the local Be transport is localized near the surface and relatively small. Still, the
distribution of the 3D plasma parameters, shadowing and other geometrical effects
are quite important. The plasma parameter simulated by Edge2D-EIRENE [3] are
extrapolated towards the surface and mapped in 3D.
These initial modelling results are consistent with the range of potentials anticipated
through RF sheath rectification (see, e.g., [4]). Shortcomings from both the modelling
and experimental side will be discussed, as will be plans for improvements in both
areas method for the upcoming 2015 - 2016 JET campaign.
[1] C.C. Klepper et al., J. Nucl. Mater. 438 (2013) S594–S598
[2] D. Borodin et al., Phys. Scr. T159 (2014) 014057
[3] M. Groth et al., Nucl. Fusion 53 (2013) 093016
[4] Jonathan Jacquot et al., Phys. Plasmas 21 (2014) 061509
*Corresponding author: presently at CCFE (UK) tel.: +44 1235 46 4304, e-mail: kleppercc@ornl.gov
**See the Appendix of F. Romanelli et al., Proc. of the 25th IAEA Fusion Energy Conference 2014,
Saint Petersburg, Russia
†Work supported, in part, by US DOE under Contract DE-AC05-00OR22725 with UT-Battelle, LLC.