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.
Highlights
Optical emission spectroscopy (OES) is used on JET ITER-Like Wall (JET-ILW) to non-intrusively study RF sheath-enhanced plasma-surface interactions (RF-PSI) in ICRF heated discharges
The aim of this paper is to provide first analyses of the data obtained from the new experimental sessions, with emphasis on the correlation between the optical emission amplification at the observation points, which include points on a limiter near the ITER-like antenna (ILA), to regions of high parallel components of the near-field (E//) at the active, magnetically connected antenna
The two important features of these responses are: (1) they both decrease to nearly the Ohmic level when the magnetic fieldline connection gets beyond the poloidal range of the limiter(s) and (2) the relative increase in the emission is similar for the two different antenna technologies
Summary
Optical emission spectroscopy (OES) is used on JET ITER-Like Wall (JET-ILW) to non-intrusively study RF sheath-enhanced plasma-surface interactions (RF-PSI) in ICRF heated discharges. In the early stages of JET-ILW (2011-12) dedicated JET pulses, combining sequential toggling of all available ICRH antennas with concurrent magnetic pitch-angle sweeping, indicated that these RF-PSIs can be traced, via magnetic field-line connection, to regions of anticipated high E// near-field at an active antenna [1 - 3] Such effects are understood as arising from local rectification of these fluctuating near-fields by the plasma near the local limiters, with the resulting potential acting on the observed (remote) limiter via magnetic connection. The aim of this paper is to provide first analyses of the data obtained from the new experimental sessions, with emphasis on the correlation between the optical emission amplification at the observation points, which include points on a limiter near the ILA, to regions of high parallel components of the near-field (E//) at the active, magnetically connected antenna.
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