Minority Ion Cyclotron Resonance Heating in H-Mode in Presence of the ITER-Like Wall in JET

An ITER-like wall (ILW) of Beryllium and a Tungsten divertor were installed in JET in 2012 and a first series of experiments in presence of this wall was performed at modest power and thus in L-mode that same year. The logical focus in the 2013 JET campaign was the characterization of high power H-mode operation. One of the critical issues to be addressed was whether the influx into the main plasma of high-Z materials coming from plasma facing components can be kept low enough so that reaching fusion-relevant temperatures is not compromised. In the past L-mode experiments, it was shown that ion cyclotron resonance heating (ICRH) or radio frequency (RF) waves tend to give rise to increased impurity sources from the main chamber, the reason believed to be that ions can be accelerated to high energies in the sheaths that form close to metallic surfaces ­ and antennas in particular ­ in contact with a plasma and give rise to increased sputtering of wall material. Although the increased level of ensuing radiation was seen as a bonus for safe operation rather than as a disadvantage (as it allowed to harmlessly radiate away a significant fraction of the power that otherwise is deposited on the divertor), one crucial question to be answered is if high RF power is beneficial for sustaining the H-mode. Just like for the earlier L-mode experiments, Hydrogen minority fundamental cyclotron heating in a Deuterium plasma was chosen as the wave heating scheme. Past experiments with the carbon wall have shown that centrally deposited RF power is able to reduce or annihilate the influx of impurities to the plasma core. Although further confirmation is needed, a series of JET studies seeking optimization of ICRH for W control suggests this is still the case in presence of the ILW.
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