Characterization of the mutual influence of Ion Cyclotron and Lower Hybrid Range of frequencies systems on EAST

Abstract

Abstract. Waves in the Ion Cyclotron (ICRF) and Lower Hybrid (LH) Range of Frequencies are efficient techniques respectively to heat the plasma and drive current. Main difficulties come from a trade-off between good RF coupling and acceptable level of impurities release. The mutual influence of both systems makes such equilibrium often hard to reach [1]. In order to investigate those interactions based on ScrapeOff Layer (SOL) plasma parameters, a new reciprocating probe was designed allying a three tips Langmuir probe with an emissive wire. The emissive filament provides a precise measure of plasma potential [2], which can be used to calibrate Langmuir probe's results. This paper reports on experimental results obtained on EAST, where there are two ICRF antennas and two LH launchers. Among others diagnostics, the new reciprocating probe enabled to evidence the deleterious influence of ICRF power on LHWs coupling in Lmode plasmas. In areas connected with an active ICRF antenna, SOL potentials increase while densities tend to decrease, respectively enhancing impurities release and deteriorating LHWs coupling. This phenomenon has mostly been attributed to RF sheath; the one that forms on top of Plasma Facing Components (PFCs) and causes ExB density convections [3]. From those experiments it seems ICRF has a strong influence on magnetically connected areas, both in the near field -influencing ICRF waves coupling -and in farther locations such as in front of LH grills. Moreover, influence of ICRF on LH system was observed both in L and H modes. Those results are consistent with RF sheath rectification process. Concerning the influence of LHWs on ICRF coupling, nothing was observed in L-mode. Besides during Hmode experiments, LHWs have been identified as having a mitigating effect on ELMs [4], which on average lowers the pedestal, increasing edge densities to the profit of ICRF waves coupling.

Optimization of discharges with ion cyclotron range of frequencies using local gas injection in EAST

“…10). Known from precedent studies [25,26,27], strong local density convections due to local × drifts affect this region when the I-port antenna is active, which is for instance visible on the significant differences between dashed and solid curves of Fig. 11; higher potential gradients and consequently lower densities are measured when the I-port antenna is powered.…”

“…7. This is believed to be the consequence of the B-port-induced near fields at the origin of × density convection away from the antenna [24,25]. In the case of gas puffed on the J-port though, as the valve is very far and not magnetically connected to the B-port, high density values are representative of a global increase.…”

“…Langmuir probes were fixed at different poloidal positions from the mid-plane to the top left corner of the antenna (figure 3). The protective head design was optimized to mitigate sensitivity to the RF sheath by constraining the plasma to be sampled by lateral holes whose inner surface is approximately parallel to the magnetic field lines, preventing large sheath expansion [24]. Despite those design efforts to reduce sensitivity to the RF sheath, ongoing-upgrades are still needed to improve the probe's data in the presence of ICRH.…”

“…Special effort was consequently put into optimizing the design of the three probes fixed to the top of the antenna. Inspired by tunnel probes [21] we added protective head that constrains the tips to sample the plasma from lateral holes, the area of which is less sensitive to sheath expansion than if the tip were entirely exposed to the plasma (figures 3(a) and (b)) [24]. Furthermore, such a design offers the advantages of being much more robust as tips do not get eroded as fast as when directly exposed to the plasma, and the data reliability is also improved as only a small part of the tip is exposed, greatly diminishing global heat loads and expected to prevent occurrence of secondary electron emission.…”

Metallic impurity content behavior during ICRH-heated L-mode discharges in EAST

“…1 and Fig. A3) [1,3,23]. The power lost in the transmission line is not measured experimentally, however calculation of the RF losses show that they are always less than 15%, resulting from uncertainties on power estimations detailed in [24].…”