Role of the antenna screen angle during ICRF heating in JET

Abstract

Two comparative heating experiments were carried out to test the role of the antenna screen angle during high power ICRF heating in JET. In the first experiment the magnetic field was practically aligned with the screen elements, whereas in the second experiment the toroidal field was reversed and the angle between the elements and the field was 25°. There were clear differences in the impurity influx, the heating efficiency and the coupling resistance between discharges with normal field and discharges with reversed field. The results indicate the impact of the Faraday screen design on the levels of RF enhanced metal influxes originating at the screen. This should be of particular relevance for the ICRF antennas in a reactor where, in contrast to small devices, most of the interaction between the RF fields and the plasma edge is expected to take place around the antenna structures because of the highly localized RF field patterns. A numerical code based on three-dimensional single-pass and full wave calculations of antenna-plasma coupling, including an arbitrary angle between the screen elements and the magnetic field, can qualitatively reproduce the observed difference in the coupling resistance. The experimental results are discussed with regard to possible mechanisms that can account for the observed differences in the two cases studied.

On the parasitic absorption in FWCD experiments in JET ITB plasmas

“…Earlier experiments in JET, using the A1 antennas with large misalignment angles of 22˚, showed small losses with the dipole phasing [16] but strong degradation of the heating with the monopole [16,17] which was consistent with losses at rectified RF-sheath potentials at the Faraday screens [17]. The power losses in these experiments were found to be proportional to the coupled power.…”

“…Such strong degradation had not been seen for the A1 antennas under normal operating conditions [15]. However, strong reduction in the heating was seen with the A1 antennas for monopole phasing with large misalignment angles between the magnetic field lines and the Faraday screens in front of the antenna straps [16], giving rise to large impurity radiation by nickel [16] and beryllium [17]. The increased impurity accumulation was explained by sputtering caused by the presence of rectified RF-sheath potentials at the Faraday screens in front of the antennas [17,18].…”

High-frequency B-dot probes used to detect characteristics of ion cyclotron range of frequency waves in EAST

“…In H-mode discharges, a different distribution of the phase difference is observed compared to the L-mode discharges. There are at least two reasons which can account for the domain parallel wavenumbers changing, namely (i) the profile of the antenna current along the conductor (Bures et al 1990); (ii) the distance between the antenna and plasma (the toroidal variation of the current density can be rewritten in a plasma σ plasma ϕ (ϕ) = σ ϕ (ϕ)e −k // d , with d is the antenna-plasma distance (Savona 2017)). The distance between the plasma and antenna and the profile of the antenna current along the conductor vary strongly during edge localized modes (ELMs) in H-mode discharges.…”

Formation of a Charged Layer in a Bounded Non-Uniform Magnetized Plasma in RF Field

“…After the re-evaluation of the role of FS [9], a number of experiments with unshielded antennas was fulfilled on different tokamaks [10][11][12][13][14][15][16][17][18]. Most of them showed the successful operation of an unshielded ICRF antenna, others gave a degradation of heating parameters.…”