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This paper presents a detailed design of the Q-shu University experimental steady state spherical tokamak’s (QUEST’s) high-field side (HFS) injection system for electron Bernstein wave (EBW) excitation and the results of an experimental comparison of the HFS eXtraordinary X-mode and low-field side (LFS) ordinary O-mode injection of 8.2 GHz radio frequency (RF) power. Waveguides, as an alternative to mirror polarizers for transmitting RF X-mode power from LFS to HFS for EBW conversion, were used instead of the installation of an RF mirror. Testing of LFS-to-HFS RF power transmission at 8.2 GHz, using an RG-50-type vacuum waveguide in a bench-scale device filled with SF6 gas at 0.03 Mpa, revealed that an RF power of 10.8 kW could traverse the fundamental electron cyclotron resonance layer for 60 s without breakdown. The short-length, open-ended waveguide antenna used in the HFS injection-induced wave diffraction reduced the efficiency of power delivery to the upper hybrid resonance (UHR) by approximately 7% at an electron temperature of 50 eV. The HFS injection was able to produce brighter camera images than the standard LFS injection. The location of the UHR, as estimated by measuring the density with an interferometer, agreed with its location as measured by plasma radiation low-field, side-edge positions shown by fast camera imaging. This indicates that the plasma was produced by mode-converted EBW. The HFS injection had an absorption efficiency of 96%, compared to 40% for LFS. A greater fluctuation of floating potential adjustable to the lower hybrid wave (LHW) was observed in the HFS case by installing a Langmuir probe, confirming that EBW conversion efficiency was higher in the HFS case. Moreover, after setting the poloidal field to BPF = 7.6 mT, plasma current (IP) in the HFS peaked at 1.3 kA, as opposed to 0.3 kA for LFS, despite LFS injection having a total power of 55 kW, compared to 40 kW for HFS. However, as the impurity level was comparatively high, it is believed that this IP is dominated by pressure-drive, which makes it difficult to analyze EBWCD. Finally, the line-integrated density in the HFS injection peaked at 1.6 × 1018 m−2, compared to 8 × 1017 m−2 in the LFS one. [ABSTRACT FROM AUTHOR]

Ion cyclotron emission (ICE) is probably excited by non-thermal ion velocity distribution. Identifying characteristics of the ion velocity distribution that excites the ICE can contribute to understanding its emission mechanism. The characteristics of fast helium-3 (3He) ion velocity distribution that excites 3He ICE [ICE(3He)] on JT-60U are investigated. First, fast deuterium ion distributions, which mainly contribute to deuterium–deuterium fusion reactions, are calculated with a fast ion orbit following Monte-Carlo code (OFMC code) under realistic conditions to evaluate birth spatial and velocity distributions of the 3He ions. Then, the fast 3He ion distributions are evaluated with the OFMC code using the birth distributions and compared between cases when the ICE(3He) is observed and not. The evaluated fast 3He ion distributions at the midplane edge of the plasma on the low field side have strong pitch-angle anisotropy in both cases. In the case with the ICE(3He) excitation, a relatively peaked bump-on tail structure in the energy direction is formed in the distributions. The formation of the relatively peaked bump-on tail structure is localized at the plasma edge on the low field side. On the other hand, the distributions at the plasma edge on the low field side have a broader structure in the energy direction in the case without the excitation. The comparison results show that the formation of the relatively peaked bump-on tail structure in the fast 3He ion distribution makes a key contribution to the excitation of the ICE(3He) on JT-60U. [ABSTRACT FROM AUTHOR]