High-Field-Side RF Injection for Excitation of Electron Bernstein Waves

Yoneda, Ryota; Hanada, K.; Elserafy, Hatem; Bertelli, N.; Ono, M.

doi:10.1585/pfr.13.3402115

Cited by 3 publications

(2 citation statements)

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“…As electrostatic waves, EBW are immune to the effects of overly dense plasmas, which can form RF cutoff layers that prevent the further enhancement of plasma density through RF injection. Potential mode conversion scenarios include ordinary-extraordinary-Bernstein (O-X-B) [12,13], direct X-B mode conversion from the low-field side (LFS) [14], and X-B mode conversion from the high-field side (HFS) [15]. A number of explanations have been given as to how to achieve EBW excitation in principle [16][17][18] through simulation [19] and experimentally in both STs and stellarators [20][21][22][23][24][25].…”

Section: Introductionmentioning

confidence: 99%

Electron Bernstein wave conversion of high-field side injected X-modes in QUEST

Elserafy

Hanada

Kojima

et al. 2020

Plasma Phys. Control. Fusion

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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 SF 6 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 modeconverted 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 B PF =7.6 mT, plasma current (I P ) 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 I P 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×10 18 m −2 , compared to 8×10 17 m −2 in the LFS one.

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Section: Introductionmentioning

confidence: 99%

Electron Bernstein wave conversion of high-field side injected X-modes in QUEST

Elserafy

Hanada

Kojima

et al. 2020

Plasma Phys. Control. Fusion

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“…One is the LFS injection of O-mode, which is converted first to X-mode in plasma, then to EBW after reaching upper hybrid resonance (UHR). These conversions are known as the O-X-B scenario [7]. Another scenario is the LFS injection of X-mode that tunnels the cutoff layer, and converts to EBW after reaching the UHR [8].…”

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confidence: 99%

HFS Injection of X-Mode for EBW Conversion in QUEST

Elserafy

Hanada

Kuroda

et al. 2019

Plasma and Fusion Research

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High field side (HFS) injection of eXtraordinary X-mode for electron Bernstein wave (EBW) conversion was conducted in the QUEST tokamak. Radio frequency (RF; 8.2 GHz) power was delivered from the low field side (LFS) to the high field side HFS through waveguides, and from the HFS placed 20 cm above the midplane of the vacuum vessel. The aim was to compare the RF launches from the LFS and HFS. The plasma brightness, measured by a fast camera, as well as the H α signal captured along the mid-plane, was noticeably higher in the HFS launch than in the LFS launch. The HFS injection achieved a plasma current of approximately 130 A, versus 35 A in the LFS injection. The electron density n e predicted from the position of the upper hybrid resonance agreed with the line-averaged n e measured by an interferometer, confirming the effective conversion and subsequent damping of the EBW mode. The RF leakage of the HFS injection was less than one-sixth that of the LFS injection. These results indicate that HFS delivers better RF coupling and conversion efficiency to EBW than LFS injection. Such efficient plasma heating via EBW will significantly enhance the plasma production.

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