HFS Injection of X-Mode for EBW Conversion in QUEST

Elserafy, Hatem; Hanada, K.; Kuroda, Kengoh; Idei, H.; Yoneda, Ryota; Huang, Chongxiang; Kojima, Shinichiro; Hasegawa, Makoto; Nagashima, Yoshihiko; Onchi, Takumi; Ikezoe, Ryuya; Higashijima, A.; Nagata, T.; Shoji, K.; Shimabukuro, Shun; Bertelli, N.; Ono, M.

doi:10.1585/pfr.14.1205038

Cited by 4 publications

(2 citation statements)

References 12 publications

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“…To verify this, the locations of the low-field-side-edge of the plasma radiation in figures 8(a) and (b) were monitored. Assuming that those edges are the UHRL positions, the corresponding density was calculated based on the deviation from the ECRL position [48], showing that the density for HFS is 7×10 17 m −3 , and the density for LFS is 4×10 17 m −3 . To verify the assumption that the plasma edges are the locations of the UHRL, the interferometer density was measured showing a HFS density of 7.5×10 17 m −3 and an LFS one of 4×10 17 m −3 .…”

Section: Hfs Injection Experiments In Questmentioning

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: Hfs Injection Experiments In Questmentioning

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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“…The O-X-B mode conversion scenario of the low-field side injection has been considered for EBWHCD experiments with the 8.2 GHz wave in the QUEST [10]. Recently, X-B mode conversion experiments of high-field injection have begun in the QUEST [11].…”

Section: Introductionmentioning

confidence: 99%

Electron heating of over-dense plasma with dual-frequency electron cyclotron waves in fully non-inductive plasma ramp-up on the QUEST spherical tokamak

et al. 2019

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A 28 GHz system with a high-power gyrotron tube has been used for the QUEST spherical tokamak to form an over-dense plasma for electron Bernstein wave (EBW) heating and current drive with an 8.2 GHz-wave. Non-inductive high-density plasma ramp-up experiments with dual-frequency (dual-f ) electron cyclotron (EC) (8.2 GHz and 28 GHz) waves were conducted. A spontaneous density jump (SDJ) to an over-dense state was first observed as a bifurcation phenomenon in the dual-f wave experiment. The over-dense plasma on the 8.2 GHz wave was non-inductively ramped up to 25 kA, and was maintained for 0.4 s under stable plasma equilibrium after two such jumps in one shot. Heating to mildly energetic electrons and bulk electrons was observed even in the over-dense region. The electrostatic EBW heating effect on the mildly energetic electrons in the over-dense region is assessed following a dispersion analysis of the 8.2 GHz wave. The bulk electron heating effect observed is explained as heat exchange from mildly energetic electrons heated by the electrostatic EBW. Remarkably, a high hard-x-ray-radiation temperature of ∼500 keV was also observed in tangential viewing for current-carrying electrons in the over-dense core region. Synergetic heating from the overlap of different 28 GHz EC harmonic resonances as well as higher harmonic heating is discussed for maintaining the highly energetic electrons in the over-dense core region. In addition, the SDJ process and mechanism are considered based on the discussion of the electron heating effects with the 8.2 GHz wave.

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Heating characteristic of electron Bernstein wave using high-field side X-mode injection in QUEST

Murakami,

Kojima,

Hanada

et al. 2024

Plasma Phys. Control. Fusion

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A comparative investigation was conducted to assess the impact of high-field side (HFS) extraordinary (X) mode (HFS X) injection and low-field side (LFS) ordinary (O) mode (LFS O) injection of 8.2 GHz radio frequency (RF) power in the Q-shu University experimental steady-state spherical tokamak (QUEST). In the case of the HFS X injection, the ratio of electron plasma frequency and RF frequency f pe/f RF was 1.3, indicating an overdense plasma, whereas in the LFS O injection, this ratio was 0.9. Distinctive features emerged in the electron temperature and density profiles between the two injection scenarios. In the HFS X injection, the profiles peaked between the fundamental electron cyclotron resonance (ECR) layer and the upper hybrid resonance layer. Conversely, in the LFS O injection, the peaks were situated near the 1st ECR layer. This implies effective excitation of the electron Bernstein wave (EBW) in the case of the HFS X injection, with the wave power being absorbed through the doppler-shifted ECR of the EBW. Density and temperature oscillations were observed only in the start-up phase of the HFS X injection, which may correlate with the presence of the left-hand cutoff of the X-mode. These findings will be contributed to understand the distinctive characteristics associated with plasma heating through EBW excitation in the HFS X injection.

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HFS Injection of X-Mode for EBW Conversion in QUEST

Cited by 4 publications

References 12 publications

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

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

Electron heating of over-dense plasma with dual-frequency electron cyclotron waves in fully non-inductive plasma ramp-up on the QUEST spherical tokamak

Heating characteristic of electron Bernstein wave using high-field side X-mode injection in QUEST

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