Direct observation of transition to electron Bernstein waves from electromagnetic mode by three mode-conversion scenarios in the dipole confinement torus plasma

Uchijima, K.; Takemoto, T.; Morikawa, Junji; Ogawa, Yuichi

doi:10.1088/0741-3335/57/6/065003

Cited by 12 publications

(7 citation statements)

References 15 publications

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“…The slow X-mode will then reach the UHR and excite the EBW. Like the O-X-B mode-conversion, the X-B mode-conversion has been welldemonstrated experimentally [32,33,34]; unlike the O-X-B mode-conversion, however, the X-B mode-conversion is difficult to study analytically, and is typically studied computationally with particle-in-cell methods [35,36,37] or full-wave solvers [38,39]. This trend can probably be attributed in large part to the inherent non-locality of the X-B mode-conversion, which will be elucidated in the following section.…”

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confidence: 99%

Mode-conversion of the extraordinary wave at the upper hybrid resonance in the presence of small-amplitude density fluctuations

Lopez

Ram

2018

Plasma Phys. Control. Fusion

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In spherical tokamaks, the electron plasma frequency is greater than the electron cyclotron frequency. Electromagnetic waves in the electron cyclotron range of frequencies are unsuitable for directly heating such plasmas due to their reduced accessibility. However, mode-conversion of the extraordinary wave to the electron Bernstein wave (X-B mode-conversion) at the upper hybrid resonance makes it possible to efficiently couple externally-launched electromagnetic wave energy into an overdense plasma core. Traditional mode-conversion models describe an X-mode wave propagating in a potential containing two cutoffs that bracket a single wave resonance. Often, however, the mode-conversion region is in the edge, where turbulent fluctuations and blobs can generate abrupt cutoffs and scattering of the incident X-mode wave. We present a new framework for studying the X-B mode-conversion which makes the inclusion of these fluctuations analytically tractable. In the new approach, the highfield cutoff is modelled as an infinite barrier, which manifests as a boundary condition applied to a wave equation involving only one cutoff adjacent to the resonance on the low-field side. The new model reproduces the main features of the previous approach, yet is more suitable for analyzing experimental observations and extrapolating to higher dimensions. We then develop an analytical estimate for the effect of small-amplitude, quasi-monochromatic density fluctuations on the X-B mode-conversion efficiency using perturbation theory. We find that Bragg backscattering of the launched X-mode wave reduces the mode-conversion efficiency significantly when the fluctuation wavenumber is resonant with the wavenumber of the incident X-mode wave. These analytical results are corroborated by numerically integrating the mode-conversion equations.to their compact size and attractive stability properties. However, since STs typically operate in an overdense regime, it is difficult to use traditional EC waves on STs for heating and current drive purposes.Fortunately, the electron Bernstein wave (EBW) provides a method to heat an overdense plasma in the EC frequency range. The EBW[10, 11] is a thermal mode which exists in the vicinity of the upper hybrid resonance (UHR) and the harmonics of the electron cyclotron resonance. It freely propagates within discrete frequency bands [12,13], which can be roughly mapped into discrete spatial bands for an inhomogeneous plasma. Importantly, the EBW which originates at the UHR propagates unimpeded towards larger magnetic field until it reaches the nearest cyclotron resonance, where it will damp strongly [14,15,16]. This feature makes the EBW ideal for heating overdense plasmas.Being a thermal mode of predominantly electrostatic polarization, the EBW does not propagate in vacuum; the excitation of the EBW via external means is non-trivial. Grills are not typically used due to the small grid spacing required to excite a wave whose wavelength is comparable to the electron gyroradius [11]. Instead, experiments often us...

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confidence: 99%

Mode-conversion of the extraordinary wave at the upper hybrid resonance in the presence of small-amplitude density fluctuations

Lopez

Ram

2018

Plasma Phys. Control. Fusion

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“…As another possible heating scenario, the mode conversion to electron Bernstein wave can access to the high density region beyond the cut off density. Although the possibility has been studied in the mini-RT [6], further studies are required to explain the over dense results of the RT-1.…”

Section: High Energy Electron Core In Dipole Confinement Systemmentioning

confidence: 99%

“…The Mini-RT with a compact levitation ring magnet operated from 2003 to 2018. The recent study was focused on the excitation of electron Bernstein wave to produce a high-density plasma in low magnetic fields [6]. The ring trap 1 (RT-1) device experiments subsequently started in 2006 [7,8].…”

Section: Introductionmentioning

confidence: 99%

Experimental analysis of self-organized structure and transport on the magnetospheric plasma device RT-1

Nishiura¹,

Yoshida²,

Kinoshita³

et al. 2019

Nucl. Fusion

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The dipole plasma exhibits strong heterogeneities in field strength, density, temperature, and other parameters, while maintaining the holistic balance. Enquiring into the internal structures, we reveal the fundamental self-organizing mechanisms operating in their simplest realization (as commonly observed in astronomical systems). Three new findings are reported from the RT-1 experiment: Creation of a high-energy electron core (similar to the radiation belts in planetary magnetospheres) is observed for the first time in a laboratory system. Highenergy electrons (3 -15 keV), produced by an electron cyclotron heating (ECH), accumulate in a "belt" located in the low-density region (high-beta value ~ 1 is obtained by increasing the high-energy component up to 70% of the total electrons). The dynamical process of the "uphill diffusion" (a spontaneous mechanism of creating density gradient) has been analyzed by perturbing the density by gas injection. The spontaneous density formation in laboratory 2 magnetosphere eluciates the self-organized plasma transport relevant to planetary magnetosphere. The coherence-imaging spectroscopy visualized the two dimensional profiles of ion temperature and flow velocity in the ion cyclotron resonance frequency (ICRF) heating.The ion temperature and flow were enhanced globally, and particularly along the magnetic field lines near the levitation magnet. These results advance our understanding of transport and self-organization not only in dipole plasmas, but also in general magnetic confinement systems relevant to fusion plasmas.

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“…Podoba et al demonstrated conversion from an O-wave to an X-wave by probe measurements of amplitude and phase of the wave field in the WEGA stellarator [11]. Uchijima et al conducted direct measurement experiments on the mode conversion to EBW in the Mini-RT by three mode-conversion scenarios, but the measured EBW wavelength is about one-order larger than the theoretical one [21].…”

Section: Introductionmentioning

confidence: 99%

Direct measurement of electron Bernstein waves in Low Aspect ratio Torus Experiment (LATE)

Guo¹,

ASHIDA²,

Noguchi³

et al. 2022

Plasma Phys. Control. Fusion

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By using a newly developed five-pin probe antenna and two-dimensional mechanical probe driving system, the 2-D wave pattern of phase and amplitude has been directly measured in Low Aspect ratio Torus Experiment (LATE), for an overdense ECR plasma with microwave obliquely injected. In the case of O-mode injection, an EBW-like wave pattern has been detected for the first time, in a localized region near the upper hybrid resonance layer. The pattern has a short wavelength of about 2 mm and is also electrostatic and backward. In the case of X-mode injection, the 2-D wave pattern is quite different and no EBW signal can be observed. By adjusting the toroidal magnetic field in O-mode injection, it is found that both the position and size of the EBW region have changed, which suggest the localized condition of efficient O-X-B conversion and high collisional damping rate of EBWs in these experiments.

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Direct observation of transition to electron Bernstein waves from electromagnetic mode by three mode-conversion scenarios in the dipole confinement torus plasma

Cited by 12 publications

References 15 publications

Mode-conversion of the extraordinary wave at the upper hybrid resonance in the presence of small-amplitude density fluctuations

Mode-conversion of the extraordinary wave at the upper hybrid resonance in the presence of small-amplitude density fluctuations

Experimental analysis of self-organized structure and transport on the magnetospheric plasma device RT-1

Direct measurement of electron Bernstein waves in Low Aspect ratio Torus Experiment (LATE)

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