Electron Bernstein Wave Emission from an Overdense Plasma at the W7-AS Stellarator

Laqua, H. P.; Hartfuss, H. J.

doi:10.1103/physrevlett.81.2060

Cited by 73 publications

(59 citation statements)

References 4 publications

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“…An important aspect of the mode-converted EBW emission is the degree of radial localization of the source of the emission. This was determined using a technique similar to that of Laqua et al 10 .…”

Section: Ebw Measurementsmentioning

confidence: 99%

“…The slow X-mode then couples directly to the electromagnetic O-mode. This process, originally proposed by Preinhaelter and Kopecky 9 for electron cyclotron resonance heating, was successfully used by Laqua et al 10 to measure EBW emission on the W7-AS stellarator. Figure 2 illustrates the typical frequency ranges relevant to our mode-converted EBW emission measurements on CDX-U, for the B o = 2.1 kG plasmas presented here.…”

Section: Introductionmentioning

confidence: 99%

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Electron Bernstein wave electron temperature profile diagnostic

Taylor¹,

Efthimion²,

Jones³

et al. 2000

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Electron cyclotron emission (ECE) has been employed as a standard electron temperature profile diagnostic on many tokamaks and stellarators, but most magnetically confined plasma devices cannot take advantage of standard ECE diagnostics to measure temperature. They are either "overdense", operating at high density relative to the magnetic field (e.g. w pe >> W ce in a spherical torus) or they have insufficient density and temperature to reach the blackbody condition (t >2). Electron Bernstein waves (EBWs) are electrostatic waves that can propagate in overdense plasmas and have a high optical thickness at the electron cyclotron resonance layers, as a result of their large k perp . This paper reports on measurements of EBW emission on the CDX-U spherical torus, where B o ~ 2 kG, 10 13 cm -3 and T e » 10 -200 eV. Results are presented for electromagnetic measurements of EBW emission, mode-converted near the plasma edge. The EBW emission was absolutely calibrated and compared to the electron temperature profile measured by a multipoint Thomson scattering diagnostic. Depending on the plasma conditions, the modeconverted EBW radiation temperature was found to be £ T e and the emission source was determined to be radially localized at the electron cyclotron resonance layer. A Langmuir triple probe and a 140 GHz interferometer were employed to measure changes in edge density profile in the vicinity of the upper hybrid resonance where the mode conversion of the EBWs is expected to occur. Initial results suggest EBW emission and EBW heating are viable concepts for plasmas where w pe >> W ce .

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Section: Ebw Measurementsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Electron Bernstein wave electron temperature profile diagnostic

Taylor¹,

Efthimion²,

Jones³

et al. 2000

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show abstract

“…4 More recently, experiments on Current Drive ExperimentUpgrade (CDX-U) and National Spherical Torus Experiment (NSTX), 5 Mega Amp Spherical Tokamak (MAST), 6 and Madison Symmetric Torus (MST) 7 have observed emission of EBWs via the mode conversion process, and studied its dependence on the edge properties of the plasma.…”

Section: Introductionmentioning

confidence: 99%

Emission of electron Bernstein waves in plasmas

2002

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] it has been shown that, in overdense plasmas of the type encountered in spherical tori, electron Bernstein waves can be excited in a plasma by mode conversion of either an externally launched X mode or an O mode. The electron Bernstein waves are strongly absorbed by electrons in the region where the wave frequency matches the Doppler broadened electron cyclotron resonance frequency or its harmonics. The strong absorption also implies that electron Bernstein waves are emitted by a thermal plasma. These waves can then mode convert to the X mode and to the O mode and be observed external to the plasma. In this paper an approximate kinetic model describing the coupling between the X mode, the O mode, and the electron Bernstein waves is derived. This model is used to study the mode conversion properties of electron Bernstein wave emission from the plasma interior. It is shown, analytically and numerically, that the energy flow conversion efficiencies of the electron Bernstein wave to the X mode and to the O mode are the same as the energy flow conversion efficiency of the X mode to electron Bernstein waves and of the O mode to the electron Bernstein waves, respectively. This has important experimental consequences when designing experiments to heat overdense plasmas by electron Bernstein waves.

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“…A key issue for EBW heating is to identify the optimum mode-conversion (MC) scenario, given that the externally injected electro-magnetic wave must be modeconverted to excite an electro-static EBW. The high-field side X-mode injection scenario [1] demonstrated on conventional tokamaks is not applicable to ST. O-mode injection from the low-field side, the so-called OXB scenario [2], is a possible candidate. In addition, the ST configuration with a low toroidal field offers the possibility of a unique MC scenario: low-field side perpendicular X-mode injection [3].…”

Section: Mode-conversion Electron Bernstein Wave Spherical Tokamakmentioning

confidence: 99%

Evidence of Electron Bernstein Wave Heating on the TST-2 Spherical Tokamak

Shiraiwa¹,

Ejiri

Hanada

et al. 2005

J. Plasma Fusion Res.

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Electron Bernstein wave heating experiments based on the X-mode to Bernstein mode mode-conversion scenario were performed on the Tokyo Spherical Tokamak -2 (TST-2). Up to 140 kW of microwave power at 8.2 GHz was injected perpendicularly from the low-field side. Evidence of electron heating was observed as increases of the stored energy (by about 15%) and soft X-ray (hν > 1 keV) emission. Keywords:mode-conversion, electron Bernstein wave, spherical tokamak, heatingThe electron Bernstein wave (EBW), which has no density cutoff and is strongly absorbed by electron cyclotron damping, is an attractive candidate for heating a spherical tokamak (ST). A key issue for EBW heating is to identify the optimum mode-conversion (MC) scenario, given that the externally injected electro-magnetic wave must be modeconverted to excite an electro-static EBW. The high-field side X-mode injection scenario [1] demonstrated on conventional tokamaks is not applicable to ST. O-mode injection from the low-field side, the so-called OXB scenario [2], is a possible candidate. In addition, the ST configuration with a low toroidal field offers the possibility of a unique MC scenario: low-field side perpendicular X-mode injection [3]. In the third scenario, the launched X-mode encounters a triplet consisting of the R-cutoff, the upper hybrid resonance, and the L-cutoff. Efficient MC is predicted when a suitably steep density gradient (small density scale length, L n ) in the triplet region is realized. Advantages of this scenario include a simple launcher design and the possibility of L n control independent of the core plasma.Thus far, only low power EBW receiving experiments have been reported using this scenario [4,5]. In order to examine its feasibility for heating ST plasmas, TST-2 (R = 0.38 m, a = 0.25 m, B t = 0.3 T, I p = 0.14 MA) [6] was temporarily moved to Kyushu University, where high power microwave sources (200 kW @ 8.2 GHz) are available.In this experiment, a launcher consisting of 8 waveguide horn antennas and a movable local limiter surrounding the antennas were installed on the low field side of the torus, below the midplane, and up to 140 kW of microwave power was injected perpendicular to the magnetic surface. The local limiter was used to change L n in front of the antennas and could be moved in the range R = 625 mm to 665 mm (the antenna aperture was located at R~750 mm). An RF leakage monitor, measuring the power leaking through a vacuum window, was also installed and used as an indicator of RF power that was neither absorbed by the plasma nor reflected back to the launcher.In some discharges, a possible indication of EBW heating was observed. Figure 1 shows an example of such a discharge. The first RF pulse was used for pre-ionization, during which the line-integrated density n e l was low (< 1 × 10 18 m -2 ) and the RF leakage was large, suggesting poor absorption. During the second RF pulse, used for heating, the density was higher than the cut-off density (n e l > 4 × 10 18 m -2 , l ~ 0.7 m) and the RF leakage became n...

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Electron Bernstein Wave Emission from an Overdense Plasma at the W7-AS Stellarator

Cited by 73 publications

References 4 publications

Electron Bernstein wave electron temperature profile diagnostic

Electron Bernstein wave electron temperature profile diagnostic

Emission of electron Bernstein waves in plasmas

Evidence of Electron Bernstein Wave Heating on the TST-2 Spherical Tokamak

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