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Effects of reflectometer asymmetries on fluctuation measurements

G D Conway1999年Plasma Physics and Controlled FusionIF 2.2出版社

Effects on reflectometer phase and power fluctuation signals due to (a) asymmetries in the transmit-plasma-receive antenna geometry (misalignments), and (b) asymmetries in the plasma cut-off layer perturbations (distortions from sinusoidal or Gaussian) are studied using a two-dimensional (2D) physical optics model. Results show the onset of phase runaway with antenna misalignment and/or sawtooth type perturbations when the perturbation amplitude exceeds some critical value. For broadband (Gaussian) turbulence antenna misalignment leads to Doppler shifts in the spectrum - provided the reflectometer beam width w and spectral width of the turbulence are sufficiently large. Misalignment also generates coherence between and at Bragg backscatter frequencies with quadrature phase difference. Sawtooth (asymmetric gradient) perturbations also generate phase-power coherence in quadrature, but at frequencies determined by w and , i.e. not Bragg. Cusped or spiky (asymmetric amplitude) perturbations generate asymmetric phase distributions (nonlinear phase offsets) and low-frequency phase-power coherence with 0 or phase difference. The simulations indicate that a combination of antenna misalignment (or plasma tilt) and cusped reflection layer perturbations can account for a wide range of experimentally reported features in reflectometer signals.

日本語訳

反射計の位相およびパワー変動信号に対する、(a)送信・プラズマ・受信アンテナ幾何学配置の非対称性(ミスアライメント)、および(b)プラズマ遮断層摂動の非対称性(正弦波またはガウス分布からの歪み)の影響を、2次元(2D)物理光学モデルを用いて研究した。結果は、摂動振幅が特定の臨界値を超えると、アンテナのミスアライメントおよび/または鋸歯状波型摂動によって位相暴走が発生することを示している。広帯域(ガウス型)乱流の場合、アンテナのミスアライメントは、反射計のビーム幅wおよび乱流のスペクトル幅が十分に大きければ、スペクトルにドップラーシフトをもたらす。ミスアライメントはまた、ブラッグ後方散乱周波数において、直交位相差を伴うコヒーレンスを生成する。鋸歯状波型(非対称勾配)摂動もまた、直交位相の位相-パワーコヒーレンスを生成するが、その周波数はwおよびによって決定され、すなわちブラッグ周波数ではない。尖頭型またはスパイク状(非対称振幅)摂動は、非対称な位相分布(非線形位相オフセット)と、0またはの位相差を伴う低周波数の位相-パワーコヒーレンスを生成する。シミュレーションは、アンテナのミスアライメント(またはプラズマの傾斜)と尖頭型反射層摂動の組み合わせが、反射計信号において実験的に報告された広範囲の特徴を説明できることを示している。

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