Electron Density Fluctuations In Plasmas Research Articles

Design of microwave broadband CMOS transmitter and receiver circuits for MIR and ECEI plasma diagnostics.

To efficiently determine the plasma electron density fluctuations using the MIR diagnostic technique, a 55-75GHz 65 nm-CMOS transmitter has been developed where four separate intermediate frequency (IF) signals are up-converted, amplified, and then combined to generate an 8-tone RF output; a broadband 90 nm-CMOS receiver has also been constructed, which consists of an RF-low noise amplifier (LNA), mixer, and IF amplifier. The circuits and their corresponding modules will soon be deployed on the DIII-D and NSTX-U fusion devices. A 110-140GHz 65 nm-CMOS receiver has also been designed, which is suitable for measuring the deep-core temperature fluctuations in the DIII-D tokamak using the electron cyclotron emission imaging diagnostic system. In addition to the RF-LNA/balun, mixer, and IF amplifier, an LO balun/tripler and driving amplifier are now included in this highly integrated circuit chip. By adopting the microwave and millimeter-wave system-on-chip concept in the front-end system design, this paper demonstrates that compact transmitter and receiver modules can be easily built, which, in turn, facilitates array implementation and maintenance.

Characterization of edge and scrape-off layer fluctuations using the fast Li-BES system on COMPASS

Recently the Lithium-Beam Emission Spectroscopy (Li-BES) system on COMPASS has reached its full diagnostic power in terms of routine automatic operation in any kind of plasma scenarios and it is normally used as a standard tool for reconstruction of ultra fast density profiles in the edge region of COMPASS plasmas. The purpose of this study is to investigate the advantages and limitations of the COMPASS Li-BES system in characterizing plasma electron density fluctuations. We show how the atomic physics of plasma-beam interactions can affect the interpretation of the measurement at different radial positions and for different electron density profiles. We also demonstrate the usability of generalized sequential probability ratio test for automatic event detection. Using non-perturbative diagnostic, we verify the validity of the stochastic Garcia-model for scrape-off layer filaments and accompanying holes (density deficits).

Calibration and test of the tangential phase contrast imaging diagnostic on CDX-U

A novel CO2 laser-based tangential imaging diagnostic was completely calibrated and the system was tested on CDX-U plasmas. It was shown that localized, two-dimensional images of the plasma electron density fluctuations in the tokamak core can be obtained from a tangential imaging beam [E. Lo, J. Wright, and R. Nazikian, Rev. Sci. Instrum. 66, 1180 (1995)]. A variation of the Zernike phase mirror is used. Test measurements made on sound waves verify that the system will image phase variations placed at the object plane. An absolute sensitivity of ≈1×107 cm−3/Hz was determined. A series of plasma measurements made on CDX-U is described. Results show that the plasma fluctuations are two to three orders of magnitude above the noise floor of the diagnostic. It is also verified that a high-pass cut-off wave number (from ≈1 to 8 cm−1) for the fluctuations can be selected by translating the phase mirror. The density fluctuation k spectrum is measured and found to peak at ≈1–3 cm−1.

Calibration and test of the tangential phase contrast imaging diagnostic on CDX-U (abstract)

The novel CO2 laser based tangential imaging diagnostic has been completely calibrated and the system has been tested on CDX-U plasmas. It has been shown that localized, two-dimensional images of the plasma electron density fluctuations in the tokamak core can be obtained from a tangential imaging beam.1 A variation of the Zernike phase mirror is used. Test measurements made on soundwaves verify that the system will image phase variations placed at the object plane. An absolute sensitivity of ≈1×107 cm−3/Hz has been determined. A series of plasma measurements made on CDX-U are described. Results show that the plasma fluctuations are 2–3 orders of magnitude above the noise floor of the diagnostic. It is also verified that a high-pass cutoff wave number (from ≈1–8 cm−1) for the fluctuations can be selected by translating the phase mirror. The density fluctuation k spectrum is measured and found to peak at ≈1–3 cm−1.

Density fluctuation measurements in ATF using correlation reflectometry

A two-frequency correlation reflectometer has been operated on the Advanced Toroidal Facility (ATF) to measure plasma electron density fluctuations. This reflectometer uses quadrature phase detection to permit true phase measurement of the reflected microwave signal (probing beam). By measuring the phase fluctuations in the reflected probing beam, the amplitude of the density fluctuations can be estimated. Simultaneous two-frequency operation makes it possible to measure the coherence between fluctuations at two radially separated cut-off layers, from which the radial correlation lengths and wavenumbers can be estimated. This reflectometer has been used to study the density fluctuations in the edge gradient region of low density ATF plasmas produced by electron cyclotron heating. These studies have revealed globally coherent turbulence with a radial correlation length of up to approximately 5 cm, a radial wavenumber kr ≈ 0 cm-1 and a poloidal wavenumber kθ ≈ 1 cm-1. The rms amplitude of the fluctuations reaches a maximum of ≈ 5% at the plasma edge (ρ = 1, where ρ is the flux surface normalized radius) and decreases with decreasing radius to a level of ≲1%. Simultaneous measurements of the fluctuations with the reflectometer, the heavy ion beam probe and the fast reciprocating Langmuir probe provide consistent results. A comparison of the measurements with simplistic mixing length estimates, ne/ne = 1/kθLn or ne/ne = 1/krLn, shows that these estimates are too high by factors of two to more than 100, while a comparison with a more detailed estimate for the pressure gradient driven resistive interchange turbulence yields reasonable agreement with the experimentally measured fluctuation characteristics

Development of Laser Imaging Method for Measurements of Electron Density Fluctuations in Plasmas

A generalized analysis of the laser imaging method is presented for measurements of long-wavelength electron density fluctuations in plasmas, so as to enable its application to practical situations, such as oblique incidence of laser light and identification of the position of the fluctuations. Experimental verification of the more salient features of the theory is also described.

Generalized turbulence space-correlation and wave-number spectrum-function pairs

A new generalized pair of space-correlation and wave-number spectrum functions, which has properties indicated by fluid dynamics and by intuition, is proposed. In contrast with previously used combinations, both are analytic functions and smooth and cover the complete range of their arguments. For example, the spectrum form can apply to all wave numbers rather than to any limited portion, such as the inertial range. Whenever convolution is necessary, it becomes a routine computer calculation. Furthermore, the correlation function has a zero first derivative and a negative second derivative at the origin, giving both an integral scale and a microscale for the turbulence. The spectrum function can have an inertial region with some power-law decay for intermediate wave numbers and has an exponential decay for very large wave numbers. There are three adjustable parameters, determined by experiment, namely, the spectral power-law decay index, the turnover point where this power law starts, and the turnover point where the power dependence changes to an exponential decay. A library of such spectrum functions is not required, since the single proposed function, with its adjustable parameters, can be made to fit most data. One can also allow any even power of wave number for the spectral dependence in the limit of zero wave number. Simple anisotropic functional forms can readily be incorporated. It is indicated that two inertial falloffs with different power laws can be provided and the resultant correlation function is derivable by convolution. The results are applied to simplified versions of plasma electron density fluctuations and to velocity fluctuations of the background fluid. Various scales of turbulence and the related pressure-correlation function are derived. Expressions of correlation functions of amplitude and phase over parallel-line-of-sight propagation paths are also deduced.The proposed pair of functions should only be considered as a generalized model of Tatarski's form, which allows easy conversion from the correlation to the spectrum function and vice versa. Perhaps even more realistic forms can be invented.

Effects of Collisions on Electron Density Fluctuations in Plasmas

The spectrum of electron density fluctuations has previously been calculated for a collisionless plasma. In this paper, the theory is extended to include collisions as represented by a Fokker-Planck equation such as the one used in the theory of Brownian motion. Numerical calculations have been carried out to show graphically the transition of the spectrum from the collisionless to the collision-dominated case. The results of the present analysis involve some thus far untabulated functions. These functions have been numerically evaluated and the results are presented in graphical form.