Takashige Tsukishima

Phillips–Tikhonov regularization of plasma image reconstruction with the generalized cross validation

The use of the Phillips–Tikhonov regularization is proposed for numerically stabilizing the ill‐conditioned plasma image reconstructions. An objective function to be minimized leads to a linear estimator of the image intensity distribution and, with the aid of the singular value decomposition, makes it possible to use the generalized cross validation for optimizing a regularization parameter. An excellent behavior of the estimator with computational facility is obtained on the Hα emission computerized tomography of a toroidal plasma.

Measurements of ion wave turbulence by microwave scattering

Ion wave turbulence spontaneously excited in a current‐carrying plasma is investigated by the microwave scattering method. The dispersion relation, the wavenumber spectra, and the angular spectra of the ion wave turbulence are determined from the angular distribution of the scattered wave. The wavenumber spectrum has a peak around k ≃ νin/ vp, and decreases as k−4.8±0.5 with increasing k, where νin is the ion‐neutral collision frequency and vp is the phase velocity of the ion wave. The angular spectrum of the wave energy is found to broaden as the total wave energy, W, increases along the electron drift. The effective collision frequency, νf, for electrons is also measured by the rf probe method as well as the dc method. An anomalous increase in νf is observed with the increase in W along the electron drift. The present experimental results are consistent with the theory of perturbed ion orbits.

Recent Heliotron E physics study activities and engineering developments

Recent studies of transport, magnetohydrodynamic stability, and divertor action on Heliotron E are summarized. A pellet injector and a new diagnostic system are developed. Moreover, the Heliotron groups is conducting research and development on heating and other new systems for the Large Helical Device.

Estimation of wave‐number spectrum parameters from fixed probe‐pair data

Spectrum parameter estimation concerning a stationary and homogeneous random field is examined in terms of quantities obtainable from a pair of fixed probes. The estimators of the mean wave number and the wave‐number spectral width are derived by relating wave‐number spectral moments to spatial derivatives of the complex covariance of the field, the effect of finite probe size being taken into account. The estimate biases associated with the probe separation can be greatly reduced by employing an algorithm based on the polar‐form representation of the complex covariance, while the variances of the estimates have a tendency to be enhanced with the decrease of the probe separation. An optimum range of the probe separation is theoretically determined.

Homodyne method for detecting asymmetric spectra

A homodyne method proposed for detecting asymmetric spectra encountered in scattering experiments is examined theoretically and experimentally. The experiments are carried out on a simulated electrical circuit, and the validity of the proposed method is demonstrated. Various kinds of error in spectra obtained by the proposed homodyne method are examined and identified.

Observation of a current-limited double layer in a linear turbulent-heating device

Time‐ and space‐resolved measurements of strong double layers (DLs) have been carried out for the first time on a linear turbulent‐heating device, together with those of fluctuation spectra and precise current measurements. A stable stong DL is formed even when the electric current through the DL is less than the so‐called Bohm value. Discussion of the formation and decay processes is given, indicating a transition from an ion‐acoustic DL to a monotonic DL.

Least‐squares autoregressive (maximum entropy) spectral estimation for Fourier spectroscopy and its application to the electron cyclotron emission from plasma

A new procedure for the maximum entropy spectral estimation is studied for the purpose of data processing in Fourier transform spectroscopy. The fitting of the autoregressive model to the interferogram is examined under a least‐squares criterion based on the Yule‐Walker equations. A practical criterion is suggested for selecting the model order. The principal advantage of the new procedure lies in the enhanced frequency resolution, particularly for small values of the maximum optical path difference of the interferogram. The usefulness of the procedure is ascertained by some numerical simulations and further by experiments with respect to a highly coherent submillimeter wave and the electron cyclotron emission from a stellarator plasma.

A correlation method for estimating wave‐number spectrum moments of fluctuations

The covariance function of a stationary and homogeneous random field, represented by an analytic signal, is related to its wave‐number spectrum moments. By using a pair of fixed probes with an appropriately small separation, one can estimate the mean wave number and the wave‐number spectral spread. This method may be useful for the experimental study of fluctuation phenomena in plasmas, fluids, etc.

Microwave Reflection by Uniform Dense Plasmas

Convenient formulas are deduced, which relate electron densities and collision frequencies of uniform dense plasmas to two standing wave signals of a probing microwave reflected at a plane boundary of the plasma. In an experiment with a uniform afterglow plasma produced in a waveguide, the electron density from 2 x 10/sup 13/ cm/sup -3/ to 3 x 10/sup 11/ cm/sup -3/ is measured with reasonable value of the collision frequency by using a X-band microwave. The technique provides a mean for detecting an electron density as high as 1 x 10/sup 16/ cm/sup -3/ with the X-band microwave. (auth)

Dual homodyne detection system for measuring asymmetric spectra in the far-infrared regime

A dual homodyne detection system for measuring scattered radiation from plasmas has been fabricated. The system is operative in the far‐infrared regime, and with it the power spectral distribution of the scattered radiation in the upper and lower sidebands with respect to the frequency of the incident wave can be obtained. Theory of operation and experimental results for simulated radiation fields are given.