M A Irzak

2D Modeling of the O–X conversion in toroidal plasmas

The conversion of an ordinary (O) mode to an extraordinary (X) mode at electron cyclotron frequencies (Prienhalter and Kopecky 1973 J. Plasma Phys. 10 1) in a spherical tokamak geometry has been examined numerically. A cold plasma dielectric tensor was used; the singularity at the upper hybrid resonance (UHR) was treated by introducing an artificial collision frequency. This allowed us to model the O–X–B mode conversion scheme (B stands for the Bernstein wave) analytically reducing the wave equations to the second order. Our numerical modeling confirmed the predictions of the 2D O–X conversion theory. In particular, it was shown that, first, the pattern of the converted X wave can be correctly described by an analytical formula within its validity limits, and second, there is an asymmetry of the O–X conversion efficiency with respect to the direction of the toroidal magnetic field. The last result is the first numerical confirmation of the effect predicted earlier by the 2D O–X conversion theory. Our modeling did not confirm the discouraging result of the numerical simulation (Vdovin 2006 33rd EPS Conf. on Plasma Physics vol 30I) that the UHR plays an important role in RF power deposition when O-mode is launched at the fundamental harmonic, and no conditions for effective O–X conversion are provided.

Lower hybrid wave excitation and propagation in the spherical tokamak Globus-M

The problem of lower hybrid (LH) wave excitation and current drive (CD) in tokamaks with a small aspect ratio (spherical tokamaks) is discussed. It is proposed to solve this problem by exciting the waves slowed down in the poloidal rather than the toroidal direction. As a result, due to the strong poloidal inhomogeneity of the magnetic field in spherical tokamaks, even the waves with comparatively weak slowing down (N ? 3?5) excited by a waveguide antenna in the equatorial plane can penetrate into the dense plasma and be absorbed via the Landau mechanism. This approach was applied for modeling the LHCD experiments in the low aspect ratio tokamak Globus-M (R = 0.36?m, a0 = 0.24?m, B0 = 0.4?T, Ip = 0.25?MA, vertical elongation k = 1.6, operating frequency 2.45?GHz). The modeling was carried out using four independent codes: (i) the self-consistent antenna coupling code GRILL3D, (ii) the ray-tracing code incorporating a specially developed ray-tracing technique including some corrections necessary in strongly inhomogeneous plasma, (iii) the 2D full-wave code WAVETOP2D and (iv) the Fokker?Plank code combined with the ray-tracing code allowing simulation of the driven current density profile. The results of simulations were cross-checked and appeared to be in a good agreement. It was demonstrated that the proposed scenario provides a possibility for the efficient LHCD in a spherical tokamak.

Globus-M plasma physics research for fusion application and compact neutron source development

During the past decade, plasma physics research promoting the physics base of ITER and developing novel concepts such as a compact fusion neutron source has been conducted on the Globus-M spherical tokamak (ST) (R = 36 cm, a = 24 cm, I p ≤ 250 kA, B T ≤ 0.4 T). Tokamak reconstruction is imminent. The upgraded tokamak Globus-M2 will have the same vacuum chamber and an enhanced magnetic system to provide B T = 1 T and I p = 500 kA. In this paper we outline the most important research directions and the main results obtained on Globus-M and make some predictions about the possibilities and parameters of Globus-M2.

Poloidal inhomogeneity of turbulence in the FT-2 tokamak by radial correlation Doppler reflectometry and gyrokinetic modelling

The poloidal dependence of the drift-wave turbulence characteristics is investigated at the FT-2 tokamak by radial correlation Doppler reflectometry (RCDR) technique and using the full distribution function global gyrokinetic modelling by ELMFIRE code. The poloidal variation of the turbulence radial correlation length from 0.2–0.55 cm is demonstrated both by measurement and computation. The turbulence correlation length rapidly decreases from the top of the poloidal cross-section to the high field side and then steadily grows in the poloidal direction. A well-pronounced excess of the turbulence radial correlation length in deuterium over its value in hydrogen discharges is demonstrated.

Simulations on the Role of the Resonance of the Probing Wave on Reflectometry Measurements in Fluctuating Plasmas

In fusion plasmas for energy, turbulence is clearly associated to the anomalous transport of energy and particles. Diagnostics using electromagnetic waves is usually used to measure the plasma parameters and the turbulence characteristics. One of the diagnostics is reflectometry, which is based on the radar principle and can be used to reach these different goals: density profile, turbulence characterization, plasma positioning, etc. During the plasma probing, fast phase variations called phase jumps have been measured, and part of them can be explained by a local enhancement of the probing field. Using an analytical model to compute the probing electric field amplification, a new expression for the phase variations has been written, taking into account the amplification of the probing electric field. This formula exhibits a good agreement with the full-wave computations. A possible improvement of the reflectometer sensitivity can be done using the local enhancement of the probing electric field induced by Bragg resonant density perturbations, which build resonant cavities. The computed cases show that it is possible to improve the reflectometry measurements when the local enhancement of the probing electric field exists. To illustrate the possible improvements, simulations of the radial wavenumber spectrum reconstruction have been done, and they confirm the possibility to increase the sensitivity and the spatial resolution for a given range of wavenumber.

2D modeling of turbulence wave number spectra reconstruction from radial correlation reflectometry data

Computational analysis of radial correlation reflectometry in the framework of a model accounting for 2D probing wave propagation and scattering is performed. The procedure of the turbulence wavenumber spectra reconstruction from radial correlation reflectometry data previously proposed in the case of normal incidence of probing wave onto the 1D plasma is justified for the real geometry of a reflectometry experiment at large and modest scale fusion plasmas.

The first lower hybrid current drive experiments in the spherical tokamak Globus-M

First experiments on non-inductive current drive (CD) in the spherical tokamak using lower hybrid (LH) waves at 2.45 GHz are described. The basic characteristics of the new experimental approach utilizing poloidal LH wave slowing-down scheme obtained by modeling are presented at the beginning of the paper. The experimental results for a poloidal slowing-down scheme are described. The CD efficiency comparable or even higher than theoretically predicted is demonstrated in the experiment using a 10 waveguide grill with 120° phase shift between neighboring waveguides.

Possibility of a giant scattering enhancement due to wave trapping in a reflectometry experiment

A new scheme of reflectometry diagnostic based on the enhanced scattering effect invented by the late Professor A D Piliya is proposed and analyzed in this paper. It is shown that due to wave trapping, a strong coherent density perturbation in a reflectometry experiment can cause a giant cross-section enhancement for scattering occurring between it and cut-off. The theoretical approach is based on the method developed by Piliya for the treatment of a three-wave interaction in inhomogeneous media. Similar values of the scattering enhancement factor are obtained both numerically and analytically.

Fast synthetic X-mode Doppler reflectometry diagnostics for the full-f global gyrokinetic modeling of the FT-2 tokamak

The fast linear (Born approximation) version of the X-mode Doppler reflectometry (DR) synthetic diagnostics is developed in the framework of the ELMFIRE global gyrokinetic modeling of the FT-2 tokamak ohmic discharge. The DR signal frequency spectra and the dependence of their frequency shift and shape on the probing antenna position are computed and shown to be similar to those measured in the high magnetic field side probing DR experiment at the FT-2 tokamak. The fluctuation poloidal velocities are determined using the DR experiment and synthetic diagnostics and shown to be within 15%. However, the computed and measured dependences of the DR signal power on the antenna position appear to be different presumably due to underestimation of the small-scale trapped electron mode turbulence component in the measurement region by the code.The fast linear (Born approximation) version of the X-mode Doppler reflectometry (DR) synthetic diagnostics is developed in the framework of the ELMFIRE global gyrokinetic modeling of the FT-2 tokamak ohmic discharge. The DR signal frequency spectra and the dependence of their frequency shift and shape on the probing antenna position are computed and shown to be similar to those measured in the high magnetic field side probing DR experiment at the FT-2 tokamak. The fluctuation poloidal velocities are determined using the DR experiment and synthetic diagnostics and shown to be within 15%. However, the computed and measured dependences of the DR signal power on the antenna position appear to be different presumably due to underestimation of the small-scale trapped electron mode turbulence component in the measurement region by the code.

Transition into the improved core confinement mode as a possible mechanism for additional electron heating observed in the lower hybrid current drive experiments at the FT-2 tokamak

In experiments on lower hybrid current drive (LHCD) carried out at the FT-2 tokamak, a substantial increase in the central electron temperature Te(r = 0 cm) from 550 to 700 eV was observed. A complex simulation procedure is used to explain a fairly high LHCD efficiency and the observed additional heating, which can be attributed to a transition into the improved core confinement (ICC) mode. For numerical simulations, data obtained in experiments with deuterium plasma at 〈ne〉 = 1.6 × 1019 m–3 were used. Simulations by the GRILL3D, FRTC, and ASTRA codes have shown that the increase in the density and central temperature is apparently caused by a significant suppression of heat transport in the electron component. The mechanism for transition into the improved confinement mode at r < 3 cm can be associated with the broadening of the plasma current channel due to the lower hybrid drive of the current carried by superthermal and runaway electrons. In this case, the magnetic shear s = (r/q)(dq/dr) in the axial region of the plasma column almost vanishes during the RF pulse. In this study, the effect of lower hybrid waves on the plasma parameters, resulting in a transition into the ICC mode, is considered. New experimental and calculated data are presented that evidence in favor of such a transition. Special attention is paid to the existence of a threshold for the transition into the ICC mode in deuterium plasma.