To study the characteristics of electron scale turbulence, an enhanced scattering (ES) diagnostic has been developed on the J-TEXT tokamak. This ES diagnostic consists of 8 detection channels covering 44-58 GHz with a 2 GHz step. It launches and receives X-mode microwaves from the high-field-side. The probing wave is scattered by electron-scale turbulence at the upper hybrid resonance layer on the low-field-side, where the electric fields of both probing and scattering waves are enhanced. The ES diagnostic has been successfully applied to J-TEXT experiments. Through cross-validation with the Doppler backscattering diagnostic, it has been verified that the ES diagnostic can measure high wavenumber fluctuations, providing a new diagnostic tool for the study of electron-scale turbulence on J-TEXT.

The paper presents observations of plasma intermediate frequency range wave emission ((200--1000) MHz) performed in ohmic discharge of the FT-2 tokamak. The dependencies of the RF signal amplitude and spectrum on the plasma parameters and on the location of the probes are presented. Two possible explanations for this effect is proposed. One of them is related to the distortion of the electron distribution function due to the strong ripple of the magnetic field in the FT-2 tokamak. Second one is related to the existence of a beam of run-away electrons, the buildup of a fan instability responsible for the generation of synchrotron radiation in the range (10-80) GHz, and a complex chain of nonlinear decay instabilities leading to such a strong decrease in the frequency of the observed oscillations. Keywords: tokamak, lower hybrid current drive, high frequency oscillations, parametric decay instabilities.

The paper presents observations of plasma intermediate frequency range wave emission ((200-1000) MHz) performed in ohmic discharge of the FT-2 tokamak. The dependencies of the RF signal amplitude and spectrum on the plasma parameters and on the location of the probes are presented. Two possible explanations for this effect is proposed. One of them is related to the distortion of the electron distribution function due to the strong ripple of the magnetic field in the FT-2 tokamak. Second one is related to the existence of a beam of run-away electrons, the buildup of a fan instability responsible for the generation of synchrotron radiation in the range (10-80) GHz, and a complex chain of nonlinear decay instabilities leading to such a strong decrease in the frequency of the observed oscillations.

A detailed study of the general transport properties of the FT-2 tokamak shows significant isotopic difference in ohmically heated quasi-stationary plasma in high density regimes 〈 n e 〉 ≈ (6–9) × 10 19 m −3 . For deuterium, the signatures of a transition to the H-mode are found at a plasma density exceeding a certain value, while hydrogen plasma remains in L-mode in all comparable discharge scenarios. The origin of this isotopic discrepancy is studied both with energy transport analysis and direct turbulence and plasma velocity measurements performed with Langmuir probes and microwave diagnostics. A special, more complicated dynamic experimental series with current ramp up, performed in these high density regimes, demonstrated the stability and robustness of an improved energy confinement in deuterium, whereas a comparable confinement improvement in hydrogen plasma was observed.

Results are presented of the study of the efficiency of generation of lower hybrid current drive in the plasma of FT-2 tokamak. Experimental data, including the data of the system of poloidally spaced Mirnov coils, was processed by specialized software to analyze the observed generation of lower hybrid current drive and suppression (“locking”) of the m/n = 2/1 MHD tearing mode. Possible mechanisms responsible for the “locking” of the mode were considered, which are connected both with the broadening of the plasma current channel and with the decrease of its poloidal rotation speed Еr × ВT due to the development of the Parail‒Pogutse instability or additional toroidal rotation of the plasma column as a result of transmission of longitudinal pulse from the pumping wave to suprathermal electrons. It was noted that the end of lower hybrid current drive (LHCD) under conditions of increased microwave pulse duration (to ΔtRF ≈ 20 ms) is determined not by the end of the microwave pulse, but by the development of MHD tearing modes m/n = (3–4)/1 at the periphery of the discharge.

Prior experimental work on the FT-2 tokamak has observed electron density fluctuations at electron Larmor radius scales using the enhanced scattering (ES) diagnostic (Gusakov et al 2006 Plasma Phys. Control. Fusion 48 A371–6, Gurchenko and Gusakov 2010 Plasma Phys. Control. Fusion 52 124035). Gyrokinetic GENE simulations of conditions at the upper hybrid resonance layer probed by the ES diagnostic show the presence of the anticipated turbulence from the electron temperature gradient (ETG) driven instability in linear and nonlinear simulations. Ion-scale turbulence is responsible for majority of the transport via trapped electron modes, while impurities act to merge the spectrum of the ion and the electron scale instabilities into a continuum. The linear spectrum at electron scales is characterized by maximal growth rate at a significant ballooning angle θ 0 , and at ion scales the turbulence is broad in the ballooning angle distribution. The neoclassical shearing rate obtained from GENE breaks symmetry in nonlinear simulations of ETG turbulence, which manifests itself as an asymmetric turbulence spectrum. The electron density fluctuation spectrum obtained with GENE corresponds well to the ES measurement at electron scales, as do the fluxes obtained from the ion-scale simulations.

Experiments on the lower hybrid current drive (LHCD) were performed in plasma of the FT-2 tokamak at the microwave pulse duration of ΔtRF ≤ 20 ms covering the entire quasi-stationary stage of the plasma current Ipl(t) = 35 kA. At the relatively low plasma densities 〈ne〉 ~ (0.3–0.6) × 1019 m−3, the regimes were observed and analyzed corresponding to the full replacement of the plasma ohmic current by the non-inductive current generated using LHCD. During the LHCD in the denser plasmas, the formation of the Improved Core Confinement (ICC) regime was observed near the axis of the plasma column. New data was obtained on the threshold of transition to the ICC regime and its dependence on the input microwave power. The details of the transition to the ICC regime were analyzed for deuterium and hydrogen plasmas. It was ascertained that the highest LHCD efficiency was obtained in deuterium plasma at the densities of 〈ne〉 ~ (1.3–1.4) × 1019 m–3.

It was shown that the assumption of straight-line probing beams, which is usually used in reconstructing the plasma density profile from interferometric measurements, leads to unacceptably large errors in the case of a dense plasma. A fast method based on the assumptions of geometric optics is proposed for calculating the interferometric signal. This method takes into account the refraction of the probe wave and can be used in reconstruction of the plasma density profile. The method correctness is confirmed by comparison with the results of full-wave calculations.

The dependence was studied of the level of density fluctuations recorded at different devices on the minor and major radii of the tokamak. It is known that a high level of density fluctuations can negatively affect the operation of diagnostics, in particular, the plasma reflectometry diagnostic. The increase of density fluctuations decreases the quality of raw data by increasing the error of measuring the density profile and, when density fluctuations exceed the threshold value, it renders such measurements impossible. Based on experimental data obtained on devices with substantially different sizes, a dependence of the density fluctuation level on the major and minor tokamak radii was proposed. Since the main experiments were carried out in round limiter tokamaks in ohmic (OH) heating regimes, the extrapolation result is applicable, generally speaking, to installations of larger size with the same configuration and regimes. However, experiments with electron cyclotron heating at the T-10 tokamak also allow one to extend the obtained dependence to regimes with auxiliary heating. It was shown that the obtained dependence is applicable to limiter tokamaks Tore Supra and TFTR of larger size. The applicability of the dependence to installations with diverter configuration is discussed and the possible level of density fluctuations is extrapolated to the international reactor tokamak ITER that is being constructed in France.

Nonlinear symmetry breaking may occur in systems with two or more states whose linear dynamics displays certain symmetries, one of which is preferred nonlinearly. We have identified a regime of electron temperature gradient (ETG) instabilities in a tokamak plasma with circular concentric flux surfaces that has its largest growth rate at a finite ballooning angle, establishing a symmetry that is nonlinearly broken to favor one sign for the ballooning angle. This is the first example of nonlinear symmetry breaking in simulations of a drift instability in the absence of externally imposed flow shear or asymmetry in the plasma column.

Numerous theoretical and experimental studies have proved the important role of radial electric field inhomogeneity, or shear, in LH-transition initiation, and established heating power threshold for transition, although some experiments provide the observation of LH-transition dependency on particle source. It is necessary to apply a concerted approach to describe LH-transition initiation possibility and dynamics, considering E-r shear, particle source and turbulence properties as the main factors responsible for LH-transition initiation.

The possibility of local measurement of the fluctuation level of the radial plasma velocity using equatorial enhanced scattering of a narrow microwave beam in the upper hybrid resonance in internal regions of tokamak plasma is demonstrated. Restrictions of the proposed method at the plasma periphery where the density fluctuation amplitude increases and small-angle scattering of microwaves by the fluctuations along the path to the upper hybrid resonance and back becomes significant are clarified.

In the framework of the isotope effect studies at the FT-2 tokamak, the efficiencies were compared of the lower hybrid wave heating of the ion components of the hydrogen and deuterium plasmas with high densities (close to the Greenwald limit, 〈ne〉 ≤ 1020 m–3). It was experimentally ascertained that, in accordance with the theoretical concepts, the efficient axial heating of the deuterium plasma ions occurs, as opposed to the peripheral heating of the hydrogen plasma ions. Such an isotope effect occurs due to the different localization of the plasma-RF wave interaction regions. The distinctive feature of these experiments is the fact that, in deuterium plasma, during the preliminary ohmic heating, the dependence of the energy lifetime on density τЕ(n) is linear (LOC dependence), and, with increasing density, the transition to the improved ohmic confinement mode occurs. In hydrogen plasma, on the contrary, the transition to the saturation mode is observed. In this study, the considerable decrease in τЕ was revealed, observed during the additional lower hybrid wave heating in both hydrogen and deuterium plasmas.

The dynamics of spectra formation was studied experimentally using radar stroboscopic time-of-flight measurements. Both conventional fluctuation reflectometry spectra (obtained with probing normal to the cutoff surface) and Doppler reflectometry spectra were studied. Two mechanisms of spectrum formation were distinguished even at low time resolution that did not allow capturing the spectra formed after single radiation pass to the cutoff surface and back. The first mechanism that manifests itself for the normal probing and works for the unshifted component of the Doppler reflectometry spectrum is typical for the nonlocal formation of the fluctuation reflectometry spectra in small machines. This can be associated with the radiation propagation along the lengthy trajectory between the cutoff surface and the chamber wall, as well as with the small-angle scattering that occurs during this propagation along the trajectory. The second mechanism, which manifests itself during the formation of the bulk of the Doppler reflectometry spectrum (which is dominant in power and is frequency shifted), occurs without a time delay and most likely corresponds to the classical single-pass backscattering near the cutoff surface.