V. V. D’yachenko

Analysis of the efficiency of lower hybrid current drive in the FT-2 tokamak

Results are presented from experimental studies of the efficiency of lower hybrid current drive (LHCD) in the FT-2 tokamak. The dependence of the LHCD efficiency on the grill phasing Δφ and RF oscillator power was determined experimentally in a wide range of plasma densities. It is shown that, at high plasma currents (i.e., at sufficiently high electron temperatures), current drive is suppressed when the plasma density reaches its resonance value nLH for the pumping wave frequency, rather than when parametric decay comes into play (as was observed in regimes with lower plasma currents and, accordingly, lower electron temperatures Te). In order to analyze the experimentally observed effect of LHCD and its dependence on the value and sign of the antenna phasing, the spectra of the excited LH waves, P(Nz), were calculated. Simulations using the FRTC code with allowance for the P(Nz) spectrum and the measured plasma parameters made it possible to calculate the value and direction of the LH-driven current, which are determined by the spectrum of the excited LH waves. It is shown that the synergetic effect caused by the interaction between different spectral components of the excited RF wave plays a decisive role in the bridging of the gap in the wave spectrum.

Effect of the radial electric field on lower hybrid plasma heating in the FT-2 tokamak

Conditions for efficient ion heating in the interaction of lower hybrid waves with plasma are experimentally determined. Experiments show that efficient lower hybrid heating stimulates a transition to the improved confinement mode. The formation of internal and external transport barriers is associated with strong central ion heating, which results in a change of the radial electric field Er and an increase in the shear of the poloidal plasma velocity. The improved confinement mode in the central region of the discharge is attained under the combined action of lower hybrid heating and an additional rapid increase in the plasma current. A new mechanism for the generation of an additional field Er is proposed to explain the formation of a transport barrier.

Use of additional helium puffing for the diagnostics of plasma parameters at the FT-2 tokamak

The experiments carried out at the FT-2 tokamak in which additional pulsed puffing of helium into the hydrogen plasma was used for diagnostic purposes are considered. To estimate the necessary content of helium ions in the experiments on studying short-scale plasma oscillations, the ionization-recombination balance was simulated numerically under the assumption of a toroidally homogeneous influx of the working gas onto the boundary of the plasma column. In these simulations, the effective density of the neutral gas incident on the plasma boundary was determined by the iteration method, which made it possible to provide agreement between the obtained solution and the experimental discharge conditions. In particular, the correspondence of the determined admixture content to both the plasma quasineutrality condition and the value of the effective charge Zeff, as well as agreement between the calculated and measured plasma density profiles, was ensured. The simulations were performed under the assumption of anomalous diffusion coefficients for all plasma components. The temporal variations of the ionization-recombination balance were checked by comparing them with the measured spectra of radiation in the HeI, HeII, and Hα lines. In the current drive experiments, variations in ne(r) at the discharge periphery were examined by the method based on the proportionality of the intensity ratio of the helium spectral lines, HeI(668 nm)/HeI(728 nm), to the plasma density. In these calculations, the factors relating the intensity ratio of these lines to the plasma density were taken from the literature on spectral diagnostics.

Isotopic effect in experiments on lower hybrid current drive in the FT-2 tokamak

To analyze factors influencing the limiting value of the plasma density at which lower hybrid (LH) current drive terminates, the isotopic factor (the difference in the LH resonance densities in hydrogen and deuterium plasmas) was used for the first time in experiments carried out at the FT-2 tokamak. It is experimentally found that the efficiency of LH current drive in deuterium plasma is appreciably higher than that in hydrogen plasma. The significant role of the parametric decay of the LH pumping wave, which hampers the use of the LH range of RF waves for current drive at high plasma densities, is confirmed. It is demonstrated that the parameters characterizing LH current drive agree well with the earlier results obtained at large tokamaks.

Mechanism for the suppression of lower hybrid current drive in the FT-2 tokamak

The mechanism for the switching off of the lower hybrid current drive in the FT-2 tokamak is studied. It is shown that the lower hybrid wave-driven current is switched off when a parametric instability, which causes lower hybrid waves to decay into slowed waves interacting with plasma ions, develops at the plasma periphery. The onset of a parametric instability is attributed to the fact that the electron temperature falls off in the course of discharge, thereby lowering the instability threshold.

Particle diagnostics of ion cyclotron resonance plasma heating in the Globus-M tokamak

Basic experimental results on cyclotron heating of the ion plasma component in the Globus-M spherical tokamak obtained by means of the ACORD-12 charge-exchange ion analyzer are presented. A procedure for determining the maximum energy of fast ions confined in the plasma is described. The procedure was applied to estimate the limiting energy of hydrogen minority ions accelerated during ion cyclotron heating in the Globus-M tokamak. The experimental evaluation of the maximum hydrogen ion energy is confirmed by simulations of ion orbits. Recommendations for optimizing experiments on ion cyclotron heating in the Globus-M tokamak are formulated.

The first experimental HF heating of plasma at ion cyclotron frequencies in the globus-M spherical tokamak

We present the results of the first experiments on the HF heating of hydrogen-deuterium plasma at ion cyclotron frequencies in the Globus-M spherical tokamak. A power of 200 kW at a frequency of 9.1 MHz introduced via a single-loop antenna led to an increase in the ion temperature from 170 to 300 eV. Characteristic times of the ion temperature buildup and decay corresponded to the ion energy confinement time in the tokamak plasma.

Noninductive plasma generation and current drive in the Globus-M spherical tokamak

Experimental results on the generation and maintenance of the toroidal current in the Globus-M spherical tokamak by using waves in the lower hybrid frequency range without applying an inductive vortex electric field are presented. For this purpose, the original ridge guide antennas forming a field distribution similar to that produced by multiwaveguide grills were used. The high-frequency field (900 MHz) was used for both plasma generation and current drive. The magnitude of the generated current reached 21 kA, and its direction depended on the direction of the vertical magnetic field. Analysis of the experimental results indicates that the major fraction of the current is carried by the suprathermal electron beam.

Conception of ion cyclotron plasma heating in the Globus-M2 spherical tokamak

The impedance of the coupling between the antenna and the plasma of the Globus-M2 tokamak with an increase in the toroidal magnetic field and profiles of HF power absorption by plasma components are calculated. The concept of an antenna for the input of fast magnetosonic waves into plasma is discussed.

On the role of graphite chamber-wall lining in experiments on the ion-cyclotron resonance plasma heating in the globus-M spherical tokamak

Comparative results of experiments on the ion-cyclotron resonance (ICR) heating of plasma in the Globus-M spherical tokamak using a hydrogen-deuterium mixture are presented. The experimental data are analyzed over a long period of time during which the internal surface of the chamber walls was gradually coated with graphite plates. The observed monotonic decrease in the efficiency of ICR plasma heating in the chamber with increasing graphite coverage can be explained by a growth in the high-frequency power absorption in the coating material. Results of numerical simulation of this process are reported.