O. N. Shcherbinin

Investigation of beam– and wave–plasma interactions in spherical tokamak Globus-M

The experimental and theoretical results obtained in the last two years on the interaction of neutral particle beams and high-frequency waves with a plasma using the spherical tokamak Globus-M are discussed. The experiments on the injection of low-energy proton beam of ~300 eV directed particle energy are performed with a plasma gun that produces a hydrogen plasma jet of density up to 3 × 1022 m−3 and a high velocity up to 250 km s−1. A moderate density rise (up to 30%) is achieved in the central plasma region without plasma disruption. Experiments on high-energy (up to 30 keV) neutral beam injection into the D-plasma are analysed. Modelling results on confinement of fast particles inside the plasma column that follows the neutral beam injection are discussed. The influence of the magnetic field on the fast particle losses is argued. A neutral beam injection regime with primary ion heating is obtained and discussed. The new regime with fast current ramp-up and early neutral beam injection shows electron temperature rise and formation of broad Te profiles until the q = 1 flux surface enters the plasma column. An energetic particle mode in the range of frequencies 5–30 kHz and toroidal Alfven eigenmodes in the range 50–300 kHz are recorded in that regime simultaneously with the Te rise. The energetic particle mode and toroidal Alfven eigenmodes behaviour are discussed. The toroidal Alfven eigenmode spectrum appears in Globus-M as a narrow band corresponding to n = 1. The first experimental results on plasma start-up and noninductive current drive generation are presented. The experiments are carried out with antennae providing mostly poloidal slowing down of waves with a frequency of 920 MHz, which is higher than a lower hybrid one existing under the experimental conditions. The high current drive efficiency is shown to be high (of about 0.25 A W−1), and its mechanism is proposed. Some near future plans of the experiments are also discussed.

Numerical modelling and experimental study of ICR heating in the spherical tokamak Globus-M

In spherical tokamaks the conventional ICR plasma heating has a number of specific features and therefore requires additional investigations. For this aim the modelling of wave propagation and absorption was performed by the 1-D code developed at the Ioffe institute. All possible mechanisms of RF absorption (cyclotron absorption at fundamental and second harmonics, TTMP, Landau) were taken into account. The calculations demonstrated the possibility of effective RF power absorption both by ions and electrons in a broad range of plasma parameters (including relative hydrogen concentration).The ICRH experiments were performed on the low aspect ratio tokamak Globus-M (R = 0.36 m, a = 0.24 m, B0 = 0.3–0.4 T, Ip = 0.15–0.25 MA, vertical elongation 1.2–2, at RF power input level up to 200 kW at frequencies of 7.5–9.2 MHz. A 12-channel neutral particle analyser measured simultaneously hydrogen and deuterium fluxes and relative concentration of ion components. In the experiment the ion temperature increases twice, but the ion heating efficiency depends on the location of the second hydrogen cyclotron harmonic and on the concentration of the light ion component. It is shown that the position of the second hydrogen harmonic in front of the antenna decreases the efficiency of the ion heating. The increase in H-concentration in deuterium target plasma from 10% up to 70% does not influence ion heating efficiency essentially but seems to increase it moderately. The first results on 1.5-D transport ASTRA Modelling are described. They are in reasonable agreement with experimental data. The electron heating was not detected in the experiment due to comparatively low absorbed power with respect to OH one.

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.

Simulation of the conditions of wave excitation for the optimization of the lower hybrid current drive in the Globus-M spherical tokamak

At present, the method of current drive by means of lower hybrid waves is not applied to low-aspect-ratio tokamaks, because, in the traditional approach, it would be necessary to use waves with a very high slowing-down factor. However, studies of new transparency regions for waves in a nonuniform magnetized plasma, performed earlier at the Ioffe Physical Technical Institute, Russian Academy of Sciences, made it possible to develop an approach in which slow waves are excited in the poloidal (rather than toroidal) direction. In this approach, moderately slowed-down waves first propagate in the poloidal direction, but then turn in the toroidalal direction and get into the dense plasma. In this work, this approach is further developed using numerical methods. In particular, the influence of the density profile in the edge plasma on the efficiency of wave excitation for given antenna parameters is studied in detail.

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.

Review of ICR Heating on the Spherical Tokamak Globus‐M

The results of experiments on the RF heating of hydrogen‐deuterium plasma at ion cyclotron frequencies in the Globus‐M spherical tokamak are presented. RF power up to 200 kW at frequencies about 7.5–9 MHz fed via a single‐loop antenna led to an increase in the ion temperature from 180 eV up to 400 eV in the best case. High heating efficiency is maintained at high H‐concentration (up to 70%). Characteristic times of the ion temperature buildup and decay corresponded to the ion energy confinement time. If the second hydrogen harmonic is arranged in front of antenna, on‐axis ion heating becomes worse. It is shown, that in Globus conditions graphite shielding of an inner surface of a vacuum chamber decreases heating due to surface absorption of RF power. Simulation of processes of RF power absorption is performed also. For modeling 1‐D wave code was used including all possible mechanisms of RF power absorption.