V. B. Minaev

Geodesic acoustic mode observations in the Globus-M spherical tokamak

The results of geodesic acoustic mode (GAM) studies in the spherical torus Globus-M via Doppler reflectometry are presented. The intermittent character of the GAM evolution is similar to the limit-cycle oscillation behavior of zonal flows. The evident correlation between the GAM rotational velocity and both Dα emission and plasma density oscillations is exhibited and discussed. The obtained experimental results are compared with those from tokamaks with large aspect ratios.

Review of Globus-M spherical tokamak results

The first experiments on noninductive current drive (CD) using lower hybrid waves in a spherical tokamak are described. Waves at 2.45 GHz were launched by a 10 waveguide grill with 120° phase shift between neighbouring waveguides. The experimental results for a novel poloidal slowing-down scheme are described. The CD efficiency is found to be somewhat larger than that predicted theoretically whilst at the same time being somewhat less than that for standard tokamak lower hybrid CD. Geodesic acoustic modes (GAM) have been discovered in Globus-M. GAMs are localized 2–3 cm inside the separatrix. The GAM frequency agrees with theory. The mode structures of plasma density and magnetic field oscillation at the GAM frequency have been studied. Fast particle confinement during neutral beam injection has been investigated and numerically simulated. Alfven instabilities excited by fast particles were detected by a toroidal Mirnov probe array. Their excitation conditions are discussed and the dynamics of fast ion losses induced by Alfven eigenmodes is presented. Preliminary experiments on the isotopic effect influence on global confinement in the ohmic heating (OH) regime are described. Scrape-off layer (SOL) parameters were measured and compared with results from self-consistent integrated transport modelling. Results showed that SOL width scales inversely proportional to plasma current. The behaviour of an a priori damaged tungsten divertor plate mock-up exposed to plasma flows was investigated. Preliminary conclusions are that the initial damage gives rise to a loose layer formation with low thermal conductivity right beneath the surface. Finally, engineering design issues of the next step—Globus-M2 (1 T, 500 kA) and the status of component manufacture are described.

Density limits and control in the Globus-M spherical tokamak

The results of the experimental campaign on Globus-M (R = 0.36 m, a = 24 m) devoted to investigating density limits and density control are reported. The experiments were performed at Btor = 0.4 T, Ip = 0.18–0.25 MA, q95 = 3.5–5 and plasma vertical elongation, κ ~ 1.5–1.7. The density limits achieved with the gas puffing method of density control in the previous periods in ohmic heating (OH) regime are discussed. The progress made in OH scenario optimization helped the density to approach the Greenwald limit. Co-current neutral beam of deuterium with the power in the range of 0.45–0.6 MW at the beam energy of 28–29 keV was injected into deuterium target plasma at the early stage of the discharge, which allowed the density to overcome the Greenwald limit. Line averaged densities in excess of 1.5 × 1020 m−3 were achieved, during the external gas puff. An ion temperature increase, measured by NPA was accompanied by a definite increase in the electron energy content, registered by Thomson scattering. Injection of a pure, highly ionized hydrogen plasma jet with a density up to 1022 m−3, total number of accelerated particles (1–5) × 1019 and a flow velocity of ~110 km s−1 was used as another instrument for density control. It increased plasma particle inventory in the Globus-M by ~50% (from 0.65 × 1019 to 1 × 1019) in a single shot without target OH plasma parameter degradation. The injection resulted in a fast density increase with the time much shorter than with gas puff fuelling, which was confirmed by Thomson scattering measurements.

Geodesic acoustic mode investigation in the spherical Globus-M tokamak using multi-diagnostic approach

The results of a multi-diagnostic approach for geodesic acoustic mode (GAM) studies in the spherical torus Globus-M are presented. The GAM oscillations of radial electric field, plasma density and magnetic field were simultaneously observed. The spatial structures of crucial plasma parameters are exhibited and discussed. The intermittent character of the GAM manifests itself as a modulation at two time-scales: equilibrium time scale and the characteristic time for limit-cycle oscillations. The mutual influence of GAM oscillations and turbulence is demonstrated by bicoherence analysis.

Studying the interaction of high-energy deuterons with plasma in the Globus-M spherical tokamak

Plasma parameters in the Globus-M spherical tokamak have been measured upon injection of a deuteron beam with energy about 30 keV and a power of up to 700 kW. Dependences of the ion temperature and neutron yield on the plasma current and density and the gap width between the wall and plasma are considered. Possible characteristics of fast-ion confinement in the modified Globus-M2 tokamak are discussed.

Observation of geodesic acoustic modes in the Globus-M spherical Tokamak

The first results of an investigation of geodesic acoustic modes (GAMs) in the Globus-M spherical tokamak are presented. The experiments were performed using Doppler reflectometry. The conditions of the appearance of GAMs, the nature of their development, and their radial localization have been studied. The obtained data are compared to experimental results obtained in tokamaks with large aspect ratios.

THOMSON SCATTERING DIAGNOSTICS UPGRADE AT THE GLOBUS-M TOKAMAK

Диагностика томсоновского рассеяния (ТР) является ключевой диагностикой при проведении экспериментальных исследований на токамаках. Изучение динамики профилей температуры и концентрации электронов, измеренных методом ТР, позволяет получать сведения о режимах удержания и термоизоляции высокотемпературной плазмы, что является одной из важнейших проблем УТС. На токамаке Глобус-М выполнена модернизация диагностики томсоновского рассеяния. Число пространственных каналов увеличено до 10, что позволило измерить профили температуры и концентрации электронов от внутренней до внешней границы плазмы. Благодаря применению новой системы сбора света и регистрации рассеянного излучения увеличена точность измерений. В статье обсуждаются особенности калибровки диагностики и обработки сигналов рассеяния, приводятся первые результаты эксперимента. К настоящему времени модернизированная система ТР задействована в программе исследований режимов с инжекцией высокоэнергетичной плазменной струи, нейтральной инжекцией и режимов с внутренними транспортными барьерами.

The globus-m diagnostics design

Abstract Diagnostics which is currently installed or under active development for a newly commissioned spherical tokamak (ST) Globus-M is described. Among the commonly implemented plasma diagnostics the priority is given to those of the enhanced locality and repetition rate response. Technical summaries of each diagnostics are given with special emphasis on innovative approaches.

The Globus-M2 spherical tokamak: the first results

The Globus-M2 spherical tokamak is the considerably upgraded Globus-M facility. Its technical parameters were increased as much as possible to achieve the promising range of physical parameters (sub-fusion temperatures and collisionality of much less than unity). These parameters will be achieved in a compact magnetic configuration similar to that of the Globus-M tokamak, the plasma current and toroidal magnetic field amounting to 0.5 MA and 1 T, respectively. The demand to increase the magnetic field and plasma current in the Globus-M2 resulted in the need for a complete redesign of the electromagnetic system because the plasma equilibrium requirements have changed and the mechanical and thermal loads have considerably increased as compared to the Globus-M. The vacuum vessel and the in-vessel components of the new Globus-M2 tokamak remain the same. Power supplies were upgraded to provide the required currents in the toroidal field coil and the central solenoid. The Globus-M2 tokamak was build up and preliminary tests were carried out. New auxiliary heating systems and diagnostics were developed and installed to be used in future experiments. Fist plasma was achieved at the Globus-M2 in April 2018.

A ZERO-DIMENTIONAL MODEL TO STUDY PLASMA HEATING EFFICIENCY AND THERMAL INSULATION AT THE GLOBUS-M TOKAMAK IN OHMIC HEATED REGIMES

Экспериментальное исследование термоизоляции плазмы в установках с магнитным удержанием является фундаментальной задачей физики плазмы, имеющей прикладное значение. В работе представлен метод обработки данных кинетических магнитных диагностик токамака Глобус-М, на основании которого создана расчётная программа (0-мерный код), позволяющая непосредственно во время эксперимента (между разрядами токамака) определять основные характеристики разряда — электронный и ионный тепловой энергозапас плазмы, а также эффективный заряд и энергетическое время жизни плазмы. Получено хорошее соответствие 0-мерных расчётов с результатами моделирования разряда кодом ASTRA, что открывает возможность использования разработанного подхода для рутинного применения на токамаке Глобус-М с учётом особенностей диагностического комплекса установки.