Представлены результаты экспериментов по подъему и поддержанию тороидального тока в сферическом токамаке Глобус-М с помощью волн нижнегибридного диапазона частот без приложения индукционного вихревого электрического поля. Для этой цели использовались оригинальные антенны типа “гребенка”, создающие распределение полей подобное многоволноводным гриллам. Высокочастотная мощность (900 МГц) использовалась как для создания плазмы, так и для генерации тока. Величина полученного тока достигала 21 кА, его направление зависело от направления вертикального магнитного поля. По совокупности экспериментальных данных можно предположить, что основная доля тока переносится пучком надтепловых электронов.

The targeted plasma parameters of the compact spherical tokamak (ST) Globus-M have basically been achieved. The reasons that prevent further extension of the operating space are discussed. The operational limits of Globus-M together with an understanding of the limiting reasons form the basis for defining the design requirements for the next-step, Globus-M2. The recent experimental and theoretical results achieved with Globus-M are discussed, the operational problems and the research programme are summarized and finally, the targeted Globus-M2 parameters are presented. The magnetic field and plasma current in Globus-M2 will be increased to 1 T and 0.5 MA, respectively. The plasma dimensions will remain unchanged. With auxiliary heating at a high average plasma density, the temperatures will be in the keV range and the collisionality parameter with v* << 1 will define the operational conditions. Noninductive current drive will be a major element of the programme. The engineering design issues of Globus-M2 tokamak are discussed and the technical tokamak parameters are confirmed by thermal load and stress analysis simulations. The experimental results obtained on Globus-M2 and the limits of its performance should clarify the feasibility of an ST-based super compact neutron source.

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.

Non-inductive plasma start-up and current ramp-up without the central solenoid is an important problem on the way to compact fusion reactors. There are several experiments demonstrating such possibility due to absorption of electromagnetic waves in vicinity of the electron cyclotron resonance for plasma formation and then current start-up [1, 2, 3]. In our case the non-resonant scenario of plasma and current generation by RF power at 920 MHz frequency was used in toroidal vessel at standard toroidal and weak poloidal magnetic fields. Experimental data indicate that the plasma current could be determined by high energy “tail” in electron energy distribution function. Experiment. An experiment was carried out on the spherical tokamak Globus-M (R=36cm, a=24cm, A=1.5) [4]. The RF generator with 920 MHz frequency and 100 kW power was used in the experiment. The pulse duration was up to 100 ms. First results were published in [5]. The wave excitation was performed by two comb-like antennae: one of them excited mostly poloidally slowed waves (PSA) and the other – toroidally slowed waves (TSA) (Fig. 1). The RF power was fed through the one end of the antenna, whereas the other end was loaded by matching resistance. Two gas valves placed far from antennae were used in experiments. Gas puffing (H2, D2) was programmed and optimized. Inductive toroidal electric field was not excited. Quasi-stationary toroidal magnetic field (Bt0 = 0.4T) and weak vertical magnetic field (BV ≈ 2–2.5 mT) were applied before the RF pulse. The value of BV at the breakdown stage was chosen to obtain maximal rate of the current increase at the discharge start. The evolution of the plasma parameters during the discharge is shown in Fig.2. After gas ionization and plasma formation the toroidal current arises up to the value of 4-5 kA. The current direction was dependent on the direction of BV. If BV is kept to be constant the current is saved on this level. At the next stage of the discharge, the vertical magnetic field was enhanced to increase the plasma current. The rate of current increase occurred to be 38 EPS Conference on Plasma Physics (2011) P4.098

The magnetic sensors (coils and loops), made of mineral insulated (MI) cables, are intended for ITER. In MI-cables the radiation initiates electric charge separation, emergence of the radiation induced electromotive force (RIEMF) and leakage current between cable strand and sheath. This effect may impact the magnetic measurements and other tokamak diagnostics using MI-cables. In this study a set of MI-cables (with different sizes and strand materials) were irradiated on the pulse reactor BARS-6 with dose rates from 10(3) Gy/s up to 10(5) Gy/s in pulse maximum. The analysis of the obtained data and the transient electric processes under pulse action permits us to propose a model of radiation-induced separation of electric charge in MI-cables between the insulated central cable strand and the grounded metal sheath.

The paper describes a diagnostic system for studying MHD plasma perturbations in the Globus-M spherical tokamak (a major radius of 0.36 m, a minor radius of 0.24 m, and an aspect ratio of 1.5). The system includes a poloidal and a toroidal array consisting of 28 and 16 Mirnov probes, respectively, as well as a 32-channel proportional soft X-ray detector. Methods are described for calculating the poloidal and toroidal numbers of the dominant helical perturbations by using data from probe measurements. Results are presented of processing the experimental data from some tokamak discharges with a plasma current of 150–250 kA, an average electron density of up to 10 20 m −3 , and a toroidal magnetic field of 0.4 T. Specific features of MHD perturbations and their influence on the parameters of the plasma column in different stages of a discharge are briefly discussed.

A measuring system has been developed to investigate the characteristics of radiation-induced optical absorption and radioluminescence in the spectral range of 400–1000 nm for a set of optical fibers being irradiated by fast neutrons and γ rays. This system has been used on the IR-8 research nuclear reactor to perform comparative tests of eight types of silica optical fibers from Heraeus (Germany), Fujikura (Japan), Mitsubishi (Japan), and the Fiber Optics Research Center of the Russian Academy of Sciences’ Prokhorov General Physics Institute. The measured dependences of the induced absorption and radioluminescence intensity versus the nuclear radiation wavelength, flux, and fluence, as well as the temperature, are analyzed. The microscopic mechanisms of induced absorption and radioluminescence are discussed. The values of induced optical absorption and radioluminescence in glass fibers containing H 2 gas and having a hermetic coating (the technology for producing such fibers has been developed by the Fiber Optics Research Center) is found to be many times smaller than in other fibers.

A magnetic diagnostics allowing one to reliably reconstruct equilibrium plasma configurations in a tokamak over a wide range of operating parameters is developed. The accuracy of determining the geometrical parameters and thermal energy of the tokamak plasma is analyzed in detail. The experimental data obtained in the Globus-M tokamak are processed the with help of the EFIT code. The influence of the plasma configuration on the intensity of the main impurity lines is investigated.

The results of studying the level of charge separation between the core and sheath in cables with mineral insulation and nickel and stainless-steel conductors are presented. The cables were irradiated in a BARS-6 pulsed reactor with dose rates of 3 × 103 and 4 × 103 Gy/s at temperatures of 270–650 K. A mechanism for elucidating the complex temperature dependence of the level of temperature-induced charge separation is proposed.

The results of the last two years of plasma investigations at Globus-M are presented. Described are improvements helping to achieve high performance OH plasmas, which are used as the target for auxiliary heating and fuelling experiments. Increased energy content, high beta poloidal and good confinement are reported. Experiments on NBI plasma heating with a wide range of plasma parameters were performed. Some results are presented and analyzed. Experiments on RF plasma heating in the frequency range of fundamental ion cyclotron harmonics are described. In some experiments which were performed for the first time in spherical tokamaks, promising results were achieved. Noticeable ion heating was recorded at low launched power and a high concentration of hydrogen minority in deuterium plasmas. Simulations of RF wave absorption are briefly discussed. Described also are modification of the plasma gun and test-stand experiments. Fuelling experiments performed at Globus-M are discussed.

A fast algorithm is elaborated for determining the position and shape of the plasma column from measurements performed with magnetic probes located outside the vacuum vessel of the GLOBUS-M tokamak. The algorithm is based on the modeling of the plasma current by movable current filaments and allows one to take into account the effect of eddy currents induced in the vacuum vessel. The algorithm was tested in a series of model discharges under conditions characteristic of the GLOBUS-M tokamak and serves now as a software component of its magnetic diagnostic system. By employing a conventional PC (Pentium 1 GHz, 200-MHz data bus), the calculation time of the plasma column parameters at one instant in time does not exceed 3 ms, which offers the possibility of controlling the plasma parameters during a discharge.

New results from the Globus-M spherical tokamak are presented. High plasma current of 0.36 MA, high toroidal magnetic field of 0.55 T and other important plasma characteristics were achieved. Described are the operational space and plasma stability limits in the OH regime. The factors limiting operational space (MHD instabilities, runaway electrons, etc.) are discussed. New experiments on plasma fuelling are described. First results of experiments with a coaxial plasma gun injector are presented. Initial results of a plasma – wall interaction study are outlined. First results obtained with new diagnostic tools installed on the tokamak are presented. An auxiliary heating system test was performed. Preliminary results of simulations and experiments are given.

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.

In this paper the magnetic diagnostics designed for plasma experiments on Globus-M tokamak is described, The main parameters, design and arrangement of the magnetic sensors are presented. The calibration technique and the method of determination of magnetic probe coordinates are explained.

The Globus-M spherical tokamak placed in A.F. Ioffe Institute is in operation since March 1999. The Globus-M tokamak peculiarity is its small aspect ratio plasma properties (R/a=1.5). Major parameters of the Globus-M tokamak are: plasma major radius 0.36 m, plasma minor radius 0.24 m, toroidal magnetic field 0.5 T, plasma current 0.3 MA and pulse length 0.3 s [1]. Manufacturing of toroidal field coils required high accuracy of processing and high quality of contact surfaces. Winding uniformity of the conductor, both between turns, and on layers in the radial direction was required for the central solenoid. The ability to change a toroidal field coil together with providing access to the vacuum vessel manholes without disassembly of the machine is discussed. Calculations indicating dependence of magnetic flux distribution from central solenoid winding quality are presented.

Results on radiation testing of the mineral insulated cables (MI-cables) with the sheaths and center leads of different size and different material are presented. Possible cause of the radiation induced electromotive force (RIEMF) and leakage currents arising in MI-cables under irradiation are discussed.