A set of magnetic coils used to correct the error fields at the Globus-M2 spherical tokamak, which appear due to the imperfections of the production and assembly of the tokamak magnetic system, is described. The magnetic sensors that are used to monitor the locked helical MHD modes are also described. The results of experiments on detecting the locked modes in the discharges with plasma heating by neutral beam injection are presented. A correlation is found between the appearance of the locked modes accompanied by the loss of fast ions and the confinement of the main plasma.

This paper highlights the most important results achieved at the spherical tokamak Globus-M2 with a high magnetic field. This paper also covers the most important topics of fusion research: thermal energy confinement in regimes with neutral beam injection, toroidal Alfvén eigenmode and correspondent fast ions confinement issues, L-H transition, turbulence suppression and edge-localized modes' behavior, experimental and theoretical study of regimes with nitrogen seeding that allow to significantly reduce thermal loads on the divertor plates, and experiments and simulations of lower hybrid current drive. The research results provide the basis for the next step toward a fusion neutron source—the development of the Globus-3 spherical tokamak.

The thermal energy stored in plasma Wp, normalized internal plasma inductance li and currentbeta bi are calculated via the free-boundary equilibrium PET code. The equilibrium reconstruction algorithmis iterative method of minimizing two parameters, the distance between the reconstructed plasmaboundary and that simulated by the PET code, as well as the difference between the plasma diamagnetic fluxfrom PET and the experimental one. The discharges from the Globus-M2 tokamak with a toroidal magneticfield up to 0.9 T and a plasma current of 0.3–0.4 MA in a mode with auxiliary heating by two atomic injectorsare analyzed. The possibility of using approximate formulas for estimating and is considered. The measureddiamagnetic flux is used to determine bdia and then to calculate Wdia . The normalized internal plasmainductance is additionally determined from the measured vertical magnetic field under the assumption that bp=bdia.

Cреди наблюдаемых на Глобус-М2 периферийных неустойчивостей выделяют краевые неустойчивости двух типов: синхронизированные и десинхронизированные с пилообразными колебаниями. Десинхронизированные срывы появляются в режимах, характеризующихся высокими значениями давления в пьедестале p ped > 3 кПа, и наблюдаются в разрядах с тороидальным магнитным полем B T > 0.6 Т и током по плазме I P > 0.3 МА. Десинхронизированные срывы краевой неустойчивости относятся к типу 3/5 с доминирующим влиянием пилинг-моды. Синхронизированные срывы наблюдались в более широком диапазоне параметров разряда, в том числе при B T < 0.6 Т и I P < 0.3 МА. Расчеты устойчивости пилинг-баллонной (ПБ) моды показали, что при ширине пьедестала ψ norm = = 0.09 и p ped > 3.5 кПа возможна дестабилизация ПБ-моды без дополнительного воздействия. Экспериментальные данные указывают на доминирующую роль микротиринговой неустойчивости в пьедестале. Микротиринговая мода не позволяет пьедесталу Глобуса-М2 достичь состояния неустойчивой кинетической баллонной моды, что объясняет низкую предиктивную силу модели EPED в токамаке Глобус-М2.

Исследован нагрев плазмы одним и двумя инжекторами быстрых нейтральных атомов в сферическом токамаке Глобус-М2 при тороидальном магнитном поле 0.8–0.9 Тл и токе плазмы 0.35–0.4 МА. Измерение пространственных распределений температуры и концентрации электронов, выполненные диагностикой томсоновского рассеяния лазерного излучения, показало двукратный нагрев электронов плазмы при инжекции нейтральных частиц с энергией до 45 кэВ при мощности пучка 0.75 МВт по сравнению с омическим режимом. Дополнительное включение второго пучка с энергией частиц до 30 кэВ и мощностью до 0.5 МВт позволило получить режим с горячими ионами в диапазоне значений средней плотности плазмы 1.6–10 × 10 19 м –3 . По данным активной спектроскопии и корпускулярной диагностики температура ионов достигла величины 4 кэВ при плотности плазмы 8 × 10 19 м –3 в горячей зоне, превысив температуру электронов более чем в 2.5 раза.

This work describes the real-time application of the Thomson scattering diagnostics. The upgraded data acquisition system of the Globus-M2 spherical tokamak provides real-time data processing with the delay <2.4 ms for 128 scattering signals from 11 spatial points. The achieved processing performance meets the requirements for Thomson scattering diagnostics of modern thermonuclear facilities and ITER in particular. The paper demonstrates the possibility of plasma electron density control, using Thomson scattering data in the feedback loop.

Using a specially designed time-of-flight mass reflectron with a heating element inside the device, which makes it possible to heat solid samples and analyze the gases released from them, protective graphite tiles covering almost the entire plasma-facing surface of the Globus-M2 tokamak vacuum chamber were studied. Mass spectrometric analysis made it possible to determine the composition of the released gases to estimate their quantities, and thus to quantitatively characterize the quality of cleaning and degassing of tiles. It is shown that as a result of the applied method for processing tiles, the content of the light hydrogen isotope - protium, water, carbon monoxide and dioxide decreased several times, and the content of deuterium and helium isotopes decreased almost to zero.

A neutron diagnostic system was developed at the Ioffe Institute as part of the Globus-M2 tokamak to optimize NBI heating conditions and evaluate heating efficiency. The system contains two compact neutron spectrometers based on the liquid organic scintillator BC-501A and two gas-discharge counters based on a 10B isotope. The BC-501A spectrometers were calibrated by measuring neutron emission produced in a 9Be(α,n)12C nuclear reaction on the cyclotron facility at the Ioffe Institute. In addition, in situ calibrations of the system, including the neutron spectrometers and the gas-discharge counters, was carried out using an Am–Be neutron source to provide accurate measurements of the total neutron yield from the plasma of the Globus-M2 tokamak. During the plasma experiments at the Globus-M2 tokamak, a deuterium beam was injected into the deuterium plasma that causes a yield of the DD-neutrons with ∼2.45 MeV energy. The neutron spectrometry diagnostic system was used to provide neutron measurements and detect the DD-neutrons in these experiments. The neutron yield and the DD-reaction rate during plasma discharges were evaluated. The energy distributions of neutrons emitted from plasma during discharges with neutron beam injection were reconstructed from the measured neutron spectra.

The work presents the results of the energy confinement study carried out on the compact spherical tokamak Globus-M2 with a toroidal magnetic field as high as 0.8 T. A reproducible and stable discharge was obtained with the average plasma density (5–10) × 10 19 m −3 . Despite the increase in the magnetic field, the neutral beam injection (NBI) led to clear and reproducible transition to the H-mode accompanied by a decrease in the turbulence level at the plasma edge. NBI allowed effectively heat the plasma: electron and ion temperatures in the plasma core exceeded 1 keV. Compared to the previous experiments carried out with a toroidal magnetic field as high as 0.4 T plasma total stored energy was increased by a factor of 4. The main reason or this phenomenon is a strong dependence of the energy confinement time on the toroidal magnetic field in the spherical tokamak. It was experimentally confirmed that such kind of dependence is valid for ST with magnetic field up to 0.8 T. It has also been shown that the enhancement of the energy confinement in Globus-M2 with collisionality decrease is associated with an improvement of both electron and ion thermal insulation.

Hot ion mode was recently achieved at the Globus-M2 spherical tokamak when the toroidal magnetic field was as high as 0.9 T and plasma current 0.35 MA. The injection of two neutral beams with a total power of 0.7 MW and an energy of 30 keV into a plasma with major radius 0.36 m and minor radius 0.22 m (aspect ratio 1.6) made it possible to heat the plasma ions up to 4 keV, while electron density in the plasma center was as high as 8 × 1019 m−3. The achievement of high temperatures became possible due to the good thermal insulation of the plasma.

Studies were performed on the possibility of using the coaxial plasma jet accelerator to facilitate breakdown and the plasma current start-up in the Globus-M2 and KTM tokamaks. The results of experiments on the jet injection into the Globus-M2 tokamak with a magnetic field of 0.7 T showed that the discharge current rise began 1–2 ms earlier than it did in experiments using gas puffing and inductive breakdown of the working gas. In this case, the breakdown voltage decreased approximately 2 times. The results of experiments on plasma jet injection through the equatorial diagnostic duct into the KTM tokamak with a magnetic field of 0.9 T showed that the gas breakdown occurred up to 6 ms earlier than the breakdown initiated using the inductive method. In this case, the loop voltage decreases approximately 1.5 times. In almost all shots using the accelerator, the gas breakdown occurred, while in its absence, sometimes the gas breakdown could not be obtained.

on the spherical tokamak Globus-M2 V.B. Minaev, V.K. Gusev, Yu.V. Petrov, N.V. Sakharov, V.I. Varfolomeev, P.A. Bagryansky, N.N. Bakharev, F.V. Chernyshev, I.N. Chugunov, M.V. Iliasova, A.A. Kavin, E.M. Khilkevitch, I.A. Khodunov, N.A. Khromov, E.O. Kiselev, G.S. Kurskiev, M.M. Larionova, A.D. Melnik, I.V. Miroshnikov, A.N. Novokhatskii, K.Yu. Oshuev, M.I. Patrov, A.E. Shevelev, P.B. Shchegolev, I.V. Shikhovtsev, A.L. Solomakhin, A.Yu. Telnova, V.A. Tokarev, S.Yu. Tolstyakov, E.G. Zhilin 1 Ioffe Institute, St. Petersburg, Russia 2 BINP SB RAS, Novosibirsk, Russia 3 JSC «NIIEFA», St. Petersburg, Russia 4 Ioffe Fusion Technology Ltd., St. Petersburg, Russia

Recent research at three small tokamaks with different parameters located at the Ioffe Institute—the spherical tokamak Globus-M, the large aspect ratio tokamak FT-2 and the compact tokamak TUMAN-3M—are reviewed. This overview covers energy confinement (Globus-M and FT-2), L–H transition (TUMAN-3M and FT-2), Alfvén waves (Globus-M and TUMAN-3M), ion cyclotron emission (TUMAN-3M), major plasma discharge disruption (Globus-M) and scrape-off layer (Globus-M) studies. A full-f global gyrokinetic modeling benchmark using synthetic diagnostics in FT-2 is described. Anomalous absorption and emission in electron cyclotron resonance heating experiments due to the parametric excitation of localized upper hybrid waves are analyzed theoretically. Progress in the development of the neutral particle analysis, gamma-ray spectrometry and divertor Thomson scattering combined with laser-induced fluorescence diagnostics for ITER is discussed. The status of the new Globus-M2 spherical tokamak is reported.

Abstract The study of elements of rail electromagnetic accelerator (railgun), aimed at studying the mechanism of formation of the dense and free from impurities plasma jet with high kinetic energy is presented. The accelerator was tested with pulsed gas inlet, different shape and length of electrodes, as well as with an additional magnetic field created by external conductors with current. The method controlling of plasma jet parameters on test bench with the use of pressure sensor and infrared video camera was developed. The dependence of the pressure of deuterium plasma flow on distance to the accelerator was investigated. The kinetic energy of the jet was estimated.

Поступило в Редакцию 20 марта 2017 г

We present the results of studies of the plasma source based on the coaxial accelerator with the slothole channel geometry for plasma acceleration and working gas inlet into the accelerator via the electrodynamic valve. The plasma parameters at the output of the accelerator are measured. The slot-hole channel of the accelerator created higher jet pressure, as compared to the coaxial channel, especially at large distances from the source. The jet pressure reached 106 N/m2 at a distance of 0.7 m. The source created moderately pure plasma for a current below 80 kA. The density was (2.5–5) × 1022 m–3, which was higher than the density obtained with the coaxial gun.

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.