Characteristics of major plasma discharge disruption in the Globus-M spherical tokamak

The characteristics of the major disruption of plasma discharges in the Globus-M spherical tokamak are analyzed. The process of current quench is accompanied by the loss of the vertical stability of the plasma column. The plasma boundary during the disruption is reconstructed using the algorithm of movable filaments. The plasma current decay is preceded by thermal quench, during which the profiles of the temperature and electron density were measured. The data on the time of disruption, the plasma current quench rate, and the toroidal current induced in the tokamak vessel are compared for hydrogen and deuterium plasmas. It is shown that the disruption characteristics depend weakly on the ion mass and the current induced in the vessel increases with the disruption time. The decay rate of the plasma toroidal magnetic flux during the disruption is determined using diamagnetic measurements. Such a decay is a source of the poloidal current induced in the vessel; it may also cause poloidal halo currents.     

Structure of the controlled halo-current magnetic field in the T-10 tokamak

In the T-10 tokamak, the magnetic field spatially resonant with a helical MHD perturbation is generated using the controlled halo current supplied using a contact method in the scrape-off-layer plasma. This paper is concerned with studying the spatial structure of the halo current and its magnetic field. For this purpose, the magnetic field of the halo current was measured in one of the cross sections of the torus near the tokamak vacuum vessel wall. The spatial distribution of the magnetic field as a function of the halo current configuration was calculated in the cylindrical approximation. The terms proportional to the plasma pressure were disregarded. The configuration of the halo current and the spatial structure of its magnetic field were determined by comparing the calculated and experimental results.     

Analysis of Dynamics of Plasma Current Quench in the Globus-M Spherical Tokamak

Data on the dynamics of the plasma current quench in the Globus-M tokamak are presented. The main current quench characteristics at different toroidal magnetic fields are compared. The distribution of the toroidal current induced in the vessel wall is determined from magnetic measurements, and the electromagnetic loads acting on the vessel wall during the current quench are calculated. By extrapolating the experimental data, the additional pressure on the vessel wall during the current quench in the upgraded Globus-M2 tokamak is estimated. It is shown that the current quench results in the appearance of bending stresses in the vessel domes. Using numerical simulations, it is shown that the best agreement between the measured and calculated plasma current dynamics during the current quench corresponds to the linear (in time) influx of the carbon impurity.     

Devices for Diagnostics and Lithium Collection on the T-11M and T-10 Tokamaks. First Results

The use of lithium as a material of the tokamak in-vessel plasma-facing components made it necessary to develop appropriate diagnostic instruments. For the T-10 and T-11M tokamaks, devices have been developed that allow one to investigate the processes of lithium transport in the tokamak scrape-off layer, the dynamics of lithium deposition at different temperatures of the collecting surface in real time by using a piezoelectric quartz detector, adsorption and desorption of the plasma-forming gas by lithium, and the influence of the electric field on the process of lithium collection. The plasma parameters are monitored using Langmuir probes. The developed devices can be used to extract lithium deposited on the tokamak vessel wall without breaking vacuum conditions. For these purposes, a gateway and a vacuum input without bellows have been designed on the basis of an innovative liquid-metal coupling.     

Injection of a high-density plasma into the Globus-M spherical tokamak

A new type of plasma source with titanium hydride granules used as a hydrogen accumulator was employed to inject a dense, highly ionized plasma jet into the Globus-M spherical tokamak. The experiments have shown that the jet penetrates through the tokamak magnetic field and increases the plasma density, without disturbing the stability of the plasma column. It is found that, when the plasma jet is injected before a discharge, more favorable conditions (as compared to those during gas puffing) are created for the current ramp-up at a lower MHD activity in the plasma column. Plasma injection at the instant of maximum current results in a more rapid growth in the plasma density in comparison to gas puffing.     

Topology of drift trajectories of charged particles in a tokamak

The topology of drift orbits in a tokamak is analyzed in the entire cross section of the device both near the magnetic axis and at the periphery of the plasma column. The use of invariants of the drift equations (the generalized momentum, magnetic moment, and total energy) as variables for the entire cross section of the plasma column and self-similar variables near the magnetic axis makes it possible to comprehensively classify closed drift orbits in a tokamak. When describing orbits of different types and domains of their existence, discriminant and locus curves obtained by the methods of differential geometry are used to determine the ranges in which the invariants vary. The influence of the nonuniformity of the longitudinal current on the drift trajectories of fast particles is studied. The works in which, together with known types of orbits, trajectories along which particles leave the plasma column and can fall on the chamber wall are analyzed.     

Estimation of the radial force on the tokamak vessel wall during fast transient events

The radial force balance in a tokamak during fast transient events with a duration much shorter than the resistive time of the vacuum vessel wall is analyzed. The aim of the work is to analytically estimate the resulting integral radial force on the wall. In contrast to the preceding study [Plasma Phys. Rep. 41, 952 (2015)], where a similar problem was considered for thermal quench, simultaneous changes in the profiles and values of the pressure and plasma current are allowed here. Thereby, the current quench and various methods of disruption mitigation used in the existing tokamaks and considered for future applications are also covered. General formulas for the force at an arbitrary sequence or combination of events are derived, and estimates for the standard tokamak model are made. The earlier results and conclusions are confirmed, and it is shown that, in the disruption mitigation scenarios accepted for ITER, the radial forces can be as high as in uncontrolled disruptions.     

Collisionless current generation in the center of the tokamak plasma by an isotropic source of α-particles

The density of the noninductive current generated due to collisionless motion of α-particles in the tokamak magnetic field is calculated. The analysis is based on fully three-dimensional calculations of charged particle trajectories without simplifying assumptions typical for drift and neoclassical approaches. The current is calculated over the entire cross section of the plasma column, including the magnetic axis. It is shown that the current density is not a function of a magnetic surface and is strongly polarized over the poloidal angle. The current density distribution in the tokamak poloidal cross section is obtained, and the current density as a function of the safety factor, the tokamak aspect ratio, and the ratio of the particle Larmor radius on the axis to the tokamak minor radius is determined. It is shown that, when the source of α-particles is spatially nonuniform, the current density in the center of the tokamak is nonzero due to asymmetry of the phase-space boundary between trapped and passing particles. The current density scaling in the tokamak center differs from the known approximations for the bootstrap current and is sensitive to the spatial distribution of α-particles.     

Modeling of the feedback stabilization of the resistive wall modes in a tokamak

The problem of feedback stabilization of the resistive wall modes (RWMs) in a tokamak is discussed. An equilibrium configuration with the parameters accepted for the stationary ITER scenario 4A is considered as the main scenario. The effect of the vacuum chamber's shape on the plasma stability is studied. Ideal MHD stability is analyzed numerically by using the KINX code. It is shown that, in a tokamak with the parameters of the designed T-15M tokamak, RWMs can be stabilized by a conventional stabilizing system made of framelike coils. However, the maximum possible gain in β in such a tokamak is found to be smaller than that in ITER. It is shown that, in this case, a reduction in the plasma—wall gap width by 10 cm allows one to substantially increase the β limit, provided that RWMs are stabilized by active feedback.     

MHD diagnostics in the T-11M tokamak

An MHD diagnostic system for investigating the dynamics of disruption and the preceding phase of the discharge in the T-11M tokamak is described. This system makes it possible to study the structure of magnetic fluctuations in the plasma column. The diagnostic system includes a set of magnetic pick-up loops (Mirnov coils) arranged in several poloidal cross sections of the tokamak, a data acquisition system that provides synchronous recording of Mirnov coil signals, a synchronization system for triggering the data acquisition system during a disruption, and a system for processing and representation of the experimental data on magnetic fluctuations in the plasma column. Examples of how the MHD diagnostic system operates in the T-11M tokamak are presented.     

Diagnostics of MHD instability in the Globus-M spherical tokamak

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 10²⁰ 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.     

Numerical analysis of the near field of ICRH antennas in a tokamak

Results are presented from numerical calculations of the near fields of ICRH antennas in the quasisteady current approximation in two-dimensional geometry. The distributions of the vacuum electric and magnetic fields as well as of the surface current density in the antenna elements and inside the tokamak chamber are obtained. The electrotechnical characteristics of the antennas are analyzed numerically as functions of their geometric parameters.     

Parameters of turbulent structures at the periphery of the FT-2 tokamak

Results are presented from probe measurements carried out in the scrape-off layer of the FT-2 tokamak in the course of additional lower hybrid heating, during which an L-H transition was observed. The objective of this study was to obtain information on the parameters of blobs-turbulent structures with enhanced plasma density. The measurements were performed not only on the low-field side of the torus, but also on the high-field side, which is still poorly studied. Coherent structures with radial velocities directed both toward the vessel wall and into the plasma column were revealed at the tokamak periphery. Blobs propagating toward the vessel wall were found to prevail both before and after the L-H transition. The average radial velocity of blobs in the L- and H-modes was determined experimentally. The dependence of the radial blob velocity on the transverse size and density of the structure agrees with the ballooning mode model. It is found that the average value of the poloidal blob velocity is four to five times higher than the average radial velocity. The results of measurements carried out on both sides of the torus indicate the presence of internal poloidal polarization of blobs. The average drift velocity of such polarized structures is directed toward the vessel wall. The L-H transition is accompanied by a reduction in the radial velocity. At the same time, the average plasma density inside the structures observed on the low-field side increases appreciably during the transition. The obtained dependences of the radial blob velocity on the plasma density inside the structure generally agree with predictions of the ballooning mode model.     

Investigation of the Edge Plasma Parameters and Measurements of the Plasma Longitudinal Rotation Velocity by a Mach Probe in a Lithium Experiment on the T-11M Tokamak

The edge plasma parameters were measured by means of a Mach probe in a lithium experiment on the T-11M tokamak. The angular and radial distributions of the ion saturation current, along with the radial distribution of the electron temperature, were obtained in different modes of tokamak operation. The radial distributions of the electron temperature and ion saturation current in the main operating mode (L-mode) revealed a peak in the scrape-off-layer of the vertical limiter (lithium emitter), which can indicate the formation of a magnetic island in this region. The measured plasma flow velocity along the magnetic field was found to be close to one-half of the ion sound velocity for Li⁺ ions.     

Tokamak with Reactor Technologies (TRT): Preliminary Analysis of Nuclear Energy Release in Toroidal Field Coils

Plasma Physics Reports - Results of calculation of nuclear heat deposition rate in the vacuum vessel and magnetic coils of the tokamak with reactor technologies are presented. Results obtained for...     

Influence of the safety factor inhomogeneity on collisionless current generation in tokamaks

An improved scaling for the current density generated due to collisionless motion of α-particles in a tokamak is proposed. The dependence of the current density on the radial profile of the safety factor q is investigated. Monotonically increasing q profiles are considered, as well as q profiles with a minimum in the axial region of the plasma column. It is shown that the current density depends on the variation in q along the charged particle trajectories, rather than on the q value at the starting magnetic surface. The dependence of the current density on the gradient of q is strongest in the plasma core because of the large deviation of the drift surfaces from the magnetic ones in this region. At the plasma edge, the larger the second derivative of the plasma density, the greater the contribution of the gradient of q. For conventional plasma density profiles, the poloidal-angle-averaged current density calculated for a varying safety factor q is always lower than that calculated for a constant q. The effect of the nonuniformity of the safety factor on the current generation at the magnetic axis of a tokamak is investigated.     

Structure of global tokamak scalings

In empirical tokamak scalings, not all of the standard engineering parameters are independent. Thus, the larger the tokamak, the higher the required plasma current and input power. Also, by using higher magnetic fields, it is possible to raise the plasma density. Instead of the plasma density, plasma current, and input power, it is proposed to use such combinations of engineering parameters whose values are essentially the same for different tokamaks. With this approach, the number of free scaling parameters can be reduced from six to three, thereby improving the reliability of the scaling.     

Physical nature of the electric current produced by the asymmetry of particle motion in toroidal magnetic confinement systems

A new type of longitudinal electric current is revealed by analyzing the drift trajectories of charged particles in a tokamak—the current that may be referred to as the asymmetry current because it is associated with the asymmetry of the boundary between trapped and transit particles in phase space. The generation of this current is explained by the fact that the motions of the particles that cross the magnetic surface at a given point in opposite directions are qualitatively different. The asymmetry current results from the toroidal variations of the magnetic field and is maintained by the radial momentum flux of transit particles. The contribution of the particles of different species to the asymmetry current density is proportional to their pressure, is independent of the gradients of the plasma parameters, is maximum at the magnetic axis, and decreases toward the plasma periphery. In contrast to standard neoclassical theory, the asymmetry current can be found only from exact particle trajectories. The asymmetry current is calculated for tokamaks with differently shaped magnetic surfaces and for a model stellarator. By exploiting the newly revealed asymmetry current, together with the bootstrap current, it may be possible to substantially simplify the problem of creating a tokamak reactor.     

Study of runaway electrons using the conditional average sampling method in the Damavand tokamak

Some experiments for studying the runaway electron (RE) effects have been performed using the poloidal magnetic probes system installed around the plasma column in the Damavand tokamak. In these experiments, the so-called runaway-dominated discharges were considered in which the main part of the plasma current is carried by REs. The induced magnetic effects on the poloidal pickup coils signals are observed simultaneously with the Parail–Pogutse instability moments for REs and hard X-ray bursts. The output signals of all diagnostic systems enter the data acquisition system with 2 Msample/(s channel) sampling rate. The temporal evolution of the diagnostic signals is analyzed by the conditional average sampling (CAS) technique. The CASed profiles indicate RE collisions with the high-field-side plasma facing components at the instability moments. The investigation has been carried out for two discharge modes—low-toroidal-field (LTF) and high-toroidal-field (HTF) ones—related to both up and down limits of the toroidal magnetic field in the Damavand tokamak and their comparison has shown that the RE confinement is better in HTF discharges.     

Measurements of the plasma radiative loss profile in the M-11M tokamak with the help of a tangential-view AXUV photodiode array

The plasma radiative loss profile in the T-11M tokamak operating with a lithium limiter was measured using a sixteen-channel absolute extreme-ultraviolet photodiode array. The field of view of the detector was set in a vertical plane tangential to the plasma column axis. The radiative loss profile was recovered by solving an inverse problem under the assumption of toroidal and poloidal symmetry of the plasma column. A stable algorithm is developed for solving the problem with this geometry, and the possible errors of the method are evaluated. The radiative loss profiles and their evolution in various tokamak regimes are derived.