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.     

On the nature of dark spots arising in a tokamak plasma during a disruption

The sudden appearance of bright and dark spots against the background lithium emission from the central regions of the plasma column was observed during major plasma disruptions in the T-11M tokamak with a lithium limiter. The measurements were performed with the help of an AXUV photodiode array operating in the energy range 1–5000 eV. Such spots in the plasma core arise in the fast transient stage of a major disruption (during the onset of the positive pulse of the plasma current) and are rather narrowly localized over r (in particular, over the vertical axis). It is supposed that the observed dark spots are related to the development of magnetic islands induced in the plasma core by an outer MHD perturbation. This effect can be used as a tool for studying specific forms of MHD resonance.     

Characteristics of discharge disruptions in the T-10 tokamak

The results of experimental studies of discharge disruptions in the T-10 tokamak at the limiting plasma density are presented. On the basis of measurements of the generated soft X-ray emission, for a group of “slow” disruptions, the dynamics of the magnetic configuration of the central part of the plasma column is studied and the possible role of the m/n = 1/1 mode in the excitation of predisruptions or the final stage of disruption is analyzed. It is shown that the characteristics of plasma electron cooling in predisruptions correspond to those of electron cooling upon pellet injection into T-10 and in discharge predisruptions occurring in regimes with the “quiet mode.” It is found that, in the latter case, the reason for predisruptions and fast electron cooling in the plasma core is the instability of the m/n = 2/1 mode, its spontaneous spatial reconstruction, and the generation of a “cooling wave” during this process. Measurements of the electron temperature (determined from the plasma radiation intensity at the second electron cyclotron harmonic) in the zone of the m/n = 2/1 mode have shown that the transformation of the m/n = 2/1 mode leads to the excitation of predisruptions and the final phase of disruption not only in regimes with the “quiet mode,” but also in disruptions of ordinary ohmic discharges. The experimental results obtained in this work make it possible to determine the scenario of the development of “slow” discharge disruptions in the T-10 tokamak at the limiting plasma density.     

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.     

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.     

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.     

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.     

Real-time determination of the position and shape of the plasma column from external magnetic measurements in the GLOBUS-M tokamak

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.     

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.     

Hydrodynamic model of a dielectric-barrier discharge in pure chlorine

A one-dimensional hydrodynamic model of a dielectric-barrier discharge (DBD) in pure chlorine is developed, and the properties of the discharge are modeled. The discharge is excited in an 8-mm-long discharge gap between 2-mm-thick dielectric quartz layers covering metal electrodes. The DBD spatiotemporal characteristics at gas pressures of 15–100 Torr are modeled for the case in which a 100-kHz harmonic voltage with an amplitude of 8 kV is applied to the electrodes. The average power density deposited in the discharge over one voltage period is 2.5–5.8 W/cm³. It is shown that ions and electrons absorb about 95 and 5% of the discharge power, respectively. In this case, from 67 to 97% of the power absorbed by electrons is spent on the dissociation and ionization of Cl₂ molecules. Two phases can be distinguished in the discharge dynamics: the active (multispike) phase, which follows the breakdown of the discharge gap, and the passive phase. The active phase is characterized by the presence of multiple current spikes, a relatively high current, small surface charge density on the dielectrics, and large voltage drop across the discharge gap. The passive phase (with no current spikes) is characterized by a low current, large surface charge density on the dielectrics, and small voltage drop across the discharge gap. The peak current density in the spikes at all pressures is about 4 mA/cm². In the multispike phase, there are distinct space charge sheaths with thicknesses of 1.5–1.8 mm and a mean electron energy of 4.3–7 eV and the central region of quasineutral plasma with a weak electric field and a mean electron energy of 0.8–3 eV. The degree of ionization of chlorine molecules in the discharge is ~0.02% at a pressure of 15 Torr and ~0.01% at 100 Torr. The DBD plasma is electronegative due to the fast attachment of electrons to chlorine atoms: e + Cl₂ → Cl + Cl^–. The most abundant charged particles are Cl ₂ ⁺ and Cl^? ions, and the degree of ionization during current spikes in the active phase is (4.1–5.5) × 10^–7. The mechanism of discharge sustainment is analyzed. The appearance of a series of current spikes in the active phase of the discharge is explained.     

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.     

Analysis of electron thermal diffusivity and bootstrap current in ohmically heated discharges after boronization in the HT-7 tokamak

Significant improvements of plasma performance after ICRF boronization have been achieved in the full range of HT-7 operation parameters. Electron power balance is analyzed in the steady state ohmic discharges of the HT-7 tokamak. The ratio of the total radiation power to ohmic input power increases with increasing the central line-averaged electron density, but decreases with plasma current. It is obviously decreased after wall conditioning. Electron heat diffusivity χ_e deduced from the power balance analysis is reduced throughout the main plasma after boronization. χ_e decreases with increasing central line-averaged electron density in the parameter range of our study. After boronization, the plasma current profile is broadened and a higher current can be easily obtained on the HT-7 tokamak experiment. It is expected that the fact that the bootstrap current increases after boronization will explain these phenomena. After boronization, the plasma pressure gradient and the electron temperature near the boundary are larger than before, these factors influencing that the ratio of bootstrap current to total plasma current increases from several percent to above 10%.     

Plasma Cooling During Disruptions of Ohmic Discharges at the T-10 Tokamak

Plasma Physics Reports - Experimental data on plasma cooling during slow discharge disruptions at the T-10 tokamak are analyzed. It is shown that, in the initial phase of thermal quench at T-10,...     

Evidence of cold bubble-like structure in START density limit plasmas

Cold bubble (CB) structures were observed in START density limit studies for the first time in a low aspect ratio tokamak. They seem related to minor and major disruption processes, clearly identified here as a trigger to those events. Enormous discrepancies on the CB velocities in several devices are reported. This shows that the physical mechanisms related to the time scales for its propagation should be revised. Several models related to CB formation and its role in the disruptive process or just in a plasma with the presence of sawteeth qualitatively predict a great part of the observations. Published in Russian in Fizika Plazmy, 2008, Vol. 34, No. 9, pp. 786–790. The text was submitted by the authors in English.     

Behavior of hard X-ray emission in discharges with current disruptions in the DAMAVAND and TVD tokamaks

Results are presented from studies of the behavior of hard X-ray emission in discharges with current disruptions in the DAMAVAND and TVD tokamaks. The current disruptions are caused by either an MHD instability or the instability related to the vertical displacement of the plasma column. Experiments were conducted at a fixed value of the safety factor at the plasma boundary (q _a ? 2.3). Experimental data show that, during a disruption caused by an MHD instability, hard X-ray emission is suppressed by this instability if the amplitude of the magnetic field fluctuations exceeds a certain level. If the disruption is caused by the instability related to the vertical displacement of the plasma column, then hard X-ray emission is observed at the instant of disruption. The experimental results show that the physical processes resulting in the generation and suppression of runaway electron beams are almost identical in large and small tokamaks.     

Characteristic properties of fluctuating particle fluxes near the last closed flux surface in the course of lower hybrid heating and during a transition into the improved confinement mode in the FT-2 tokamak

Results are presented from experimental studies of the time behavior of the transport processes in the edge plasma of the FT-2 tokamak during auxiliary lower hybrid heating when an internal transport barrier and then an external transport barrier form in the plasma. An analysis of the data on turbulent particle transport in the edge plasma shows that the radial electric field generated inside the plasma column during auxiliary heating plays an important role in both the formation of a transport barrier and the suppression of anomalous transport at the plasma periphery in the postheating phase of the discharge. The mechanism for the formation of a negative radial electric field E _r near the last closed flux surface after the end of the lower hybrid heating pulse is considered. Fluctuation spectra of the particle density and poloidal electric field are presented that characterize the process of suppression of microturbulence at the plasma periphery. The experimental data were obtained with the use of movable multielectrode Langmuir probes.     

On the nature of global plasma oscillations in a tokamak. Part I: Kinetic properties of oscillations observed at the T-10 tokamak

Large-scale plasma oscillations (so-called MHD oscillations) in the T-10 tokamak are investigated. The central electron cyclotron heating was used to enhance oscillations at the m/n = 1/1 mode with the goal of determining the internal characteristics of the process. Spatially resolved measurements of electron cyclotron emission in a wide frequency range for two polarizations allowed for analyzing a number of effects indicating the kinetic nature of plasma oscillations. The major part of the electron distribution over longitudinal velocities in the plasma core experiences strong periodic oscillations accompanied by synchronous spikes of radiation emitted by high-energy electrons at the edge of the plasma column.     

A van der Pol coupled-oscillator model as a basis for developing a system for suppressing MHD instabilities in a tokamak

The main parameters of tokamak discharges are known to be limited by large-scale MHD instabilities. Sometimes, the instabilities lead to a rapid (on time scales of tens of microseconds) disruption of the discharge current and to the release of all the energy stored in the plasma column at the discharge chamber wall. This process, which is called the disruptive instability, may have irreversible catastrophic consequences for the operation of a fusion reactor. In the present paper, a study is made of the dynamics of self-oscillations in systems of two and six van der Pol coupled oscillators. A van der Pol coupled-oscillator model is used to develop a multivariable feedback controller based on the combined principle of compensating for internal cross feedbacks within the object and introducing damping feedbacks in each control channel. By using mathematical simulation methods, it is shown that the controller designed guarantees the suppression of self-oscillations in a system of van der Pol oscillators over a fairly broad range of parameters of the object under control (and thereby provides the structural stability of the object). The nonlinear control system model makes it possible to suppress coupled MHD perturbations developing in a tokamak plasma.     

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.