Characterization of intermittent bursts at the edge of the CASTOR tokamak

Temporal characteristics of density bursts in the scrape-off layer of the CASTOR tokamak were investigated. Intermittent positive bursts of density are observed by means of a radial array of Langmuir probes. Globally (comparing the closest and furthermost radial positions with respect to the center of the discharge chamber) we observe a radial decrease of average burst rate together with a radial increase of the average burst duration. Recently, we observed a monotonic radial decrease of average burst rate together with an increase of the average burst duration in the Tore Supra tokamak [1, 2], but the decay length is significantly shorter than in CASTOR. This is most probably due to radially elongated turbulent events (streamers), which govern the radial transport at the edge of the CASTOR tokamak [3] and are responsible for the appearance of density bursts. Published in Russian in Fizika Plazmy, 2008, Vol. 34, No. 9, pp. 781–785. The text was submitted by the authors in English.     

Improvement of plasma energy confinement in tokamak under radiative cooling of the edge plasma

Improvement of plasma energy confinement in the T-10 tokamak by injection of impurity gases was studied experimentally. Injection of Ne and He in the ohmic and ECR heating regimes allows one to separate the dependences of energy confinement on the plasma density and on the edge plasma cooling rate. It is shown that the well-known dependence of the energy confinement time on the plasma density is, in fact, the dependence on the radiative loss power. This phenomenon can be explained by plasma self-organization. The experiments are described by a thermodynamic model for self-organized plasma in which the transport coefficient depends on the difference between the actual and self-consistent pressure profiles. The reduction in the heat flux at the plasma edge due to radiative cooling leads to a decrease in the transport coefficient in this region and, accordingly, improves energy confinement. Results of approximate model calculations for experiments with Ne injection are presented.     

Turbulent dynamics of helical perturbations in the edge plasma of the T-10 tokamak

Turbulent dynamics of the edge plasma in the T-10 tokamak is simulated numerically by solving nonlinear MHD equations in the framework of the four-field {?, n, p _e , p _i } reduced two-fluid Braginskii hydro-dynamics. It is shown that the transition from ohmic to electron-cyclotron heating is accompanied by a decrease in the amplitudes of turbulent fluctuations in plasma. This is caused by the enhancement of longitudinal dissipation due to the increase in the electron temperature. However, phase relations between potential fluctuations of different modes change in such a way that the Reynolds turbulent force increases, which leads to an increase in the poloidal velocity in the direction of ion diamagnetic drift. Since the poloidal and ion diamagnetic drift velocities enter into the equation of the radial force balance for ions with different signs, the radial electric field decreases. The simulation results agree qualitatively with the results of experiments in the T-10 tokamak. The dependence of the radial electric field on the plasma density, ion pressure, and neutral density is also calculated.     

Numerical simulation of drift-resistive ballooning turbulence in the presence of the GA mode in the plasma edge tokamak

Drift-resistive ballooning turbulence is simulated numerically based on a quasi-three-dimensional computer code for solving nonlinear two-fluid MHD equations in the scrape-off layer plasma in a tokamak. It is shown that, when the toroidal geometry of the magnetic field is taken into account, additional (geodesic) flux terms associated with the first poloidal harmonic (∼sinθ) arise in the averaged equations for the momentum, density, and energy. Calculations show that the most important of these terms is the geodesic momentum flux (the Stringer-Windsor effect), which lowers the poloidal rotation velocity. It is also shown that accounting for the toroidal field geometry introduces experimentally observed, special low-frequency MHD harmonics—GA modes—in the Fourier spectra. GA modes are generated by the Reynolds turbulent force and also by the gradient of the poloidally nonuniform turbulent heat flux. Turbulent particle and heat fluxes are obtained as functions of the poloidal coordinate and are found to show that, in a tokamak, there is a “ballooning effect” associated with their maximum in the weak magnetic field region. The dependence of the density, temperature, and pressure on the poloidal coordinate is presented, as well as the dependence of turbulent fluxes on the toroidal magnetic field.     

Numerical modeling of L-H transitions in the presence of a radial current at the edge of a tokamak plasma

Hydrodynamic equations describing wall plasma turbulence are analyzed numerically using a two-dimensional four-field model. Turbulent transport coefficients are calculated with consideration of the radial current. Numerical analysis revealed a possible scenario for L-H transitions that is associated with the radial current driven by nonambipolar processes. It is shown that the transition of a plasma to an improved confinement mode can also be triggered by other mechanisms.     

Resonant magnetic perturbations and edge ergodization on the COMPASS tokamak

Results of calculations of resonant magnetic perturbation spectra on the COMPASS tokamak are presented. Spectra of the perturbations are calculated from the vacuum field of the perturbation coils. Ergodization is then estimated by applying the criterion of overlap of the resulting islands and verified by field line tracing. Results show that for the chosen configuration of perturbation coils an ergodic layer appears in the pedestal region. The ability to form an ergodic layer is similar to the theoretical results for the ELM suppression experiment at DIII-D; thus, a comparable effect on ELMs can be expected. Published in Russian in Fizika Plazmy, 2008, Vol. 34, No. 9, pp. 808–811. The text was submitted by the authors in English.     

Determination of the electron density in the tokamak edge plasma from the time evolution of a laser-induced fluorescence signal from atomic helium

The time evolution of a laser-induced fluorescence signal from neutral helium He I in the edge and divertor plasmas in modern magnetic confinement devices is considered. Computations are performed on the basis of a nonstationary collisional-radiative model involving ten singlet and nine triplet states of helium that affect the time evolution of the fluorescence signal. A new method is proposed for determining the electron density from measurements of the time derivative of the profile of the fluorescence spectral line.     

Bifurcations of axisymmetric plasma equilibrium in a tokamak

Bifurcation of solutions to the Grad–Shafranov-type equation for helically symmetric plasma near the threshold for tearing instability are analyzed. Quadratic and cubic nonlinearities were added to the linear dependence of the current density on the helical flux. Depending on the character of nonlinearity, two types of bifurcation can be observed, the “small” and the “large” ones. The small bifurcation is typical of cubic nonlinearity and reveals itself in the growth of the magnetic island from zero as the profile parameter increases above the instability threshold. The large bifurcation is typical of quadratic nonlinearity and causes jumplike formation of a large-scale magnetic island upon exceeding the instability threshold. As the profile parameter decreases below the instability threshold, the large-scale island continues to persist for some time (the hysteresis effect) and then suddenly disappears.     

Simulation of plasma density evolution in the T-10 tokamak

Analysis of the experimental profiles of the plasma density and pressure in the T-10 tokamak shows that in the plasma core they are close to the corresponding canonical profiles. This allows one to construct an expression for the particle flux in terms of the canonical profile model. T-10 experiments performed with ohmic discharges have revealed transitions from improved to low particle confinement, similar to the effect of the density pump-out from the central part of the plasma upon switching-on of the electron cyclotron resonance heating (ECRH). It is shown that such a change in the particle confinement is associated with the deviation of the radial pressure profile from the canonical one. A nonlinear model of particle transport in discharges with density variations that allows for the transition effects is proposed. The plasma density evolution is numerically simulated for a number of ohmic and ECRH T-10 discharges.     

Reduced kinetic models of neutral transport in the tokamak plasma

A method for constructing reduced models of neutrals transport for problems with a reduced dimension has been proposed on the basis of the kinetic equation. The case of a cylindrically symmetric plasma column, which is a good approximation for the tokamak geometry, has been thoroughly analyzed. For this geometry, the kinetic model of neutrals in the isotropic approximation is implemented using an algorithm based on energy grouping of neutrals; this algorithm for atomic hydrogen isotopes is integrated in the ASTRA code.     

On gas desorption from the tokamak first wall during edge localized modes

The effect of gas desorption from the tokamak first wall on the pedestal recovery in the H-mode after an edge-localized-mode burst is considered. Results of FACE code simulations of hydrogen desorption from a beryllium wall are presented. It is found that the wall has a significant effect on plasma processes only at sufficiently low temperatures (of about 400 K), which agrees with qualitative estimates obtained earlier in the zero-dimensional approximation.     

Mechanisms governing radial heat fluxes in tokamak plasma

A method for analyzing the characteristics of turbulence responsible for radial heat transport is proposed. The method is based on the previously proposed hypotheses (to a great extent, confirmed experimentally) concerning the consistency of normalized pressure profiles in tokamak plasmas and the mechanism of internal transport barrier formation. Using the proposed approach, it is shown that, under an external action on the plasma, when the plasma heat flux onto the wall grows, the spectrum of turbulent modes broadens due to the excitation of modes with lower poloidal numbers m. Thus, in contrast to the conventional diffusion approach, the transport coefficient depends on the flux intensity. A mechanism of formation of internal transport barriers is proposed.     

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.     

Time-of-flight measurements of the plasma density in the T-11M tokamak

The average plasma density in the T-11M tokamak is determined by means of an O-mode time-of-flight refractometer measuring the propagation time τ of microwave pulses through the plasma. Since the front duration τ_fr of these pulses is shorter than 2 ns, filtering the measured signal cannot reduce the signal-to-noise ratio below a certain level. This circumstance impedes the use of this diagnostics in larger devices, where the signals may be substantially attenuated because of the larger chamber size and larger waveguide losses. There are several ways to overcome these difficulties: to raise the microwave power, to increase the sensitivity of the receivers, etc. In this paper, a technique is described that is based on the differential method for determining the propagation time of a microwave signal through the plasma. In this method, the plasma is probed by two continuous microwaves with close frequencies and the phase difference between them Δφ₁₂ is measured. As long as the condition Δφ₁₂ < 2π is satisfied, the measurements are unambiguous, because there are no phase jumps by a value multiple of 2π, as is usually the case in conventional interferometers at an increased level of MHD activity, in regimes with a rapid density growth, etc. This method allows the signal to be filtered, thereby ensuring an appreciable improvement in the signal-to-noise ratio in comparison with the pulsed methods. The first measurements of the average density along the +3-cm chord were performed with the help of this new differential time-of-flight refractometer in the T-11M tokamak. The refractometry data agree well with the interferometric data and are used to recover the plasma-density profile.     

Identification of the plasma boundary shape and position in the Damavand tokamak

A series of experiments and numerical calculations have been done on the Damavand tokamak for accurate determination of equilibrium parameters, such as the plasma boundary position and shape. For this work, the pickup coils of the Damavand tokamak were recalibrated and after that a plasma boundary shape identification code was developed for analyzing the experimental data, such as magnetic probes and coils currents data. The plasma boundary position, shape and other parameters are determined by the plasma shape identification code. A free-boundary equilibrium code was also generated for comparison with the plasma boundary shape identification results and determination of required fields to obtain elongated plasma in the Damavand tokamak.     

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.     

Noninductive plasma generation and current drive in the Globus-M spherical tokamak

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.     

Drift-Alfvén turbulence in a tokamak wall plasma

The behavior of turbulent fluxes in the vicinity of a resonant point m/n=q(x _res) in a plane wall plasma layer in a tokamak is studied by numerically analyzing the nonlinear MHD equations in a four-field electromagnetic model. Simulations show that, as the electron temperature at the plasma edge increases, the intensity of turbulent particle flux decreases, reaching its minimum value, and then increases. Such behavior is found to be due to the stabilizing effect of the electron drift velocity (V _y0∼dT _e0/dx) in the equation for the longitudinal component of the magnetic potential. It is shown that, at a strong toroidal magnetic field, turbulent transport processes conform to the gyro-Bohm scaling, which gradually passes over to the Bohm scaling as the field decreases. __________ Translated from Fizika Plazmy, Vol. 30, No. 5, 2004, pp. 387–397. Original Russian Text Copyright ￠ 2004 by Shurygin.     

Particle diagnostics of ion cyclotron resonance plasma heating in the Globus-M tokamak

Basic experimental results on cyclotron heating of the ion plasma component in the Globus-M spherical tokamak obtained by means of the ACORD-12 charge-exchange ion analyzer are presented. A procedure for determining the maximum energy of fast ions confined in the plasma is described. The procedure was applied to estimate the limiting energy of hydrogen minority ions accelerated during ion cyclotron heating in the Globus-M tokamak. The experimental evaluation of the maximum hydrogen ion energy is confirmed by simulations of ion orbits. Recommendations for optimizing experiments on ion cyclotron heating in the Globus-M tokamak are formulated.