Geodesic plasma compressibility effects on the magnetic islands in a tokamak

It is shown that the stability of rotating magnetic islands in a tokamak plasma is affected by plasma compressibility related to the geodesic curvature in an inhomogeneous magnetic field. A robust contribution has been found to the Rutherford evolution equation. It is shown that the sign of the geodesic curvature contribution is opposite to the sign of the polarization term. It is suggested that this mechanism plays a crucial role in the stability of small scale magnetic islands.     

Self-organization in a tokamak plasma

Conclusions The observed tendency for certain optimal current-density and pressure profiles to be maintained can be interpreted if one assumes a relaxed state in the plasma, which corresponds to minimal total energy for a given current. With optimal power deposition, when the current-density and pressure profiles also remain optimal, ohmic scaling may be maintained up to fairly high densities, i.e., fairly high _p. If the power deposition profile deviates from optimal, the plasma resists deviation from the optimal profiles, and the more so the higher _p. The thermal-conductivity profile is modified to maintain the relaxed state. Nonoptimal profiles may lead to degradation in the global energy lifetime _E, which can be related phenomenologically to pumping of magnetic noise by the excess free energy F. Our argument provides a qualitative explanation of the self-organization physics.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 29, No. 9, pp. 1032–1040, September, 1986.     

Transport induced by large scale convective structures in a dipole-confined plasma

Convective structures characterized by E×B motion are observed in a dipole-confined plasma. Particle transport rates are calculated from density dynamics obtained from multipoint measurements and the reconstructed electrostatic potential. The calculated transport rates determined from the large-scale dynamics and local probe measurements agree in magnitude, show intermittency, and indicate that the particle transport is dominated by large-scale convective structures.     

Toroidal plasma rotation induced by the dynamic ergodic divertor in the TEXTOR tokamak

The first results of the Dynamic Ergodic Divertor in TEXTOR, when operating in the m/n=3/1 mode configuration, are presented. The deeply penetrating external magnetic field perturbation of this configuration increases the toroidal plasma rotation. Staying below the excitation threshold for the m/n=2/1 tearing mode, this toroidal rotation is always in the direction of the plasma current, even if the toroidal projection of the rotating magnetic field perturbation is in the opposite direction. The observed toroidal rotation direction is consistent with a radial electric field, generated by an enhanced electron transport in the ergodic layers near the resonances of the perturbation. This is an effect different from theoretical predictions, which assume a direct coupling between rotating perturbation and plasma to be the dominant effect of momentum transfer.     

Transport bifurcation in a rotating tokamak plasma

The effect of flow shear on turbulent transport in tokamaks is studied numerically in the experimentally relevant limit of zero magnetic shear. It is found that the plasma is linearly stable for all nonzero flow shear values, but that subcritical turbulence can be sustained nonlinearly at a wide range of temperature gradients. Flow shear increases the nonlinear temperature gradient threshold for turbulence but also increases the sensitivity of the heat flux to changes in the temperature gradient, except over a small range near the threshold where the sensitivity is decreased. A bifurcation in the equilibrium gradients is found: for a given input of heat, it is possible, by varying the applied torque, to trigger a transition to significantly higher temperature and flow gradients.     

Intense geodesic acousticlike modes driven by suprathermal ions in a tokamak plasma

Intense axisymmetric oscillations driven by suprathermal ions injected in the direction counter to the toroidal plasma current are observed in the DIII-D tokamak. The modes appear at nearly half the ideal geodesic acoustic mode frequency, in plasmas with comparable electron and ion temperatures and elevated magnetic safety factor (q_{min}>or=2). Strong bursting and frequency chirping are observed, concomitant with large (10%-15%) drops in the neutron emission. Large electron density fluctuations (n[over ]_{e}/n_{e} approximately 1.5%) are observed with no detectable electron temperature fluctuations, confirming a dominant compressional contribution to the pressure perturbation as predicted by kinetic theory. The observed mode frequency is consistent with a recent theoretical prediction for the energetic-particle-driven geodesic acoustic mode.     

Suppression of the magnetic surface shift by plasma rotation in a tokamak

The equilibrium of a toroidally rotating plasma in an axisymmetric tokamak-like system is considered. The equilibrium equation is represented in the form of the Grad-Shafranov equation in which, in contrast to the static case (with no plasma rotation), the plasma pressure depends on both the flux surface label and major radius. It is shown that the dependence of the pressure on the major radius makes it possible to choose the profile of the plasma rotation velocity so as to minimize the effect of the plasma pressure on the shift of the flux surfaces, in which case it might be anticipated that the maximum pressure of the confined plasma will be higher. This result was derived analytically and tested numerically for a number of typical tokamak configurations with a fixed plasma boundary.     

Analytic expression for the influence of plasma birefringence on Faraday-rotation measurements in a tokamak plasma

Summary An approximate analytic expression is given for the influence of plasma birefringence on Faraday rotation in the tokamak and the range of validity is determined. The expression is of interest for polarimetric measurements and for the analysis of electron cyclotron emission.     

"Snowflake" H mode in a tokamak plasma

An edge-localized mode (ELM) H-mode regime, supported by electron cyclotron heating, has been successfully established in a "snowflake" (second-order null) divertor configuration for the first time in the TCV tokamak. This regime exhibits 2 to 3 times lower ELM frequency and 20%-30% increased normalized ELM energy (ΔWELM/Wp) compared to an identically shaped, conventional single-null diverted H mode. Enhanced stability of mid- to high-toroidal-mode-number ideal modes is consistent with the different snowflake ELM phenomenology. The capability of the snowflake to redistribute the edge power on the additional strike points has been confirmed experimentally.     

Eddy current effects on plasma equilibrium in a single-turn tokamak

In a fusion experiment based on the single-turn tokamak concept, the plasma is surrounded by a massive conducting structure composed of several layers of material with different resistivities. This conducting shell is located near the plasma edge and is magnetically coupled to the plasma column. The plasma magnetohydrodynamic (MHD) equilibrium is studied by neglecting the effect of structural induced currents. Eddy current effects are then analyzed. Poloidal uniformization of the poloidal field magnet current distribution required for plasma equilibrium is demonstrated. The possibility of continuous-limiter discharges in a single-turn tokamak configuration is pointed out. The significance of these results for the operation of a high-current tokamak experiment is discussed     

Heating by an electron Bernstein wave in a spherical tokamak plasma via mode conversion

The first successful high power heating of a high dielectric constant spherical tokamak plasma by an electron Bernstein wave (EBW) is reported. An EBW was excited by mode conversion (MC) of an mode cyclotron wave injected from the low magnetic field side of the TST-2 spherical tokamak. Evidence of electron heating was observed as increases in the stored energy and soft x-ray emission. The increased emission was concentrated in the plasma core region. A heating efficiency of over 50% was achieved, when the density gradient in the MC region was sufficiently steep.