No abstract

The profile and dissipation of the field excited by dynamic ergodic divertor (DED) coils in tokamak plasmas are calculated, and an estimate is made of the poloidal/toroidal velocities driven by this field. The coils are idealized as an inboard sheet current composed of a toroidal sequence of helical line current segments expanded in Fourier series with poloidal/toroidal mode numbers M/N, and mode amplitudes depending on feeding. Numerical calculations with cylindrical and toroidal codes show maxima of field dissipation due to Alfvén wave mode conversion effect taking place at the rational magnetic surfaces where q = M/N. The effects of toroidicity and ion collisions in the dielectric tensor in the upper DED frequency range described (f = 5-10 kHz) are found to be very important in absorption calculations. At the q = 3 resonant magnetic surface typical for DED coil design, it is estimated that ponderomotive forces produced by 20 kW of dissipation can drive local toroidal and poloidal flows of respective orders 8 km s −1 and 10 km s −1 in the TEXTOR tokamak.

The strong nonlocal effect seen about cyclotron harmonics in a previous numerical study accounting for magnetic inhomogeneity along B, is described analytically and interpreted in terms of an echo-like mechanism. Though this effect is found to cause a redistribution of power deposition in the resonance zone, agreement is found with earlier work insofar as the total power deposition is unaltered.WHEN deriving a wave equation for inhomogeneous plasma, one replaces J by a (generally nonlocal) expression involving field amplitudes. In the case of parallel magnetic inhomogeneity (B,.VIB,I if 0, e.g.

After introduction of the experimental options available with the Dynamic Ergodic Divertor (DED) and a discussion of the static aspects of the ergodic and laminar zones, the dynamic aspects of the rotating DED field are emphasized. The rotating perturbation field induces a shielding current which is modelled under different assumptions. Interaction of the shielding current with that of the DED coils results in a torque exerted by the coils on the plasma. The location of the maximum of this torque with respect to the frequency depends critically on the width of the shielding current, and for the TEXTOR-DED conditions it is in the frequency band of 1-30 kHz. The DED will have the option of operation with full power in this band so that the basic investigations on the field line penetration can be attempted. The force transferred to the plasma has two components, a weaker toroidal one and a dominant poloidal one. The toroidal force component has about the same value as the one from NBI; from the experience with NBI induced plasma rotation, a substantial plasma acceleration in the toroidal direction is expected. For neoclassical reasons it is not yet clear whether the dominant poloidal force component will result in a poloidal plasma rotation or a radial force. If the poloidal rotation is inhibited, a static radial electric field is estimated on the basis of a revisited neoclassical theory to be of the order of several kilovolts per metre.

The electrostatic shields now commonly employed to shield antennas in the heating of plasma in the ion cyclotron frequency range are shown to reduce the specific radiation resistance of a long narrow antenna in the presence of plasma by a significant factor (on the order of2/3 for a typical double-array shield) due to the effect of magnetic shielding of the magnetosonic polarization. An allied change in antenna specific inductance is also found. These effects are shown to diminish with increase in antenna width and should pose no major problem for the wide antennas projected for use in fusion experiments. In addition to the foregoing effects which are not ohmically dissipative in essence, electrostatic shields are also shown to introduce surprisingly high ohmic loss, this being of potential importance in shield design. The dependences of the above magnetic and ohmic phenomena on shield parameters are given and a shield design minimizing them is presented. Their repercussion on coupling efficiency and on the excitation voltage necessary for a given power flux from the antenna is discussed.