Interaction of externally applied rotating helical field with tokamak plasma

Abstract

Abstract. The penetration process of a rotating magnetic perturbation into a rotating tokamak plasma has been investigated, taking account of the mode structure of the perturbations, and using a small magnetic probe in the small research tokamak CSTN-IV. It was observed that the radial component of the perturbation was amplified in the plasma when magnetic islands are formed, while the poloidal component was attenuated but, deep inside the plasma, amplified. Using a resistive MHD code, such modifications of external perturbation were found to be caused by a redistribution of plasma current due to the formation of magnetic islands. Considering the phase velocity of both the perturbation and the intrinsic plasma rotation, it was also found that the sideband component substantially affects the penetration process with magnetic islands forming. Therefore, in dynamic ergodic divertor operation, it is necessary to put the emphasis not only on the frequency of the perturbation fields but also on their direction, mode structure and the radial profile of the intrinsic plasma rotation at the same time.

Low frequency heating and flow driven by the dynamic ergodic divertor in tokamaks

“…In figures 5(a) and (b), the distribution of the E r component around the q = 3 resonance surface in the toroidal code calculations for [3,1] coil settings appears very similar to the cylindrical code calculations in figure 4(a). We again observe two maxima of the E r component at the local Alfvén resonance points, but some difference in the field behaviour resonant field and dissipation at the q = 3 surface are absent for the 2/1 antenna current in the cylindrical calculations, this type of LF field excitation and absorption being produced by the coupling of poloidal modes due to plasma toroidicity.…”

“…To study the toroidicity effects, we use the toroidal code for analyses of the [3,1] coil setting in TEXTOR. In figure 5 we show the distribution of radial and poloidal LF fields as a function of the normalized minor radius r/a from the magnetic axis in the outboard equatorial plane for the 2/1 and 3/1 antenna modes.…”

“…Here, we ignore any driving effect in the skin layer (order of ion Larmor radius ρ i ) because of large surface friction from the limiter and scrape-off layer. We assume a dissipated LF power in the plasma of about W ≈ 20 kW, which from figure 3 can be seen to be produced by a current I d in each coil of the order of 2 kA amplitude for the [3,1] q r = 3 in the volume V r ≈ 4π 2 r r s R 0 , where r is the half-width of the power absorption profile P (r). Since the normed 'velocity change without transverse viscosity' plotted in figure 8 is computed in fixed proportion to F θ P , hence to P (r), all three quantities have the same normed profile.…”

“…[1]), dynamic ergodic divertor (DED) coils have been proposed as a means to favourably influence heat exhaust, edge cooling, impurity screening, plasma confinement and stability at the plasma boundary (see [2] for motivation and detailed description of DED coils for the TEXTOR tokamak). Related earlier experimental and theoretical studies on penetration of rotating low frequency (LF) fields into tokamak plasmas [3,4] continues in recent experiments on the HYBTOK-II tokamak [5]. More exclusively theoretically oriented study is an active endeavour [6][7][8][9][10][11][12] which includes modelling of DED coils for the Tokamak Chauffage Alfvén Brésilien (TCABR) [9].…”

Characteristics of local helical coil system for dynamic ergodic divertor

“…Penetration processes and the dynamic behavior of RHF in tokamak plasmas have been investigated. In the case of resistive plasmas (Lundquist number S ~ 230), the redistribution of the plasma current due to magnetic field reconnection induced by RHF was predominantly observed [9,10]. We have started a new DED experiment on a small tokamak device, HYBTOK-II [11,12] to study the fundamental processes of DED in a higher-temperature region (S ~ 5000) compared with CSTN-IV.…”