The ergodic magnetic limiter is a device designed to generate a cold boundary layer of chaotic magnetic field lines at the peripheral region of a tokamak, with the main purpose of reducing the deleterious effects of the plasma-wall interaction. In the TCABR tokamak an ergodic limiter was constructed and recently installed inside the vacuum chamber. We developed a theoretical model for the action of an ergodic magnetic limiter in a large aspect-ratio tokamak taking into account the finite width of the limiter. The theoretical results are in good agreement with measurements of the vacuum magnetic field created by the limiter. Poincaré maps of field line flow are computed to reveal the resulting magnetic field line structure due to the ergodic limiter and show that the operation of the ergodic limiter in the TCABR tokamak is feasible and results in a chaotic boundary layer for limiter currents of about 6% of the plasma current.

It is shown that fast drift Alfvén waves can be excited in tokamak plasmas by an external antenna operating in the low-frequency band. The dispersion of these waves depends on the derivative of the q-profile or drift terms in the case of low or negative shear. The wave absorption is determined by the Alfvén continuum dissipation and has an oscillating increase with frequency. Typical global Alfvén wave resonances are found at the plasma core for different signs of poloidal/toroidal mode numbers (N∕M<0) for a standard tokamak safety factor profile with central value q0>1.

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The rotating low frequency (RLF) field penetration and dissipation and the effect of ponderomotive forces driven by Ergodic Magnetic Limiter (EML) on the poloidal/ toroidal flow in tokamak plasmas are discussed. EML coils are represented as a sheet current expanded in Fourier series with poloidal/toroidal wave numbers M/N depending on coil shape and feeding. The Alfvén wave mode conversion effect in the RLF range is found responsible for wave dissipation at the rational magnetic surfaces q r = −M/N = 3 typical for EML coil design. Analytical and numerical calculations show maximums of LF field dissipation at the local Alfvén wave resonance ω = |k || cA| near the rational magnetic surface qr = 3 in Tokamak Chauffage Alfvén Brésilien. The poloidal rotation velocity U , taken into account in the dielectric tensor, can strongly modify the LF field and dissipated power profiles. Even stationary EML fields can dissipate at the local Alfvén wave resonance (U M/r A = k c A ). Preliminary estimations show that the stationary EML fields can decelerate the plasma rotation.

The effect of plasma rotation on low frequency (LF) field penetration, absorption and ponderomotive forces in TEXTOR and in Tokamak Chauffage Alfvén Brésilien (TCABR) is investigated in the frequency band of 1–10 kHz. The LF fields are driven by the dynamic ergodic divertor in TEXTOR and the ergodic magnetic limiter in TCABR. Alfvén wave mode conversion is responsible for the LF field absorption at the rational magnetic surface where q = −M/N is the integer. Analytical and numerical calculations show the maxima of the LF field absorption at the local Alfvén wave resonance |ω − k · U| = |k∥| cA, where ω and k are the frequency and the wave vector, respectively, and cA is the Alfvén velocity at the rational magnetic surface q = 2, 3 in TEXTOR and TCABR. The rotation velocity U along the magnetic surfaces, taken into account in the dielectric tensor, can strongly modify the LF field and dissipated power profiles. The absorption in the local AW resonances begins to be non-symmetric in relation to the resonance surface. Calculations show that coil impedance has a maximum related to excitation of some stable (possibly Suydam) modes for waves travelling in the direction of plasma rotation.