A helical magnetic limiter for boundary layer control in large tokamaks

Abstract

In a tokamak configuration, superposition of the magnetic field of resonant helical windings which surround the toroidal plasma current outside the first wall destroys the magnetic surfaces in the boundary layer (ergodization). A transport model is analysed, where convective flow of the plasma from the boundary layer to the first wall permits elevated particle densities in the boundary layer and leads to very high particle and energy transport. The convective flow is driven by the pressure gradient along the field lines which intersect the toroidal wall at an oblique small angle ϵ. The required thickness Δ of the boundary layer is around 1015 n−1·cm−2. As a result, the plasma temperature there can be reduced towards the threshold of critical plasma-wall-interaction processes, the plasma core can be shielded against impurities from the wall and, at the same time, a very short life-time of all particles in the boundary layer can be achieved (use of pumpholes and/or scrape-off-limiters for removing ash). Thus, this model also improves the concepts of edge radiation cooling. An estimate is given of the parameters of INTOR using only a weak helical perturbation field which conserves the magnetic surfaces in the plasma core: one can reach wall temperatures Tw between 20 and 30 eV in the presence of wall densities nw approaching 1014cm−3.

Effects of the resonant modes on the magnetic footprint patterns in a tokamak wall

“…3 Hence, various ingenious techniques have been developed in order to uniformize the spatial distribution of particle and heat transport in the tokamak outer region as the use of chaotic magnetic field lines generated by an ergodic limiter. [4][5][6][7] However, this initial expectation has been rather frustrated by a series of experimental results obtained in many tokamaks in which ergodic limiters were mounted, which often pointed out to a nonuniform distribution of field lines hitting the tokamak wall. [8][9][10][11][12][13] The term magnetic footprints has been coined to indicate the deposition pattern of field lines in the tokamak wall.…”

Magnetic surfaces in a steady-state tokamak

“…In addition to the questions treated herein, this model may also be applicable to the problem of the "ergodic limiter" [14], in which the role of the limiter in a tokamak is played by stochastic field lines. The filamentary features of currents and magnetic fields are wellnoted of late in astrophysical plasmas as well [15, 1_].…”

“…(14), while random local deviations from this current are represented by the parameters ai's which take on individual values centred around 0 and whose interaction is represented by the second terms in the right-hand side of (14).…”

“…The portion of the filament motion due to the externally applied current in (14) (that is, the first terms of the right-hand side) can be represented by the "standard map," a well-known non-linear map which has been studied both for its own interest and as a model for various physical systems (See, e.g. [9] and [10]).…”

“…(14). Filaments are given random strengths between +a and -tc with mean zero, and were initially distributed randomly throughout 0 < y < 1 , -1/2 < x < 1/2.…”