Analytic model for coaxial helicity injection in tokamak plasmas

作者：Weening R H^*

来源：Physics of Plasmas, 2011, 18(12): 122503.

DOI：10.1063/1.3665088

Using a partial differential equation for the time evolution of the mean-field poloidal magnetic flux that incorporates resistivity eta and hyper-resistivity Lambda terms, an exact analytic solution is obtained for steady-state coaxial helicity injection (CHI) in force-free large aspect ratio tokamaks. The analytic mean-field Ohm's law model allows for calculation of the tokamak CHI current drive efficiency and the plasma inductances at arbitrary levels of magnetic fluctuations, or dynamo activity. The results of the mean-field model suggest that CHI approaching Ohmic efficiency is only possible in tokamaks when the size of the effective current drive boundary layer, delta equivalent to (Lambda/eta)(1/2), becomes greater than half the size of the plasma, delta > a/2, with a the plasma minor radius. The electron thermal diffusivity due to magnetic fluctuation induced transport is obtained from the expression chi(e) = Lambda/mu(0)d(e)(2), with mu(0) the permeability of free space and d(e) the electron skin depth, which for typical tokamak fusion plasma parameters is on the order of a millimeter. Thus, the ratio of the energy confinement time to the resistive diffusion time in a tokamak plasma driven by steady-state CHI approaching Ohmic efficiency is shown to be constrained by the relation tau(E)/tau(eta) < (d(e)/a)(2) approximate to 10(-6). The mean-field model suggests that steady-state CHI can be viewed most simply as a boundary layer of stochastically wandering magnetic field lines.

• 出版日期2011-12