A functional form is presented for the efficiency of current drive in toroidal geometry with waves at frequencies below the electron cyclotron frequency. By fitting constants in order to duplicate numerical results for the efficiency, an accurate function is obtained which will be useful for computer calculations of current drive.

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In the absence of an emf, the parallel current, j II, in a steady-state tokamak will consist of a neoclassical portion plus a wave-driven contribution. Using the drift kinetic equation, the quasi-linear (wave driven) current is computed for high-phase-speed waves in a torus, and this is combined with the neoclassical term to obtain the general expression for the flux surface average