Density clamping and power deposition profile in rf heated plasma in toroidal helical systems

Radial transport driven by rf heating in toroidal helical systems with the large helical ripple is studied. The cyclotron resonance heating changes the pitch angle of resonating particles, so that the transition from the transit orbit to the trapped one occurs. The deviation of the average minor radius of the particle orbit from the magnetic surface strongly depends on the pitch angle of the particle velocity. Owing to this dependence, the slight change in the pitch angle by heating can generate a large step in the radial location. The approximate forms of the driven radial flow, which is rf-power-dependent, and the deformation of the heating profile are obtained. It is found that the deposition profile in this configuration is hardly made sharper than a critical gradient length. The injection of the heating power from the inside of the torus is preferable to improve the heating efficiency and to overcome the density clamping. (author)