Passive thermal-stability control in ignited tokamaks by ripple transport and radial motion

A methodology is developed to study the use of the combination of ripple-induced ion conduction loss and radial motion for passive thermal stability control of an ignited tokamak plasma. Radial fluctuations are coupled to temperature fluctuations by MHD equilibrium requirements. A 0-dimensional model with parabolic profiles is employed. Illustrative calculations are made for tokamak plasmas where the electron energy confinement time is described by the empirical scaling law tausub(e) approximately na2. The ripple-induced thermal conduction is modelled by the addition of an ion energy loss channel to a plasma described by neoclassical ion energy transport; this additional ion loss channel is represented by an energy confinement time scaling tausub(i,ripple) approximately Tsub(i)sup(-epsilon), where Tsub(i) is the ion temperature and epsilon is a variable parameter. The amount of additional ion power loss required for the achievement of thermal stability is determined. The corresponding increase in the value of ntausub(e) required for ignition is computed. The combined effects of radial motion and a ripple-induced transport loss with tausub(i,ripple) approximately Tsub(i)sup(-7/2), as might be the case for ripple trapping, can be used to obtain thermal stability for central ion temperatures higher than 20 keV with a very small (<2%) additional power loss by ripple transport; this loss could be provided by peak-to-average ripple at the boundary of approximately 1%. (author)