Tokamak transport in the presence of multiple mechanisms

Abstract

Some success has been achieved by F.W. Perkins, W.M. Tang, R.E. Waltz, A. Rogister and others in explaining tokamak transport using drift wave turbulence as the source of the losses in the confinement region, 1 ⪅ q ∼ 2. They obtain solutions to the transport equations with the assumption that some unspecified mechanism — profile consistency — restricts the temperature in the edge region of the plasma. In this paper their work is extended, to include an edge region dominated by rippling modes and resistive ballooning modes, with a confinement zone controlled by trapped electron modes, ηi modes and resistive ballooning modes. Analytic solutions are obtained for the situation in which the source of particles decays exponentially in the plasma and [(1/T) (dT/dr)]/[(l/n) (dn/dr)] ≃ 2 in the confinement region. It is shown that differences in neutral penetration can contribute to a positive mass dependence of confinement for hydrogen isotopes, but these contributions are not enough to explain the strong dependence seen in many circumstances. The combination of the two sets of modes also leads to a form of profile consistency. Although the trapped electron modes and ηi modes can contribute many of the features of Ohmic and auxiliary heated transport — such as τE ∝ n̄, L3, confinement saturation at high densities and some power degradation — they do not lead to the correct density, current and atomic mass dependence to explain all of the features of L-mode scaling. A candidate mode to supplement them is the resistive ballooning mode, which has a strong favourable current dependence and an unfavourable density dependence which, when combined with the more favourable dependence of the other modes, may be the cause of the weak density dependence seen experimentally in the L-mode. The addition of the resistive mode typically reduces the confinement for the drift wave model by a factor of 1.5–2; thus, suppression of resistive modes may be the cause of the L-H transition.