Electrostatic drift waves in tokamaks: a numerical study of instability and transport

A numerical study of drift modes in the electrostatic slab approximation over a range of parameters consistent with Ohmically heated tokamaks is described. With this model it is found that the trapped-electron mode may be excited only at low collisionality (ν *min <1), whereas for moderate to high collisionality (1 <ν *min < 10), where many tokamaks have anomalous transport, the mode is shear-stabilized. The current-driven, resonant-electron mode is strongly stabilized for practical current drifts. The ion-temperature-gradient mode is unstable for d ln T i /d ln n > 1 at all collisionalities. From the electrostatic quasi-linear theory of transport, it is found that the electron modes (except near marginal stability) and the ion mode have ion thermal conductivity comparable to or larger than the electron thermal conductivity, in contrast to experimental results. It is concluded that the quasi-linear, electrostatic slab model of drift waves does not adequately describe anomalous transport in tokamaks.