ELECTRON-TEMPERATURE GRADIENT INDUCED INSTABILITY IN TOKAMAK SCRAPE-OFF LAYERS

An electron temperature instability driven by the Kunkel-Guillory sheath impedance has been applied to the scrape-off layer of tokamaks. The formalism has been generalized in order to more fully account for parallel wavelength dynamics, to differentiate between electromagnetic and electrostatic perturbations and to account for particle recycling effects. It is conjectured that this conducting wall instability leads to edge fluctuations in tokamaks that produce scrape-off layers tens of ion gyroradii thick. The predicted instability characteristics have similarities to experimental edge fluctuation data, and the scrape-off layer width in the DIII-D experiment agrees with theoretical estimates that can be derived from mixing length theory.