Onset condition of thermoelectric instability in divertor tokamaks

The onset condition of a thermoelectric instability in divertor tokamaks has been studied analytically and numerically by using a five point model based on SOL and divertor plasmas. The SOL current due to the thermoelectric effect causes the thermoelectric instability and destabilizes the symmetric equilibrium of the SOL and divertor plasmas below a critical divertor plasma temperature. The critical temperature of the thermoelectric instability is equal to or higher than that of the thermal instability due to the divertor radiation, and depends on the midplane SOL plasma temperature and the divertor radiation loss. Below the critical temperature, stable asymmetric equilibria are obtained. As the degree of the asymmetry in the divertor plasma temperature increases, the SOL current has no effect on the instability, or even stabilizes the equilibrium. The SOL current generates spontaneously the divertor asymmetry through the thermoelectric instability. (author)