Divertor experiments on the dite tokamak

Abstract

A set of equations governing plasma parameters (electron density, electron temperature, and ion temperature) within the burial chamber of a duble divertor, including potential steps in the divertor throat and in front of the divertor plate, is derived. In this approach the hydrogen ion density is equal to the electron density, since the impurity ions are neglected. Secondary electron emission at the divertor plate, the particle, and energy flow reduction due to the mirror are taken into account. The modelling is based mainly on the assumption that strong relaxation processes due to various microinstabilities establish a Maxwellian distribution function in both the scrape-off region and the divertor chamber for all particle species involved. Particle sink and energy loss terms due to the divertor are derived suitable for 1-D transport code calculations. The main results are as follows. In the DITE case the unloading efficiency ηp = 25% and the exhaust efficiency ηE = 75% are obtained if the calculation is based on 40% of the Bohm diffusion coefficient. The extrapolation to the TEXTOR-device, equipped with a slightly modified DITE Mark II divertor [1], results in ηp = 13% and ηE = 54%, if the optimistic assumption is made that 20% of the Bohm diffusion coefficient are appropriate in this case.