Measurements of cross-field plasma transport to an object in a tokamak edge plasma

Abstract

When a material object is inserted into the scrape-off layer of a tokamak, it creates a wake which extends parallel to the magnetic field. Experiments are described which are the first direct test of the concept of a collection length, i.e the parallel scale length for this disturbance. The disturbed zone is a consequence of the equilibrium between parallel and cross-field particle transport. Two adjacent probes on the DITE tokamak were used. The large 'disturber' probe is a rectangular graphite plate which creates the main particle flow pattern and it can be rotated to change its projected width normal to the magnetic field. The small 'search' probe is located 0.5 m away on the same magnetic field line. It consists of a back-to-back pair of single probes which are used to measure the poloidal variations of the ion saturation current and Mach number in the wake of the disturber probe. Results are presented from experiments in which the disturber probe is rotated to change its natural collection length. The disturber probe was also biased with respect to the torus. The data obtained demonstrate the importance of considering non-ambipolar transport processes which are seldom discussed in the literature.