Experimental investigation of free and bounded presheaths in weakly magnetized plasmas

Transports of plasmas in the edge of fusion devices have similarities in terms of formation of a free presheath and unclear explanation on the transport process relating the diffusion coefficient (D⊥) to characteristic length of perturbing for flux tube (Lc). D⊥ and Lc are investigated by generating perturbations in various free presheaths due to a perturbing object located at the axial center of a linear plasma device, called DiPS (Divertor Plasma Simulator). Free presheaths are generated due to a tungsten perturbing object by changing the magnetic flux density. Bounded presheaths are also formed due to a limiting structure of a magnetic nozzle and due to the given geometry of DiPS. In terms of plasma discharge currents, radial plasma profiles were measured by using a fast scanning probe system. D⊥ and Lc within the free presheath regions were calculated by using the measured plasma parameters and compared with those of bounded presheaths near the chamber walls. Decay length of plasma density was introduced to calculation of D⊥. To calculate the perturbation length (L) of free presheaths, a theoretical scale factor K was introduced as L=KLc using a fluid model. Normalized factor δ=D⊥/DB, where DB= Bohm diffusion coefficient, were obtained as 8 at free presheaths and 11 at bounded presheaths.