Fig. 2-4 Arrangement for the measurement of plasma flow by two reciprocating Mach probes A Mach probe is an electrostatic probe designed specifically to measure plasma flows, reciprocating across the SOL horizontally. The probes are located at the midplane and the divertor region. Simultaneously plasma flow along the field lines is measured.

Fig. 2-5 Plasma flow in the SOL estimated from the measured flow at the midplane and the divertor region The plasma flow in the SOL is shown schematically based on the simultaneous measurement of flow at the midplane and the divertor region, considering the poloidal flow of ion drift.

In a tokamak with a divertor a thin boundary layer of low density plasma is formed surrounding the main plasma, called scrape-off layer, SOL, and in the SOL a plasma flow along the magnetic field line occurs generally in the direction towards the divertor. Since the plasma flow in the SOL will affect the penetration of impurities into the main plasma and also helium ash exhaust from it, understanding of the basic physics of the flow is important for divertor design. In JT-60 we have studied the spatial distribution of flow in the SOL systematically by multi-point and simultaneous measurements using electrostatic probes at the midplane of the main plasma and the divertor region as shown in Fig. 2-4. We observed for the first time flow reversal in the SOL at the midplane, while the plasma flows directing towards the divertor plates in the divertor region. Figure 2-5 shows schematically the flow in the SOL based on the analysis of measurements of the distribution of ion and electron temperatures, densities and electric potentials. The observed flow reversal occurring at the midplane near the main plasma boundary can be explained as a resultant flow of a poloidal flow of ions (ion drift) and a flow driven along the field lines to keep the pressure constant along the field lines. This result can be introduced to a detailed evaluation of plasma physical processes in the SOL and the divertor plasma by simulation, and is expected to contribute to a more reliable and realistic physics basis for the divertor design of the next step fusion experimental reactor such as ITER.

Reference N. Asakura et al., Heat and Particle Transport of SOL and Divertor Plasmas in the W-Shaped Divertor on JT-60, Nucl. Fusion, 39, 1983 (1999).

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