Retarding Field Analyser Measurements in the JET Plasma Boundary
Ion temperatures, Ti, and velocity distributions, f(v), in the plasma edge of fusion devices are notoriously difficult to measure and thus rarely available but yet are important plasma boundary quantities. Though requiring the insertion of a material probe into the Scrape-Off Layer (SOL), the Retarding Field Analyser (RFA) is one experimental approach that can access the plasma ion or electron velocity distribution directly. By designing a "bi-directional" probe, ie. one which can intercept particles on both sides of a plane oriented perpendicular to the total field line direction at a particular location in the SOL, the device may also be used to simultaneously characterise any net flow that may be present there. In fact, according to recent theoretical work, it turns out that the presence of an external flow field should have a strong influence on the parallel field ion velocity distributions (and hence on the derived ion temperatures) at the upstream and downstream locations of any surfaces inserted into the flow. This contribution will offer the first experimental evidence for this effect, describing recent results obtained with an RFA in the JET plasma boundary.