High-spatial resolution edge electrostatic probe system for RFX

The RFX reversed field pinch (RFP) is undergoing major modifications of the load assembly and will be equipped with a large number of external coils for active magnetohydrodynamics mode control. The investigation of plasma parameters is then of paramount importance to study the effects on the plasma of both the new passive structure and the action of the external active coils. The study of turbulence is also very important because of its influence on plasma transport. A system of electrostatic probes is needed, fulfilling very challenging requirements in terms of spatial and temporal resolution, which are characteristic features of RFP plasmas. This system should also be integrated with other in-vessel sensors (magnetic and calorimetrical probes) and with other diagnostics placed outside the vessel. A description of the diagnostic system is presented. The global layout of the probes along the first wall is shown: The system allows a two-dimensional investigation of the plasma; both single and triple electrostatic probes have been constructed. Each sensor is embedded in the graphite tiles which completely cover the vessel. The design of the probes is displayed, describing the original solutions adopted to cope with several technological issues. In particular, due to the high number of signals and to limited space available within the vessel, the probe dimensions were minimized. Since RFX possesses no iron core, care has been devoted to avoid pick-up of stray time-varying magnetic fields. Under particularly high thermal loads onto the wall, due to the absence of limiters and to phase-locked magnetic modes, thermal requirements are provided by a suitable choice of materials, at the same time meeting the necessary standards of electrical insulation. Mechanical tests on the carbon tiles modified for inserting the probes were also needed; thermal and electrical tests on prototypes were performed.