A change in the radial electric field (Er) at the plasma edge using a biased electrode is a promising technique to achieve a regime of improved energy and particle confinement in fusion experiments. For this purpose and to unlock stationary disturbances in the plasma configuration a movable in-vessel electrode has been designed for the Reversed Field Pinch (RFP) experiment RFX (major radius R = 2 m minor radius a = 0.457 m).

In order to modify Er, the current and voltage ratings of the electrode are of primary concern. It has been assumed, as a preliminary estimate, that the force applied to the plasma resulting from the interaction of the radial current injected by the electrode with the poloidal magnetic field should balance the plasma viscous damping force in the toroidal direction, so that for a low toroidal plasma current of 250 kA, a current of a few kA is estimated. The electrode operating voltage can be derived on the basis of the results obtained with other RFP and tokamak experiments. Thus, the design requirements have been set for 5 kA, 2 kV.

The power will be injected into the plasma with a duration of the order of ten milliseconds. A capacitor bank will be pre-charged and, at the selected time, an ignitron will be fired, connecting an L-C circuit to the electrode. The radial insertion depth is remotely adjustable between two subsequent discharges, up to a maximum of 200 mm.

The electrode head consists of a mushroom shaped limiter made of a carbon-carbon composite, sustained by a 200 mm long molybdenum alloy (TZM) shaft, that is protected by a boron nitride (BN) tube 9.5 mm thick. The head is sustained by a stainless steel AISI 304L shaft (a tube, 2 m long, 56 mm outer diameter), isolated by an outer tube (same length as the stainless steel shaft, 70 mm outer diameter) in technopolymer material. Static and dynamic analyses carried out with a finite element model led to an optimal design of the shaft and of the drive system which guarantees the required stiffness (maximum edge displacement of the shaft less than 10 mm).

Particular care has been devoted to protect the system from overvoltage and overcurrent, by use of ZnO varistors and a crowbar system. The crowbar system is fired either when the plasma current is larger than 300 kA (to prevent excessive stress on the electrode), or when the plasma current is less than 100 kA (end of the discharge).

The paper describes the design of the movable in-vessel power electrode, and the tests performed on critical materials.