A fast moving diagnostic system on the RFX experiment

A fast rotating graphite arm equipped with two Langmuir probes has been dev eloped to investigate the temperature, density, and plasma potential profil es at the edge in the reversed field pinch experiment RFX. The system, driv en by an electromagnetic torque, allows a 5 cm radial insertion in about 20 ms with a magnetic field of 0.3 T. The motion of the arm takes place on a plane having an angle of 45 degrees with respect to the direction of the po loidal magnetic field. The probe collecting area is constant during the mov ement. The initial and final positions of the graphite arm are monitored by electrical contacts. To reconstruct the motion of the equipment, an equiva lent lumped-constant network has been developed. The equivalent resistance and inductance are measured before each discharge. By this system radially resolved measurements of edge plasma parameters in a single RFX plasma disc harge have been obtained. (C) 1999 American Institute of Physics. [S0034-67 48(99)50201- 3].