Hydrogen Recycling in the RFX Reversed Field Pinch

Abstract

RFX (Reversed Field experiment) is a large toroidal device (a=0.46 m, R= 2 m) designed to explore the properties of the Reversed Field Pinch configuration up to 2 MA of plasma current [1]. So far it has been operated in hydrogen, with plasma currents up to 1.2 MA, electron densities between 1 10¹⁹ and 1 10²⁰ m^-3 and electron temperatures between 200 and 400 eV. The first wall is made of graphite tiles covering all over the inconel vessel and has a total surface of approximately 36 m². Neither protruding limiters nor divertors are present (see Fig.l). The average particle confinement time is of the order of few milliseconds and in order to sustain the electron density the fluxes of atomic hydrogen are of the order of 10²³ s^-1. In most of the circumstances the hydrogen flux required to refuel the plasma originates from the recycling processes at the wall. An amount of pre-filling gas adequate to allow the breakdown may in fact be sufficient to drive a whole discharge, whose typical duration is about 150 ms, much shorter than the characteristic pumping time (≈1 s). In absence of any wall conditioning procedures too large filling pressures may easily saturate the wall, leading to a complete loss of density control. Thus, the capability to control the plasma density strictly reflects the capability to control the hydrogen concentration on the tile surfaces facing the plasma. Various techniques have been experimented to condition the wall: hot wall operations, (up to 280°C), Glow Discharge Cleaning (GDC) in He, boronisation with GDC in Diborane. In the following the degree of density control gained with such methods is described in detail.