An experimental investigation of electron confinement in reversed-field pinches

Abstract: Experimental studies aimed at an improved understanding ofelectron confinement in reversed-field pinch (RFP) plasmas arepresented. Different diagnostic methods have been employed:Thomson scattering and soft x-ray continuum measurements wereused to detect ektrons in the core of the plasma, x-rayemission from an insertable target was used to detect energeticelectrons in the edge region. The latter technique has alsobeen calibrated for the first time by means of an electron gun.The measurements were conducted on the Extrap Tl and Extrap T2experiments in Stockholm.Regarding the core electrons, empirkal scaling relationsbetween electron temperature and density on the one hand andother global parameters such as plasma current on the otherhand have been determined. From the data two importantconfinement parameters: the poloidal ß value and theenergy confinement time are also deduced. Further the radialdistributions of electron density and temperature have alsobeen studied and it has been found that these are alsodependent quantities. Flatter density profiles are observed inhigh density plasmas as well as plasmas where the density isrising. These effects are proposed to be due to changes in theionisation profiles. In addition flatter temperature profilesare seen in an analogue manner at higher temperautres.In the edge region an energetic non-thermal component ispresent in the electron distribution function. In Tl thenon-thermal component has been determined to constitute around10% of the electron density with an energy characteristic ofthe core ektrons. The parallel heat flux associated with theseelectrons has heen estimated as a function of plasma currentand the associated fluctuation induced radial heat flux hasbeen found to constitute an important energy loss channel.Keywords: Plasma, magnetic confinement, reversed-fieldpinch (RFP), Thomson scattering, x-ray continuum, superthermalelectrons, hot ektrons, Extrap Tl, Extrap T2