Electron density and temperature measurements by repetitively pulsed 90 degrees Thomson scattering from a steady state, low-density plasma with superimposed magnetic field are reported. The density is calibrated by Rayleigh scattering from argon; a pressure scan shows that the stray-light signal is the same as the Rayleigh scattering from 2.7 Pa argon. The detection sensitivity of the Thomson experiment reaches 1010 electrons and an accuracy of 10% is achieved by a nonlinear three-parameter fitting procedure. Electron data are presented resulting from measurements in the middle of the arc. To study plasma-wall interaction the vicinity of the plane anode is investigated, too.