On the Use of Sweeping Langmuir Probes in Cutting Arc Plasmas—Part I: Experimental Results

The first study of Langmuir probes applied to cutting arcs using a sweeping-probe system is presented. It is found that, under a relatively broad range of experimental conditions (changes in the probe material, in the probe radii, or in the sweeping frequency of the probes), no probe damage is registered, notwithstanding the large value of the power flux present with these arcs. In practice, probes with radii down to 63 mum and with sweeping rotation frequencies down to 8.7 s^-1 (probe transit time of ap140 mus through the arc) were used without noticeable alterations. In the measurements of the ion current collected by negatively biased probes, the following two unexpected features are found: the lack of a current plateau in the ion branch of the I-V probe characteristic and the independence of the signal amplitude on the probe radius. According to the experimental evidence, as well as several estimations, we have neglected electron emission of the probe surface as a relevant mechanism in modifying the ion branch of the characteristic. On the contrary, some arguments on which a collection model will be based are presented.