On the use of the metallic nozzle of a cutting arc torch as a Langmuir probe

Abstract

The region inside the nozzle (bore diameter ≈ 1 mm) of a cutting arc torch is inaccessible to most plasma diagnostics, and numerical simulations are the only means to find out the relative importance of several physical processes. In this work, a study of electrostatic (Langmuir) probes applied to the inside of a high energy density 30 A cutting arc torch nozzle is presented. The metallic nozzle was used as a Langmuir probe, so the plasma flow is not perturbed by the probe as a solid body. Biasing the nozzle through an electric circuit that employs appropriate resistors together with the arc power source, the i–V nozzle characteristic was built. It was found that under a large positively biased nozzle, the electron current drained from the arc was relatively small, ≈ 1 A, notwithstanding the fact that the size of the nozzle was relatively large. On the other hand, an almost linear ion current was found for the ion branch for nozzle voltages well below the floating value. Based on the magnitude of inverse slope of the ion current, an estimation of the average electron temperature of the plasma in the vicinity of the nozzle wall was estimated from an ion sheath resistance model using a non-equilibrium two-temperature Saha-equation. An average electron temperature of about 4200 K and a corresponding plasma density of 4 × 1017 m- 3 were found.