Comparative analysis of the plasma parameters of ECR and combined ECR + RF discharges in the TOMAS plasma facility

Abstract

The toroidal magnetized system (TOMAS) plasma facility aims at complementary research on wall conditioning methods, plasma production and plasma–surface interaction studies. This paper explores for the first time the parameters in helium electron-cyclotron resonance (ECR) plasma and combined ECR + radio-frequency (RF) discharges in TOMAS. The ECR discharge in this work, at 2.45 GHz and 87.6 mT, is the main one for creating and maintaining the plasma, while the addition of RF power at 25 MHz allows to broaden the achievable electron temperature and density at a given gas flow, as evidenced by triple Langmuir probe measurements. This effect of the combined ECR + RF discharge provides flexibility to study particular aspects of wall conditioning techniques relevant to larger devices, or to approach plasma conditions relevant to fusion edge plasmas for particular surface interaction studies.

Overview of TOMAS plasma diagnostics

“…The device is equipped with various methods of plasma diagnostics as indicated in figure 1. The Langmuir probes system [3,4] consists of one single (SP) and two triple (TP) movables probes. The optical diagnostics include video diagnostics (VD) [3], a photodetector (PD) and an optical emission spectroscopy (OES) [5].…”

“…One of the movable probes is a SP manufactured by Impedans Ltd. It is installed on the same horizontal port of the vacuum vessel as the TP [3,4]. The probe uses an RF and DC compensation scheme.…”

“…The probe tips have a length of 4 mm and a diameter of 0.8 mm. The TPs work in the voltage mode (direct-display method) [4]. The TPs are used to measure the electron density and temperature, and the floating potential.…”

“…A typical time evolution of the plasma parameters measured by the horizontal TP is shown in figure 2(a). Previously, a first comparison of plasma parameters measured by the SP and the TP was made in [4]. New experiments were carried out comparing electron temperature measurements with the SP and the TP in helium ECWC plasma at a pressure of 6.5 × 10 −2 Pa and a microwave power of 1.8 kW.…”