Diamond-Coated Plasma Probes for Hot and Hazardous Plasmas.

Codrina Ionita,Huiqian Wang,Roman Schrittwieser,Doris Steinmüller-Nethl,Volker Naulin,Ning Yan,Guosheng Xu,Jens Juul Rasmussen

doi:10.3390/ma13204524

Abstract

Plasma probes are simple and inexpensive diagnostic tools for fast measurements of relevant plasma parameters. While in earlier times being employed mainly in relatively cold laboratory plasmas, plasma probes are now routinely used even in toroidal magnetic fusion experiments, albeit only in the edge region, i.e., the so-called scrape-off layer (SOL), where temperature and density of the plasma are lower. To further avoid overheating and other damages, in medium-size tokamak (MST) probes are inserted only momentarily by probe manipulators, with usually no more than a 0.1 s per insertion during an average MST discharge of a few seconds. However, in such hot and high-density plasmas, their usage is limited due to the strong particle fluxes onto the probes and their casing which can damage the probes by sputtering and heating and by possible chemical reactions between plasma particles and the probe material. In an attempt to make probes more resilient against these detrimental effects, we tested two graphite probe heads (i.e., probe casings with probes inserted) coated with a layer of electrically isolating ultra-nano-crystalline diamond (UNCD) in the edge plasma region of the Experimental Advanced Superconducting Tokamak (EAST) in Hefei, People’s Republic of China. The probe heads, equipped with various graphite probe pins, were inserted frequently even into the deep SOL up to a distance of 15 mm inside the last closed flux surface (LCFS) in low- and high-confinement regimes (L-mode and H-mode). Here, we concentrate on results most relevant for the ability to protect the graphite probe casings by UNCD against harmful effects from the plasma. We found that the UNCD coating also prevented almost completely the sputtering of graphite from the probe casings and thereby the subsequent risk of re-deposition on the boron nitride isolations between probe pins and probe casings by a layer of conductive graphite. After numerous insertions into the SOL, first signs of detachment of the UNCD layer were noticed.

Highlights

Plasma probes are well established diagnostic tools for the determination of various relevant plasma parameters, such as electron and ion density, electron temperature, floating potential and plasma potential
The Hα-radiation stems from neutral deuterium in the edge region, which is excited by the plasma electrons
Hα-line intensity during the L-mode, followed by strong fluctuations in the intermediate mode (I-mode), which is characteristic for the I-mode

Summary

Introduction

Plasma probes are well established diagnostic tools for the determination of various relevant plasma parameters, such as electron and ion density, electron temperature, floating potential and plasma potential. Probes are small additional electrodes of a refractory material (usually graphite or tungsten), enclosed in heat-resistant isolating tubes (usually alumina (Al2 O3 ) or boron nitride (BN)), inserted into plasma and biased with a variable voltage to register their current–voltage (IV-) characteristic, or their floating potential. But relatively easy to construct and to handle and are inexpensive. They have good spatial and temporal resolution. An exact theory of probes is very complicated but usually not necessary, since even with simple approximations valuable information can be drawn from a probe’s IV- characteristic

Methods

Results

Conclusion