Initial wave-field measurements in the Material Diagnostic Facility (MDF)

1. Initial wave-field measurements in the Material Diagnostic Facility (MDF) Juan F. Caneses, B.D. Blackwell, Cormac Corr, Cameron Samuell, John Wach,.Plasma Research Laboratory, Research School of Physics and Engineering, Australian National University, Canberra 0200, Australia Motivation: Some aspects of Plasma-Material Interaction relevant to Fusion devices are not fully understood, in particular the behaviour of Plasma Facing Components under steady state ion and heat fluxes. Erosion processes and mechanical property degradation due to steady state particle and heat fluxes will limit the lifetime of Plasma Facing Components in a Fusion reactor. The motivation for building MDF is to provide a facility capable of producing plasma densities and heat fluxes relevant to fusion reactors, enabling us to develop advance optical diagnostics to remotely asses the interaction between the plasma and different test materials, improve the understanding of Plasma-Material Interactions and to test/characterize materials for Fusion applications. Linear Plasma Devices like MDF offer the possibility to explore these interactions at reduced cost while providing simpler geometries and improved access for diagnostics compared to Toroidal Plasma Devices. Introduction: The Plasma Research Laboratory at the Australian National University has recently constructed a prototypical Linear Plasma Device to study Plasma-Material Interactions at Fusion relevant conditions. This device is referred to as MDF (Material Diagnostics Facility) and generates RF discharges through helicon waves in a non-uniform magnetic field. To achieve the plasma parameters required to study Plasma-Material Interaction in MDF it is first necessary to optimize the plasma production. MDF produces its plasma using an RF Helicon plasma source in a non-uniform magnetic field. We start the optimization process by observing the formation of the plasma and the wave fields produced. We present some initial results of the Helicon wave field structure in the target region of MDF. a) b) Experimental setup: MDF consists of a (1)Vacuum chamber divided into a source and target region, (2) magnetic system that produces an axially non-uniform field and (3) an RF helicon plasma source. MDF launches Helicon waves (type of electromagnetic wave) using a Left-Handed helical antenna to produce Argon and Hydrogen plasma which is magnetically confined and transported to the target region. The axially non-uniform magnetic field is used to produce a magnetic mirror, which increases the plasma density. The magnetic system consists of a set of 10 water cooled electromagnets of 0.3 m internal diameter capable of producing a maximum of 0.19 T and 0.90 T at the target and source region respectively. MDF was designed and constructed to produce the magnetic field strength depicted in Figure 1c. c) Figure 1, a) isometric view of MDF and its components, b) schematic of MDF and helical antenna (insert), c)axially non-uniform magnetic field in MDF Wave field measurements: Electromagnetic waves in plasmas can be measured using RF magnetic probes. The probes use small inductors inserted in the plasma in order to couple with the magnetic component of the wave as seen in figure 2. The radial variations of the Helicon wave fields where measured 20 cm downstream of the antenna at the following conditions: • Magnetic field: 0.09 and 0.008 Tesla (target and source region respectively) • Pressure: 3.1 mTorr in Argon • RF power: 2.1 and 0.6 kW forward power The formation of a blue cored plasma , associated with the production of Ar II ions, was observed during 2.1 kW operation. The radial profiles corresponding to the blue core mode are seen to be peaked at the centre and show indication of a second radial mode being excited. At 0.6 kW operation the blue core is no longer excited, the corresponding radial wave profiles are broader and only the first radial mode is seen to be excited. Further investigation is required, with RF magnetic and Langmuir probes, to observe the behaviour of the wave fields and the plasma density a) b) Figure 2, a) inductors at the tip of a RF magnetic probe b) schematic of the RF magnetic probe used Radial variation of Helicon wave fields 20 cm downstream of antenna: a) Future work: • Upgrade of the Magnetic and RF system in order to achieve fields in the order of ~1T and RF powers of 10 kW. • Operation of hydrogen and optimization of plasma production • Implementation of a sample holder in collaboration with ANSTO. • Implementation of advanced optical diagnostics in order to characterize plasma dynamics and Plasma-Material Interactions using Laser Induced Fluorescence, Thomson Scattering and Motional Stark Imaging. b) Figure 3, Magnitude and phase of radial wave fields at a) 2.1 kW and b) 0.6 kW