Temperature response of laser heated emissive probe materials under vacuum and free atmospheric conditions

Abstract

Precise temporal and spatial knowledge of plasma potential has been a challenging task for decades. Gradient in values of plasma potential govern local electric fields providing insight into many other bulk plasma properties like particle drifts, confinement, transport barriers etc and plays a crucial role in determining stability of magnetically confined high temperature plasmas. In high temperature devices like tokamaks, plasma tends to develop edge bifurcations and results in edge transport barriers, which are a key tool for enhancing the plasma confinement properties in magnetic fusion devices, which in turn requires knowledge of plasma potential. Conventional emissive probes (CEPs) in high temperature magnetically confined plasmas are not advisable owing to their inherent properties and tokamak parameters like high magnetic field, ultra-high vacuum pressure etc as well as tokamak geometry. A new type of emissive probe is becoming popular in recent times in such devices called the laser heated emissive probe (LHEP). Mostly, LaB6 and graphite are used as a LHEP tip owing to their inherent properties of thermal conductivity, low work function, high emissivity, higher lifetime etc. Similar with LaB6 in its mechanical and electrical properties, CeB6 is emerging as a promising candidate for LHEP. CeB6 is a better electron emitter than graphite and LaB6 at comparatively low power due to its lower work function. In this work, the heating dynamics of LaB6 and CeB6 heated by a CW CO2 laser with maximum power of 55 W have been reported. Theoretical and simulation models using Matlab and ANSYS have been developed to understand and explain the temperature gaining process of the probes. Simulation results are further validated by comparing them with experimentally measured data using an infrared camera.