Janus, a bidirectional, multifunctional plasma diagnostic

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Abstract
Janus is a multiple-function edge probe used to diagnose the Alcator C limiter shadow plasma whose components include two sets of identical diagnostics, each set facing a different direction. Included within each set of diagnostics are a retarding-field energy analyzer (RFEA), Langmuir probe, and calorimeter. Janus is constructed to make measurements both parallel and antiparallel to a magnetic field. It can withstand high heat fluxes and can be scanned perpendicular to the magnetic field in the limiter shadow. The RFEA can alternatively sample both the ion and electron parallel energy distribution functions during a tokamak discharge. From the Langmuir probe, one can infer electron temperature, density, and the plasma floating potential. The calorimeter independently detects the total parallel heat flux incident on an electrically floating plate. Together these three diagnostics enable detailed, localized edge plasma studies on Alcator C. This paper presents the design considerations for each of the diagnostics along with a brief summary of the analysis techniques. Some experimental results obtained using Janus will also be presented.