title: A two photon absorption laser induced fluorescence diagnostic for fusion plasmas.
abstract: The quality of plasma produced in a magnetic confinement fusion device is influenced to a large extent by the neutral gas surrounding the plasma. The plasma is fueled by the ionization of neutrals, and charge exchange interactions between edge neutrals and plasma ions are a sink of energy and momentum. Here we describe a diagnostic capable of measuring the spatial distribution of neutral gas in a magnetically confined fusion plasma. A high intensity (5 MW/cm(2)), narrow bandwidth (0.1 cm(-1)) laser is injected into a hydrogen plasma to excite the Lyman β transition via the simultaneous absorption of two 205 nm photons. The absorption rate, determined by measurement of subsequent Balmer α emission, is proportional to the number of particles with a given velocity. Calibration is performed in situ by filling the chamber to a known pressure of neutral krypton and exciting a transition close in wavelength to that used in hydrogen. We present details of the calibration procedure, including a technique for identifying saturation broadening, measurements of the neutral density profile in a hydrogen helicon plasma, and discuss the application of the diagnostic to plasmas in the DIII-D tokamak.
authors.affiliation: Physics Department, West Virginia University, Morgantown, 26506, USA. richard.magee@mail.wvu.edu
authors.forename: R M
authors.initials: RM
authors.lastname: Magee
authors.forename: M E
authors.initials: ME
authors.lastname: Galante
authors.forename: D
authors.initials: D
authors.lastname: McCarren
authors.forename: E E
authors.initials: EE
authors.lastname: Scime
authors.forename: R L
authors.initials: RL
authors.lastname: Boivin
authors.forename: N H
authors.initials: NH
authors.lastname: Brooks
authors.forename: R J
authors.initials: RJ
authors.lastname: Groebner
authors.forename: D N
authors.initials: DN
authors.lastname: Hill
authors.forename: G D
authors.initials: GD
authors.lastname: Porter