title: Applying the new gamma ray imager diagnostic to measurements of runaway electron Bremsstrahlung radiation in the DIII-D Tokamak (invited).
abstract: A new gamma ray imager (GRI) is developed to probe the electron distribution function with 2D spatial resolution during runaway electron (RE) experiments at the DIII-D tokamak. The diagnostic is sensitive to 0.5-100 MeV gamma rays, allowing characterization of the RE distribution function evolution during RE growth and dissipation. The GRI consists of a lead "pinhole camera" mounted on the DIII-D midplane with 123 honeycombed tangential chords 20 cm wide that span the vessel interior. Up to 30 bismuth germanate (BGO) scintillation detectors capture RE bremsstrahlung radiation for Pulse Height Analysis (PHA) capable of discriminating up to 20 000 pulses per second. Digital signal processing routines combining shaping filters are performed during PHA to reject noise and record gamma ray energy. The GRI setup and PHA algorithms will be described and initial data from experiments will be presented. A synthetic diagnostic is developed to generate the gamma ray spectrum of a GRI channel given the plasma information and a prescribed distribution function. Magnetic reconstructions of the plasma are used to calculate the angle between every GRI sightline and orient and discriminate gamma rays emitted by a field-aligned RE distribution function.
authors.affiliation: Oak Ridge Associated Universities, Oak Ridge, Tennessee 37830, USA.
authors.forename: C M
authors.initials: CM
authors.lastname: Cooper
authors.affiliation: General Atomics, P.O. Box 85608, San Diego, California 92186-5608, USA.
authors.forename: D C
authors.initials: DC
authors.lastname: Pace
authors.affiliation: General Atomics, P.O. Box 85608, San Diego, California 92186-5608, USA.
authors.forename: C
authors.initials: C
authors.lastname: Paz-Soldan
authors.affiliation: Oak Ridge National Laboratory, Oak Ridge, Tennessee 37830, USA.
authors.forename: N
authors.initials: N
authors.lastname: Commaux
authors.affiliation: General Atomics, P.O. Box 85608, San Diego, California 92186-5608, USA.
authors.forename: N W
authors.initials: NW
authors.lastname: Eidietis
authors.affiliation: University of California, San Diego, La Jolla, California 92093-0533, USA.
authors.forename: E M
authors.initials: EM
authors.lastname: Hollmann
authors.affiliation: Oak Ridge National Laboratory, Oak Ridge, Tennessee 37830, USA.
authors.forename: D
authors.initials: D
authors.lastname: Shiraki