Two Photon Absorption Laser Induced Fluorescence for Neutral Hydrogen Profile Measurements

Technical Report · Fri Sep 23 00:00:00 EDT 2016

DOI:https://doi.org/10.2172/1326021· OSTI ID:1326021

Scime, Earl E. ^[1]

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1. West Virginia Univ., Morgantown, WV (United States)

The magnitude and spatial dependence of neutral density in magnetic confinement fusion experiments is a key physical parameter, particularly in the plasma edge. Modeling codes require precise measurements of the neutral density to calculate charge-exchange power losses and drag forces on rotating plasmas. However, direct measurements of the neutral density are problematic. In this work, we proposed to construct a laser-based diagnostic capable of providing spatially resolved measurements of the neutral density in the edge of plasma in the DIII-D tokamak. The diagnostic concept is based on two-photon absorption laser induced fluorescence (TALIF). By injecting two beams of 205 nm light (co or counter propagating), ground state hydrogen (or deuterium or tritium) can be excited from the n = 1 level to the n = 3 level at the location where the two beams intersect. Individually, the beams experience no absorption, and therefore have no difficulty penetrating even dense plasmas. After excitation, a fraction of the hydrogen atoms decay from the n = 3 level to the n = 2 level and emit photons at 656 nm (the H_α line). Calculations based on the results of previous TALIF experiments in magnetic fusion devices indicated that a laser pulse energy of approximately 3 mJ delivered in 5 ns would provide sufficient signal-to-noise for detection of the fluorescence. In collaboration with the DIII-D engineering staff and experts in plasma edge diagnostics for DIII-D from Oak Ridge National Laboratory (ORNL), WVU researchers designed a TALIF system capable of providing spatially resolved measurements of neutral deuterium densities in the DIII-D edge plasma. The laser systems were specified, purchased, and assembled at WVU. The TALIF system was tested on a low-power hydrogen discharge at WVU and the plan was to move the instrument to DIII-D for installation in collaboration with ORNL researchers. After budget cuts at DIII-D, the DIII-D facility declined to support installation on their tokamak. Instead, after a no-cost extension, the apparatus was moved to the University of Washington-Seattle and successfully tested on the HIT-SI3 spheromak experiment. As a result of this project, TALIF measurements of the absolutely calibrated neutral density hydrogen and deuterium were obtained in a helicon source and in a spheromak, designs were developed for installation of a TALIF system on a tokamak, and a new, xenon-based calibration scheme was proposed and demonstrated. The xenon-calibration scheme eliminates significant problems that were identified with the standard krypton calibration scheme.

• Research Organization: West Virginia Univ., Morgantown, WV (United States)
• Sponsoring Organization: USDOE Office of Science (SC)
• Contributing Organization: Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States); General Atomics, San Diego, CA (United States)
• DOE Contract Number: SC0004736
• OSTI ID: 1326021
• Report Number(s): DOE_WVU-SC0004736; 3042920830; TRN: US1700288
• Country of Publication: United States
• Language: English

Related Subjects

70 PLASMA PHYSICS AND FUSION TECHNOLOGY
FLUORESCENCE
ROTATING PLASMA
SPHEROMAK DEVICES
HYDROGEN
PHOTONS
DENSITY
DEUTERIUM
ABSORPTION
XENON
LASER RADIATION
TRITIUM
HYDROGEN 1
FAR ULTRAVIOLET RADIATION
BEAMS
CALIBRATION
WASHINGTON
GROUND STATES
DESIGN
EXCITATION
SPACE DEPENDENCE
DE-EXCITATION
LYMAN LINES
PULSES
PLASMA DIAGNOSTICS
POWER RANGE 100-1000 KW
TOKAMAK DEVICES
HELICON WAVES
two photon laser induced fluorescence
neutral density measurements