First Results from the Thomson Scattering Diagnostic on the Large Plasma Device

Abstract

We present the first Thomson scattering measurements of electron density and temperature in the Large Plasma Device (LAPD), a 22 m long magnetized linear plasma device at the University of California Los Angeles (UCLA). The diagnostic spectrally resolves the Doppler shift imparted on light from a frequency-doubled Nd:YAG laser when scattered by plasma electrons. A fiber array coupled to a triple-grating spectrometer is used to obtain high stray light rejection and discriminate the faint scattering signal from a much larger background. In the center of the plasma column, the measured electron density and temperature are about ne≈1.5×1013 cm−3 and Te≈ 3 eV, respectively, depending on the discharge parameters and in good agreement with Langmuir probe data. Optical design considerations to maximize photon count while minimizing alignment sensitivity are discussed in detail and compared to numerical calculations. Raman scattering off of a quartz crystal probe is used for an absolute irradiance calibration of the system.

Development of the Thomson scattering measurement system for cascade arc device with indirectly heated hollow cathode

“…The triple grating spectrometer consisted of three gratings and a notch filter. The first and second grating (both 1800 grooves/mm) dispersed and inversely dispersed the incident light to eliminate the stray light from the out of the region of interest [14][15][16][17]. The notch filter between the two gratings occluded the stray light spectrum around 532 nm to improve the signal-to-noise ratio.…”

Two-Dimensional Thomson Scattering in Laser-Produced Plasmas

“…This optical design with a relatively small collection angle is used to minimize alignment sensitivity, accommodate a range of probe beam diameters, and to assure that alignment between the beam and fiber projection is maintained during a motorized raster scan over a large area and for the duration of an experimental run lasting several hours. The scattered light collection efficiency is best evaluated using the optical extent G = A Ω = AΩ, which is the same for the source and detector [23]. Here, Ω is the collection solid angle and A is the source area defined by the intersection of the circular fiber projection and the pencil beam.…”

“…The instrument described here extends the 1D setup from Kaloyan et al [22] to 2D, and includes a motorized auto-alignment technique, which autonomously maps out the probe-laser beam height and inclination and steers the collection optics and scattering volume in three dimensions (3D) accordingly, as the laser-plasmas are produced repeatedly at 1 Hz. The technique is enabled by an optical design that maximizes alignment stability and optical extent [23], a triple grating spectrometer with notch filter for stray light suppression [24] and a motorized auto-align scattering system [22], as well as a laser target and data acquisition system that produces LPPs at high repetition rate [25]. Unlike other multi-dimensional Thomson scattering systems [3][4][5][6][7], this new technique significantly increases efficiency and spatial resolution and can scan planar regions of arbitrary shape and size.…”

Thomson scattering on the Large Plasma Device

“…This lens projects a 3.7× demagnified image of the beam onto a linear fiber bundle. Using relatively slow focusing and collection lenses outside the vacuum chamber allows the diagnostic to move from port to port and minimizes alignment sensitivity, and yet results in high light-collection efficiency with an effective optical extent of 0.9×G max , where G max is the spectrometer extent 13 . The fiber array consists of 40 linearly arranged 200 µm diameter core optical fibers with a numerical aperture of 0.22, mapped end-to-end with combined slit dimensions of 8.8 mm × 0.22 mm.…”