Page 4 - Jet outflow and open field line measurements on the C-2W advanced beam-driven field-reversed configuration plasma experiment

10D120-4 Sheftman et al.
D. Spatial heterodyne spectroscopy
Measurement of the jet outflow into the inner divertor
is of particular interest and can provide much insight into
the physics of the field expanded plasma. However, particle
density in the divertor is expected to be <1012 cm−3, result-
ing in weak radiation output. Thus, in order to analyze ion
dynamics in the divertor, a high throughput spectrometer is
required. A field widened Spatial Heterodyne Spectrometer
(SHS) is being developed to answer these requirements. SHS
is a Fourier transform spectrometer, of compact size and high
6
􏰀∞ 0
where σ is the wavenumber, θL is the Littrow angle, x is the position on the detector, and σ0 is the Littrow wavenumber. As can be seen in Eq. (4), the spectrum is the Fourier trans- form of the fringe pattern. Field widening prisms have been introduced, to increase the light throughput by two orders of magnitude. Through the use of the prisms, the gratings appear perpendicular to the optical axis and are well focused by the imaging lens. A prototype SHS was built to demonstrate capa- bility on an experimental compact toroid injector (CTI) test
FIG.6. DesignofSHSplannedfortheC-2Winnerdivertor.
Rev. Sci. Instrum. 89, 10D120 (2018)
throughput.
optics, a beam splitter, diffraction gratings, imaging optics, and a fast camera (2 kHz frame rate for full frame size). Incident light is split by the beam splitter and recombined to extract an interference pattern, created by the crossing of wave fronts as they reflect from the gratings (see Ref. 6 for more details):
I(x) =
B(σ){1 + cos[2π(4(σ − σ0) tan θLx)]}dσ, (4)
The system (see Fig. 6) consists of collimating
FIG. 7. (a) and (b) Image of the interference pattern and resulting spectrum of Dβ radiation from the CTI test stand, respectively. (c)–(e) Image of the interference pattern, inferred fringe pattern, and resulting spectrum of CTI plasma radiation using a 10 nm FWHM filter, respectively.
stand. The prototype was optimized at the wavelength range of 450-500 nm and did not include an image intensifier, and a single frame camera was used, with a minimal frame time of 160 ms. Thus, velocity measurements were not possible for a compact toroid of 100 km/s translation velocity. Instead, time averaged spectra were obtained. Interference filters were introduced to select the spectrum under observation. Spectra of deuterium neutrals and C2+ ions were obtained (see Fig. 7). The field angle of the prototype SHS was approximately 4◦, determined by the distance between the collection optics and
FIG.8. SHSlineofsightinterlaidwitharadialcrosssectionviewoftheinner divertor. P1 represents the relatively dense, central portion of the jet plasma, and P2 represent the field expanded plasma.