A compact multichannel spectrometer for Thomson scattering

Abstract

The availability of high-efficiency volume phase holographic (VPH) gratings and intensified CCD (ICCD) cameras have motivated a simplified, compact spectrometer for Thomson scattering detection. Measurements of T(e) < 100 eV are achieved by a 2971 l∕mm VPH grating and measurements T(e) > 100 eV by a 2072 l∕mm VPH grating. The spectrometer uses a fast-gated (~2 ns) ICCD camera for detection. A Gen III image intensifier provides ~45% quantum efficiency in the visible region. The total read noise of the image is reduced by on-chip binning of the CCD to match the 8 spatial channels and the 10 spectral bins on the camera. Three spectrometers provide a minimum of 12 spatial channels and 12 channels for background subtraction.

Progress on Thomson scattering in the Pegasus Toroidal Experiment

“…This extension provides approximately a 30 ns delay in stray light signal originating from the beam dump. Finally, a cut-on filter at the laser wavelength was tested in anticipation of using it to mitigate stray light in the wavelength domain [4]. It was designed to filter the collimated light just before the VPH grating in the spectrometers, with angle tuning providing adjustment of the cut-on location in wavelength-space.…”

“…Figure 1(a) depicts the laser beam path, beam dump, and collection optics location, and figure 1(b) gives a schematic of the spectrometer. A description of the overall system [3] and spectrometer [4] has been given previously, and the following subsections provide details needed to interpret results presented herein and on improvements to the initial implementation.…”

Design of a multichannel polychromator for KTX Thomson scattering diagnostic

“…The scattered light is de-polarized after passing through multimode fiber bundles, but traditional reflective gratings used in a concave grating spectrometer, Czerny-Turner spectrometer [13], or Littrow spectrometer [14] could not have satisfactory diffraction efficiency for unpolarized light. Recently transmission volume phase holographic (VPH) grating spectrometer has been successfully used in the TVTS system at Pegasus Toroidal Experiment [7,15,16]. The diffraction efficiency of the VPH grating can achieve as high as 90% for unpolarized light [17].…”

“…JT-60U (1999) [33] Nd-YAG laser GS ICCD Pegasus Toroidal [7,15,16], TVTS (532 nm) HT-7 [34] D3-D (1990) [2,3], JT-60U (1999) [23], Alcator C-Mod (1997) [35], JET (2004) [36], Nd-YAG laser IFP APD MAST (2006) [37], RTS (1064 nm) RFX-mod (2007) [4], MST (2008) [21], EAST (2011) [38], NSTX-U (2012) [39] GS APD TdeV (1988) [11] Alcator C-Mod (1990) [12] Ruby laser IFP MCP-PMT JET (1988) [8,9] LIDAR (short pulse) An incoherent TS system has been designed at the Keda Torus eXperiment (KTX) facility [10] based on a high repetition rate (200 Hz), and high pulse energy (5 J/6.6 ns) Nd-YAG laser. The typical plasma density of KTX is about 𝑛 𝑒 = 1 × 10 19 m −3 and the electron temperature 𝑇 𝑒 ranges…”

A Thomson scattering diagnostic on the Pegasus Toroidal experiment

“…Once the ICCD's image intensifier switches off, the CCD can be read out as slowly as needed to reduce amplifier noise and charge transfer efficiency losses. 7 This built-in digitization of CCD cameras provides very cost-effective data acquisition. Instead of requiring standalone GHz-class digitizers for each spatial channel, this implementation gangs up to 8 spatial channels together on a single CCD camera's ultra-low noise AD converter.…”