A Thomson scattering diagnostic on the Pegasus Toroidal experiment

Abstract

By exploiting advances in high-energy pulsed lasers, volume phase holographic diffraction gratings, and image intensified CCD cameras, a new Thomson scattering system has been designed to operate from 532 - 592 nm on the Pegasus Toroidal Experiment. The system uses a frequency-doubled, Q-switched Nd:YAG laser operating with an energy of 2 J at 532 nm and a pulse duration of 7 ns FWHM. The beam path is < 7m, the beam diameter remains ≤ 3 mm throughout the plasma, and the beam dump and optical baffling is located in vacuum but can be removed for maintenance by closing a gate valve. A custom lens system collects scattered photons from 15 cm < R(maj) < 85 cm at ~F∕6 with 14 mm radial resolution. Initial measurements will be made at 12 spatial locations with 12 simultaneous background measurements at corresponding locations. The estimated signal at the machine-side collection optics is ~3.5 × 10(4) photons for plasma densities of 10(19) m(-3). Typical plasmas measured will range from densities of mid-10(18) to mid-10(19) m(-3) with electron temperatures from 10 to 1000 eV.

Design of a multichannel polychromator for KTX Thomson scattering diagnostic

“…Usually, a single IFP can give Te and ne at one local position in plasma and at several different times during single plasma discharge. By using a set of IFPs the spatial profile of Te and ne can be obtained in the RTS diagnostic; 2) The television-like TS (TVTS) system [5][6][7] utilizes visible laser and grating spectrometer coupled with an ICCD to achieve spatial profile and high-resolution spectral measurements; 3) The light detection and ranging TS (LIDAR) system [8,9] uses a short laser pulse and one single IFP coupled with MCP-PMTs. During plasma discharges, the LIDAR can obtain the spatial profile along the laser beam through the time-of-flight of the short pulse at only one single time due to the low repetition rate of the high-energy ruby laser.…”

“…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 compact multichannel spectrometer for Thomson scattering

“…1 A central feature is a novel spectrometer optimized to operate in the visible region with high efficiency and low noise. The switch to visible wavelengths has been motivated by improved technology, including high efficiency volume phase holographic (VPH) gratings and fast gated image intensified CCD (ICCD) cameras containing new Gen III image intensifiers.…”

First results from the Thomson scattering diagnostic on proto-MPEX

“…In particular, as with all TS diagnostics, great care must be taken to reduce and eliminate unwanted "background" photons that might be present in the collection optics. [3][4][5][6] These stray photons arise from errant ray trajectories introduced by inherent laser beam divergence, scattering centers on the optics and mirrors, and reflections at airto-vacuum interfaces. Since the number of TS "signal" photons is linearly proportional to plasma electron density, it is helpful to commission the diagnostic in electron dense plasmas.…”