LIDAR TS for ITER core plasma. Part I: layout &amp; hardware

et al. 2019

Self Cite

In spite of its very high reliability, LIDAR Thomson Scattering (LIDAR-TS) has so far only been installed on JET. The new Divertor Tokamak Test (DTT) facility at Frascati will be similar in size to JET and thus is clearly suited for a LIDAR-TS system. The proposal here follows the concept of a no vignetting region, which circumvents the need for relative density calibrations and provides much better signals than those on the JET system, particularly at the outer boundary. The scattered spectrum reaches the spectrometer through a train of relay lenses. The spectrometer uses fast MCP-PMTs for detection of the scattered signal. The fast detectors are generally limited to a spectral range below 900 nm. Hence, a laser at a single wavelength slightly below 900 nm would be ideal. Here we investigate using an Nd:YAG laser, emitting simultaneously both the fundamental and the second harmonic wavelength. The expected performance is analyzed in a simulation program, confirming the ability of this choice to cover the full temperature range. The spatial resolution of the system on JET was about 7 cm. For DTT it is possible to improve this value both by using more modern hardware and by deconvoluting the measured signals. Using a commercially available Nd:YAG two wavelength system with 100 ps pulses in combination with 180 ps MCP-PMTs a resolution of less than 4 cm is achieved. Deconvolution works well at the outer boundary of an H-mode plasma. With deconvolution of the spectral channel signals, the effective spatial resolution in this region can be reduced to ∼ 2 cm. K: Nuclear instruments and methods for hot plasma diagnostics; Plasma diagnosticsinterferometry, spectroscopy and imaging 1Corresponding author.

mentioning

confidence: 88%

“…In this proposal we are assuming that the laser is a Nd:YAG laser emitting simultaneously at both the fundamental and the frequency doubled wavelength. [7]. We are aware that high power lasers emitting around 900 nm are technically feasible, but to our knowledge not commercially available.…”

Section: Lasers and Detectorsmentioning

confidence: 99%

A LIDAR Thomson Scattering System for the Divertor Tokamak Test (DTT) facility

et al. 2019

Self Cite

“…We have extended the simulation code used previously [1] to include the case of launching two laser pulses of different wavelengths, simultaneously through the ITER machine using the ITER LIDAR optical set up described in reference [2] (this conference).…”

Section: Introductionmentioning

confidence: 99%

“…This is because scattered light from the 2 nd harmonic wavelength boosts the scattered signal at the blue end of the spectrometers's spectral range making low temperatures measurable. (See [2] for a brief discussion of suitable Nd:YAG lasers). Other authors have also discussed launching two different wavelength laser pulses in a conventional TS system, but separated in time by a few microseconds [3,4] and [5].…”

Section: Introductionmentioning

confidence: 99%

LIDAR TS for ITER core plasma. Part II: simultaneous two wavelength LIDAR TS

2017

We have shown recently, and in more detail at this conference (Salzmann et al) that the LIDAR approach to ITER core TS measurements requires only two mirrors in the inaccessible port plug area of the machine. This leads to simplified and robust alignment, lower risk of mirror damage by plasma contamination and much simpler calibration, compared with the awkward and vulnerable optical geometry of the conventional imaging TS approach, currently under development by ITER. In the present work we have extended the simulation code used previously to include the case of launching two laser pulses, of different wavelengths, simultaneously in LIDAR geometry. The aim of this approach is to broaden the choice of lasers available for the diagnostic. In the simulation code it is assumed that two short duration (300 ps) laser pulses of different wavelengths, from an Nd:YAG laser are launched through the plasma simultaneously. The temperature and density profiles are deduced in the usual way but from the resulting combined scattered signals in the different spectral channels of the single spectrometer. The spectral response and quantum efficiencies of the detectors used in the simulation are taken from catalogue data for commercially available Hamamatsu MCP-PMTs. The response times, gateability and tolerance to stray light levels of this type of photomultiplier have already been demonstrated in the JET LIDAR system and give sufficient spatial resolution to meet the ITER specification. Here we present the new simulation results from the code. They demonstrate that when the detectors are combined with this two laser, LIDAR approach, the full range of the specified ITER core plasma Te and ne can be measured with sufficient accuracy. So, with commercially available detectors and a simple modification of a Nd:YAG laser similar to that currently being used in the design of the conventional ITER core TS design mentioned above, the ITER requirements can be met.

“…The LIDAR Thomson scattering system proposed at this conference is injecting two simultaneous laser beams at the fundamental and the second harmonic wavelengths respectively of an Nd:YAG laser [1,2]. This paper only deals with issues relating to calibration and edge resolution of this particular choice of laser.…”

mentioning

confidence: 99%

LIDAR TS for ITER core plasma. Part III: calibration and higher edge resolution

2017

Self Cite

Calibration, after initial installation, of the proposed two wavelength LIDAR Thomson Scattering System requires no access to the front end and does not require a foreign gas fill for Raman scattering. As already described, the variation of solid angle of collection with scattering position is a simple geometrical variation over the unvignetted region. The additional loss over the vignetted region can easily be estimated and in the case of a small beam dump located between the Be tiles, it is within the specified accuracy of the density. The only additional calibration is the absolute spectral transmission of the front-end optics. Over time we expect the transmission of the two front-end mirrors to suffer a deterioration mainly due to depositions. The reduction in transmission is likely to be worse towards the blue end of the scattering spectrum. It is therefore necessary to have a method to monitor such changes and to determine its spectral variation. Standard methods use two lasers at different wavelength with a small time separation. Using the two-wavelength approach, a method has been developed to determine the relative spectral variation of the transmission loss, using simply the measured signals in plasmas with peak temperatures of 4–6 keV . Comparing the calculated line integral of the fitted density over the full chord to the corresponding interferometer data we also have an absolute calibration. At the outer plasma boundary, the standard resolution of the LIDAR Thomson Scattering System is not sufficient to determine the edge gradient in an H-mode plasma. However, because of the step like nature of the signal here, it is possible to carry out a deconvolution of the scattered signals, thereby achieving an effective resolution of ∼ 1–2 cm in the outer 10–20 cm.