APD detector electronics for the NSTX Thomson scattering system

Johnson, D. W.; LeBlanc, B.; Long, Donald; Renda, G.

doi:10.1063/1.1321751

Cited by 20 publications

(4 citation statements)

References 7 publications

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“…The DC amplifier bandwidth was estimated to be 23.5 MHz using equation (3.2) on the 'DC' model response function. Combining with the value of δVar(s) δ s = 2.29 × 10 −5 V from figure 9, the resulting value of F = 3.2 is near the values calculated for similar APDs by other Thomson scattering researchers [11,12].…”

Section: Constant Signal Modelsupporting

confidence: 79%

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Detailed modeling of the statistical uncertainty of Thomson scattering measurements

2013

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The uncertainty of electron density and temperature fluctuation measurements is determined by statistical uncertainty introduced by multiple noise sources. In order to quantify these uncertainties precisely, a simple but comprehensive model was made of the noise sources in the MST Thomson scattering system and of the resulting variance in the integrated scattered signals. The model agrees well with experimental and simulated results. The signal uncertainties are then used by our existing Bayesian analysis routine to find the most likely electron temperature and density, with confidence intervals.In the model, photonic noise from scattered light and plasma background light is multiplied by the noise enhancement factor (F) of the avalanche photodiode (APD). Electronic noise from the amplifier and digitizer is added. The amplifier response function shapes the signal and induces correlation in the noise. The data analysis routine fits a characteristic pulse to the digitized signals from the amplifier, giving the integrated scattered signals. A finite digitization rate loses information and can cause numerical integration error.We find a formula for the variance of the scattered signals in terms of the background and pulse amplitudes, and three calibration constants. The constants are measured easily under operating conditions, resulting in accurate estimation of the scattered signals' uncertainty. We measure F ⇡ 3 for our APDs, in agreement with other measurements for similar APDs. This value is wavelength-independent, simplifying analysis. The correlated noise we observe is reproduced well using a Gaussian response function. Numerical integration error can be made negligible by using an interpolated characteristic pulse, allowing digitization rates as low as the detector bandwidth. The effect of background noise is also determined.

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Section: Constant Signal Modelsupporting

confidence: 79%

“…The DC amplifier bandwidth was estimated to be 23. figure 9, the resulting value of F = 3.2 is near the values calculated for similar APDs by other Thomson scattering researchers [11,12]. M and F are expected to be independent of wavelength for this type of APD which has a "reach-through" structure [13].…”

Section: Constant Signal Modelsupporting

confidence: 71%

Detailed modeling of the statistical uncertainty of Thomson scattering measurements

2013

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“…An auto-zero process is included to zero the output by an external TTL (Transistor-Transistor Logic) trigger, so DC offset drift is eliminated to reduce the influence on DC measurement. The channel of high-frequency output is optimized for measuring electric pulses similar to the scattered light pulse [4][5][6], and the contribution of plasma background light is rejected by using a low-pass filter. The other channel of normal output is used to provide the composite signal of plasma background light and the scattered light [7].…”

Section: Introductionmentioning

confidence: 99%

Progress of Thomson scattering diagnostic on HL-2A tokamak

et al. 2017

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“…More information about the MPTS detection electronics can be found elsewhere. 4 Similarly one finds the following expression for the evaluation of G from Raman scattering measurement.…”

Section: Global Geometric Factormentioning

confidence: 99%

Thomson Scattering Density Calibration by Rayleigh and Rotational Raman Scattering on NSTX

LeBlanc¹

2008

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The multi-point Thomson scattering (MPTS) diagnostic measures the profiles of the electron temperature T e (R) and density n e (R) on the horizontal midplane of NSTX.Normal operation makes use of Rayleigh scattering in nitrogen or argon to derive the density profile. While the Rayleigh scattering n e (R) calibration has been validated by comparison with other density measurements and through its correlation with plasma phenomena, it does require dedicated detectors at the laser wavelength in this filter polychromator based diagnostic. The presence of dust and/or stray laser light precludes routine use of these dedicated spectral channels for Thomson scattering measurement.Hence it is of interest to investigate the use of Raman scattering in nitrogen for the purpose of density calibration, since it could free up detection equipment, which could then be used for the instrumentation of additional radial channels. In this paper the viewing optics "geometrical factor" profiles obtained from Rayleigh and Raman scattering are compared. While both techniques agree nominally, residual effects on the order of 10% remain and will be discussed. leblanc@pppl.gov* Contributed paper, published as part of the

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APD detector electronics for the NSTX Thomson scattering system

Cited by 20 publications

References 7 publications

Detailed modeling of the statistical uncertainty of Thomson scattering measurements

Detailed modeling of the statistical uncertainty of Thomson scattering measurements

Progress of Thomson scattering diagnostic on HL-2A tokamak

Thomson Scattering Density Calibration by Rayleigh and Rotational Raman Scattering on NSTX

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