Study of the effects of photoelectron statistics on Thomson scattering data

Hart, Grant W.; Levinton, F. M.; Mcneill, D. H.

doi:10.1063/1.1138685

Cited by 7 publications

(7 citation statements)

References 2 publications

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“…A recent paper on a single channel counting process (radionuclide metrology) illustrates that problems remain even in the very well established field of radioactivity counting measurements. [3] Simulations are often used to track errors where analytic calculations are impossible [4]. Simple analytic representation of a measurement can, on the other hand, aid general understanding (limits of measurement, identification of interference, systematic errors, etc.)…”

Section: The Problemmentioning

confidence: 99%

“…The results of Thomson scattering measurements are often presented in the form of a histogram, a plot of the binned number of temperature readings within specified intervals as a function of the characteristic temperature of the bins [4].…”

Section: Distribution Of Temperature Measurements -From Histogram To ...mentioning

confidence: 99%

“…Here f (r) is calculated from the distributions of I 1 and I 2 (r = I 1 /I 2 ) and then is used to calculate the distributions of h(T e = const/ln r) for three distributions of the signal ratio for plasmas with a nominal temperature T 0 = 40 eV, an average signal ratio r 0 = 1 1 /I 2 = 4, and variances σ 2 r of that ratio equal to 2 (a), 0.5 (b), and 0.1 (c) for three different scattering systems on plasmas with densities ranging over more than 4 orders of magnitude. They are (a) ELMO Bumpy Torus, with n e ∼ 10 18 m −3 [6,7], (b) S-1, with n e ∼ 10 20 m −3 [4], and (c) Columbia University Shock Tube, with n e ∼ 10 22 m −3 [12]. The three measurements have similar signal ratios r 0 in the two channels (and, therefore, values of T e ), but widely different variances (because of differences in heating mechanism, plasma background light [13], and electron density) and S/N, which leads to the different calculated variances σ 2 r and, thereby, shapes of the probability densities h(T e ) in figure 3.…”

Section: Distribution Of Temperature Measurements -From Histogram To ...mentioning

confidence: 99%

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Statistical characteristics of Thomson scattering data

Mcneill¹

2019

J. Inst.

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Thomson scattering is used to measure the electron temperature of plasmas. It is essentially a counting experiment. The distribution of detected photons (photoelectrons) in a number of channels yields an estimate of the scattered light spectrum and, thereby, of the plasma electron temperature and density. Repeated measurements at fixed plasma parameters yield a range of values of the measured electron temperature. The spread in these values is typically broader toward high temperatures and the peak (mode) of the temperature density function is lower than the nominal plasma temperature, while the mean is higher. This spread is intrinsic to these measurements, as actual data samples and simulations show. Here a two-channel analytic model of the distribution of temperature readings from non-relativistic, thermal plasmas is used to obtain an intuitive picture of the significance of data from this means of measurement. The model is applied to data from three experiments with electron densities ranging from 5 • 10 17 m −3 to 7 • 10 22 m −3 , with similar temperatures in widely different profiles of the probability density function. The model is consistent with observed scattered and background light and can, for example, help identify artifacts of diagnostic response, such as apparent high temperatures, and, supplemented by data from additional detector channels, deviations from a thermal electron population, electron drift, and relativistic effects. This is a chance to look in a modern way at some metrological features of this well-established measurement technique. K: Analysis and statistical methods; Detector modelling and simulations II (electric fields, charge transport, multiplication and induction, pulse formation, electron emission, etc); Plasma diagnostics -interferometry, spectroscopy and imaging

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Section: The Problemmentioning

confidence: 99%

Section: Distribution Of Temperature Measurements -From Histogram To ...mentioning

confidence: 99%

Section: Distribution Of Temperature Measurements -From Histogram To ...mentioning

confidence: 99%

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Statistical characteristics of Thomson scattering data

Mcneill¹

2019

J. Inst.

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“…Thomson scattering electron temperature measurements in spheromaks always seem to show large scatter, presumably beyond the uncertainties attributed to photon statistics [27]. We do not know what causes this, and we cannot directly correlate the scatter to any measurement of radiation.…”

Section: Database Of Thomson Scattering Resultsmentioning

confidence: 93%

Improvement of Sensor for Noncontact Capacitance/Voltage Measurement and Lifetime Measurement of Bare Silicon(100)

Sakai¹,

Kohno²,

Okada³

et al. 1997

Jpn. J. Appl. Phys.

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The paper presents experiments and analysis of energy confinement on the CTX spheromak. Compared to previous published results from 0.4 m radius flux conservers, in a 0.67 m radius mesh flux conserver (with the current density kept constant), the magnetic field increases while the plasma density is kept the same. However, the electron temperature does not rise, and hence (fi) yol drops. The plasma resistivity remains constant (the resistance drops as the size increases), and the energy confinement time stays the same. Plasma energy content results from spheromaks during sustainment by helicity injection are also presented and show confinement equivalent to that during the decay phase. Increased magnetic field in the same size experiment produces very little improvement in electron temperature and a decrease in confinement time. The resistive decay time is found to be empirically independent of the core electron temperature. It is, instead, proportional to the strength of the magnetic field at constant plasma density, while the ratio of magnetic field to decay time depends on plasma density, consistently with ionization breakdown at the edge of the spheromak dominating helicity dissipation. The possible causes of this observed confinement are examined separately in detailed quantitative and qualitative studies. Absolutely calibrated multichord bolometry shows that impurity radiation is not the cause of the low electron temperatures. The particle confinement time has increased with size, but does not show an increase with increasing magnetic field. At the lower (/3) vo) of the larger experiment, the particle replacement power cannot explain the unaccounted energy losses. Any important role of pressure driven modes in the CTX energy balance is shown to be inconsistent with the available CTX data. The possibility that rotating coherent current driven kink modes can seriously degrade energy confinement is evaluated and discounted owing to the lack of improvement when the modes are absent. The role of anomalous ion heating is examined, and the available data are presented. Finally, a hypothesis explaining these results is presented, suggestions for future work are made, and the results are summarized.

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“…Pi directly. Instead, we use the fact that repeated measurements of the intensity of a stable light source will be distributed in a Poisson statistical distribution [7,10]. Conversion factors Ci are then simply given by…”

Section: Detection Efficiencymentioning

confidence: 99%

A simple, high-performance Thomson-scattering diagnostic for high-temperature plasma research

Hartog

Cekić

1994

Meas. Sci. Technol.

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This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States nor any agency thereof, nor any of their employees, makes any warranty, expressed or implied, or assumes any legal liability or responsibility for any third party's use or the results of such use of any information, apparatus, product or process disclosed in this report, or represents that its use by such third party would not infringe privately owned rights.

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Study of the effects of photoelectron statistics on Thomson scattering data

Cited by 7 publications

References 2 publications

Statistical characteristics of Thomson scattering data

Statistical characteristics of Thomson scattering data

Improvement of Sensor for Noncontact Capacitance/Voltage Measurement and Lifetime Measurement of Bare Silicon(100)

A simple, high-performance Thomson-scattering diagnostic for high-temperature plasma research

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