Gaussian process tomography for soft x-ray spectroscopy at WEST without equilibrium information

International audienceGaussian process tomography (GPT) is a recently developed tomography method based on the Bayesian probability theory [J. Svensson, JET Internal Report EFDA-JET-PR(11)24, 2011 and Li et al., Rev. Sci. Instrum. 84, 083506 (2013)]. By modeling the soft X-ray (SXR) emissivity field in a poloidal cross section as a Gaussian process, the Bayesian SXR tomography can be carried out in a robust and extremely fast way. Owing to the short execution time of the algorithm, GPT is an important candidate for providing real-time reconstructions with a view to impurity transport and fast magnetohydrodynamic control. In addition, the Bayesian formalism allows quantifying uncertainty on the inferred parameters. In this paper, the GPT technique is validated using a synthetic data set expected from the WEST tokamak, and the results are shown of its application to the reconstruction of SXR emissivity profiles measured on Tore Supra. The method is compared with the standard algorithm based on minimization of the Fisher information

Construction and measurements of a vacuum-swing-adsorption radon-mitigation system

Long-lived alpha and beta emitters in the $^{222}$Rn decay chain on (and near) detector surfaces may be the limiting background in many experiments attempting to detect dark matter or neutrinoless double-beta decay, and in screening detectors. In order to reduce backgrounds from radon-daughter plate-out onto the wires of the BetaCage during its assembly, an ultra-low-radon cleanroom is being commissioned at Syracuse University using a vacuum-swing-adsorption radon-mitigation system. The radon filter shows ~20$\times$ reduction at its output, from 7.47$\pm$0.56 to 0.37$\pm$0.12 Bq/m$^3$, and the cleanroom radon activity meets project requirements, with a lowest achieved value consistent with that of the filter, and levels consistently < 2 Bq/m$^3$.Comment: 5 pages, 3 figures, Proceedings of Low Radioactivity Techniques (LRT) 2013, Gran Sasso, Italy, April 10-12, 201

Comparison of two regularization methods for Soft x-ray tomography at Tore Supra

International audienceSoft x-ray (SXR) emission in the range 0.1-20 keV is widely used to obtain valuable information on tokamak plasma physics, such as particle transport, magnetic configuration or magnetohydrodynamic activity. In particular, 2D tomography is the usual plasma diagnostic to access the local SXR emissivity. The tomographic inversion is traditionally performed from lineintegrated measurements of two or more cameras viewing the plasma in a poloidal cross-section, like at Tore Supra (TS). Unfortunately, due to the limited number of measured projections and presence of noise, the tomographic reconstruction of SXR emissivity is a mathematical ill-posed problem. Thus, obtaining reliable results of the tomographic inversion is a very challenging task. In order to perform the reconstruction, inversion algorithms implemented in present tokamaks use a priori information as additional constraints imposed on the plasma SXR emissivity. Among several potential inversion methods, some of them have been identified as well suited to tokamak plasmas. The purpose of this work is to compare two promising inversion methods, i.e. the minimum fisher information method already used at TS and planned for WEST configuration, and the alternative 2nd order Phillips-Tikhonov regularization with smoothness constraints imposed on the second derivative norm. Respective accuracy of both reconstruction methods as well as overall robustness and computational time are studied, using several synthetic SXR emissivity profiles. Finally, a real case is studied through tomographic reconstruction from TS SXR database

Measurement of the 0.511 MeV gamma ray line from the Galactic Center

The detection of the 0.511 MeV electron positron annihilation line coming from the Galactic Center to provide the means to estimate the rate of positron production and to test some theoretical sources of positrons is addressed. The results of the measurements of the 0.511 MeV line flux made with a gamma ray experiment on board a stratospheric balloon are presented. The detector field of view looked at the galactic longitude range -31 deg l(II) +41 deg. The observed flux is 0.0067 (+ or - 0.0005) photons 1/cm(2)5 which is in very good agreement with the expected flux when assuming that the Galactic Center is a line source emitting uniformly

Incorporating magnetic equilibrium information in Gaussian process tomography for soft X-ray spectroscopy at WEST

Paper published as part of the Proceedings of the 22nd Topical Conference on High-Temperature Plasma Diagnostics, San Diego, California, April 2018International audienceGaussian process tomography (GPT) [J. Svensson, JET Internal Report EFDA-JET-PR(11)24, 2011 and D. Li, J. Svensson, H. Thomsen, F. Medina, A. Werner, and R. Wolf, Rev. Sci. Instrum. 84, 083506 (2013)] is a recently developed tomography method applied earlier to soft X-ray (SXR) spectroscopy on WEST---Tungsten (W) Environment in Steady-state Tokamak. The short execution time of the algorithm makes GPT an important candidate for providing real-time information on impurity transport and for fast MHD control. In earlier work, GPT has shown its flexibility by providing good reconstruction results without background information about the magnetic equilibrium. On the other hand, information about the magnetic flux surface geometry can in general be useful for additional regularization of the solution. In this paper, we develop a way to take into account the equilibrium information, by constructing a covariance matrix of the prior Gaussian process depending on the flux surface geometry. The GPT method is validated using synthetic SXR emissivity profiles relevant to WEST plasmas and compares favorably with the classical algorithm based on minimization of the Fisher information

Design of a cold neutron source for the MIT reactor

Also issued as a Sc. D. thesis in the Dept. of Nuclear Engineering, MIT, 1970Includes bibliographical references (leaves 211-212

The reactor physics of the Massachusetts Institute of Technology reactor redesign

"August, 1970."Also written as a Ph. D. thesis by the first author and supervised by the second and third author, MIT, Dept. of Nuclear Engineering, 1970Includes bibliographical references (pages 284-289)An H20 cooled compact MITR-II core, reflected by D20 has been designed for the MITR to increase the reflector thermal neutron flux at tips of beam ports by a factor of 3 or better, without changing the operating power level of the reactor. The diffusion approximation to the neutron transport equation has been used. A three neutron energy group scheme, that retains essential spatial effects, used in the studies has yielded satisfactory agreement with measured data. The factors which affect the intensity as well as the quality of the reflector thermal neutron flux have been studied. These studies show that the permanent features of the MITR limit the maximum power densities in the MITR-II core to factors between 4.5 and 12 below the corresponding values in reactors employing a similar core concept Nevertheless, the predicted unperturbed reflector thermal neutron flux of 1.lXlO14 n/cm 2-sec in MITR-II yields a reflector flux per unit power that is competitive with the corresponding values available in reactors of its type and a factor of 5.0 higher than that in MITR-I

Analytical and experimental investigations of the behavior of thermal neutrons in lattices of uranium metal rods in heavy water

Statement of responsibility on title-page reads: R. Simms, I. Kaplan, T. J. Thompson, D. D. Lanning"October 11, 1963.""NYO-10211."Also issued by the first author as a Ph. D. thesis, Massachusetts Institute of Technology, Dept. of Nuclear Engineering, 1964Includes bibliographical references (leaves 193-199)Measurements of the intracellular distribution of the activation of foils by neutrons were made in lattices of 1/4-inch diameter, 1.03% U-235, uranium rods moderated by heavy water, with bare and cadmium-covered foils of gold, depleted uranium, lutetium, europium and copper. The measurements were made in the M.I.T. Heavy Water Lattice Facility with source neutrons from the M.I.T. Reactor. Two lattices were studied in detail in this work. The more closely packed lattice had a triangular spacing of 1.25 inches, and the less closely packed lattice had a triangular spacing of 2.5 inches. The results of the experiments were compared to one-dimensional, 30-energy group, THERMOS calculations based on the available energy exchange kernels. The comparison indicated that the approximation that the hexagonal cell may be replaced by an equivalent circular cell (the Wigner-Seitz approximation) can lead to serious discrepancies in closely packed lattices moderated b! y heavy water.A modified one-dimensional, and a two-dimensional, calculation were shown to predict the intracellular activation distribution in the closely packed lattice. An analytical treatment of the problem of the flux perturbation in a foil was developed and compared to the experimental results obtained by using gold foils of four different thicknesses in the lattice cell; the method was shown to be adequate. An analytical method to treat the effect of leakage from an exponential assembly was formulated; the results indicated that only in small exponential assemblies would leakage be a significant problem in intracellular flux measurements. A method was developed to predict the cadmium ratio of the foils used in the lattice cell; comparison with available measurements with gold foils indicated good agreement between theory and experiment, except for a lattice having very large ratios of moderator volume, to fuel volume, e.g., 100:1.Calculations of the fuel disadvantage factor by the method of successive generations for gold, lutetium and europium detector foils were compared to the results of THERMOS calculations, because THERMOS was shown to predict the experimental distributions. The comparison indicated that the method of successive generations is a good alternative to the THERMOS calculation, if all that is required is 17 and the thermal utilization.U.S. Atomic Energy Commission contract AT(30-1)234

Use of neutron absorbers for the experimental determination of lattice parameters in subcritical assemblies

Statement of responsibility on title-page reads: J. Harrington, D. D. Lanning, I. Kaplan, T. J. Thompson"February 1966."AEC Research and Development ReportMIT-2344-6Includes bibliographical references (leaves 188-192)U.S. Atomic Energy Commission contract AT(30-1)234