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  • Rachlew, Elisabeth, 1944-, et al. (författare)
  • Measuring fast ions in fusion plasmas with neutron diagnostics at JET
  • 2019
  • Ingår i: Plasma Physics and Technology. - : IOP PUBLISHING LTD. - 2336-2626 .- 2336-2634. ; 61:1
  • Tidskriftsartikel (refereegranskat)abstract
    • Fast ions in fusion plasmas often leave characteristic signatures in the neutron emission from the plasma. In this paper, we show how neutron measurements can be used to study fast ions and give examples of physics results obtained on present day tokamaks. The focus is on measurements with dedicated neutron spectrometers and with compact neutron detectors used in each channel of neutron profile monitors. A measured neutron spectrum can be analyzed in several different ways, depending on the physics scenario under consideration. Gross features of a fast ion energy distribution can be studied by applying suitably chosen thresholds to the measured spectrum, thus probing ions with different energies. With this technique it is possible to study the interaction between fast ions and MHD activity, such as toroidal Alfven eigenmodes (TAEs) and sawtooth instabilities. Quantitative comparisons with modeling can be performed by a direct computation of the neutron emission expected from a given fast ion distribution. Within this framework it is also possible to determine physics parameters, such as the supra-thermal fraction of the neutron emission, by fitting model parameters to the data. A detailed, model-independent estimate of the fast ion distribution can be obtained by analyzing the data in terms of velocity space weight functions. Using this method, fast ion distributions can be resolved in both energy and pitch by combining neutron and gamma-ray measurements obtained along several different sightlines. Fast ion measurements of the type described in this paper will also be possible at ITER, provided that the spectrometers have the dynamic range required to resolve the fast ion spectral features in the presence of the dominating thermonuclear neutron emission. A dedicated high-resolution neutron spectrometer has been designed for this purpose.
  • Salewski, M., et al. (författare)
  • Bayesian Integrated Data Analysis of Fast-Ion Measurements by Velocity-Space Tomography
  • 2018
  • Ingår i: Fusion science and technology. - : TAYLOR & FRANCIS INC. - 1536-1055 .- 1943-7641. ; 74:1-2, s. 23-36
  • Tidskriftsartikel (refereegranskat)abstract
    • Bayesian integrated data analysis combines measurements from different diagnostics to jointly measure plasma parameters of interest such as temperatures, densities, and drift velocities. Integrated data analysis of fast-ion measurements has long been hampered by the complexity of the strongly non-Maxwellian fast-ion distribution functions. This has recently been overcome by velocity-space tomography. In this method two-dimensional images of the velocity distribution functions consisting of a few hundreds or thousands of pixels are reconstructed using the available fast-ion measurements. Here we present an overview and current status of this emerging technique at the ASDEX Upgrade tokamak and the JET toamak based on fast-ion D-alpha spectroscopy, collective Thomson scattering, gamma-ray and neutron emission spectrometry, and neutral particle analyzers. We discuss Tikhonov regularization within the Bayesian framework. The implementation for different types of diagnostics as well as the uncertainties are discussed, and we highlight the importance of integrated data analysis of all available detectors.
  • Salewski, M., et al. (författare)
  • Diagnostic of fast-ion energy spectra and densities in magnetized plasmas
  • 2019
  • Ingår i: Journal of Instrumentation. - : Institute of Physics Publishing (IOPP). - 1748-0221 .- 1748-0221. ; 14
  • Tidskriftsartikel (refereegranskat)abstract
    • The measurement of the energy spectra and densities of alpha-particles and other fast ions are part of the ITER measurement requirements, highlighting the importance of energy-resolved energetic-particle measurements for the mission of ITER. However, it has been found in recent years that the velocity-space interrogation regions of the foreseen energetic-particle diagnostics do not allow these measurements directly. We will demonstrate this for gamma-ray spectroscopy (GRS), collective Thomson scattering (CTS), neutron emission spectroscopy and fast-ion D-alpha spectroscopy by invoking energy and momentum conservation in each case, highlighting analogies and differences between the different diagnostic velocity-space sensitivities. Nevertheless, energy spectra and densities can be inferred by velocity-space tomography which we demonstrate using measurements at JET and ASDEX Upgrade. The measured energy spectra agree well with corresponding simulations. At ITER, alpha-particle energy spectra and densities can be inferred for energies larger than 1.7 MeV by velocity-space tomography based on GRS and CTS. Further, assuming isotropy of the alpha-particles in velocity space, their energy spectra and densities can be inferred by 1D inversion of spectral single-detector measurements down to about 300 keV by CTS. The alpha-particle density can also be found by fitting a model to the CTS measurements assuming the alpha-particle distribution to be an isotropic slowing-down distribution.
  • Smirnov, A. N., et al. (författare)
  • Application of thin-film breakdown counters for characterization of neutron field of the VESUVIO instrument at the ISIS spallation source
  • 2012
  • Ingår i: Nuclear Instruments and Methods in Physics Research Section A. - 0168-9002 .- 1872-9576. ; 687, s. 14-22
  • Tidskriftsartikel (refereegranskat)abstract
    • The high-energy neutron field of the VESUVIO instrument at the ISIS facility has been characterized using the technique of thin-film breakdown counters (TFBC). The technique utilizes neutron-induced fission reactions of U-nat and Bi-209 with detection of fission fragments by TFBCs. Experimentally determined count rates of the fragments are approximate to 50% higher than those calculated using spectral neutron flux simulated with the MCNPX code. This work is a part of the project to develop Chiplr, a new dedicated facility for the accelerated testing of electronic components and systems for neutron-induced single event effects in the new Target Station 2 at ISIS. The TFBC technique has shown to be applicable for on-line monitoring of the neutron flux in the neutron energy range 1-800 MeV at the position of the device under test (DUT).
  • Tardocchi, M., et al. (författare)
  • Modeling of neutron emission spectroscopy in JET discharges with fast tritons from (T)D ion cyclotron heating
  • 2006
  • Ingår i: Review of Scientific Instruments. - 0034-6748 .- 1089-7623. ; 77:12, s. 126107-
  • Tidskriftsartikel (refereegranskat)abstract
    • The measurement of fast ion populations is one of the diagnostic capabilities provided by neutron emission spectroscopy (NES). NES measurements were carried out during JET trace tritium campaign with the magnetic proton recoil neutron spectrometer. A favorable plasma scenario is (T)D where the resulting 14 MeV neutron yield is dominated by suprathermal emission from energetic tritons accelerated by radio frequency at their fundamental cyclotron frequency. Information on the triton distribution function has been derived from NES data with a simple model based on two components referred to as bulk (B) and high energy (HE). The HE component is based on strongly anisotropic tritium distribution that can be used for routine best-fit analysis to provide tail temperature values (THE). This article addresses to what extent the THE values are model dependent by comparing the model above with a two-temperature (bi-) Maxwellian model featuring parallel and perpendicular temperatures. The bi-Maxwellian model is strongly anisotropic and frequently used for radio frequency theory.
  • Zhang, Xing, et al. (författare)
  • Diagnosing NB plasmas on the EAST tokamak with new time-of-flight neutron spectrometer
  • 2014
  • Ingår i: Nuclear Fusion. - 0029-5515 .- 1741-4326. ; 54:10, s. 104008-
  • Tidskriftsartikel (refereegranskat)abstract
    • The new neutron spectrometer time-of-flight enhanced diagnostics (TOFED) for the EAST tokamak is presented and its characteristics are described in terms of simulation results, as well as the interface in the torus hall along with new neutral beam (NB) injectors. The use of TOFED for studies of the slowing down of NB-injected deuterons is illustrated. The implications of measuring the neutron emission on a long pulse machine are discussed together with the experimental challenges and diagnostic possibilities approaching those to be encountered in continuous operation.
  • Zhang, X., et al. (författare)
  • Second generation fusion neutron time-of-flight spectrometer at optimized rate for fully digital data acquisition
  • 2014
  • Ingår i: Review of Scientific Instruments. - 0034-6748 .- 1089-7623. ; 85:4, s. 043503-
  • Tidskriftsartikel (refereegranskat)abstract
    • The progress on high-rate event recording of data is taken as starting point to revisit the design of fusion neutron spectrometers based on the TOF (time-of-flight) technique. The study performed was aimed at how such instruments for optimized rate (TOFOR) can be further developed to enhance the plasma diagnostic capabilities based on measurement of the 2.5 MeV dd neutron emission from D plasmas, especially the weak spectral components that depend on discrimination of extraneous events. This paper describes a design (TOFOR II) adapted for use with digital wave form recording of all detector pulses providing information on both amplitude (pulse height) and timing. The results of simulations are presented and the performance enhancement is assessed in comparison to the present.
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