| 1. |
- Binda, Federico, 1987-
(författare)
-
Neutron emission spectroscopy of fusion plasmas with a NE213 liquid scintillator at JET
- 2015
-
Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Neutron diagnostics will play a fundamental role in future fusion plasma machines,where the harsh environment will make the use of many other type of diagnos-tics practically impossible. Complex techniques to measure the neutron spectrumemitted from tokamk plasmas have been developed over the years, producing stateof the art neutron spectrometers. However, recently compact neutron spectrom-eters have been gaining the interest of the research community. They are muchsimpler to operate and maintain, have lower cost and they can be employed in thechannels of a neutron camera, providing profile measurements. The drawbacks arethat they have a worse resolution and a response to neutrons that is not optimalfor spectroscopy.The goal of the work presented in this thesis is to estimate to which extenta compact detector such as a NE213 liquid scintillator can be used to performneutron emission spectroscopy analysis.The detector used for this study was installed in the back of the MPRu spec-trometer at JET in 2012. The characterization of the response of the detector wasdone using a combination of MCNPX simulations and real measurements. Thedata analysis was performed using the forward fitting approach: a model of theneutron spectrum is produced, then folded with the response of the detector andfinally compared with the data. Two types of plasma scenarios were analyzed, onewith NBI heating only, and another with NBI and third harmonic radio-frequencyheating. In both cases the TOFOR spectrometer was used as a reference to esti-mate the parameters in the model of the neutron spectrum.The results are promising and suggest that neutron spectroscopy can be per-formed with NE213 scintillators although the quality of the results, as given byperformance indicators such as uncertainties, is much lower than the performanceof high resolution spectrometers.
|
|
| 2. |
|
|
| 3. |
- Eriksson, Jacob
(författare)
-
Fuel ion densities and distributions in fusion plasmas : Modeling and analysis for neutron emission spectrometry
- 2012
-
Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Neutrons produced in fusion reactions in a magnetically confined plasma carry information about the distributions and densities of the fuel ions in the plasma. This thesis presents work where various theoretical models of different fuel ion distributions in the plasma are used to calculate modeled components of the neutron energy spectrum. The calculated components can then be compared with measured data, either to benchmark and validate the model or to derive various plasma parameters from the experimental data. Neutron spectra measured with the spectrometers TOFOR and the MPR, which are both installed at the JET tokamak in England, are used for this purpose. The thesis is based on three papers.The first paper presents the analysis of TOFOR data from plasmas heated with neutral beams and radio frequency waves tuned to the third harmonic of the deuterium cyclotron frequency, which creates fast (supra thermal) ions in the MeV range. It is found that effects of the finite Larmor radii of the fast ions need to be included in the modeling in order to understand the data. These effects are important for fast ion measurements if there is a gradient in the fast ion distribution function with a scale length that is comparable to - or smaller than - the width of the field of view of the measuring instrument, and if this scale length is comparable to - or smaller than - the Larmor radii of the fast ions.The second paper presents calculations of the neutron energy spectrum from the T(t,n)4He reaction, for JET relevant fuel ion distributions. This is to to form a starting point for the investigation of the possibility to obtain fast ion information from the t-t neutron spectrum, in a possible future deuterium-tritium campaign at JET. The t-t spectrum is more challenging to analyze than the d-d and d-t cases, since this reaction has three (rather than two) particles in the final state, which results in a broad continuum of neutron energies rather than a peak. However, the presence of various final state interactions - in particular between the neutron and the 4He - might still allow for spectrometry analysis.Finally, in Paper III, a method to derive the fuel ion ratio, nt/nd, is presented and applied to MPR data from the JET d-t campaign in 1997. The trend in the results are consistent with Penning trap measurements of the fuel ion ratio at the plasma edge, but the absolute numbers are not the same. Measuring the fuel ion ratio in the core plasma is an important task for fusion research, and also a very complicated one. Future work should aim at measuring this quantity in several independent ways, which should then be cross checked against each other.
|
|
| 4. |
- Höök, Lars Josef, 1979-
(författare)
-
Variance reduction methods for numerical solution of plasma kinetic diffusion
- 2012
-
Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Performing detailed simulations of plasma kinetic diffusion is a challenging task and currently requires the largest computational facilities in the world. The reason for this is that, the physics in a confined heated plasma occur on a broad range of temporal and spatial scales. It is therefore of interest to improve the computational algorithms together with the development of more powerful computational resources. Kinetic diffusion processes in plasmas are commonly simulated with the Monte Carlo method, where a discrete set of particles are sampled from a distribution function and advanced in a Lagrangian frame according to a set of stochastic differential equations. The Monte Carlo method introduces computational error in the form of statistical random noise produced by a finite number of particles (or markers) N and the error scales as αN−β where β = 1/2 for the standard Monte Carlo method. This requires a large number of simulated particles in order to obtain a sufficiently low numerical noise level. Therefore it is essential to use techniques that reduce the numerical noise. Such methods are commonly called variance reduction methods. In this thesis, we have developed new variance reduction methods with application to plasma kinetic diffusion. The methods are suitable for simulation of RF-heating and transport, but are not limited to these types of problems. We have derived a novel variance reduction method that minimizes the number of required particles from an optimization model. This implicitly reduces the variance when calculating the expected value of the distribution, since for a fixed error the optimization model ensures that a minimal number of particles are needed. Techniques that reduce the noise by improving the order of convergence, have also been considered. Two different methods have been tested on a neutral beam injection scenario. The methods are the scrambled Brownian bridge method and a method here called the sorting and mixing method of L´ecot and Khettabi[1999]. Both methods converge faster than the standard Monte Carlo method for modest number of time steps, but fail to converge correctly for large number of time steps, a range required for detailed plasma kinetic simulations. Different techniques are discussed that have the potential of improving the convergence to this range of time steps.
|
|
| 5. |
- Martinik, Tomas
(författare)
-
Development of Differential Die-Away Instrument for Characterization of Swedish Spent Nuclear Fuel
- 2016
-
Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The Next Generation Safeguards Initiative Spent Fuel (NGSI-SF) project was established in 2009 by the U.S. Departmentof Energy with main objective to investigate, and potentially develop and test new technologies for spent nuclearfuel (SNF) characterization. In Sweden the SNF is currently being considered to be verified and encapsulated in canistersand deposited into a geological repository. The need for an independent instrument for SNF verification by theSwedish operator turned into the collaborative effort with NGSI-SF to develop an instrument for future deployment inSweden.One of the techniques investigated within this project is the differential die-away (DDA) technique, which followingthe theoretical investigation by means of high fidelity Monte Carlo simulations indicated the potential to be applied fordetermining of various spent fuel assembly (SFA) parameters.This work introduces the first deployable DDA instrument which was designed to be used for characterizing ofSwedish SFAs currently stored in the Central Interim Storage Facility for Spent Nuclear Fuel (Clab). All the instrumentcomponents relevant for DDA design functionality were evaluated to ensure reliable operation in Clab. Although mostof the components were tuned with special consideration given to concerns from the operator (The Spent Nuclear FuelandWaste Management Company) , several post-simulation modification of the design were made. These modificationsare described in this work.A complementary study of the detector responses to asymmetrically burned SFAs indicated a different detector responses,depending on which of the four different orientations was used to assay individual SFAs. This study illustratedthe sensitivity of detectors with respect to the SFA orientation if there is a strong burn-up gradient across the SFA andhence a strong asymmetry in isotopic distribution in the SFA. In addition, the study of asymmetry provided the informationon different operational scenarios of the DDA instrument. The DDA instrument may provide general informationabout the complete SFA as well as give local information about certain parts of the SFA.
|
|
