| 1. |
- Goriaev, A., et al.
(författare)
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The upgraded TOMAS device : A toroidal plasma facility for wall conditioning, plasma production, and plasma-surface interaction studies
- 2021
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Ingår i: Review of Scientific Instruments. - : AMER INST PHYSICS. - 0034-6748 .- 1089-7623. ; 92:2
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Tidskriftsartikel (refereegranskat)abstract
- The Toroidal Magnetized System device has been significantly upgraded to enable development of various wall conditioning techniques, including methods based on ion and electron cyclotron (IC/EC) range of frequency plasmas, and to complement plasma-wall interaction research in tokamaks and stellarators. The toroidal magnetic field generated by 16 coils can reach its maximum of 125 mT on the toroidal axis. The EC system is operated at 2.45 GHz with up to 6 kW forward power. The IC system can couple up to 6 kW in the frequency range of 10 MHz-50 MHz. The direct current glow discharge system is based on a graphite anode with a maximum voltage of 1.5 kV and a current of 6 A. A load-lock system with a vertical manipulator allows exposure of material samples. A number of diagnostics have been installed: single- and triple-pin Langmuir probes for radial plasma profiles, a time-of-flight neutral particle analyzer capable of detecting neutrals in the energy range of 10 eV-1000 eV, and a quadrupole mass spectrometer and video systems for plasma imaging. The majority of systems and diagnostics are controlled by the Siemens SIMATIC S7 system, which also provides safety interlocks.
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| 2. |
- Moon, Sunwoo, et al.
(författare)
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Characterization of neutral particle fluxes from ICWC and ECWC plasmas in the TOMAS facility
- 2021
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Ingår i: Physica Scripta. - : IOP PUBLISHING LTD. - 0031-8949 .- 1402-4896. ; 96:12
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Tidskriftsartikel (refereegranskat)abstract
- Electron- (ECWC) and ion- (ICWC) cyclotron wall conditioning are essential means for controlled fusion to modify the surface state of plasma-facing components in order to reduce impurity generation and fuel accumulation in the wall. Development of ECWC and ICWC requires characterization of neutral particle fluxes generated in discharges, because neutrals enhance the homogeneity of the conditioning, which may contribute to remote or shadowed areas, especially in the presence of a permanent magnetic field (e.g. W7-X, ITER). A time-of-flight neutral particle analyzer (ToF-NPA) with 4.07 m flight distance is employed to measure time- and energy-resolved low energetic (<1 keV) neutral particle distributions. The ToF-NPA setup tested at the EXTRAP T2R reversed field pinch was installed at the TOMAS toroidal plasma facility to determine low energy neutral particle fluxes while investigating the impact of the gas pressure in the instrument and compatibility with low count rates during EC- and ICWC discharges. TOMAS has a major radius of 0.78 m and provides various plasma operation conditions: toroidal magnetic field up to 0.12 T, EC frequency 2.45 GHz with the power of 0.6-6 kW, IC frequency of 10-50 MHz with the power of up to 6 kW. Early results on the characterization of three phases (EC only, EC + IC, and IC only) of hydrogen discharges demonstrate: (i) the low energy (10-725 eV) neutrals distribution has been determined by the NPA system, (ii) the mixed EC + IC phase produces the highest population of neutral particles, while the EC only provides one order of magnitude lower rate, (iii) the neutrals produced in IC only have higher average energy (28 eV) than EC only (7 eV) and EC + IC (16 eV).
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| 3. |
- Saad, Erik, et al.
(författare)
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Experimental characterization and modelling of the resistive wall mode response in a reversed field pinch
- 2022
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Ingår i: Plasma Physics and Controlled Fusion. - : IOP Publishing. - 0741-3335 .- 1361-6587. ; 64:5
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Tidskriftsartikel (refereegranskat)abstract
- Model-based control algorithms have potential advantages for resistive wall mode (RWM) feedback control. In this study, a physics model of the RWM response to externally applied perturbation fields is validated against experiments in a reversed field pinch (RFP). The experimental characterization of the RWM plasma response is performed in the EXTRAP T2R device by the excitation of nonaxisymmetric perturbation magnetic fields utilizing an external array of saddle coils for RWM control. The modelling and experimental validation is carried out with an extended sensor array, resolving a wider spectrum of RWM compared to earlier studies, covering the relevant poloidal m = 1 and toroidal -32 < n < 32 modes for this high aspect ratio RFP device. In addition to the nonresonant unstable modes, which are the primary target of RWM feedback control, this spectrum also includes a wide range of resonant modes. The validated resistive magnetohydrodynamics (MHD) model includes the passive stabilization effect on these modes from intrinsic plasma rotation. The inclusion of resistivity and plasma rotation in the present model provides a substantially better agreement between modelled and experimental growth rates than that observed in earlier studies using the ideal MHD model. The present model provides a realistic description of the plasma response for both nonresonant and resonant modes, which is both relatively simple and compatible with the computing capabilities and latency limitations encountered in practical implementations of model-based control algorithms.
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| 4. |
- Saad, Erik, et al.
(författare)
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Model based optimal magnetic feedback control of multiple resistive wall modes with discrete coil and sensor arrays
- 2024
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Ingår i: Fusion engineering and design. - : Elsevier BV. - 0920-3796 .- 1873-7196. ; 198
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Tidskriftsartikel (refereegranskat)abstract
- A model-based optimal control method for multiple resistive wall mode (RWM) feedback stabilization has been developed and tested in plasma experiments at the EXTRAP T2R reversed field pinch (RFP) device. The controller is designed to target issues that arise in connection with RWM magnetic feedback stabilization systems based on discrete control coil and sensor arrays in tokamak and reversed field pinch devices. Multi-mode control capabilities in these systems is limited due to coupling of modes induced by the control system. The coupling originates from the generation of side-band control field harmonics and from aliasing of multiple harmonics in the sensor measurements. These couplings naturally leads to a multiple-input, multiple-output (MIMO) control problem. A model based state space controller has been designed based on a relatively simple physics model of the RWM plasma response. The physics model, which is applicable for the high aspect ratio RFP, is the basis for Fourier decoupling of the MIMO control problem into a set of single-input, multiple-output (SIMO) systems using the discrete Fourier transform (DFT). The linear, time-invariant physics model allows for the design of a state space model with states representing physical quantities; in this case the Fourier harmonics of the radial field at the resistive wall. Since the states cannot be directly measured, a Kalman filter is used for estimation of the states from the aliased sensor array measurements. A linear–quadratic (LQ) optimal state controller has been implemented. Design parameters, such as the LQ control cost function state weights and the Kalman filter input error covariances have been used to optimize the control operation in various ways. The controller has enhanced multi-mode control capabilities compared to earlier designs. For example it allows the prioritizing of suppression of one of the multiple magnetic field Fourier harmonics produced by a given control current DFT component. The controller has been tested in plasma experiments at EXTRAP T2R device, utilizing a newly installed extended sensor array, and the enhanced capabilities for multiple RWM feedback stabilization has been demonstrated.
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| 5. |
- Schmuck, Stefan, 1980-
(författare)
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Bayesian Inference for Microwave Diagnostics at Joint European Torus
- 2022
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The major goal of this thesis is to perform Bayesian inference jointly for electron kinetic profiles, a toroidal magnetic field correction, diagnostic sensitivities, and wall reflection properties given noisy measurements of four microwave diagnostics at the tokamak JET. Besides the measurements, this kind of inference considers prior knowledge, like profile length-scales, to update objectively the information about the physics parameters. Such a probabilistic update, i.e. a posterior probability distribution or posterior, states the plausibility of parameter combinations and captures parameter uncertainties and correlations. The already existing Bayesian framework Minerva was used to carry out the inference.Contrary to standard approaches, plasma physics and diagnostic models and physics parameters are used to make predictions which are compared objectively with the data provided by one reflectometer and three electron cyclotron emission (ECE) diagnostics; two broadband Martin-Puplett interferometers and one heterodyne radiometer. In addition, these models represent reality more closely, for instance: (i) the smoothnesses of temperature and density profiles are modelled and estimated by length-scale parameters, (ii) the models SPECE and ECEPT, predicting each broadband ECE spectra, take into account relativistic and density effects like the finite optical thickness, (iii) the working principle and measurement uncertainty of each diagnostic are considered. As an example for the latter point, the two interferometers supply convoluted ECE spectra over several harmonic ranges up to 500 GHz and in mainly ordinary and extra-ordinary wave mode polarisations. The uncertainties on these spectra originate mostly from absolute calibrations for which a dedicated, robust and reliable procedure had to be established during this thesis.Further intermediate achievements of this thesis are for example: (i) the analytical derivation of the generalised square-exponential covariance function, enabling the estimation of multiple length-scales for electron temperature and density profiles in the plasma core and edge domains, (ii) the already existing ray-tracer SPECE, which predicts accurately but slowly ECE spectra, was parallelised successfully by a client-server approach, (iii) the derivation of the model ECEPT to predict quickly and sufficiently accurately broadband ECE spectra for an Ohmically heated plasma, and (iv) the extension of the multi-reflection model to allow different properties for the high-field and low-field side walls of a fusion device, affecting substantially the predictions of ECE spectra at frequencies for which the plasma has a low optical thickness.For a low temperature and low density plasma, the joint inference of more than 200 parameters was carried out for a given flux surface geometry, using either SPECE or ECEPT as predictor for the measured ECE spectra from 50 GHz to 280 GHz (first three harmonic ranges at least) with contributions from the ordinary and extra-ordinary wave-modes. For the ECEPT case, the shape of the joint posterior could be explored numerically. Related findings are, for example, smooth electron temperature and density profiles with values at the centre of 1.5 keV and 1.75x1e19 m^-3 and at the separatrix of 80 eV and 2x1e18 m^-3 with uncertainties of the order of 10 eV and few 1e17 m^-3. Furthermore, the smoothness of each core and edge profile originates in the inferred length-scale. These results were confirmed by the most likely parameter combination of the posterior when the more accurate predictor SPECE is used. The only exceptions are the correction to the magnetic field, which increased from 1.4% to 2%, and the reflectivity of the ITER-like wall, which elevated from 0.72 to 0.92.With ECEPT in use, multiple posterior correlation features could be revealed of which many could be explained. For instance, each profile smoothness, due to the estimated length-scale, implies considerable correlationsbetween nearby locations. In addition, a global correlation feature is evidentfor edge and core density profiles, likely caused by the measurement principle of the reflectometer.The second part of this thesis focusses on Bayesian inference about spectra in the field of Fourier transform spectroscopy. An example application has been carried out for the calibration data measured with one of the Martin-Puplett interferometers at JET. Compared to standard analysis techniques, more information could be extracted about the spectra from the so-called double-, single- and zero-sided data domains. The spectra have been modelled as Brownian bridge processes a priori, by which the global trend of the data located in double- and single-sided domains is modelled. This enables the estimation of the posterior uncertainties of the spectra due to non-probed data domains, especially the zero-sided domain. Such consideration is not made in any form by conventional techniques, because no method has been developed to estimate the seemingly lost information contained in the zero-sided domain. The lower and upper limits of the spectra have been estimated by (33+/-1.7) GHz and (913+/-2.9) GHz. These limits are rated to be more likely by a factor of 1e277 than the limits at 0 GHz and 3747 GHz (Nyquist frequency) assumed by conventional analysis approaches but never checked for plausibility.
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| 6. |
- Stefanikova, Estera, 1987-
(författare)
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Pedestal structure and stability in JET-ILW and comparison with JET-C
- 2020
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Controlled thermonuclear fusion offers a promising concept for safe and sustainable production of electrical energy. However, there are still many issues to be investigated on the way to a commercial fusion reactor. An important point for detailed studies is connected to wall materials surrounding hot thermonuclear plasma. The JET tokamak (the largest fusion experiment in the world) in the United Kingdom has completed a major upgrade in 2011 in which the materials of the vessel surrounding the fusion fuel have been changed from a carbon-fibre-composite (or JET-C wall) to Beryllium and Tungsten. These new materials are the same as those that will be used in a next step fusion device International Thermonuclear Experimental Reactor ITER (hence the name ITER-like wall or JET-ILW), designed to demonstrate the feasibility of fusion reactor based on the tokamak concept. One of the goals of JET with the ILW is to act as a test bed for ITER technologies and for ITER operating scenarios.The overall purpose of the thesis work is to characterise the effect of the ILW on the structure and stability of edge plasma phenomenon called the pedestal, a steep pressure gradient associated with the H-mode, an operational regime with improved confinement. The aim is to contribute to the understanding of the difference in the pedestal performance between JET-C and JET-ILW.The work is focused on experimental characterisation of the pedestal structure in deuterium discharges by analysing the experimental data (radial profiles of electron temperature and density measured in H-mode plasmas) from Thomson scattering diagnostics at JET and on investigating the differences in pedestal stability between JET-ILW and JET-C plasmas in terms of the pedestal modelling. The pedestal structure is determined using a modified hyperbolic tangent fit to the experimental Thomson scattering profiles. The modelling is performed with the pedestal predictive code Europed, based on the EPED model commonly used to predict the pedestal height in JET.The experimental analysis has shown several differences in the pedestal structure of comparable JET-ILW and JET-C discharges. One of the key differences introduced in this work is the pedestal relative shift (a separation between the middle of the pedestals of the electron density and temperature) that plays a major role in the difference in the pedestal performance between JET-C and JET-ILW. The work shows that the relative shift can vary significantly from pulse to pulse and that, on average, JET-C plasmas have lower relative shift than JET-ILW plasmas. The pedestal relative shift tends to increase with increase in the gas fuelling and the heating power. Furthermore, the increase in the relative shift has been empirically correlated with the degradation of the experimental normalized pressure gradient αexp.To understand the differences in the JET-C and JET-ILW pedestal stability, parameters that affect the pedestal stability and that tend to vary between comparable JET-C and JET-ILW discharges have been identified. These parameters are the pedestal relative shift, pedestal density neped, effective charge number Zeff, pedestal pressure width wpe, and normalized pressure βN. The modelling performed with the predictive Europed code has shown that these five parameters are sufficient to explain the difference in the pedestal performance between JET-C and JET-ILW.Furthermore, the modelling has shown that the relative shift and neped play a major role in affecting the critical normalized pressure gradient αcrit (normalized pressure gradient expected by the model comparable to αexp), while the relative shift, wpe and Zeff have a major impact on the pedestal pressure height. Finally, a possible mechanism that has led to the degradation of the pedestal pressure from JET-C to JET-ILW is proposed.
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