Pedestal Characteristics and MHD Stability of H-Mode Plasmas in TCV

The tokamak à configuration variable (TCV) is unique in its ability to create a variety of plasma shapes and to heat the electron population in high density regimes using microwave power at the third harmonic of the electron cyclotron frequency. In the frame of this thesis, the impact of plasma shaping and heating on the properties of the edge transport barrier (ETB) in the high confinement mode (H-mode) was studied. This mode of operation is foreseen as one of the reference scenarios for ITER, the International Tokamak Experimental Reactor, which is being built to demonstrate the feasibility of thermonuclear fusion using magnetic confinement. A feature of H-mode regime operation are edge localized modes (ELMs), instabilities driven by the steep pressure gradients that form in the plasma edge region due to a transport barrier. During an ELM event, energy and particles are expelled from the plasma in a short burst. This will cause serious problems with respect to the heat load on plasma facing components in a tokamak of the size of ITER. Understanding of the phenomena associated with ELMs is thus required and dedicated investigations of their theory and experimental observations are carried out in many laboratories worldwide. This thesis presents several experimental and numerical investigations of tokamak behavior for configurations where the plasma edge plays an important role. From the experimental viewpoint, studies of transport barriers are challenging, as plasma parameters change strongly within a narrow spatial region. As part of the work presented here, the TCV Thomson scattering system was upgraded to meet the requirements for diagnosing electron temperature and density with high spatial resolution in the region of internal and external transport barriers. Simultaneously, the data analysis was significantly improved to cope with statistical uncertainties and alleviate eventual systematic errors. For measurements of the time evolution of density and temperature profile during the ELM cycle, the low repetition rate of the lasers used for Thomson scattering is a limiting. Although the system on TCV comprises 3 laser units that may be triggered in sequence with time separations down to 1 ms, time evolution over longer periods can only be reconstructed from repetitive events. In this context, an adjustment of the laser trigger to improve the synchronization with the ELM event is an advantage. A method was developed and implemented to generate a synchronizing trigger sequence, by a real-time monitoring of the D-alpha emission, which provides a marker for the ELM event. Recently, a "snowflake" (SF) divertor configuration, proposed as a possible solution to reduce the plasma-wall interaction by changing the divertor's poloidal magnetic field topology, was generated, for the first time, in TCV. A numerical code (KINX), based on a magnetohydrodynamic model (ideal MHD), was used to investigate the stability limits of this configuration under H-mode conditions and compare them with a similar standard single-null equilibrium. In a series of experiments, improved energy confinement was found and explained by improved stability of the edge region in the SF configuration. The influence of the pedestal structure in ELMy H-mode plasmas on the energy confinement and on ELM energy losses was investigated. The different ELM regimes found in TCV were analyzed, in particular the transition between type-III to type-I ELMs. The operational boundary of each ELM regime was characterized and verified by ideal MHD stability simulations for the ETB region. Recent studies on the scaling of the pedestal width with normalized poloidal pressure were confirmed. Using the capabilities of TCV, the influence of plasma shaping on pedestal parameters and MHD stability limits was investigated. In the past, models were developed to describe the onset of type-I ELMs, which are associated with modes in the ETB region arising from a coupling of pressure- and current-driven instabilities (coupled kink-ballooning modes). Experimental studies were performed to trace the temporal evolution of pedestal parameters characterizing the ETB during an ELM cycle. The results of these experiments were analyzed using information from MHD stability calculations. It is concluded that these models are capable of predicting limits as necessary conditions for ELM activity, but are not sufficient to fully explain ELM triggering

Transport and turbulence reduction with negative triangularity : Correlation ECE measurements in TCV

Turbulence and Transport Reduction with Negative Triangularity : Correlation ECE Measurements in TCV Due to turbulence, core energy transport in fusion devices such as tokamaks generally exceeds collisional transport by at least an order of magnitude. It is therefore crucial to understand the instabilities driving the turbulent state and to find ways to control them. Plasma shape is one of these fundamental tools. In low collisionality plasmas, such as in a reactor, changing the plasma shape from Dee-shape to inverse Dee-shape (from positive to negative triangularity δ) reduces the energy transport by a factor two: the heat flux necessary to sustain the same profiles and stored energy in a discharge with δ=-0.4 is only half of that at δ=+0.4. This is significant, since it opens the possibility of having Hmode-like confinement time within an L-mode edge; or at least with smaller ELMs. Recent correlation ECE measurements show that this reduction of transport at negative δ is reflected in a reduction by a factor of two of both 1) the amplitude of temperature fluctuations in the broadband frequency range 30-150 kHz, and 2) the fluctuation correlation length, measured at mid-radius (ρv~0.6). In addition, the fluctuations amplitude is reduced with increasing collisionality, consistent with theoretical estimates of the collisionality effect on Trapped Electron Modes (TEM). The correlation ECE results are compared to gyrokinetic code results: 1) global linear gyrokinetic simulations (LORB) have predicted shorter radial TEM wavelength λ⊥ for negative triangularity plasmas, consistent with the shorter radial turbulence correlation length λc observed. 2) At least close to the strongly shaped plasma boundary, local nonlinear gyrokinetic simulations with the GS2 code predict that the TEM induced transport decreases with decreasing triangularity and increasing collisionality, in fair agreement with the experimental observations. 3) Calculations are now extended to global nonlinear simulations (ORB5). This work was supported in part by the Swiss National Science Foundatio

Use of portable gamma spectrometers for triage monitoring following the intake of conventional and novel radionuclides

Current internal dosimetry monitoring programmes generally feature periodic measurements that are defined for the most commonly-encountered radionuclides. These programmes are not directly applicable to research centres that produce novel and short-lived radionuclides which are then used for the manufacture of radiopharmaceuticals, such as the CERN-MEDICIS facility hosted at CERN. This work presents an in vivo internal dosimetry programme based on the concept of triage monitoring. The programme allows to comply with the annual committed effective dose limit of E50=1 mSv by performing rapid gamma-spectroscopy screening measurements. Two portable spectrometers (HPGe- and NaI-based) were characterised using two different phantoms: a simplified model of the human torso and an anthropomorphic phantom allowing for customised source-filling geometries. The efficiencies of the spectrometers were determined using both phantoms and the minimum detectable activities were computed as a function of the measuring time for a selection of 21 among novel and conventional radionuclides. The minimum detectable activity was then used to calculate the minimum committed effective dose associated to each measurement for a realistic intake scenario. For a single screening measurement of 30 s performed at the end of the working day, the minimum detectable committed effective dose resulting from a radionuclide inhalation ranged between few uSv and hundreds of uSv for the majority of the considered radionuclides. The suggested approach allows to set up pragmatic in vivo measurements to monitor the workers’ internal contamination in research centres and industries where unsealed conventional and/or novel radionuclides may be handled

Determination of the radiance of cylindrical light diffusers: design of a one-axis charge-coupled device camera-based goniometer setup

A one-axis charge-coupled device camera-based goniometer setup was developed to measure the three-dimensional radiance profile ( longitudinal, azimuthal, and polar) of cylindrical light diffusers in air and water. An algorithm was programmed to project the two-dimensional camera data onto the diffuser coordinates. The optical system was designed to achieve a spatial resolution on the diffuser surface in the submillimeter range. The detection threshold of the detector was well below the values of measured radiance. The radiance profiles of an exemplary cylindrical diffuser measured in air showed local deviations in radiance below 10% for wavelengths at 635 and 671 nm. At 808 nm, deviations in radiance became larger, up to 45%, most probable due to the manufacturing process of the diffuser. Radiance profiles measured in water were less Lambertian than in air due to the refractive index matching privileging the radial decoupling of photons from the optical fiber. (C) 2017 Society of Photo-Optical Instrumentation Engineers (SPIE

Impact of the phantom geometry on the evaluation of the minimum detectable activity following a radionuclide intake: From physical to numerical phantoms

The establishment of an in vivo internal monitoring programme requires the use of phantoms to represent an activity distribution of an incorporated radionuclide within the body. The aim of this study was to quantify the impact of the phantom geometry on the minimum detectable activity (MDA) of an incorporated radionuclide. The MDA was assessed for two instruments: a conventional radiation protection instrument and a portable gamma spectrometer. Four phantoms were considered: two physical phantoms, a simplified torso phantom and a commercial whole body phantom, as well as two numerical phantoms, the reference adult male and female voxel phantoms published by the International Commission on Radiological Protection (ICRP). The phantoms were loaded with activity at the level of the thorax and abdomen using reference sources of Co-57, Ba-133, Cs-137, Co-60 and Eu-152. The MDA for both instruments was experimentally assessed using the two physical phantoms. The experimental setup was modelled in GEANT4 and the simulated instrument responses were validated by the experimental data. The Monte Carlo model was then used to compute the instruments response and corresponding MDA when using the ICRP voxel phantoms. The simplified torso phantom provided one of the highest MDA estimates, up to a factor of 5 higher than the ones obtained with the voxel phantoms when considering a Co-57 source. Depending on the considered source distribution within the phantoms, physical phantoms may lead to an underestimation of the MDA when compared to more complex and anatomically accurate numerical phantoms. This work presents a quantitative comparison between the MDA obtained with different phantoms and radionuclide distributions

Optical properties of rabbit brain in the red and near-infrared: changes observed under in vivo, postmortem, frozen, and formalin-fixated conditions

The outcome of light-based therapeutic approaches depends on light propagation in biological tissues, which is governed by their optical properties. The objective of this study was to quantify optical properties of brain tissue in vivo and postmortem and assess changes due to tissue handling postmortem. The study was carried out on eight female New Zealand white rabbits. The local fluence rate was measured in the VIS/NIR range in the brain in vivo, just postmortem, and after six weeks' storage of the head at -20 degrees C or in 10% formaldehyde solution. Only minimal changes in the effective attenuation coefficient mu(eff) were observed for two methods of sacrifice, exsanguination or injection of KCl. Under all tissue conditions, mu(eff) decreased with increasing wavelengths. After long-term storage for six weeks at -20 degrees C, mu(eff) decreased, on average, by 15 to 25% at all wavelengths, while it increased by 5 to 15% at all wavelengths after storage in formaldehyde. We demonstrated that mu(eff) was not very sensitive to the method of animal sacrifice, that tissue freezing significantly altered tissue optical properties, and that formalin fixation might affect the tissue's optical properties. (C) 2015 Society of Photo-Optical Instrumentation Engineers (SPIE)