Browsing by Subject "Nuclear fusion"

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Explosive evolution of near-threshold kinetic instabilities
(2017-08) Sanz Orozco, David; Berk, H. L.; Breizman, Boris N.; Morrison, Philip J; Waelbroeck, Francois L; Gamba, Irene M
In the past, studies of waves close to marginal stability have revealed a rich variety of behavior in different physical contexts. One of the possible outcomes is an explosive growth of the mode amplitude, which forms the core of this thesis. This outcome has been predicted in both the fluid mechanics and the plasma literature. While we make some comments regarding the fluids context, in this work we focus on the near-threshold waves that are excited in kinetic systems (such as plasmas). With a few exceptions, the explosive behavior is found to asymptote to an attractor that depends on a system parameter that we shall discuss. When the mode amplitude is sufficiently large, the explosive growth loses physical meaning, and here we explore the transition between the weakly-nonlinear regime where the explosive description holds, and the strongly-nonlinear phase where the mode amplitude saturates. By investigating the phase space dynamics associated with the kinetic response, we find a link between a local flattening, or folding, of the particle distribution function and the breakdown of the explosive description. Since the explosive growth sets the stage for long-term frequency chirping modes, it is hoped that the present work can be of relevance for the prediction of the variety of chirping modes that have been observed in many experimental situations. These modes are expected to have a very significant effect on the confinement properties of fusion plasmas.
MHD spectroscopy of tokamak plasmas using Alfvén waves
(2020-03-16) Oliver, Henry James Churston; Breizman, Boris N.; Sharapov, S. E. (Sergei E.); Hazeltine, Richard D; Mahajan, Swadesh M; Morrison, Philip J
Alfvén waves are electromagnetic waves that occur in magnetised plasmas. Alfvén waves are routinely observed in tokamaks — toroidal devices that confine plasma using magnets. Energetic ions that are used to heat the plasma within tokamaks can drive the Alfvén waves unstable. Additionally, alpha particles produced in fusion reactions may destabilise the wave. Alfvén waves with sufficiently high amplitudes can eject energetic particles from the plasma, damaging the reactor and decreasing fusion efficiency. When these waves are not strong enough to eject particles from the plasma, their benign behaviour can be used to diagnose the plasma. This technique is known as magnetohydrodynamic (MHD) spectroscopy. In this thesis, we outline three new techniques of MHD spectroscopy that we have developed. The first new method of MHD spectroscopy was developed in plasmas composed of hydrogen and deuterium in the Mega Ampere Spherical Tokamak (MAST). Compressional Alfvén eigenmodes (CAEs) and global Alfvén eigenmodes (GAEs) were suppressed in plasmas with high hydrogen concentrations. At the highest hydrogen concentrations investigated, high frequency ion-ion hybrid waves appeared. We used a 1D model of the refractive index and wave-particle resonances to explain these observations and estimate the relative ion concentration at which the spectrum of excited waves changed. These estimates agreed with experimental observations, suggesting the spectrum of excited waves can be used to diagnose the relative ion concentrations for plasmas with two ion species. The second new form of MHD spectroscopy was developed using observations of axisymmetric modes in experiments on the Joint European Torus (JET). Axisymmetric modes do not change in the toroidal direction and are driven unstable by energy gradients in the fast particle distribution function. Therefore, we can use observations of the axisymmetric mode to infer information about the gradient of the fast particle energy distribution function. We explained how these axisymmetric modes can exist without heavy damping. We also examined how the elongation of the plasma column modifies the mode using numerical and analytical tools. The final MHD spectroscopic technique was developed for JET plasmas injected with pellets of frozen deuterium, which are used to refuel the plasma core. We demonstrated how key pellet parameters can be inferred from dramatic changes to the Alfvén eigenfrequencies that we observed in JET. MHD spectroscopy of pellet injected plasmas was enabled by generalising two 3D MHD codes to incorporate 3D density profiles. 3D density profiles were generated using a model for the expansion of the pellet wake along a magnetic field line derived from the fluid equations. From the change in mode frequency, we estimate the density of the pellet wake and the time-scale for poloidal homogenisation of the wake. Before presenting these studies, we introduce the basics of fusion, tokamaks, and the models used to describe tokamak plasmas. We then discuss the MHD waves that we will use for MHD spectroscopy of tokamak plasmas, and how these waves are excited by fast particles. The three new methods of MHD spectroscopy are then discussed.
Simulations and reduced models for microtearing modes in the Tokamak pedestals
(2022-07-26) Curie, Max Tian; Tenerani, Anna; Hatch, David R.; Fitzpatrick, Richard; Michoski, Craig; Morrison, Philip J
Renewable energy can not only help to clean the environment but also create a more peaceful world. Fusion has the potential to provide clean energy with abundant resources. The high energy density (per-footprint) nature of fusion makes it appealing in highly urbanized areas, such as Singapore, which complements wind and solar power. Magnetic confinement fusion (MCF) is one of the most promising routes to thermonuclear fusion energy. Among the prospective MCF configurations, the Tokamak is the most widely implemented scheme. A host of instabilities are suppressed in H-mode (high-confinement mode) plasmas in Tokamaks due to high flow shear and/or steep density gradients in the pedestal (the edge of the plasma). This produces higher confinement and thus better performance than L-mode (low-confinement mode) operation. Transport and instabilities in the pedestal of the plasma are studied more intensively using gyrokinetic simulations thanks to the improvement of computational and experimental capabilities. Recent studies show that the magnetic fluctuations from microtearing modes (MTM) can be commonly observed in magnetic spectrograms [1– 12]. and contribute significant electron heat transport [13–15]. This thesis further investigates MTMs in the pedestal through 3 projects: • Direct comparison between nonlinear gyrokinetic simulations (GENE) and a newly installed magnetic diagnostic Faradayeffect Radial Interferometer-Polarimeter (RIP) [2, 16, 17]. Such a comparison provides strong evidence of the MTM’s importance in the Tokamak pedestal. • A package based on a global reduced model for MTM stability [18, 19] called the slab-like MTM (SLiM) package [20]. This model provides a tool for rapid MTM stability assessment. Applications of its usage will be described in the thesis: determining the stability of MTM, poloidal mode numbers, and equilibrium reconstruction. • Equilibrium reconstruction based on the SLiM model. Neural networks were trained for faster MTM stability assessment. This allows for extensive variations of nominal equilibrium quantities in order to better match the experimentally-observed magnetic frequencies in discharges and hopefully produce more accurate equilibrium reconstructions.
Statistical characterization of turbulence intermittency in SOL-like regimes in the Helimak
(2019-12) Taylor, Edward I.; Gentle, Kenneth W.; Breizman, Boris; Waelbroeck, Francois; Hazeltine, Richard; Raja, Laxaminarayan
Plasma turbulence intermittency is investigated in the Texas Helimak, a simple magnetic torus which is a physical approximation to the sheared cylindrical slab. The statistical properties of the intermittency are examined through a conditional analysis of probe signal bursts and the existence of so called "blobs", coherent plasma objects which transport large amounts of particles and heat, is explored. It is shown that the conditional analysis most likely extracts artifacts due to the finite ensemble size and that no coherent part of the bursts is detected throughout the Helimak radii for all the externally applied biases. The probe signal bursts rarely follow Poisson statistics, and a match with a recently developed stochastic model based upon this is more than likely due to the large number of parameters.