Magnetohydrodynamic Waves and Instabilities in Rotating Tokamak Plasmas

One of the most promising ways to achieve controlled nuclear fusion for the commercial production of energy is the tokamak design. In such a device, a hot plasma is confined in a toroidal geometry using magnetic fields. The present generation of tokamaks shows significant plasma rotation, primarily in the toroidal direction. This plasma flow has an important impact on stability and confinement, aspects of which can be described quite well by the theory of magnetohydrodynamics (MHD). This work contains a comprehensive theoretical analysis, supported by numerical simulations, of the MHD equilibrium, waves, and instabilities of rotating tokamak plasmas. A new general description of the thermodynamic state of the equilibrium is presented. Next, a stability criterion is derived that generalizes various previous results by including toroidal rotation. This criterion shows that a radially decreasing rotation profile can be stabilizing. The previously unknown origin of this stabilization is shown to be the Coriolis effect, with a mediating role for the pressure. Various factors that affect stability also influence stable waves and eigenmodes of the plasma. New modes that are created by rotation are found, and the effect of rotation on a type of experimentally well-known modes is described. Finally, the step to nonlinear magnetohydrodynamics is made by extending an existing reduced MHD code to the full viscoresistive MHD equations. This allows a study of the nonlinear evolution of the equilibria, waves, and instabilities described in this thesis

Magnetohydrodynamic Effects on Insulating Bubbles and Inclusions in the Continuous Casting of Steel

The magnetohydrodynamic effects associated with a magnetic field perpendicular to the movement of insulating inclusions or bubbles in a conducting liquid are investigated in this article. An increase in drag coefficient as a result of the presence of a magnetic field is argued to have a significant effect on their terminal rise velocity. Inside a continuous steel caster, this lower terminal velocity has a potentially negative effect on the removal rate of unwanted inclusions, degrading the steel quality. Simulations of an insulating rigid sphere moving in the presence of an electrical current show an electromagnetophoretic force per unit volume of $-\psi \mathbf{J} \times \mathbf{B}$, with a shape factor $\psi \approx 1$. Numerical fluid and dispersed gas phase simulations of the flow inside a submerged entry nozzle show that, because of this force, inhomogeneous magnetic fields can cause nonuniform gas distributions in accordance with a theoretical analysis. In particular, the magnetic field can be tailored to increase or decrease the amount of gas near the side walls

Computational Simulations of Magnetic Particle Capture in Arterial Flows

The aim of Magnetic Drug Targeting (MDT) is to concentrate drugs, attached to magnetic particles, in a specific part of the human body by applying a magnetic field. Computational simulations are performed of blood flow and magnetic particle motion in a left coronary artery and a carotid artery, using the properties of presently available magnetic carriers and strong superconducting magnets (up to B $\approx$ 2 T). For simple tube geometries it is deduced theoretically that the particle capture efficiency scales as $\eta \sim \sqrt{\textrm{Mn}_p}$ , with $\textrm{Mn}_p$ the characteristic ratio of the particle magnetization force and the drag force. This relation is found to hold quite well for the carotid artery. For the coronary artery, the presence of side branches and domain curvature causes deviations from this scaling rule, viz. $\eta \sim \textrm{Mn}_p ^ {\beta}$, with $\beta>1/2$. The simulations demonstrate that approximately a quarter of the inserted 4 $\mu$m particles can be captured from the bloodstream of the left coronary artery, when the magnet is placed at a distance of 4.25 cm. When the same magnet is placed at a distance of 1 cm from a carotid artery, almost all of the inserted 4 $\mu$m particles are captured. The performed simulations, therefore, reveal significant potential for the application of MDT to the treatment of atherosclerosis

Magnetic particle motion in a Poiseuille flow

The manipulation of magnetic particles in a continuous flow with magnetic fields is central to several biomedical applications, including magnetic cell separation and magnetic drug targeting. A simplified twodimensional 2D equation describing the motion of particles in a planar Poiseuille flow is considered for various magnetic field configurations. Exact analytical solutions are derived for the particle motion under the influence of a constant magnetization force and a force decaying as a power of the source distance, e.g., due to a current carrying wire or a magnetized cylinder. For a source distance much larger than the transversal size of the flow, a general solution is derived and applied to the important case of a magnetic dipole. This solution is used to investigate the dependence of the particle capture efficiency on the dipole orientation. A correction factor to convert the obtained 2D results to a three-dimensional cylindrical geometry is derived and validated against computational simulations. Simulations are also used to investigate parameter ranges beyond the region of applicability of the analytical results and to investigate more complex magnetic field configurations

Magnetohydrodynamics of insulating spheres

The effect of electric and magnetic fields on a conducting fluid surrounding an insulating object plays a role in various industrial, biomedical and micro-fluidic applications. Computational simulations of the magnetohydrodynamic flow around an insulating sphere, with crossed magnetic and electric fields perpendicular to the main flow, are performed for Rm << 1 in the ranges 0.1 &#8804; Re &#8804; 100, 1 &#8804; Ha &#8804; 20 and 0.01 &#8804; N &#8804; 1000. Careful examination of this fundamental three-dimensional flow reveals a rich physical structure with surface charge on the sphere neighbouring volume charge of opposite sign. Hartmann layers, circulating current and asymmetric velocity and current profiles appear as a result of the applied magnetic and electric field. A parametric study on the magnetic field’s influence on the drag coefficient is performed computationally. The obtained results bridge a gap between various analytical solutions of limiting cases and show good correspondence to earlier work. Correlations for the drag coefficient are proposed that can be valuable for the description of insulating inclusions in various flow applications with magnetic fields

Het grootste kernfusieproject ter wereld: ITER : De Wereld Leert Door, 15.01.2013, VARA, Ned. 3, 22.30-22.40u [11:39]

Researcher Willem Haverkort will be tonight's guest in the brand-new science show 'De Wereld Leert Door'. He will tell about his research on tokamak plasma physics, aimed at achieving controlled nuclear fusion. Haverkort is PhD student at CWI in the Scientific Computing group since 2009. His research is funded by FOM institute DIFFER. Haverkort is the second guest in the show: 'De Wereld Leert Door', spin-off of the popular talk show 'De Wereld Draait Door' was only launched yesterday. Every day from Monday to Friday, a scientist will be interviewed by presenter Matthijs van Nieuwkerk about his or her research