Particle Confinement in Astrophysical and Tokamak Environments: Insights from Magnetic Fields

Professor William Newman

IN ARENAS RELEVANT to Physics and Astronomy, Professor William Newman continues to focus on the response of charged particles to magnetic fields in energetic astrophysical environments and now to “tokamaks” where charged particles are injected in order to be aligned with toroidal magnetic field lines. The prevailing question is how well these magnetic fields can collimate and confine the particles in both their velocities and their positions. In astrophysical environments, such as the bipolar flows emerging from Active Galactic Nuclei, the particle flow and presumably the confining fields extend over many kiloparsecs in distance. Thus, magnetic fields can spatially confine the particles even if their gyroradii are very large. The situation arising with tokamak-like devices is more delicate.

Without going into technical detail, the accompanying figure demonstrates how the localization of particles is reduced as a dimensionless velocity derived from the field and particle properties transitions from small to large. When it is small, the particle position is confined to a very slender torus, not unlike a “hula hoop,” as observed in a reference frame comoving with the (field) gradient drift. When the dimensionless velocity becomes large, the hula hoop morphs into a very thick and wide donut-like shape with a very small “donut hole.” An important outcome of this work is that it demonstrates that break-down of the guiding center approximation, which is foundational to all numerical magnetohydrodynamic simulations, particularly relativistic ones. The figure illustrates the transition that occurs as the dimensionless velocity becomes much larger than 1 and the gyrations of the particles go on to exceed the radius of curvature of the field lines.

During the past two years, Prof. Newman has served as Chair of the American Physical Society’s Group on the Physics of Climate, when the Nobel Prize was awarded to two pioneers in the field which helped legitimize the application of physics-based methods to climate research.

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