Understanding the Plasma Universe through Laboratory Experiments and Related Models

Citation

Zhang, Yang (2024) Understanding the Plasma Universe through Laboratory Experiments and Related Models. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/gc8w-6n85. https://resolver.caltech.edu/CaltechTHESIS:06032024-042422418

Abstract

Laboratory experiments and the models they inspire are powerful tools for studying the plasma universe. This dissertation details possible solutions to two important problems in the plasma universe, namely how solar flares are generated and how accretion disks transport angular momentum and generate astrophysical jets.

Addressing the first problem, solar coronal loop physics is simulated in a laboratory experiment. The loop structure composed of braided strands is replicated. The MHD kink instability and the magnetic Rayleigh Taylor instability (MRTI) are observed to disrupt the loop structure. The dependence of the MRTI wavelength on the axial magnetic field is studied. Transient, localized 7.6-keV X-ray bursts and a several-kilovolt voltage spike are observed to be associated with the breaking of braided magnetic flux ropes containing 2 eV plasma. These spikes occur when the braid strand radius is choked down to be at the kinetic scale by either MHD kink or magnetic Rayleigh–Taylor instabilities. The observed sequence reveals an MHD to non-MHD cross-scale coupling that is likely responsible for generating solar energetic particles and X-ray bursts. All the essential components of this mechanism have been separately observed in the solar corona.

Magnetic flux ropes, the fundamental building block of magnetohydrodynamic plasma configurations, have often been observed to wrap around each other to form a helical braided structure with net axial current as observed from the laboratory experiment and solar coronal loops. Braiding phenomena extend to astrophysical jets, double helix nebula, and fusion plasma experiments. The equilibrium of braided flux ropes is more complicated than familiar axisymmetric systems because it requires balancing forces between the individual braids. A novel method for constructing these equilibria is developed. This method generates a double helix equilibrium with net axial current which is characteristic of observed solar loops and of laboratory-produced braided magnetic flux ropes. To the best of our knowledge, no previous model has been able to describe braided structures with net axial current. The net-axial-current equilibrium presented here reproduces the observed braided structure of the double helix nebula and is expected to be a powerful tool in other contexts.

Addressing the second problem, the dissertation introduces a first-principles angular momentum transport mechanism based only on collisions between neutrals and charged particles in the presence of gravitational and magnetic fields. The mechanism is demonstrated by a 2D N-body simulation of a weakly-ionized system. It is found that ions and electrons drift in opposite radial directions as a result of colliding with Kepler-motion neutrals. This reduces the ordinary angular momentum of neutrals and increases the canonical angular momentum of charged particles in a manner such that the net global canonical angular momentum is conserved. The accumulation of ions at small radius and electrons at large radius produces a radially outward electric field, while current from the separation of ions and electrons is radially inward. Consequently, this process provides a gravitational dynamo converting gravitational energy into the electric energy that powers an astrophysical jet. Because this neutral angular momentum loss depends only on neutrals colliding with charged particles, it should be ubiquitous. The model predicts an accretion rate of 3 × 10−8 solar mass per year in good agreement with observed accretion rates.

Based on the conservation of canonical angular momentum and dynamics of charged particles under collisions with infalling neutrals, the dissertation also investigates the origin of angular momentum in astrophysical systems. A weakly-ionized, initially non-rotating cloud of neutral particles is shown to spontaneously start rotating when infalling. Quantitative scaling predicts an angular momentum generation rate sufficient to convert neutral infall motion into neutral Keplerian rotation in the outer region of a protoplanetary accretion disk.

Thesis Files