Electron transport and azimuthal oscillations in hall thrusters

The Hall Effect Thruster (HET) is a type of satellite electric propulsion device initially developed in the 1960’s independently by USA and the former USSR. The development continued in the shadow during the 1970’s in the Soviet Union to reach a mature status from the technological point of view in the 1980’s. In the 1990’s the advanced state of this Russian technology became known in western countries, which rapidly restarted the analysis and development of modern Hall thrusters. Currently, there are several companies in USA, Russia and Europe manufacturing Hall thrusters for operational use. The main applications of these thrusters are low-thrust propulsion of interplanetary probes, orbital raising of satellites and stationkeeping of geostationary satellites. However, despite the well proven in-flight experience, the physics of the Hall Thruster are not completely understood yet. Over the last two decades large efforts have been dedicated to the understanding of the physics of Hall Effect thrusters. However, the so-called anomalous diffusion, short name for an excessive electron conductivity along the thruster, is not yet fully understood as it cannot be explained with classical collisional theories. One commonly accepted explanation is the existence of azimuthal oscillations with correlated plasma density and electric field fluctuations. In fact, there is experimental evidence of the presence of an azimuthal oscillation in the low frequency range (a few kHz). This oscillation, usually called spoke, was first detected empirically by Janes and Lowder in the 1960s. More recently several experiments have shown the existence of this type of oscillation in various modern Hall thrusters. Given the frequency range, it is likely that the ionization is the cause of the spoke oscillation, like for the breathing mode oscillation. In the high frequency range (a few MHz), electron-drift azimuthal oscillations have been detected in recent experiments, in line with the oscillations measured by Esipchuk and Tilinin in the 1970’s. Even though these low and high frequency azimuthal oscillations have been known for quite some time already, the physics behind them are not yet clear and their possible relation with the anomalous diffusion process remains an unknown. This work aims at analysing from a theoretical point of view and via computer simulations the possible relation between the azimuthal oscillations and the anomalous electron transport in HET. In order to achieve this main objective, two approaches are considered: local linear stability analyses and global linear stability analyses. The use of local linear stability analyses shall allow identifying the dominant terms in the promotion of the oscillations. However, these analyses do not take into account properly the axial variation of the plasma properties along the thruster. On the other hand, global linear stability analyses do account for these axial variations and shall allow determining how the azimuthal oscillations are promoted and their possible relation with the electron transport.

Induced radioisotopes in a linac treatment hall

When linacs operate above 8MV an undesirable neutron field is produced whose spectrum has three main components: the direct spectrum due to those neutrons leaking out from the linac head, the scattered spectrum due to neutrons produced in the head that collides with the nuclei in the head losing energy and the third spectrum due to room-return effect. The third category of spectrum has mainly epithermal and thermal neutrons being constant at any location in the treatment hall. These neutrons induce activation in the linac components, the concrete walls and in the patient body. Here the induced radioisotopes have been identified in concrete samples located in the hall and in one of the wedges. The identification has been carried out using a gamma-ray spectrometer.

Quantum Hall effect in gapped graphene heterojunctions

We model the quantum Hall effect in heterostructures made of two gapped graphene stripes with different gaps, Δ1 and Δ2. We consider two main situations, Δ1=0,Δ2≠0, and Δ1=−Δ2. They are different in a fundamental aspect: only the latter features kink states that, when intervalley coupling is absent, are protected against backscattering. We compute the two-terminal conductance of heterostructures with channel length up to 430 nm, in two transport configurations, parallel and perpendicular to the interface. By studying the effect of disorder on the transport along the boundary, we quantify the robustness of kink states with respect to backscattering. Transport perpendicular to the boundary shows how interface states open a backscattering channel for the conducting edge states, spoiling the perfect conductance quantization featured by the homogeneously gapped graphene Hall bars. Our results can be relevant for the study of graphene deposited on hexagonal boron-nitride, as well as to model graphene with an interaction-driven gapped phase with two equivalent phases separated by a domain wall.

University Hall; old dome (1872-1896)

Jenison, Edward S., architect. View is from the east. In 1879 two circular corner turrets and two turrets at base of dome were removed. On verso: Back of main building where the literary students hold forth.

University of Michigan. Main Building. Lit. Dept. [Literature Department]

Jenison, Edward S., architect. View from the west.

Haven Hall. Fire. June 1950. Students removing records from burning building

Jordan & Anderson, architect (1863); Spier & Rohns (1898). The old Law Building was renamed Haven Hall in 1933. It became one of the main buildings for LS&A used by Departments of History, Sociology and Journalism. The old Law Library became a study hall and Bureau of Government Library. Extension Division also had offices in Haven Hall.

Haven Hall Fire

Jordan & Anderson, architect (1863); Spier & Rohns (1898). The old Law Building was renamed Haven Hall in 1933. It became one of the main buildings for LS&A used by Departments of History, Sociology and Journalism. The old Law Library became a study hall and Bureau of Government Library. Extension Division also had offices in Haven Hall.

Haven Hall fire

Jordan & Anderson, architect (1863); Spier & Rohns (1898). The old Law Building was renamed Haven Hall in 1933. It becomes one of the main buildings for LS&A used by Departments of History, Sociology and Journalism. The old Law Library became a study hall and Bureau of Government Library. Extension Division also had offices in Haven Hall.

Haven Hall Fire June 1950

Jordan & Anderson, architect (1863); Spier & Rohns (1898). The old Law Building was renamed Haven Hall in 1933. It becomes one of the main buildings for LS&A used by Departments of History, Sociology and Journalism. The old Law Library became a study hall and Bureau of Government Library. Extension Division also had offices in Haven Hall. Two images from contact sheet.

Haven Hall Fire -- June 1950. Aftermath. Remains of Bureau of Gov't. Library

Jordan & Anderson, architect (1863); Spier & Rohns (1898). The old Law Building was renamed Haven Hall in 1933. It became one of the main buildings for LS&A used by Departments of History, Sociology and Journalism. The old Law Library became a study hall and Bureau of Government Library. Extension Division also had offices in Haven Hall.

Haven Hall Fire -- June 1950. Aftermath

Jordan & Anderson, architect (1863); Spier & Rohns (1898). The old Law Building was renamed Haven Hall in 1933. It becomes one of the main buildings for LS&A used by Departments of History, Sociology and Journalism. The old Law Library became a study hall and Bureau of Government Library. Extension Division also had offices in Haven Hall.

Haven Hall -- Fire June 1950. Wrecking

Jordan & Anderson, architect (1863); Spier & Rohns (1898). The old Law Building was renamed Haven Hall in 1933. It became one of the main buildings for LS&A used by Departments of History, Sociology and Journalism. The old Law Library became a study hall and Bureau of Government Library. Extension Division also had offices in Haven Hall.

Haven Hall -- Fire June 1950.

Jordan & Anderson, architect (1863); Spier & Rohns (1898). The old Law Building was renamed Haven Hall in 1933. It becomes one of the main buildings for LS&A used by Departments of History, Sociology and Journalism. The old Law Library became a study hall and Bureau of Government Library. Extension Division also had offices in Haven Hall.

Haven Hall -- Fire June 1950. Aftermath

Jordan & Anderson, architect (1863); Spier & Rohns (1898). The old Law Building was renamed Haven Hall in 1933. It becomes one of the main buildings for LS&A used by Departments of History, Sociology and Journalism. The old Law Library became a study hall and Bureau of Government Library. Extension Division also had offices in Haven Hall.

Haven Hall -- Fire June 1950. Wrecking

Jordan & Anderson, architect (1863); Spier & Rohns (1898). The old Law Building was renamed Haven Hall in 1933. It becomes one of the main buildings for LS&A used by Departments of History, Sociology and Journalism. The old Law Library became a study hall and Bureau of Government Library. Extension Division also had offices in Haven Hall.