Compact Electron Gun Based on Secondary Emission Through Ionic Bombardment
We present a new compact electron gun based on the secondary emission through ionic bombardment principle. The driving parameters to develop such a gun are to obtain a quite small electron gun for an in-flight instrument performing Electron Beam Fluorescence measurements (EBF) on board of a reentry vehicle in the upper atmosphere. These measurements are useful to characterize the gas flow around the vehicle in terms of gas chemical composition, temperatures and velocity of the flow which usually presents thermo-chemical non-equilibrium. Such an instrument can also be employed to characterize the upper atmosphere if placed on another carrier like a balloon. In ground facilities, it appears as a more practical tool to characterize flows in wind tunnel studies or as an alternative to complex electron guns in industrial processes requiring an electron beam. We describe in this paper the gun which has been developed as well as its different features which have been characterized in the laboratory.
Particle in cell modelling of the observed modes of a dc wire discharge
Low-pressure dc wire induced plasma sources exhibit two stable modes of discharge—constricted below a threshold pressure and diffuse above. Starting from experimental measurements, we conduct two-dimensional particle in cell (PIC) modelling of a dc low-pressure (10−4–10−2 mbar), low-current (∼1 mA) wire discharge in helium. 2D PIC modelling is required to capture longitudinal non-uniformity of the diffuse mode. PIC simulations reproduce the two discharge modes. The voltage versus pressure curve obtained from simulations matches fairly well the experimental data, including the transition region. Discharge voltage dependence on pressure is analysed in consideration of electron impact ionization rates' evolution with energy. In light of the PIC findings, a model of the discharge mode transition based on the Child–Langmuir theory for ions is proposed. Confrontation with simulated data shows good agreement and validates the model for mode transition prediction. Simulations show that the diffuse mode is a space-charge-dominated regime.
Miniband formulation in Ge/Si quantum dot array
In this work, we estimate the coupling effect of miniband structure and density of states (DoS) resulting from different qunatum-dot physical parameters in a well-aligned Ge/Si quantum dot (QD) array fabricated by neutral beam etching technology. The density of QDs dominates the coupling effect and miniband’s bandwidth, the radius of QDs affects the magnitude of energy levels and miniband bandwidth, and the thickness of QDs has a great impact on the magnitude of energy levels. Among the different shapes of Ge/Si QDs, discoid QDs exhibit the most band crossing and broadest bandwith under the same physical parameters.