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.
Robust solutions of inverse problems in electromagnetic non-destructive evaluation
Non-destructive testing (NDT) is the application of physical measurement technology based on energy interaction with the material and its nonconformities. The material's response is sensed by transducers and sensors which?in most cases?scan the component and document the results in inspection images. However, NDT measures a physically defined quantity or even an intrinsic property. The difference between non-destructive evaluation (NDE) and NDT is in the interpretation of the inspection data. NDE has to discuss the inspection results in terms of quality elements and characteristics which are relevant to describe the fitness of the material for use. In the case of macroscopic defects these are the kind of defect (cracklike, globular) and its size and orientation to the main stress directions; in the case of material property determination the parameters are mainly mechanical properties. Therefore, in NDE one has to solve inverse problems. The solution of inverse problems based on mathematical procedures such as integral equations is a strong developing discipline and most of the articles prepared for this special issue of the journal have the objective of discussing the latest state of the art in that field. However, practical NDE needs robust and quick solutions which are to be applied mainly online. Therefore, we present here inversion procedures based on multiple linear regression algorithms applied to inspection data. We describe the calibration procedure to fit the free parameters of the model functions and give examples of practical applications in industry.