Glow discharge | EPFL Graph Search

A glow discharge is a plasma formed by the passage of electric current through a gas. It is often created by applying a voltage between two electrodes in a glass tube containing a low-pressure gas. When the voltage exceeds a value called the striking voltage, the gas ionization becomes self-sustaining, and the tube glows with a colored light. The color depends on the gas used. Glow discharges are used as a source of light in devices such as neon lights, cold cathode fluorescent lamps and plasma-screen televisions. Analyzing the light produced with spectroscopy can reveal information about the atomic interactions in the gas, so glow discharges are used in plasma physics and analytical chemistry. They are also used in the surface treatment technique called sputtering. Electrical conduction in gas Conduction in a gas requires charge carriers, which can be either electrons or ions. Charge carriers come from ionizing some of the gas molecules. In terms of current flow, glow

Experiments and modelling for glow discharge plasmas applied to niobium sputter deposition in superconducting radiofrequency cavities

Thibaut François Hugo Richard

Thin film coatings are ubiquitous in modern daily lives, from the semi-conductor industry to aesthetic applications, and rely on the modification of a given material to enhance its surface properties. At CERN (European Organization for Nuclear Research), thin film coatings are used for several applications related to the accelerator technology field, such as electron cloud mitigation, continuous pumping or radio-frequency (RF) superconductivity. This PhD thesis aims at studying the use of a Particle-in-Cell Monte Carlo/ Direct Simulation Monte Carlo (PICMC/DSMC) numerical code applied to the modelling of deposition processes. The present study focuses on niobium thin film deposition on copper RF cavities of peculiar geometries and large sizes. Numerical simulations can help to understand and improve process parameters in existing systems, and guide the design of future coating systems such that thin ï¬lm characteristics can be predicted and expensive times of Research and Development can be reduced. In the first part of this thesis, critical parameters are identified to provide an accurate modelling of glow discharge plasmas commonly used in coating applications with the PICMC module of the numerical code. A dedicated experimental system is designed to enable both DC diode and DC magnetron operation with a coaxial cylindrical plasma source. Suitable ion-induced secondary electron emission yield and energy distribution are numerically assessed by matching simulated discharge voltages and currents with experimental ones. Simulated local plasma parameters, such as electron density and energy, are compared with experimental measurements obtained with a Langmuir probe. In the second part, transport simulations of sputtered neutral niobium atoms performed with the DSMC method are validated by comparing simulated deposition rates on a substrate with experimental ones with a compact hollow cathode magnetron sputtering source. At last, the validated ab initio methodology for coating process modelling, from plasma simulation and extraction of sputtering profiles to sputtered niobium atoms transport, is applied to typical coating systems used at CERN. The scalability of low discharge power simulation results to realistic powers is first demonstrated for a planar magnetron source in terms of cathode erosion and deposited thin film thickness profiles. Then, an elliptical RF cavity is used as a case study to apply the full methodology in a real substrate of complex shape. PICMC and DSMC simulation results are further extended towards thin film growth modelling, and simulated thin film morphology is compared with experimental Focused Ion Beam/Scanning Electron Microscopy imaging of the niobium layer.

EPFL2019

DC Breakdown in gases for complex geometries from high vacuum to atmospheric pressure

Ralph Schnyder

This thesis presents an experimental investigation and a numerical simulation of breakdown in a ring assembly. Previous works are mostly limited to breakdown in simple geometries such as parallel plates or pin-to-plate. Here we discuss the effect of more complex geometries for DC breakdown in gases over a large pressure range from high vacuum to atmospheric pressure. The breakdown voltage versus pressure curves shows a similar shape as Paschen curves but with a wide flat plateau between the low and high pressure thresholds. The low pressure threshold determines the limit between gas and vacuum discharges. Additional optical emission spectroscopy confirms the presence of two different kinds of discharges: Gas and vacuum dis- charges. Moreover the global shape of the gas breakdown voltage curve in the ring assembly has been fully understood by a complementary numerical simulation. Fur- ther current-voltage study showed that voltage only is the most significant factor for breakdown and that current determines the kind of discharge after breakdown. As the breakdown voltages are lower for gas discharges than for vacuum discharges, a numerical simulation model for gas breakdown using a fluid model was developed in order to support the experimental conclusions. Starting as simple as possible with par- allel plates (1 mm and 100 mm gap width representing approximatively the shortest and longest electric field path length in the ring assembly geometry) and extending to double gap and multi-gap geometries, an understanding of the overall shape of the breakdown voltage versus pressure curve is established: The high (low) pressure thresholds of gas discharge are determined by the shortest (longest) electric field path length in a complex geometry. Moreover, the availability of multiple path lengths leads to a breakdown voltage minimum over a wide range of intermediate pressure because breakdown can occur in the most favorable gap. Finally, the numerical simulation in the ring assembly shows the importance of parameters such as the secondary electron emission coefficient which play a major role in determining the breakdown voltage value.

EPFL2013

Real-time feedback control system for ADITYA-U horizontal plasma position stabilisation

Shivam Gupta, Rohit Kumar, Harshita Raj

The ADITYA-U tokamak (R-0 = 0.75 m, a = 0.25 m) is designed to shape plasma column in both single and double null diverter configurations. It is quite well known that sustaining a shaped plasma in tokamak requires very good plasma column position control, both horizontal and vertical. An FPGA-based proportional-integral-derivative (PID) control system has been designed and operated to achieve horizontal plasma position control in ADITYA-U tokamak. The complete system has been rigorously tested with sample signals before implementing to the ADITYA-U plasma discharges. The control system is integrated and time-synchronized with the plasma discharge operation of ADITYA -U. Furthermore, the system has been trained to take appropriate actions during the disruption or plasma failure in the tokamak operation. Detailed experimental results have been obtained by the operation of the digital PID controller. The complete design, installation, operation, tuning of the system along with all the relevant testing and operating experience of the digital PID controller for real-time horizontal plasma position control is presented in the paper.

2021