L. Vušković

Excessive Balmer line broadening in microwave-induced discharges

Results of a hydrogen Balmer line-shape study on microwave-induced plasma discharges operated with pure hydrogen and with argon–hydrogen or helium–hydrogen mixtures are reported. Plasma is generated in a rectangular or coaxial microwave cavity in two separate experiments. In both cases, the emission profiles of the Balmer lines did not show excessive broadening as reported by Mills et al. [J. Appl. Phys. 92, 7008 (2002)].

Characterization of a supersonic microwave discharge in Ar/H2/Air mixtures

Detailed characterization of a microwave cavity discharge in the supersonic flow of Ar/H2/Air mixtures at static pressures of 1–10 Torr and Mach number 2 is mostly based on emission spectroscopy techniques. In the conditions close to real combustion environments, effects of hydrogen and air admixture to plasma parameters and population of excited species in the discharge are demonstrated. The effects resulting in ionization loss are discussed from the aspects of dominant mechanisms and consequences for the plasma assisted hydrogen oxidation.

Etching mechanism of niobium in coaxial Ar/Cl2 radio frequency plasma

The understanding of the Ar/Cl2 plasma etching mechanism is crucial for the desired modification of inner surface of the three dimensional niobium (Nb) superconductive radio frequency cavities. Uniform mass removal in cylindrical shaped structures is a challenging task because the etch rate varies along the direction of gas flow. The study is performed in the asymmetric coaxial radio-frequency (rf) discharge with two identical Nb rings acting as a part of the outer electrode. The dependence of etch rate uniformity on pressure, rf power, dc bias, Cl2 concentration, diameter of the inner electrode, temperature of the outer cylinder, and position of the samples in the structure is determined. To understand the plasma etching mechanisms, we have studied several factors that have important influence on the etch rate and uniformity, which include the plasma sheath potential, Nb surface temperature, and the gas flow rate.

Characterization of a CO2/N2/Ar supersonic flowing discharge

In this paper we are presenting the full characterization of a supersonic flowing CO2/N2/Ar discharge at static pressures of 1–20 Torr and Mach number 2.15. In all aspects besides the flow speed and gas temperature, these conditions correspond to Martian entry plasma. Plasma parameters were determined by optical emission spectroscopy techniques. The gas and vibrational temperature were found from analysis of the rotational and vibrational spectra of the CO B Σ1+−A Π1 Angstrom system, respectively. The electron density was determined from the absolute intensity of the N2 C Π3u−B Π3g second positive system. A kinetic model for the discharge was developed to calculate the electron density and compared with experimental data.

VOLUMETRIC NEAR-FIELD MICROWAVE PLASMA GENERATION

A periodic series of microwave-induced plasmoids is generated using the outgoing wave from a microwave horn and the reflected wave from a nearby on-axis concave reflector. The plasmoids are spaced at halfwavelength separations according to a standing-wave pattern. The plasmoids are enhanced by an “effective focusing” in the near field of the horn (Fresnel region) as a result of a diffractive narrowing. Optical imaging, electron density, and rotational temperature measurements characterize the near field plasma region. Volumetric microwave discharges may have application to combustion ignition in scramjet engines. A. Introduction

Apparatus and method for plasma processing of SRF cavities

Abstract An apparatus and a method are described for plasma etching of the inner surface of superconducting radio frequency (SRF) cavities. Accelerator SRF cavities are formed into a variable-diameter cylindrical structure made of bulk niobium, for resonant generation of the particle accelerating field. The etch rate non-uniformity due to depletion of the radicals has been overcome by the simultaneous movement of the gas flow inlet and the inner electrode. An effective shape of the inner electrode to reduce the plasma asymmetry for the coaxial cylindrical rf plasma reactor is determined and implemented in the cavity processing method. The processing was accomplished by moving axially the inner electrode and the gas flow inlet in a step-wise way to establish segmented plasma columns. The test structure was a pillbox cavity made of steel of similar dimension to the standard SRF cavity. This was adopted to experimentally verify the plasma surface reaction on cylindrical structures with variable diameter using the segmented plasma generation approach. The pill box cavity is filled with niobium ring- and disk-type samples and the etch rate of these samples was measured.

Aerodynamic Effects in Weakly Ionized Gas: Phenomenology and Applications

Aerodynamic effects in ionized gases, often neglected phenomena, have been subject of a renewed interest in recent years. After a brief historical account, we discuss a selected number of effects and unresolved problems that appear to be relevant in both aeronautic and propulsion applications in subsonic, supersonic, and hypersonic flow. Interaction between acoustic shock waves and weakly ionized gas is manifested either as plasma‐induced shock wave dispersion and acceleration or as shock‐wave induced double electric layer in the plasma, followed by the localized increase of the average electron energy and density, as well as enhancement of optical emission. We describe the phenomenology of these effects and discuss several experiments that still do not have an adequate interpretation. Critical for application of aerodynamic effects is the energy deposition into the flow. We classify and discuss some proposed wall‐free generation schemes with respect to the efficiency of energy deposition and overall gene...

Ionization-recombination model for the supersonic flow in weakly ionized gas

A one-dimensional f ow model for weakly ionized g&‘is described. It is based on a conventional model, modified to account for the separation of charge and detailed description of non-steady state ionization-recombination process. The model was applied to simulate excited-state population profile across the shock structure in ionizing neon. Excited-state population of precursor and relaxation region of the shock structure obtained in the experiment was several orders of magnitude higher tbau predicted by conventional model. Present analysis has shown that the dominant mechanism in the modification of precursor and relaxation regions is the difbrsion of radiation from the equilibrium region, across the shock structure.

Secondary Electron Yield of Electron Beam Welded Areas of SRF Cavities

Secondary Electron Emission (SEE) is a phenomenon that contributes to the total electron activity inside the Superconducting Radiofrequency (SRF) cavities during the accelerator operation. SEE is highly dependent on the state of the surface. During electron beam welding process, significant amount of heat is introduced into the material causing the microstructure change of Niobium (Nb). Currently, all simulation codes for field emission and multipacting are treating the inside of the cavity as a uniform, homogeneous surface. Due to its complex shape and fabricating procedure, and the sensitivity of the SEE on the surface state, it would be interesting to see if the Secondary Electron Yield (SEY) parameters vary in the surface area on and near the equator weld. For that purpose, we have developed experimental setup that can measure accurately the energy distribution of the SEY of coupon-like like samples. To test the influence of the weld area on the SEY of Nb, dedicated samples are made from a welded plate using electron beam welding parameters common for cavity fabrication. SEY data matrix of those samples will be presented.

Shock Wave Propagation and Dispersion in a Microwave Cavity Discharge

We studied shock wave propagation and dispersion through a transverse electric field cavity microwave discharge. Electrodeless discharges are convenient active plasma media for shock dispersion studies in weakly ionized gas since there are no solid objects in the path of the shock wave that may distort the shock structure. Discharge was generated in a cylindrical cavity powered by one or two magnetrons operating at the commercial S-band frequency. Shock diagnostics include planar laser sheet deflection system, designed to determine radial and axial distribution of shock front velocity, as well as the transversal dispersion effects. Discharge diagnostics included spectroscopic determination of electron density and temperature, electric field, and vibrational and rotational temperatures of the nitrogen molecules. Our measurements indicate a correlation between the distribution of plasma density and the intensity of dispersion and propagation effects in the discharge.