Showing papers by "Y.Y. Lau published in 2007"

Transition of dielectric window breakdown from vacuum multipactor to collisional microwave discharge: a general scaling law

Abstract: Summary form only given. Dielectric window breakdown remains a significant issue for high power microwave systems. Using a particle-in-cell model with Monte Carlo collisions, we investigate the transition of dielectric window breakdown from a vacuum multipactor discharge to a collisional microwave discharge in a number of noble gases. At low pressure, the dominant mechanism of electron creation is the single-surface multipactor, and the mean energy of the electron population is hundreds of eV. As pressure increases to 10-50 Torr, an intermediate regime is obtained in which electrons generated in volumetric ionization compete with the multipactor electrons generated at the dielectric window surface. In this regime, the mean energy declines significantly, and we observe two distinct electron populations: a surface population participating in the multipactor process, and a detached population shielded from the surface fields by ions. Approaching atmospheric pressure, the volumetric ionization dominates, and the multipactor process is extinguished as the electron mean energy drops to a few eV. In this collisional regime, the nearly neutral discharge detaches from the dielectric window surface, and the surface charge and field that drives electrons into the window is also eliminated. The electron energy probability function changes from a bi-Maxwellian at low pressure to a Druyvesteyn at high pressure. Multidimensional effects, such as waveguide field structure and electron-absorbing transverse walls, are considered. The time to achieve breakdown is described across a broad range of pressures for Ne, Ar, and Xe, and a general analytic scaling law is deduced. The scaling law compares well with the simulation results, and work is presently underway to extend the scaling law to complicated discharges such as air.

Effect of Random Circuit Fabrication Errors on Small Signal Gain and Phase in Helix Traveling Wave Tubes

Abstract: Motivated by current interest in submillimeter and terahertz (THz) slow-wave vacuum electronic amplifiers, which employ miniature, difficult-to-manufacture slow-wave circuits, we evaluate the effects of small random fabrication errors on the small-signal characteristics of a traveling wave tube. The classical 1-D small-signal theory of Pierce, generalized to allow axially varying circuit characteristics, is applied. Random, axially varying perturbations are introduced in the circuit phase velocity mismatch , the gain parameter , and the cold-tube circuit loss , in Pierce notation. Results from a first-order perturbation analysis of the small-signal equations, which are confirmed by numerical analysis, show that the standard deviations in the output phase and in the small-signal gain are linearly proportional to the standard deviations of the individual perturbations in , , and . Our study confirms that the effects of perturbations in the circuit phase velocity dominate the effects of perturbations in and .

Magnetic Priming of a Relativistic Magnetron Utilizing Ferromagnetic Wires in the Cathode and Anode

Abstract: Magnetic priming experiments on the UM/ L-3-Titan relativistic magnetron (100 MW in L-band, -300 kV, ~3 kGauss), have shown suppression of unwanted modes and major reduction in starting currents for the pi-mode. Data from continuing experiments on magnetic priming at the cathode will be presented, as well as preliminary data on magnetic priming at the cathode and anode. Azimuthally-varying, axial-magnetic-perturbations are generated by three, 4 cm or 6 cm-long Mu-metal wires located just below the surface of 0.86 mm wall-thickness stainless steel tubing. The electron-emitting surface of the stainless steel is laser machined for field emission. Magnetic perturbations applied only at the cathode decay with increasing radius, hi order to maintain magnetic perturbations across the entire A-K gap, we also install three, magnetic-priming, Mu-metal wires inside holes drilled in the anode structure. Magnetostatics calculations have been performed for the case of magnetic wires embedded in the cathode and in the anode. Simulation results show strong perturbations at the cathode surface, which fall off slightly at small radii, but grow in intensity as the anode surface is approached. Data demonstrate that magnetic priming at the cathode significantly lowers (average factor of 2.5) the range of starting currents for pi-mode generation. The percentage of pi-mode shots was also increased by magnetic priming at the cathode by as much as 60% over unprimed shots. Experiments are reported concerning the effects of magnetic priming at both cathode and anode.

Effects of Circuit Manufacturing Errors on Small Signal Gain and Phase in a Traveling Wave Tube

Abstract: Summary form only given. Motivated by the current interest in vacuum slow wave THz sources, where difficult-to-manufacture miniature slow wave circuits are required, we evaluate the effects of geometrical random errors introduced during the circuit manufacturing processes on the small signal gain and phase characteristics of a traveling wave tube amplifier. A continuum model is used. Random perturbations are introduced through the gain parameter C, the velocity mismatch b, and the cold tube circuit loss d, in Pierce notation. The perturbations are incorporated in the differential equations describing the beam-circuit interaction in such a way that the Pierce dispersion relation is recovered for an ideal tube free of manufacturing errors. We employ these differential equations in a Monte-Carlo study to determine the rms effects on small signal gain and phase of the different types of errors. Our study shows that effects of a perturbation in the circuit phase velocity dominate those due to perturbations in the gain parameter C and loss d. More importantly, we find that the effects on gain and phase depend not only on the magnitude of the manufacturing errors, but also on the axial distribution of these errors. The analytic theory and our numerical results, including the statistics of the variations in the output gain and output phase, will be presented.

Recent Advances in Magnetron Phase Locking: Effects of Frequency Chirps and Locking of Multiple Magnetrons

Abstract: Summary form only given. Phase-locking is utilized today in many important applications, ranging from small scale devices such as cardiac pacemakers to large scale devices such as radar. We have recently developed a magnetron-specific model to qualitatively explain the various regimes observed in magnetron injection-locking experiments, which utilize two continuous wave magnetrons: one functions as an oscillator and the other as a driver. We have applied this model to study injection locking when the driver has a frequency chirp. The model has also recently been extended to analyze peer-to-peer locking of two magnetrons of comparable powers and frequencies. The feasibility of locking will be explored in terms of the variations in these parameters of the individual magnetrons, as well as the degree of coupling among them. Locking of a larger number of such magnetrons will be explored. A preliminary experiment is being conducted on the peer-to-peer locking of two commercial KW magnetrons.