Showing papers by "Y.Y. Lau published in 2008"

Analysis and Experiments On Peer-to-Peer Locking of Magnetrons

Abstract: Experiments on peer-to-peer locking of 2 kW magnetrons are performed. These experiments verify the recently developed theory on the condition under which the two nonlinear oscillators may be locked to a common frequency. Dependent on the coupling, the frequency of oscillation when locking occurs does not necessarily lie between the free running frequencies of the two isolated, stand-alone magnetrons. Likewise, when the locking condition is violated, the beat frequency is not necessarily equal to the difference between these free running frequencies.

Electron Emission near a Triple Point

Abstract: Triple point, defined as the junction of metal, dielectric, and vacuum, is the location where electron emission is favored in the presence of a sufficiently strong electric field. In addition to being an electron source, the triple point is generally regarded as the location where flashover is initiated in high voltage insulation, and as the vulnerable spot from which rf breakdown is triggered. In this paper, we focus on the electric field distribution at a triple point of a general geometry, as well as the electron orbits in its immediate vicinity. We calculate the orbit of the first generation electrons, the seed electrons. We found that, despite the mathematically divergent electric field at the triple point, significant electron yield most likely results from secondary electron emission when the seed electrons strike the dielectric. The analysis gives the voltage scale in which this electron multiplication may occur. It also provides an explanation on why certain dielectric angles are more favorable to electron generation over others, as observed in previous experiments.

Z-pinch wire-contact resistance reduction using clamped and soft metal contacts

Abstract: The contact resistance in a wire array z-pinch has a significant affect on both the level and uniformity of energy deposition in the wires. Typically wires are held taut against the electrodes by wire weights (-1-10 g depending on wire material and diameter). This can lead to contact resistance values of -90-99% of the load resistance. Previous techniques to reduce the contact resistance (i.e. soldering) do not scale to large wire number arrays (such as Z-Machine). UM contact resistance experiments used 13 micron diameter Al 5056 wires. Bench measurements of the load resistance for double-ended weighted contact (similar to the method used on the Z-Machine) gave values in the range of 100-3000 ohms per wire. The actual wire resistance was 6.5 ohms. A second set of measurements in which the z-pinch wire was clamped to the electrodes using an indium gasket showed a reduction in load resistance to -10 ohms. Using a silver gasket further reduced the load resistance to 7 ohms. This clamping technique reduced the contact resistance by 2-3 orders of magnitude. Additionally, the contact resistance for clamped contact is significantly less than the resistance of the wire itself. Experiments performed on a 20 kA, 100 kV, 150 ns risetime Marx bank showed that the energy deposition and uniformity of the plasma expansion profile was significantly affected by the contact resistance. An open pinhole camera was used to observe a time integrated image of the plasma emission, and dark-field laser Schlieren photos were used to monitor the expansion profile of the plasma. The cases with clamped contact had more intense wire-plasma emission and more uniform expansion profiles. The weighted cases exhibited distinct, bright emission regions at the electrodes, indicative of contact arcing. Additional experiments are being conducted using 11.5 micron diameter W wire. Three cases are being examined: baseline weighted case, clamped contact case, and weighted case with soft metal (In or Ag) electrode gaskets. Preliminary results show that the electrode material may have a significant impact on contact resistance.

B-field perturbation effects on magnetic priming of a relativistic magnetron

Abstract: Experiments have been performed testing magnetic priming at the cathode of a relativistic magnetron to study the effects on high power microwave performance. Three high permeability wires were embedded beneath the emission region of a 1.27 cm diameter cathode, spaced 120 degrees apart (for pi-mode symmetry in an 6 vane magnetron) to perturb both the axial and radial magnetic fields near the emission region of the cathode. Magnetic priming was demonstrated to increase the percentage of pi-mode shots by 15% over the baseline case. Mean peak power for pi-mode shots was found to be higher in the magnetically primed case by almost a factor of 2. Increases in mean microwave pulse width were also observed in the magnetically primed case when compared to the unprimed case (66 ns primed versus 50 ns unprimed). Magnetron starting current for the magnetically primed pi-mode exhibited a reduction to 69% of the unprimed baseline starting current. Earlier research by Neculaes (2005) and recent simulation work performed utilizing MAGIC PIC and the Magnum magnetostatics code suggest that using permanent magnets with radially-directed remanence fields centered under the cathode emission region instead of high permeability wires can yield improved magnetron performance. Simulations of magnetically primed magnetrons utilizing permanent magnets with radially-directed remanence fields demonstrated improved performance as compared to simulations of axially-directed remanence fields. Both simulation and experimental results will be presented for the magnetic priming cases described.

Magnetron phase locking: Effects of frequency chirp 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 adopted 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. The model has been applied 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 among 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.

Triple-point cathodes for high current vacuum electron devices

Abstract: Summary form only given. Recent experiments at the University of Michigan have explored the mechanism of electron emission from triple points (vacuum-conductor-dielectric interface) for application to high current cathodes. Metal-oxide junction (MOJ) cathodes have been developed and tested; these consist of hafnium oxide (HfO2) or magnesium oxide (MgO) coatings over metal (#304 stainless steel) substrates. To fabricate MOJ cathodes, high dielectric constant HfO2 coatings (or high secondary electron emission coefficient MgO coatings) are deposited by pulsed laser deposition (PLD) using a KrF laser at 248 nm and 50 J/cm2 fluence. Cathodes were tested on the Michigan Electron Long-Beam Accelerator (MELBA), with a relativistic magnetron, at parameters V=-300 kV, currents 1-15 kA, and pulse-lengths of 0.3-0.5 microseconds. Six variations of the MOJ cathode were tested, and were compared against five baseline cases. It was found that particulate formed during the ablation process improves the electron emission properties of the cathodes by forming additional triple points. Due to extensive electron back- bombardment during magnetron operation, secondary electron emission also appears to play a significant role. MOJ cathodes exhibit increases in current densities of up to 80 A/cm2, and up to 15% improvement in current start up time, as compared to polished stainless steel cathodes. An analytic theory is developed to assess electron multiplication in the immediate vicinity of a triple point, and predict the dielectric angles most susceptible to breakdown. These predictions are shown to agree with previous experiments. Latest experiments are exploring stacked- washer, triple-point cathodes consisting of insulating (BN) washers alternating with oxygen-free copper (OFC) washers. The stacked-washer, triple-point cathode utilizes the theory to predict dielectric angles favorable for electron emission.

A higher dimensional theory of contact resistance

Abstract: Electrical contact is an important issue for wafer evaluation of manufactured integrated circuits. It has also repeatedly surfaced in our ongoing studies of high power microwave sources, wire-array Z pinches, field emitters, and metal- insulator-vacuum junctions. Because of the surface roughness on a microscopic scale, true contact between two pieces of metal occurs only on the asperities of the two contacting surfaces. Current flows only through these asperities, which occupy a small fraction of the area of the nominal contacting surfaces. This gives rise to contact resistance. We have recently developed an analytic theory of the contact resistance of an asperity that has a finite axial length (h) connecting two metal blocks. We construct a scaling law for arbitrary aspect ratios of the asperity, with the h = 0 limit reducing to the classical "a-spot theory" of Holm and Timsit. While asperities of finite length may be a small step closer to reality, this higher-dimensional treatment could link the contact resistance to the geometrical deformations in response to an applied pressure, and eventually to the hardness of the material. Potential applications and extensions of the theory, such as the RF contact resistance, will be presented.

Advances in modeling microwave window breakdown

Abstract: Summary form only given. Dielectric window breakdown remains a significant issue for high power microwave systems. Using a particle-in-cell (PIC) 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, 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 higher order kinetic form at high pressure. The time to achieve breakdown is described across a broad range of pressures for Ne, Ar, and Xe, and a general analytic scaling law was deduced. Using the electron generation rates from the kinetic PIC model, we developed a global model with an enhanced electron energy distribution function form along with an energy equation which matches the kinetic results well across a broad range of pressure. We are now extending that model to air, a broader range of frequency and microwave power levels.