Showing papers by "Y.Y. Lau published in 2014"

Coaxial all cavity extraction in the Recirculating Planar Magnetron

Abstract: Recent experiments on the first Recirculating Planar Magnetron (RPM) prototype, the L-band RPM-12a, have demonstrated successful generation of microwave power in the vicinity of pi-mode at 1 GHz using a -300 kV pulsed voltage and 0.2 T axial magnetic field. However, these initial experiments were designed primarily to demonstrate feasibility and validate the RF dispersion relationship. This prototype has not demonstrated the efficient extraction of microwave power into waveguides. A novel approach to power extraction using a coaxial adaptation of the “all cavity extractor” is currently under development at the University of Michigan. The Coaxial All Cavity Extractor (CACE) is a highly compact axial extraction technique that is specifically designed to accommodate the planar slow wave structures of the RPM. Concepts and design of the extractor are presented as well as a new experimental prototype RPM-CACE, a 12-cavity device simulated to yield 450 MW with 60% efficiency at 1.9 GHz.

Design of a prototype Multi-Frequency Recirculating Planar Magnetron

Abstract: The Multi-Frequency Recirculating Planar Magnetron (MFRPM) is a type of Recirculating Planar Magnetron1,2 (RPM) designed for tunable or simultaneous oscillation at more than one primary frequency. Like the RPM, the MFRPM is a crossed-field, high power microwave source that integrates the same advantages over traditional cylindrical cavity magnetrons as UM's original RPM, such as a large cathode surface area for supplying higher electron current, with the added benefit of producing multiple frequencies using a single HPM device. Previous experimental research3 involved demonstration of the L-band RPM-12A, the first RPM prototype. The RPM-12A was specifically designed for use on the Michigan Electron Long Beam Accelerator with a ceramic insulator (MELBA-C), which drives the RPM by applying a −300kV, 0.3–1.0 µs pulse to the cathode.

Microwave extraction in the recirculating planar magnetron

Abstract: The recirculating planar magnetron (RPM) [1, 2] is a crossed-field device that combines the advantages of high-efficiency recirculating devices with those of planar devices: both large-area cathode (high current) and anode (improved thermal management). Experiments using the RPM-12a, the first L-band prototype, have successfully produced high power microwave pulses 50–300 ns in length at approximately 1 GHz [3]. The device is driven using the Michigan Electron Long Beam Accelerator with Ceramic insulator (MELBA-C), which delivers a pulsed cathode bias of −300 kV for durations of 0.3–1.0 µs, and a Helmholtz electromagnet capable of producing 0.15–0.2 T axial magnetic field.

A study of absolute instability in TWTs

Abstract: This paper will present results of our analysis of the onset of absolute instability at the lower band edge of a traveling wave tube (TWT). We begin with the study of an electron beam propagating in a dielectric waveguide because the exact dispersion relation is easily formulated, and it is similar to that of a gyrotron-traveling wave amplifier in which the occurrence of an absolute instability was previously established [1]. When the Briggs-Bers criterion [2] is applied to the dispersion relation for the dielectric waveguide TWT, however, there does not seem to be an absolute instability regardless of the current level for such a dielectric waveguide TWT. That is, all frequencies that satisfy the pole-pinch criterion of Briggs and Bers are real, regardless of the beam current. Adapting the dispersion relation to a coupled-cavity TWT, we tentatively conclude that there is no absolute instability in the sense of Briggs and Bers at the lower band edge of each pass band. Our preliminary conclusion is therefore that oscillation at the lower band edge is more likely due to reflection at the ends of the tube, and not to the dispersion of the waveguide modes that can produce an absolute instability. The potential for absolute instability at the upper band edges will also be addressed.

Feedthrough of the Magneto-Rayleigh-Taylor instability in the presence of a shock

Abstract: The success of the Magnetized Liner Inertial Fusion (MagLIF) campaign requires successful mitigation of the Magneto-Rayleigh-Taylor instability (MRT). Initially, the exterior of the liner is MRT unstable; as the accelerated liner compresses a fill gas, it is eventually decelerated by the back pressure of the fill gas causing the inner surface to become MRT unstable. It is thought that feedthrough of MRT from the outer to inner surface can provide a seed for disruptive growth in the deceleration phase. A common method of studying MRT is by machining sinusoidal ripples on the exterior of metal targets (Al, Be in particular). This seeding provides a known wavelength for MRT whose growth can then be compared with linear theory. For the exterior of Al liners, linear MRT theory works quite well [1] and thus, feedthrough theory should apply equally well to the inner surface for an unshocked seeded liner. However, in most shots on the Z-machine a shock is driven through the liner. 2D HYDRA simulations show the shock is rippled with the seeded wavelength and imprints this ripple on the inner surface as it breaks-out. The amplitude of this ripple could be much larger than what would be expected from the feedthrough of MRT theory [2]. Thus, to study feedthrough directly either isentropic compression is required, or we must include the effect of the shock on feedthrough. We propose a simple model to determine the imprinted ripple amplitude from a shock and then calculate the evolution of the inner surface ripple using traditional MRT [2] and Richtmyer-Meshkov theory and then compare to 2D HYDRA results. This model allows for simultaneous study of outer/inner surface growth from an exterior seeded perturbation and includes the effect of feedthrough. We also examine the impact of axial magnetic fields and fill gases (cool and pre-heated) on feedthrough and shock imprinting by applying our model and compare with 2D simulations.

Zeeman splitting measurements of local magnetic fields in wire z-pinch plasmas

Abstract: Wire z-pinch experiments at the University of Michigan are in progress to test potential spectral lines for use as a local magnetic field diagnostic in dense plasmas driven by high currents. The goal of these experiments is to determine several spectral lines that can be used to reliably characterize the magnetic field and current density profiles of high energy density plasmas. Initial feasibility experiments were performed on a compact pulser, in which 50–60 kA currents were conducted in a single wire with 400 ns risetime. A lower-inductance, higher-current pulser is currently under construction. An optical fiber collected visible light emission from the wire ablation plasma for measurement by a timegated ICCD coupled to a 0.75-m optical spectrograph. Spectra have been collected for several plasmas including W, Mo, Na, and Al. Magnetic fields of 5–6 T have been measured using Zeeman splitting.