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The electrothermal instability (ETI) arises whenever a current-carrying material has a resistivity that depends on temperature. When resistivity, η, increases with increasing temperature, ETI causes striations to form perpendicular to the direction of current. On pulsed-power-driven, ablating metallic loads, this process can cause sections of the target to ablate earlier than the bulk material, creating...
Double Planar Wire Arrays (DPWA), which consist of two parallel rows of wires, have previously demonstrated high radiation efficiency, compact size, and usefulness for various applications in experiments on a University-scale high-impedance Z-pinch generator1. Recently, we successfully performed two experimental campaigns with PWAs on the University of Michigan's low-impedance MAIZE (Linear Transformer...
Experiments are underway to study the stabilizing effects of an axial magnetic field on the magneto Rayleigh-Taylor (MRT), sausage, and kink instabilities in imploding liner-plasmas at the Michigan Accelerator for Inductive Z-pinch Experiments (MAIZE) facility at the University of Michigan (UM). The liners were fabricated from ultrathin aluminum foils and had thicknesses of 400 nm and 6.55 mm diameters,...
The RITS-6 accelerator at Sandia National Laboratories has been used to test the Self Magnetic Pinch (SMP) diode. The SMP diode is a flash x-ray radiography source with a radiation pulse length of 30–50 ns, dependent on the diode configuration. A hollow cathode emits electrons through an approximately 1 cm vacuum A-K gap onto a high Z material. The electron beam pinches from its own magnetic field...
Experiments are underway to study the effects an axial magnetic field on the magneto Rayleigh-Taylor instability (MRT) in ablating planar foils on the 1-MA LTD at the Michigan Accelerator for Inductive Z-pinch Experiments (MAIZE) facility at the University of Michigan. In planar foil ablation experiments at UM, MRT is observed when the expanding plasma-vacuum interface decelerates as the magnetic...
Single Planar Wire Arrays (SPWA) and Double Planar Wire Arrays (DPWA), which consist of one or two parallel rows of wires, respectively, have demonstrated high radiation efficiency (up to 30 kJ), compact size (1.5–3 mm), and usefulness for various applications in experiments on the high-impedance Zebra (1.9Ω, 1 MA, 100 ns). For example, DPWAs are very suitable for the new compact multi-source hohlraum...
Experiments are underway on the 1-MA, 100-kV MAIZE linear transformer driver (LTD) z-pinch experiment to explore the characterization and stabilization of the magneto-Rayleigh-Taylor (MRT) and electrothermal instabilities (ETI)1. Instability experiments at UM utilize 400 nm thick, planar Al foil loads and Al-backed Mylar foils. Theory has shown that axial magnetic fields and magnetic shear can reduce...
Self-Magnetic Pinch (SMP) diode experiments are in progress at the RITS-6 accelerator (5–11MV, 100–200kA) at Sandia National Laboratories. Te SMP diode is a flash x-ray radiography source with a radiation pulse length of 30–45 ns, dependent on the diode configuration. A hollow cathode emits electrons through an approximately 1cm vacuum A-K gap onto a high Z material. The high current (∼150kA) compresses...
This paper contrasts the very different spreading resistance and current crowding behaviors between horizontal and vertical thin film contact. Both Cartesian and circular contacts are analyzed. Accurate analytical scaling laws have been obtained for arbitrary aspect ratios and resistivity ratios, and validated against known limits, numerical simulations and experiments.
This paper explores several mechanisms to enhance the coherent Smith-Purcell radiation (SPR) at THz frequencies. A simple analytical dispersion relation is derived to predict the operating frequency for both open grating and Orotron-like devices. The analytical predictions are in excellent agreement with particle-in-cell simulations. It is found that the start current for a continuous electron beam...
We use the Briggs-Bers criterion to examine the absolute instability at the lower and upper band edges of a coupled cavity TWT. We find that the lower band-edge is not subjected to absolute instability. At the upper band-edge, we find a threshold beam current beyond which absolute instability is excited. In general, an absolute instability would occur in a linear beam tube if the cold-tube circuit...
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...
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...
This paper analyzes the effects of an axial magnetic field on the magneto-Rayleigh-Taylor instability (MRT) of an imploding cylindrical liner. In recent MagLIF experiments with a premagnetized axial magnetic field [1], helical perturbations on the outer surface of the liner were found to persist despite the severe winding up of the magnetic field streamline. The inner surface of the liner exhibited...
Motivated by the recent invention of the recirculating planar magnetron (RPM) [1], we exploit the possible technique of developing a dual-frequency high power microwave (HPM) amplifier. We first study the multi-spectral output characteristics of a novel slow wave structure (SWS) which consists of both upper and lower periodic vane arrays. The SWS is designed such that the operating frequency of the...
A voltage scale, Vs that characterizes electro-thermal runaway, is deduced from the heat conduction equation, Vs = √k/σ'0, where k-is the thermal conductivity and σ'0 is the rate of change of the electrical conductivity with respect to temperature. Vs depends only on material properties and is independent of geometry and the operating voltage. Vs measures the intrinsic tolerance of the material to...
Contact problems account for 40 percent of all electrical/electronic failures. Severe heating due to local current constrictions at thin film contacts [1–3] and at bulk contacts [2,4] is also a concern to high power microwave sources, pulsed power systems, field emitters, thin film devices and integrated circuits, and interconnects, etc. In this paper, we investigate one aspect of electro-thermal...
This paper concentrates on the effects of cylindrical geometry on the magneto-Rayleigh-Taylor instability (MRT), a major concern in the magnetized liner inertial fusion concept (MagLIF) [1]. Several issues are being studied, such as the Bell-Plesset effect [2], the effects of magnetic shear and feedthrough [3], and the nonzero MRT growth rate that remains (but was hardly noticed) in the k = m = 0...
Multipactor breakdown on a dielectric may provide automatic protective isolation of electronic circuits such as transmitter-receiver (TR) switches in radar. Prior Monte Carlo simulations suggest the application of a DC bias across a dielectric window in a vacuum-gas environment can lower the threshold for the onset of RF-initiated multipactor1. Variation of the DC bias may therefore provide a mechanism...
We examine the Brillouin flow in the conventional magnetron, inverted magnetron, and planar magnetron, with respect to the equilibrium, stability, and operating conditions. This renewed interest was prompted by our recent invention of the recirculating planar magnetron (RPM)1, where rapid start up utilizes the negative mass instability in the inverted magnetron configuration2. Given that Brillouin...
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