E. Cruz

Magnetic Priming at the Cathode of a Relativistic Magnetron

Experiments have been performed in testing magnetic priming at the cathode of a relativistic magnetron to study the effects on high-power microwave performance. Magnetic priming consists of N/2 azimuthal magnetic perturbations applied to an N-cavity magnetron for rapid generation of the desired number of electron spokes for the pi-mode. Magnetic perturbations were imposed by utilizing three high-permeability nickel-iron wires embedded beneath the emission region of the cathode, spaced 120 apart. Magnetic priming was demonstrated to increase the percentage of pi-mode shots by 15% over the baseline case. Mean peak power for -mode shots was found to be higher in the magnetically primed case by almost a factor of two. Increases in mean microwave pulsewidth 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.

Experiments on peer-to-peer locking of magnetrons

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.

Analysis of peer-to-peer locking of magnetrons

The condition for mutual, or peer-to-peer, locking of two magnetrons is derived. This condition reduces to Adler’s classical phase-locking condition in the limit where one magnetron becomes the “master” and the other becomes the “slave.” The formulation is extended to the peer-to-peer locking of N magnetrons, under the assumption that the electromagnetic coupling among the N magnetrons is modeled by an N-port network.

Magnetic Priming of a Relativistic Magnetron

Magnetic priming [1] experiments performed on the UM/Titan relativistic magnetron (6-vane, -300kV, 5-10kA, 0.3-0.5 mus) have shown improvements in magnetron performance over baseline operation. In the current experimental setup, three, 4-cm long magnetic wires (Mu-Metal) are located within the cathode structure, centered beneath the emission region, and spaced 120 degrees apart. These wires produce magnetic perturbations with N/2 azimuthal-symmetry (for pi-mode in an N vane magnetron). Because of the close proximity of the priming structures to the cathode surface, the magnetic perturbations are strongest in the region where the electrons are emitted into the magnetron interaction space.

Temporal and spatial locking of nonlinear systems

Peer-to-peer locking of two magnetrons is analyzed including the effects of a frequency chirp and of low frequency noise. It is found that complete phase locking cannot be achieved in either case. However, as long as the locking condition is well satisfied instantaneously, a high degree of locking occurs. This analysis in the time domain is adapted to locking in the spatial domain, in particular to the interpretation of some recent experiments on the spatial correlation of two ablating current-carrying wires that are placed sufficiently close to each other.

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

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.

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

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.

Coaxial guns for the ARPA-E PLX-α project — Design and initial experimental results

Summary form only given. We describe the ongoing effort to design, build, and test coaxial plasma guns [1] appropriate for a scaling study of spherically imploding plasma liners as a standoff magneto-inertial-fusion driver under ARPA-Es Accelerating Low-Cost Plasma Heating And Assembly (ALPHA) program. HyperV joins LANL, UAH, UNM, BNL, and Tech-X to develop, build, operate and analyze a 60-plasma-gun experiment using the existing PLX facility [2] at LANL. The guns are being designed to operate over a range of operating parameters: 0.5-5.0 mg of Ar, Ne, N2, Kr, and Xe; 20-60 km/s; 1016-1017 cm-3 muzzle density; and up to 7.5 kJ stored energy per gun. Each coaxial gun incorporates a contoured gap designed to suppress the blow-by instability, fast dense gas injection and triggering, and innovative integral sparkgap switching. The switch and pfn configurations are designed to reduce inductance, cost, and complexity, and to increase efficiency and system reliability. Each gun is driven by a 600μF, 5kV capacitor bank mounted directly onto the back of the gun to reduce inductance. The pfn is sufficiently low weight to allow mounting of the gun/pfn module directly on the vacuum-tank port without any additional supports. This also eliminates the need for racks and thick-cable bundled transmission lines to 60 guns, resulting in vastly improved experimental access to the vacuum tank, guns, and diagnostics. We will provide a brief overview of the PLX-α project, describe the overall design approach for the guns and pulsed-power systems, the projected performance over the parameter ranges mentioned above, and experimental results from testing of the first gun, AlphaGun-1.

Effects of perturbing B-field orientation on magnetic priming of a Relativistic Magnetron

Experiments have been performed testing magnetic priming at the cathode of a relativistic magnetron to study the effects on high power microwave performance. Magnetic perturbations were imposed utilizing three, high-permeability nickel-iron wires embedded beneath the emission region of a 1.27 cm diameter cathode, spaced 120 degrees apart (for N/2 symmetry in an N (6) cavity magnetron). These three, high-permeability wires perturb both the axial and radial magnetic fields near the emission region of the cathode. Magnetic priming was demonstrated at UM to increase the percentage of p-mode shots by 15% over the baseline case in the relativistic magnetron. Improvements in microwave power, pulse width and start-oscillation time were also observed. Earlier experimental research by Neculaes and recent simulation work 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.

Designs, tests and plans for MAIZE: a 1 MA LTD-driven z-pinch

We present designs, resistive-load test results and experimental plans of the first 1 MA z-pinch in the USA to be driven by a Linear Transformer Driver (LTD). The Michigan Accelerator for Inductive Z-pinch Experiments, (MAIZE), is based on the LTD developed at the Institute for High Current Electronics in collaboration with Sandia National Labs. This LTD utilizes 80 capacitors and 40 spark gap switches to deliver a 1 MA, 100 kV pulse with <100 ns risetime into a matched resistive load. Resistive load test results will be presented for components and the LTD facility. Designs will be presented of a low-inductance MITL terminated in a wire- array z-pinch. Planned experiments include LTD driven time- changing inductance of imploding 4-16 wire-array z-pinches. Wire ablation dynamics, axial-correlations and instability development will be explored.