N.M. Jordan

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.

Magnetron priming by multiple cathodes

A relativistic magnetron priming technique using multiple cathodes is simulated with a three-dimensional, fully electromagnetic, particle-in-cell code. This technique is based on electron emission from N∕2 individual cathodes in an N-cavity magnetron to prime the π mode. In the case of the six-cavity relativistic magnetron, π-mode start-oscillation times are reduced up to a factor of 4, and mode competition is suppressed. Most significantly, the highest microwave field power is observed by utilizing three cathodes compared to other recently explored priming techniques.

Radio frequency priming of a long-pulse relativistic magnetron

Rapid startup, increased pulsewidth, and mode locking of magnetrons have been explored experimentally on a relativistic magnetron by radio frequency (RF) priming. Experiments utilize a -300 kV, 2-8 kA, 300-500-ns electron beam to drive a Titan six-vane relativistic magnetron (5-100 MW output power in each of the three waveguides). The RF priming source is a 100-kW pulsed magnetron operating at 1.27-1.32 GHz. Tuning stubs are utilized in the Titan structure to adjust the frequency of the relativistic magnetron to match that of the priming source. Experiments are performed on rising sun as well as standard anode configurations. Magnetron start-oscillation time, pulsewidth, and pi-mode locking are compared with RF priming versus the unprimed case. The results show significant reductions in microwave output delay and mode competition even when Adlers Relation is not satisfied

Magnetic perturbation effects on noise and startup in DC-operating oven magnetrons

Previous experiments demonstrated that imposing an azimuthally varying axial magnetic field, axially asymmetric, in dc-operating oven magnetrons causes rapid mode growth (by magnetic priming) and significant noise reduction. This configuration was previously implemented by adding five perturbing magnets on the upper existing magnet of the magnetron. Experiments reported here add five perturbing magnets on each of the two existing magnets of the magnetron, restoring the axial symmetry of the magnetic field, while maintaining the five-fold azimuthal magnetic field symmetry. Compared with the unperturbed magnetic field case, it has been observed that the noise close to the carrier is reduced by up to 20 dB, while the sidebands are not completely eliminated for medium and high currents. Magnetron start-oscillation currents are somewhat higher for this axially symmetric, azimuthally varying magnetic field as compared to the baseline unperturbed magnetic field.

Simulations of magnetic priming in a relativistic magnetron

Two-dimensional simulations have been performed on a six-vane relativistic magnetron with uniform axial magnetic fields versus azimuthally varying axial magnetic fields, defined as magnetic priming. Electron phase-space plots show rapid growth of the /spl pi/-mode when the axial magnetic field has three-azimuthal perturbations: it takes 36 ns for the /spl pi/-mode to dominate in the uniform magnetic field case versus only 13 ns for the /spl pi/-mode to dominate in the case with magnetic priming imposed. RF electric field plots versus time show the suppression of the 2/spl pi//3-mode when magnetic priming is imposed.

Seeded and unseeded helical modes in magnetized, non-imploding cylindrical liner-plasmas

In this research, we generated helical instability modes using unseeded and kink-seeded, non-imploding liner-plasmas at the 1 MA Linear Transformer Driver facility at the University of Michigan in order to determine the effects of externally applied, axial magnetic fields. In order to minimize the coupling of sausage and helical modes to the magneto Rayleigh-Taylor instability, the 400 nm-thick aluminum liners were placed directly around straight-cylindrical (unseeded) or threaded-cylindrical (kink-seeded) support structures to prevent implosion. The evolution of the instabilities was imaged using a combination of laser shadowgraphy and visible self-emission, collected by a 12-frame fast intensified CCD camera. With no axial magnetic field, the unseeded liners developed an azimuthally correlated m = 0 sausage instability (m is the azimuthal mode number). Applying a small external axial magnetic field of 1.1 T (compared to peak azimuthal field of 30 T) generated a smaller amplitude, helically oriented inst...

Technique for fabrication of ultrathin foils in cylindrical geometry for liner-plasma implosion experiments with sub-megaampere currents

In this work, we describe a technique for fabricating ultrathin foils in cylindrical geometry for liner-plasma implosion experiments using sub-MA currents. Liners are formed by wrapping a 400 nm, rectangular strip of aluminum foil around a dumbbell-shaped support structure with a non-conducting center rod, so that the liner dimensions are 1 cm in height, 6.55 mm in diameter, and 400 nm in thickness. The liner-plasmas are imploded by discharging ∼600 kA with ∼200 ns rise time using a 1 MA linear transformer driver, and the resulting implosions are imaged four times per shot using laser-shadowgraphy at 532 nm. This technique enables the study of plasma implosion physics, including the magneto Rayleigh-Taylor, sausage, and kink instabilities on initially solid, imploding metallic liners with university-scale pulsed power machines.

Discrete helical modes in imploding and exploding cylindrical, magnetized liners

Discrete helical modes have been experimentally observed from implosion to explosion in cylindrical, axially magnetized ultrathin foils (Bz = 0.2 – 2.0 T) using visible self-emission and laser shadowgraphy. The striation angle of the helices, ϕ, was found to increase during the implosion and decrease during the explosion, despite the large azimuthal magnetic field (>40 T). These helical striations are interpreted as discrete, non-axisymmetric eigenmodes that persist from implosion to explosion, obeying the simple relation ϕ = m/kR, where m, k, and R are the azimuthal mode number, axial wavenumber, and radius, respectively. Experimentally, we found that (a) there is only one, or at the most two, dominant unstable eigenmode, (b) there does not appear to be a sharp threshold on the axial magnetic field for the emergence of the non-axisymmetric helical modes, and (c) higher axial magnetic fields yield higher azimuthal modes.

RF and magnetic priming of relativistic magnetrons

Summary form only given. Research is underway to investigate two techniques for priming of relativistic magnetrons for rapid startup and reduced mode competition: 1) RF priming experiments with a 2 MW magnetron signal 2) Magnetic-priming simulations by an azimuthally-varying axial magnetic field. Experiments utilize the MELBA-C (Titan) 6-vane, relativistic magnetron which operates with parameters: V=-300 kV, I=1-10 kA, e-beam pulselength=0.5 /spl mu/sec, microwave power=100-500 MW, microwave frequency in L-band: 1-1.3 GHz. The ceramic insulator enables operation down to 8.5 E-8 Torr. The RF priming source is a 2 MW, 2.2 /spl mu/sec, pulsed magnetron from AFRL operating at 1.3 GHz. The microwaves are injected into 1 of the 3 open coupling slots in the MELBA-C relativistic magnetron. Magnetic priming consists of imposing N/2 azimuthal variations in the axial magnetic field of an N-vane magnetron. Such optimal magnetic priming has been demonstrated in low voltage experiments and high voltage simulations to cause rapid startup of magnetrons by pre-bunching the electrons into the N/2 electron spokes desired for the pi-mode. A highly idealised model of magnetic priming uncovered a parametric instability, which draws electrons into N/2 spokes that extend to the anode even in the absence of RF fields.

Microwave oscillations in the Recirculating Planar Magnetron

The Recirculating Planar Magnetron (RPM) is a crossed-field device which presents numerous potential advantages in generating High Power Microwaves (HPM). A 12-cavity, conventional-polarity, 1-GHz RPM model has been experimentally designed and tested at a -300 kV, 0.3-0.5 microsecond pulselengths and a 0.2 T axial magnetic field. The experimental results and future concepts are under investigation.