P. Pengvanich

Modeling and experimental studies of magnetron injection locking

A phase-locking model has been developed from circuit theory to qualitatively explain the various regimes observed in magnetron injection-locking experiments. The experiments utilize two continuous-wave oven magnetrons: one functions as an oscillator and the other as a driver. The model includes both magnetron-specific electronic conductance and frequency-pulling parameter. Both time and frequency domain solutions are developed from the model, allowing investigations into the growth and saturation as well as the frequency response of the output signal. This simplified model recovers qualitatively many of the phase-locking frequency characteristics observed in the experiments.

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.

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.

Rapid kinematic bunching and parametric instability in a crossed-field gap with a periodic magnetic field

Single particle orbit considerations show that the cycloidal orbits of electrons in a gap with crossed electric and magnetic fields lead to rapid spoke formation if the external magnetic field has a periodic variation. This spoke formation is primarily a result of kinematic bunching, which is independent of the radio frequency electric field and of the space charge field. A parametric instability in the orbits, which brings a fraction of the electrons from the cathode to the anode region, is discovered. These results are examined in light of the recent rapid startup, low noise magnetron experiments and simulations that employed periodic, azimuthal perturbations in the axial 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.

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.

Effects of frequency chirp on magnetron injection locking

The injection locking of a magnetron is theoretically analyzed when either the free running oscillator or the drive signal has a frequency chirp. It is found that complete phase locking of the signal cannot be achieved in either case. However, as long as the locking condition of Adler is well-satisfied instantaneously, a high degree of locking occurs during a major duration of the frequency chirps. The expected output phase variation is computed in terms of the noise in the free-running magnetron oscillator for the case of constant drive frequency.

Electron emission near a triple point

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 [1], 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. Specifically, from the orbits of the seed electrons, we derive the range of angles thetas, 0 >thetas >-9.1degtimes(epsivr + 1/epsivr)timesradicE0m/(1eV)/E1/(40eV) which most likely produces secondary electron avalanche on the dielectric by a seed electron [1]. In Eq. (1), thetas is the dielectric angle defined in Fig. 1, epsivr is the relative dielectric constant, E0m is the average emission energy of a secondary electron from the dielectric, and E1 is the first cross-over energy in the secondary electron yield curve. Figure 1 shows a comparison [2] between Eq. (1) with previous experimental results [3].

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.