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A concept for a millimetre-wave (MMW) heat exchanger (HX) featuring AlN:Mo ceramic composite structures as electromagnetic absorbing elements (susceptors) has been recently introduced as a receiving device in power beaming applications. Earlier computational studies of electromagnetic and thermal processes have shown reasonable energy efficiency and exceptional uniformity of MMW-induced temperature fields in a single cubic susceptor with concentration of Mo doping on the level of 3–4% by volume. As part of ongoing research, a MMW HX comprised of an array of cylindrical susceptors is proposed to potentially enable increased robustness against thermal stress and reduced manufacturing cost. In this paper, we computationally study the effects driven by such a change and demonstrate feasibility of the designs based on multiple cylinders. We present the output of electromagnetic and coupled electromagnetic-thermal simulations of a prospective physical prototype of a HX with five cylinders on a square metal base plate. Three alternative layouts with four, nine, and sixteen cylindrical elements that are suggested by the highest density packing of equal circles in a square are also analyzed. It is shown that, in comparison with the previously studied case of a single cubic susceptor, energy efficiency of all systems with Mo = 3–4% is down from 50–55% to 35–45%. While temperature distribution within each individual cylinder remains highly uniform, maximum temperatures of different cylinders may be different by up to 30–40 °C; when the angle of incidence deviates from normal, this difference further increases: e.g. when the angle is 10°, in the sixteen-cylinder system, it may reach 120–130 °C. [ABSTRACT FROM AUTHOR]

Experiments have demonstrated high-power amplification (100 MW class) from a traveling wave amplifier driven by a modulated electron beam. The modulated electron beam is generated by applying a modulated voltage pulse from a nonlinear transmission line (NLTL) to the cathode of the electron gun. The system showed that over the designed operating range of the disk-on-rod (DoR) TWT amplifier (1.2–1.4 GHz), the peak frequency of the amplifier output tracked the peak frequency of the NLTL to within ±2%; however, the output was not phase stable. Measured RF energy gain for this frequency regime ranged from 1.5 at 1.2 GHz up to 5.0 at 1.4 GHz). [ABSTRACT FROM AUTHOR]

An apparatus for measuring the W-band (75â€"110 GHz) complex permittivity of dielectrics at 1000 °C was developed. This apparatus allows for measurements at approximately twice the temperature of previously published high temperature free-space measurement systems while maintaining similar precision. Challenges were addressed related to high temperature measurements, including temperature uniformity, the accuracy of temperature measurements, and preventing temperature related changes to mm-wave measurement systems. The details of complex permittivity extraction from the measured S-parameters are discussed. Sources of error related to permittivity measurement and mathematical models were identified and are discussed in detail herein. Thermally-cycled, mm-wave absorbing, aluminum nitride ceramic composites containing varying levels of molybdenum additives were measured over the range of 25 °Câ€"1000 °C. These measurements were compared to the same composites before thermal cycling. It was found that ceramic composites are largely stable after thermal cycling in terms of dielectric properties despite the presence of visible surface modifications. [ABSTRACT FROM AUTHOR]

A system capable of exposing a flowing aerosol stream to short duration (2–4 ns), high-power RF waveforms is described. The system utilizes a C-band gyromagnetic nonlinear transmission line source having peak power outputs ranging as high as 80 kW at a center frequency of 4.2 GHz. RF electric field magnitudes of up to 280 kV/m ± 17% are achieved within the aerosol flow region of the RF exposure apparatus. [ABSTRACT FROM AUTHOR]

A set of three apparatus enabling RF exposure of aerosolized pathogens at four chosen frequencies (2.8 GHz, 4.0 GHz, 5.6 GHz, and 7.5 GHz) has been designed, simulated, fabricated, and tested. Each apparatus was intended to operate at high power without leakage of RF into the local environment and to be compact enough to fit within biocontainment enclosures required for elevated biosafety levels. Predictions for the range of RF electric field exposure, represented by the complex electric field vector magnitude, that an aerosol stream would be expected to encounter while passing through the apparatus are calculated for each of the chosen operating frequencies. [ABSTRACT FROM AUTHOR]

To enable the use of additively manufactured polymer materials as structural and insulating components within high-voltage pulsed power systems, a better understanding of material compatibility with common pulser material environments is required. This work examines the effect of long-duration contact of printed polymeric components with three high-voltage insulating oils (Shell Diala S2 ZX-A, Luminol TRi, and Royco 602) at room temperature (~25 °C). Test pieces printed using stereolithography (SLA) were evaluated for changes in dielectric strength, mechanical properties, and composition after submersion in insulating oil for a nine-month duration. Gas chromatography–mass spectrometry (GC–MS) was used for compositional analysis on neat and polymer-exposed oils to identify compositional changes that occurred in the oil over the course of the exposure period. [ABSTRACT FROM AUTHOR]

Field emission cathodes, comprised of commercially available carbon fiber fabrics directly brazed to metal substrates, were fabricated and tested. Cathodes fabricated in this manner eliminate the need for an epoxy bond between the carbon fibers and the substrates and can be baked, in a vacuum, at high temperatures, limited by the re-melt temperature of the braze. Preliminary testing at mildly relativistic voltages (200 kV–300 kV) yielded average current emission densities of 100's of A/cm2, which are in line with previously published results on epoxy-bonded carbon fiber field emission cathodes. [ABSTRACT FROM AUTHOR]

The recirculating planar crossed-field amplifier (RPCFA) was designed, constructed, and tested at the University of Michigan. The RPCFA was driven by a number of RF sources ranging in frequency from 2.40 to 3.05 GHz and powers of 1 to 800 kW. Pulsed voltage was delivered to the cathode by the Michigan electron long beam accelerator with ceramic insulator (MELBA-C) which was configured to supply pulses of −300 kV, 1–10 kA, with 0.3–1.0- $\mu \text{s}$ pulse lengths. The RPCFA demonstrated zero-drive stability and a bandwidth of 15%. Amplification of microwave signals, at the design frequency of 3 GHz, below 150 kW, was observed with a mean gain of 7.87 dB and high variability, $\sigma = 2.74$ dB. Filtering this data set to only include shots with identical voltage and current profiles yielded a gain of 6.6 ± 1.6 dB. The mean gain increased to 8.71 dB and the variability decreased to $\sigma =0.63$ dB when the injected microwave power increased beyond 150 kW. Peak output powers of nearly 6 MW were achieved with RF breakdown limiting the maximum output power of the device. [ABSTRACT FROM AUTHOR]

The harmonic recirculating planar magnetron (HRPM) is a frequency-agile, multispectral HPM source. The HRPM implements an $L$ -band oscillator (LBO) and an $S$ -band oscillator (SBO) near 1 and 2 GHz, respectively. The novel coaxial-all-cavity-extraction (CACE) method was implemented to extract power from the SBO. The two oscillators demonstrated harmonic frequency locking, where the SBO frequency locked to the LBO second harmonic frequency. The two oscillators are concluded to operate as a damped, driven, harmonic oscillator system. In the locked state, the LBO acts as the driving oscillator, the SBO acts as the driven oscillator, and the coupling mechanism between the two oscillators is the harmonic content in the electron spokes as they propagate directly from the LBO to the adjacent SBO. The two primary variables studied in this system are the LBO harmonic frequency and the SBO quality factor, $Q$. In isolated SBO experiments, the dominant operating state of the magnetron was the $5\pi $ /6 mode. However, when operated in tandem with the LBO, the SBO was forced to operate in the $\pi $ -mode. Output powers in HRPM experiments generated SBO powers of 9.5 ± 1.4 MW at high $Q$ , 19 ± 6 MW at moderate $Q$ , and 28 ± 9 MW at low $Q$. Output powers in the isolated SBO configuration were not significantly different from the HRPM. By experimentally manipulating the hub drift direction and altering the evolution of harmonic content received by the SBO, the locked state was significantly diminished, suggesting that the beam spokes play a crucial role. [ABSTRACT FROM AUTHOR]

Free-space measurement techniques can be contactless and are able to accommodate large, flat sheets of dielectric material, making them useful for characterization of high-temperature, millimeter-wave, window and radome candidate materials. As part of the present work, a high-temperature, W-band (75–110 GHz), free-space measurement system was developed and used to characterize complex dielectric properties of bulk material samples at temperatures ranging from 25 °C to 600 °C. Two test cases, polyvinyl chloride (PVC) and CoorsTek 92% alumina, were measured at 25 °C and found to have $\boldsymbol {\epsilon }_{r}^\prime $ values of 2.731 ± 0.005 and 8.061 ± 0.027 at 95 GHz, respectively. The 25 °C PVC sample was measured to have a $\boldsymbol {\epsilon }_{r}^{\prime \prime }$ value of 0.032 ± 0.007. At 25 °C, the $\boldsymbol {\epsilon }_{r}^{\prime \prime }$ value of the 92% alumina sample was below the uncertainty threshold achievable with the present free-space measurement apparatus and could only be bounded to <0.009. As the alumina sample was heated to 600 °C, $\boldsymbol {\epsilon }_{r}^\prime $ and $\boldsymbol {\epsilon }_{r}^{\prime \prime }$ values increased to 8.501 ± 0.028 and 0.035 ± 0.008, respectively. The high-temperature behavior of the authors’ 92% alumina ceramic was found to be similar to that previously documented for Sumitomo AKP-50 alumina over the 25 °C–600 °C temperature range. In addition to the 92% alumina sample, three commercially available ceramic substrates (zirconium oxide, boron nitride, and silicon nitride) were also characterized at temperatures ranging from 25 °C to 600 °C. [ABSTRACT FROM AUTHOR]

Spatially dispersive nonlinear transmission lines (NLTLs) have attracted interest as pulsed RF sources. The characteristics of these sources need to be further evaluated and understood to optimize their design. This paper presents the performance of a spatially dispersive nickel-zinc (NiZn) NLTL for two experiments. The first experiment evaluated the RF power and frequency generated as a function of the primary dielectric medium for SF 6 compared to the previously implemented Royco 602 transformer oil. The second experiment evaluated the RF power and frequency generated as a function of the ferrites' loss tangents, initial permeabilities, and dimensions. The NLTL's performance is presented for each experiment across a range of operational input voltage levels and ferrite bias conditions. Experimental primary dielectric performance results demonstrate approximately 4X increase in RF power at frequencies above 1.2 GHz for replacement of oil with SF 6. Experimental ferrite performance results show output frequencies from 0.9 to 1.39 GHz and peak RF powers >2X higher for the higher permeability and larger volume ferrite. [ABSTRACT FROM AUTHOR]

The recirculating planar crossed-field amplifier (RPCFA) was designed and simulated using the finite-element frequency-domain code ANSYS HFSS and the particle-in-cell (PIC) code MAGIC. The RPCFA is a high-power microwave device adapted from the recirculating planar magnetron, developed at the University of Michigan, Ann Arbor, MI, USA. Electromagnetic (EM) PIC simulations of a planar, meander line, and slow wave structure demonstrated 13.5-dB amplification of a 1.3-MW, 3-GHz signal to approximately 29 MW. The RPCFA is designed to be driven by pulsed power from the Michigan electron long beam accelerator-ceramic insulator, which is currently configured to deliver pulses at −300 kV, 1–10 kA, with 0.3– $1~\mu \text{s}$ pulse lengths. The RF input-drive signal will be provided by an MG5193 magnetron which delivers 5- $\mu \text{s}$ pulses up to 2.6 MW at 3 GHz. EM PIC simulations also demonstrated zero-drive stability of the design and were used to evaluate changes in performance resulting from variations of several experimental parameters. Variation of the drive frequency suggested that the RPCFA is expected to have a 3-dB amplification bandwidth of 300 MHz or 10%. [ABSTRACT FROM AUTHOR]

In a helix traveling-wave tube (TWT) with a bandwidth exceeding one octave, the second harmonic of an input signal near the low end of the band will experience exponential growth. In such a case, we have found that the nonlinear electron orbits in the beam, as opposed to the orbital bunching exhibited in the linear electron orbits, are the main source for second-harmonic generation. This unexpected result is due to the synchronous amplification of the second harmonic.We demonstrate this phenomenon from the solution of the nonlinear equations that we have formulated that govern evolution of the second-harmonic field; these equations may include axial variations of the Pierce parameters. In several test cases, we compare the theory with simulation using the CHRISTINE large signal TWT code. Good agreement between theory and simulation is found. [ABSTRACT FROM AUTHOR]

Performance of a 20-stage X-band sine waveguide amplifier, driven by a 40 A, 100 kV, cylindrical electron beam, is studied using numerical simulation and interpreted using Pierce’s classical traveling wave tube theory. For an input signal power level of 1.8 kW, particle-in-cell simulations predict gain and bandwidth values exceeding 14 dB and 13%, respectively. For an input signal power level of 7.2 kW, particle-in-cell simulations predict gain and bandwidth values exceeding 12 dB and 15%, respectively, with output power levels exceeding 110kW at peak gain. Also given are: an assessment of the space charge factor (Pierce’s QC parameter) for the complex circuit using simulation data, and an evaluation of the harmonic contents in the beam current. [ABSTRACT FROM AUTHOR]

Using measured waveforms from a synchronous-wave ferrite nonlinear transmission line (NLTL), particle-in-cell simulations demonstrate the potential to emit GW-class electron beams (hundreds of kilovolts, multiple kiloamperes) with current modulations as great as 30% of the average beam current, with modulations occurring at a peak frequency equal to that of the peak NLTL output frequency. The NLTL-modulated beam is then coupled to disk-on-rod slow wave structure (SWS) simulations in which it is shown that the extractable RF generated through interaction of the modulated beam with the SWS is up to ten times greater than that directly extractable from the NLTL itself. [ABSTRACT FROM PUBLISHER]

A high-power, pulsed operation, multibeam, S-band, coaxial disk-on-rod traveling-wave-tube amplifier has been simulated. In this paper, the amplifier, having an 81-period slow wave structure, utilized six 20-A beams. For 800 W input signals at frequencies between 2 to 4 GHz, gain and bandwidth values of 21.6 dB at 8% bandwidth and 15 dB at 15% bandwidth were observed for the beam voltages of 100 and 120 kV, respectively. [ABSTRACT FROM PUBLISHER]

Additively manufactured components were successfully fielded for the first time in a relativistic crossed-field device. Anode structures for a relativistic planar magnetron were 3-D printed from a photopolymer using a stereolithography printing process. One anode was electroplated with copper (RPM-12b), whereas the other was thermal sprayed with copper (RPM-12c). The coating thicknesses at the vane tips were approximately 0.18 and 0.23 mm, respectively. The performance and durability of these structures were evaluated in comparison with a solid aluminum anode (RPM-12a) fabricated via conventional machining. The experimental parameters were cathode voltages between −150 and −300 kV, voltage pulse lengths of 200 to 600 ns, axial magnetic fields of 0.13 to 0.31 T, peak anode currents from 1 to 7 kA, and a base operating pressure of 9 \times 10^-6 torr. The 3-D printed anodes demonstrated microwave performance comparable to the aluminum anode, generating microwave powers in excess of 150 MW, with an average instantaneous peak total efficiency of 27% ± 10%. After 100 shots on each structure, neither anode showed any signs of operationally induced damage. The anodes did, however, have a higher rate of postshot outgassing, emitting 32% and 23% more CO2 per shot, respectively. [ABSTRACT FROM PUBLISHER]

An apparatus for generating quasi-free-space microwave-driven plasmas has been designed, constructed, and tested. The plasma is driven by a multi-kW, ~5 GHz microwave beam focused at the center of a vacuum chamber using a Koch-type metal plate lens. Sustained plasma discharges have been generated in argon at pressures ranging from 150 to 200 mTorr, at beam power levels ranging from 5 to 10 kW, and at gas flow rates of approximately 200 SCCM. [ABSTRACT FROM AUTHOR]

Dielectric strength testing of selected Polyjet-printed polymer plastics was performed in accordance with ASTM D149. This dielectric strength data is compared to manufacturer-provided dielectric strength data for selected plastics printed using the stereolithography (SLA), fused deposition modeling (FDM), and selective laser sintering (SLS) methods. Tested Polyjet samples demonstrated dielectric strengths as high as 47.5 kV/mm for a 0.5 mm thick sample and 32.1 kV/mm for a 1.0 mm sample. The dielectric strength of the additively manufactured plastics evaluated as part of this study was lower than the majority of non-printed plastics by at least 15% (with the exception of polycarbonate). [ABSTRACT FROM AUTHOR]

In a klystron, charge overtaking of electrons leads to an infinity of ac current on the electron beam. This paper extends the klystron theory of orbital bunching to a traveling-wave tube (TWT). We calculate the harmonic content of the beam current in a TWT that results from an input signal of a single frequency. We assume that the electron orbits are governed by Pierce’s classical three-wave, linear theory. The crowding of these linear orbits may lead to charge overtaking and, therefore, harmonic generation on the beam current, as in a klystron. We analytically calculate the buildup of harmonic content as a function of tube length from the input, and compare the results with the CHRISTINE code. Good agreement is found. Also found is the surprisingly high level of harmonic contents in the electron beam current, even when the TWT operates in the small signal regime. A dimensionless bunching parameter for a TWT, X=(2P\rm{ in/(Pb C))}^{1/2} , is identified, which characterizes the harmonic content in the ac beam current, where $P_{{\textrm {in}}} is the input power of the signal, Pb is the dc beam power, and C is Pierce’s gain parameter. [ABSTRACT FROM PUBLISHER]

Calibrated microwave power and phase measurements are presented for the first recirculating planar magnetron prototype consisting of two coupled six-cavity 1-GHz planar cavity arrays. The results are presented for a solid cathode and two mode-control cathodes (MCCs) with aluminum or velvet electron emitters. The measurements were conducted using a prototype coaxial microwave power extraction scheme. The experimental operating parameters included: pulsed cathode voltages between −250 and −300 kV, voltage pulselengths of 200–600 \mu \texts , axial magnetic fields of 0.1–0.32 T, and entrance currents of 1–10 kA. The results showed improved oscillator frequency locking for the MCCs and increases in power and efficiency using the velvet electron emitter. [ABSTRACT FROM PUBLISHER]

A spatially dispersive nonlinear transmission line (NLTL) using axially biased ferrite as the nonlinear medium has been developed. The NLTL is frequency tunable from 0.95 to 1.45 GHz with adjustment of the axial biasing field. A circuit model describing the dispersion of the line has been developed and compared with experimental measurements and time and frequency domain simulations. Instantaneous peak power levels exceeding 100 MW and average power levels of 10s of MW with durations from 4 to 17 ns have been observed. [ABSTRACT FROM PUBLISHER]

Non-linear ferro-electric materials exhibit electromagnetic shock-forming properties which can be a challenge for electromagnetic modeling algorithms such as finite-difference time-domain (FDTD). If the material response time is sufficiently fast, the shock fronts steepen until the Nyquist sampling criteria is violated, and the resulting aliasing leads to spurious grid oscillations. A flux corrected transport (FCT) algorithm is adapted for use on the staggered FDTD mesh and effectively removes the spurious oscillations. Numerical results demonstrate the effectiveness of the algorithm. [ABSTRACT FROM PUBLISHER]

Microwave oscillation has been measured for the first time in a 12-cavity axial-magnetic-field recirculating planar magnetron, designed to operate in \pi mode at 1 GHz. The device operates with a -300-kV pulsed cathode voltage and a 0.2-T axial magnetic field, and oscillates at transverse currents exceeding 1 kA when driven by an electron beam pulselength between 0.5 and 1 \mu\s. Microwave pulses were measured at frequencies between 0.97–1 GHz and achieved several hundred nanoseconds in length. Mode competition was observed between the \pi and 5 \pi/6 modes. [ABSTRACT FROM PUBLISHER]

Gated field emission cathodes can provide short electron pulses without the requirement of laser systems or cathode heating required by photoemission or thermionic cathodes. The large electric field requirement for field emission to take place can be achieved by using a high aspect ratio cathode with a large field enhancement factor which reduces the voltage requirement for emission. In this paper, a cathode gate driver based on the output pulse train from a nonlinear transmission line is experimentally demonstrated. The application of the pulse train to a tufted carbon fiber field emission cathode generates short electron pulses. The pulses are approximately 2 ns in duration with emission currents of several mA, and the train contains up to 6 pulses at a frequency of 100 MHz. Particle-in-cell simulation is used to predict the characteristic of the current pulse train generated from a single carbon fiber field emission cathode using the same technique. [ABSTRACT FROM AUTHOR]

A compact axial \pi-mode extraction scheme, which is based on a patent by Greenwood, is demonstrated in conjunction with the UM/L-3 relativistic magnetron using the particle-in-cell code ICEPIC. Cases utilizing Greenwood's extraction technique were compared with power extraction using traditional radial waveguides. Average extracted power values in all simulated axial cases were found to be within +/-6.5% of the radial cases. Cases utilizing 85 ^\circ and 90^\circ sector waveguides were found to have efficiencies up to ten percentage points higher than the radial case. The best performing case was found to use a set of three axially oriented 90 ^\circ sector waveguides, shorted on the upstream side, with the short located 15 cm from the center of the magnetron apertures. [ABSTRACT FROM PUBLISHER]

We present designs and simulations of a new class of magnetron, the recirculating planar magnetron. This magnetron has numerous advantages as a high-power microwave generator, including larger cathode and anode area, fast start-up, and compact microwave extraction geometry. The following two geometries are demonstrated by electromagnetic particle-in-cell codes: 1) axial magnetic field with radial electric field and 2) radial magnetic field with axial electric field. [ABSTRACT FROM AUTHOR]

Using a hybrid approach, 3-D simulations of magnetic priming of a relativistic magnetron have been performed. The primed magnetic field values were calculated using a magnetostatics code (Magnum) and then imported into a particle-in-cell code (Magic PIC) and run for the case of a six-vane relativistic magnetron. The magnetically perturbative structures chosen for implementation in these simulations were sets of three highpermeability wires of various lengths, which would be placed within the cathode, the anode, or, in the combined case, both the cathode and anode. In the best-performing cathode-wire case (three 4-cm wires), magnetic priming was found to reduce the start-oscillation time of the magnetron to 50% that of the unprimed case. When wires were embedded in both the cathode and the anode, the best-performing case (4-cm cathode wires and 4-cm anode wires) was found to start oscillating at 30% of the start-oscillation time of the unprimed case. The cases of magnetically primed magnetrons were found to exhibit slightly reduced equilibrium power levels, compared with the unprimed case. [ABSTRACT FROM AUTHOR]

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 π-mode. Magnetic perturbations were imposed by utilizing three high-permeability nickel-iron wires embedded beneath the emission region of the cathode, spaced 1200 apart. Magnetic priming was demonstrated to increase the percentage of π-mode shots by 15% over the baseline case. Mean peak power for π-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 π-mode exhibited a reduction to 69% of the unprimed baseline starting current. [ABSTRACT FROM AUTHOR]

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 π-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 Adler's Relation is not satisfied. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Azimuthally varying axial magnetic fields have been utilized to perform "magnetic priming" of magnetrons for rapid startup, low noise, and mode control. An overview of the latest magnetic priming experimental and simulation results are presented in this paper. Magnetic priming experiments in dc-operating microwave oven magnetrons show sideband elimination, even with the cathode heater turned off. Simulations using three three-dimensional (3-D) improved concurrent electromagnetic particle-in-cell (ICEPIC) codes with two different computational algorithms recover the oven magnetron experimental results obtained with magnetic priming including fast mode growth, rapid spoke formation, and the tendency toward lower noise operation. A new, axially symmetric, azimuthally varying magnetic field geometry for oven magnetrons is explored and preliminary results are reported. Simulations using two-dimensional (2-D) MAGIC code for the University of Michigan/Titan relativistic magnetron show that the oscillation startup time can be dramatically decreased (almost by a factor of 3) and mode competition can be suppressed with magnetic priming. [ABSTRACT FROM PUBLISHER]

An electron rotating under a uniform axial magnetic field and a radial electric field exhibits an effective mass that may be negative, positive, or infinite, in response to an azimuthal electric field. This paper reports simulation results that show instability and stability when the effective mass are negative and positive, respectively, depending on the magnitude and orientation of the radial electric field. Thus, the inverted magnetron would have a much faster startup than the conventional magnetron, an important consideration for pulsed operation. When the effective mass is infinite, the electrons hardly respond to an azimuthal ac electric field. [ABSTRACT FROM AUTHOR]