Direct observation of electrons in microwave vacuum components

A high power, pulsed RF source designated for use in multipactor research is described. Four gallium nitride high electron mobility transistors from Cree/Wolfspeed, capable of 700 W in long pulse mode (500 W rated output), are combined to achieve a maximum rated output of 2.8 kW with a pulse length of ∼100 µs. Custom splitters/combiners are used due to the power levels considered in addition to a custom power and sequencing control system to ensure the proper biasing and sequencing of the relatively delicate depletion mode GaN devices. With high efficiency and small size, gallium nitride devices present a good solution for lab based sources, and this paper aims to provide information helpful in the construction of such a source. The multipactor phenomenon itself is studied within a high impedance waveguide section—achieved with a tapered impedance transformer—placed in a WR284 traveling wave ring resonator, which increases the effective power up to a factor of 20, or ∼40 kW.

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A 2 kW S-band RF source for multipactor research utilizing GaN HEMTs

Esser

Shaw

Dickens

et al. 2020

AIP Advances

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Coupled analysis to probe the effect of angular assignments on the secondary electron yield (SEY) from copper electrodes

et al. 2020

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Secondary electron emission from copper is probed utilizing Monte Carlo simulations that take account of elastic scattering based on the Mott theory and inelastic collisions based on energy-dependent energy loss functions. The loss function and stopping power were obtained through first-principles density functional theory. Angular assignment of electrons following elastic scattering or the creation of secondaries is shown to affect the energy-dependent secondary electron yield (SEY). A good match of the simulation results (with a peak SEY of ∼180% at around 300 eV and less than 10% deviation over the 0 to 1000 eV energy range) to available experimental data is shown based on an energy and momentum conservation scheme. Also, the distribution of delay times for the generation of secondaries, the SEY behavior at different incident angles, the energy distribution of emergent secondaries, and their creation profiles as a function of depth are computed to provide a more complete picture of the governing mechanisms and predicted responses.

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Analysis of multipactor in a rectangular waveguide using Spark3D software

et al. 2020

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Multipactor is a resonant nonlinear electron multiplication effect that may occur in high power microwave devices at very low pressures, such as those operating in particle accelerators and satellite subsystems. In this research, multipactor of a rectangular waveguide was analyzed using the commercially available, numerical simulation software “Spark3D.” The electromagnetic wave in the simulation was a TE10 mode-2.85 GHz wave of varying power, fed into the impedance transformer waveguide. At the lowest threshold, multipactor is generated in the minimum height region in the impedance transformer and nowhere else. More precisely, the multipacting electrons have a continuous energy distribution since the emitted secondary electrons carry a random initial velocity distribution. We observed that there are cases where the impact electron energy decreases despite an increase in power due to growing non-resonance of the microwave field and electron oscillations, resulting in not only two threshold points where secondary emission yield (SEY) = 1 but several more. As a consequence, it was uncovered that when the average SEY in the highest field region is close to or less than one, multipactor may be caused in a lower field region where the SEY is effectively higher than one. The numerical results are compared with data from the experiment. While there is some deviation between the thresholds obtained from Spark3D and the experiment, the results at higher power levels are consistent with the experiment in the view of the SEY for each power level.

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Direct observation of electrons in microwave vacuum components

Cited by 11 publications

References 34 publications

A 2 kW S-band RF source for multipactor research utilizing GaN HEMTs

A 2 kW S-band RF source for multipactor research utilizing GaN HEMTs

Coupled analysis to probe the effect of angular assignments on the secondary electron yield (SEY) from copper electrodes

Analysis of multipactor in a rectangular waveguide using Spark3D software

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