Multipactor is studied based on the coupled Vlasov-Poisson equation set and applied to a parallel plate geometry. This approach can be considered complementary to the particle-in-cell (PIC) methods that have provided excellent insight into multipactor behavior. However, PIC methods have limitations in terms of 'particle noise,' which can affect electron energy distribution functions and create re-scaling issues under conditions of charge growth. Utilizing our continuum Vlasov-Poisson approach yields susceptibility curves that are in line with reports in the literature, Spark3D PIC simulations, and experimental data. Playing to the strength of the Vlasov-Poisson approach, the differences between various multipactor orders are elucidated as they are observed in phase-space, revealing electron density dynamics without requiring increased computational resources due to electron growth. The method presented is general and can be extended to multi-input excitations and higher-dimensional phase-space.

Radio frequency (RF) breakdown in air at a frequency of particular relevance to ionospheric heating—3.3 MHz, close to the low end of the applicable frequency range—is studied at centimeter-sized gap distances and compared to the literature for small gaps. To establish a reference point, Paschen's early DC breakdown study utilizing two brass spheres of 1 cm radius was replicated following the original procedure and subsequently extended to examine RF breakdown. Various electrode combinations were tested with brass cathodes creating the highest variance in the datasets among DC tests. The greatest variation in RF breakdown arose when either electrode was brass. Gap distances of 1–10 mm were tested for both DC and RF with the slow-rise time (5 mV/μs) RF breakdown occurring at approximately 80% of the DC breakdown value, a value corroborated by Monte Carlo breakdown simulations. Pushing the envelope rise time of the applied RF voltage into the microsecond regime yielded an RF voltage of roughly 20% above the DC breakdown value accompanied by a distinct increase in breakdown amplitude fluctuations. Illuminating the gap electrodes with deep ultraviolet (280 nm and below) minimized the breakdown amplitude fluctuations due to photoemission at the electrodes as expected. Finally, to address the conditions found in real-world geometries with sharp corners or protrusions, RF corona behavior utilizing tungsten needles above a ground plane is measured. The obtained results help us define the operation limits of high-power antennas at 1–10 MHz frequencies.

Multipactor in WR-284-like geometries is measured utilizing local and global detection techniques. To emulate conditions one may find in a waveguide filter structure while maintaining the fundamental microwave mode, a standard rectangular waveguide geometry with the reduced waveguide height set to 2.1 or 5.5 mm was adopted. Two high power RF sources were used to investigate a large range of input power (few kWs to MWs): a solid state source using GaN HEMTs allowing for larger pulse widths than standard magnetrons (100 μs as opposed to ∼4 μs) and a MW level S-band coaxial magnetron for the high power end. Particular interest was taken in capturing the lower and upper limits of multipactor threshold. Lower multipactor thresholds for finite pulse duration are governed by the appearance of one or more electrons in the multipactor gap during the applied pulse as well as a minimum power (electric field) level that affects a secondary electron emission yield above unity. As shown, such initial electrons(s) may easily be seeded via an external UV source illuminating the gap. However, wall collisions of excited metastable molecules may be another source of electrons, an observation based on the experiment and prior research. A multipactor upper threshold was non-existent in the experiment, even at powers over 200 kW within a 2.1 mm test gap, which numerically yielded a gap transit time significantly shorter than one half-period of the GHz wave. This is attributed to the electric field distribution within the waveguide structure, which results in the multipactor's spatial position moving to more favorable locations within the test gap.

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