Conference Agenda

Session
Multipactor
Time:
Monday, 20/June/2022:
10:00am - 11:00am

Session Chair: Nicholas M Jordan, University of Michigan
Location: Ballroom C

Oral Session

Presentations
10:00am - 10:20am
Multipactor: 1

High Power Multipactor Suppression in S-band Waveguide

D. Wright, A. Gregory, H. Spencer, J. Mankowski, J. Stephens, J. Dickens, A. Neuber

Texas Tech University, United States of America

To investigate multipactor (MP), a rectangular waveguide testbed was designed for S-band frequencies with the broad wall dimension matching the standard WR-284 waveguide geometry. Setting the waveguide height to 5.5 mm yielded a frequency-gap product susceptible to MP. One of the test sources, a coaxial magnetron provides test input power at a frequency of 2.85 GHz with a peak power output of 4 MW and 3.5 µs pulse width. The other, a RF solid-state source using GaN HEMTs delivers a pulse width of 100 µs with a test input power of 3 kW for comparison of threshold power. With the two sources a range of 3 kW to 4 MW of input power was accessed.

For MP detection, local (electron multiplier tube) and global (phase/power) diagnostic methods were implemented. At power levels tested (MW) and a 5.5 mm gap, low multipactor orders (N = 1) are observed, whereas an order of N = 9 are observed at the lower power level. To suppress MP, previous numerical simulations of the geometry have shown that adding a grooved structure to one of the broadsides should aid in mitigating multipactor. In this case, grooves are machined into the broad wall in the direction of propagation, which avoids continuous impedance mismatching and large E-field perturbations. The efficacy of this mitigation technique was experimentally evaluated. Experimentally, there were distinct differences between the standard case (smooth broadside wall) and the case with grooves in MP delay and magnitude. The difference in power transmitted before MP onset was limited, however, an about 12% improvement was measured.

This research was supported by the Air Force Office of Scientific Research under contracts FA9550-18-1-0062 and FA9550-21-1-0367



10:20am - 10:40am
Multipactor: 2

Monte Carlo Analysis of Electron Trapping in Nested Cave Structures for Mitigation of Secondary Electron Emission for Potential Multipactor Control in Waveguide

M. Brown, W. Milestone, R. Joshi

Texas Tech University, United States of America

The multipactor phenomenon is a resonant vacuum phenomenon frequently observed in microwave systems, accelerator structures, and high power radio-frequency (RF) satellite components. Electrons produced under multipactor resonance conditions can lead to rapid charge growth that can potentially decrease power throughout, damage the structure, cause internal heating, and lead to unexpected breakdown events. Thus, mitigation of multipactor, and hence ultimately, the secondary electron yield (SEY), remains an important objective in the pulsed power context. Geometry modifications of rectangular waveguide surfaces and/or the application of an axial magnetic field have been investigated for suppressing multipactor growth both through simulations and in experiments. While the results appear promising at lower fields, the task becomes challenging with increase in the microwave power and field intensity. Besides, the problem of finding optimal combinations of suppression for a given parameter space consisting of the electric field, geometric length scales, and operating frequency remains.

Here as part of the surface geometry modification strategy for rectangular waveguides, the fabrication of nested holes to trap the emitted electrons in examined for suppressing the overall secondary electron yield. A simulation scheme based on the kinetic Monte Carlo (MC) method is employed to probe the electron swarm dynamic. Our work also examines possible SEY suppression and quantifies the time dependence of electron population growth on the operating frequency, electric fields intensity, geometric size of the nested holes, and their relative density. The energy-dependent SEY curves required for the MC simulations are taken from the literature for copper material.

The results obtained will be presented and discussed. In particular, the percentage suppression effect and possible mitigation of electron swarm growth will be analyzed for structures with two-sized nested surface holes.

*This Research was supported by the Air Force Office of Scientific Research under Contract No. FA9550-19-1-0056.



10:40am - 11:00am
Multipactor: 3

Probing Multipactor in X-band Waveguide Components

A. Gregory, D. Wright, H. Spencer, J. Mankowski, J. Stephens, J. Dickens, A. Neuber

Texas Tech University, United States of America

Multipactor is a vacuum-based resonant effect that causes detuning, heating, and ultimately component damage in microwave systems. Suppression of this effect then becomes important in high power systems such as satellite communications. To study multipactor, a plug and play setup was designed and built in to allow for quick testing in a waveguide-like structure. A tunable X-band magnetron, tuned to 9.4 GHz, with a typical pulse length of 2.5 μs delivers peak power output of 130 kW. The magnetron driver pulse duration and amplitude are freely adjustable, utilizing a modern hard-switched semiconductor-based topology. This project's primary device under test is a waveguide stepped impedance transformer that reduces the side-wall dimension of a typical WR90 waveguide down to a gap size conducive to multipactor formation.

This research evaluates the efficacy of varying methods of multipactor suppression as well as conditioning of surfaces through repeated multipactor. In this context, a residual gas analyzer is added as a diagnostic tool to check the species of gas desorbed from different surfaces. Phase detection is used as a diagnostic to determine when a multipactor event has occurred, alongside an electron multiplier tube (EMT) that allows analysis of the multipacting electron cloud.

This research was supported by the Air Force Office of Scientific Research under contracts FA9550-18-1-0062 and FA9550-21-1-0367