Chao Chang

Review of recent theories and experiments for improving high-power microwave window breakdown thresholdsa)

Dielectric window breakdown is a serious challenge in high-power microwave (HPM) transmission and radiation. Breakdown at the vacuum/dielectric interface is triggered by multipactor and finally realized by plasma avalanche in the ambient desorbed or evaporated gas layer above the dielectric. Methods of improving breakdown thresholds are key challenges in HPM systems. First, the main theoretical and experimental progress is reviewed. Next, the mechanisms of multipactor suppression for periodic rectangular and triangular surface profiles by dynamic analysis and particle-in-cell simulations are surveyed. Improved HPM breakdown thresholds are demonstrated by proof-of-principle and multigigawatt experiments. The current theories and experiments of using dc magnetic field to resonantly accelerate electrons to suppress multipactor are also synthesized. These methods of periodic profiles and magnetic field may solve the key issues of HPM vacuum dielectric breakdown.

The effect of grooved surface on dielectric multipactor

The effect of periodic rectangular grooves on vacuum multipactor has been theoretically and experimentally investigated. Dynamic calculation is applied to research the electron trajectory and impact energy under groove surface. Two-dimensional electromagnetic particle-in-cell simulation is used to analyze and compare multipactor scenario, statistic energy, and secondary emission yield on the flat surface with that on the corrugated surface. It has been found by computational and simulative analysis that grooved surface can explicitly suppress multipactor in the developmental stage of multipactor. S-band high power microwave (HPM) dielectric breakdown experiment under vacuum, with microsecond pulse length was conducted. It was confirmed by experiment that periodic grooves perpendicular to the major electric field can effectively increase transmitted power.

Suppressing high-power microwave dielectric multipactor by the sawtooth surface

This paper theoretically and experimentally researches the effect of a periodic sawtooth surface on vacuum multipactor. Dynamic calculation and particle-in-cell simulation are applied to analyze the electron impact energy and multipactor scenario on the periodic isosceles triangular surface with different slope angles and heights. It has been discovered that, with the slope angle increasing, the impact energy significantly decreases to be lower than the first crossover energy of the secondary yield curve, leading to suppressed multipactor. S-band high power microwave dielectric breakdown experiment, with microsecond pulse length, was conducted. It is confirmed by experiment that the periodic sawtooth surface with sufficiently large slope angle (such as 45°) can effectively increase the breakdown threshold of about 2 compared to the flat surface.

Enhanced window breakdown dynamics in a nanosecond microwave tail pulse

The mechanisms of nanosecond microwave-driven discharges near a dielectric/vacuum interface were studied by measuring the time- and space-dependent optical emissions and pulse waveforms. The experimental observations indicate multipactor and plasma developing in a thin layer of several millimeters above interface. The emission brightness increases significantly after main pulse, but emission region widens little. The mechanisms are studied by analysis and simulation, revealing intense ionization concentrated in a desorbed high-pressure layer, leading to a bright light layer above surface; the lower-voltage tail after main pulse contributes to heat electron energy tails closer to excitation cross section peaks, resulting in brighter emission.

Suppression of high-power microwave dielectric multipactor by resonant magnetic field

Through dynamic calculation and electromagnetic particle-in-cell simulation, high-power microwave dielectric multipactor is discovered to be suppressed by utilizing external dc magnetic field parallel to the surface, perpendicular to the rf field and satisfying the gyrofrequency close to the rf frequency Ω∼ω. It is found that multipactor electrons emitted from the surface can be resonantly accelerated to obtain the impact energy ee higher than the second crossover energy, leading to secondary emission yield lower than one. Besides, the corresponding flight time gets close to the rf period, also the period of the vector Erf×B, resulting in secondary electrons immediately pulled away without multipactoring along the surface. What is more, with the rf field increasing, suppression effect can be further enhanced due to ee rising.

Experimental verification of improving high-power microwave window breakdown thresholds by resonant magnetic field

Recently, high-power microwave (HPM) dielectric multipactor is theoretically discovered to be suppressed by utilizing external resonant magnetic field. This paper gives the related experimental demonstration of increasing the vacuum window breakdown thresholds. In the S-band HPM experiment with 0.5 μs width, the magnetic field with gyrofrequency Ω close to rf frequency ω can triple the breakdown threshold. Besides, reducing or enhancing magnetic field resulted in a relatively lower threshold in comparison of Ω∼ω, agreeing with theoretical analysis. By HPM pulse compression to 14 ns width, the threshold was also demonstrated to be significantly enhanced by magnetic field.

DESIGN AND EXPERIMENTS OF THE GW HIGH- POWER MICROWAVE FEED HORN

Design and optimization of high-power microwave (HPM) feed horn by combining the aperture fleld with radiation patterns are presented in the paper. The optimized feed horn in C band satisfles relatively uniform aperture fleld, power capacity higher than 3GW, symmetric radiation patterns, low sidelobes, and compact length. Cold tests and HPM experiments were conducted to investigate the radiation patterns and power capacity of the horn. The theoretical radiation patterns are consistent with the cold test and HPM experimental results. The power capacity of the compact HPM horn has been demonstrated by HPM experiments to be higher than 3GW.

Theory and Experiment of a Compact Waveguide Dual Circular Polarizer

A new compact and wide-band waveguide dual circular polarizer at Ka-band is presented and tested in this paper. This compact structure is composed of a three-port polarizing diplexer and a circular polarizer realized by a simple pair of large grooves. The polarizing diplexer includes two rectangular waveguides with a perpendicular H-plane junction, one circular waveguide coupled in E- plane. A cylindrical step and two pins are used to match this structure. For a LHCP or RHCP wave in the circular port, only one speciflc rectangular port outputs power and the other one is isolated. The accurate analysis and design of the circular polarizer are conducted by using full-wave electromagnetic simulation tools. The optimized dual circular polarizer has the advantage of compact size with a volume smaller than 1.5‚ 3 , broad bandwidth, uncomplicated structure, and is especially suitable for use at high frequencies such as Ka-band and above. The prototype of the polarizer has been manufactured and test, the experimental results are basically consistent with the theories.

Single and repetitive short-pulse high-power microwave window breakdown

The mechanisms of high-power microwave breakdown for single and repetitive short pulses are analyzed. By calculation, multipactor saturation with electron density much higher than the critical plasma density is found not to result in microwave cutoff. It is local high pressure about Torr class that rapid plasma avalanche and final breakdown are realized in a 10–20 ns short pulse. It is found by calculation that the power deposited by saturated multipactor and the rf loss of protrusions are sufficient to induce vaporizing surface material and enhancing the ambient pressure in a single short pulse. For repetitive pulses, the accumulation of heat and plasma may respectively carbonize the surface material and lower the repetitive breakdown threshold.

Novel Compact Waveguide Dual Circular Polarizer

A novel type of dual circular polarizer for simultaneously receiving and transmitting right-hand and left-hand circularly polarized waves is developed and tested. It consists of a H-plane T junction of rectangular waveguide, one circular waveguide as an E- plane arm located on top of the junction, and two metallic pins used for matching. The theoretical analysis and design of the three-physical- port and four-mode polarizer were researched by solving Scattering- Matrix of the network and using a full-wave electromagnetic simulation tool. The optimized polarizer has the advantages of a very compact size with a volume smaller than 0:6‚ 3 , low complexity and manufacturing cost. A couple of the polarizer has been manufactured and tested, and the experimental results are basically consistent with the theories.