Overmoded slow wave structures (SWSs) with large diameter are utilized in O-type Cerenkov high-power microwave (HPM) sources for their high power capacity. However, multi-modes may be output simultaneously in the overmoded O-type Cerenkov HPM sources. In order to achieve high mode purity, the mode composition of the output power should be analyzed quantitatively when the structure of this type of device is being optimized. Two accurate numerical methods of making quantitative analysis of the mode composition in particle-in-cell model are introduced in this paper. And then, the mode composition of an X-band O-type Cerenkov oscillator with overmoded SWSs (D/λ ≈ 2.7) is analyzed. The analysis indicates that appropriate selection of the parameters of overmoded SWSs and electron beam is important, as mentioned in previous reports, for realizing mode selection in beam-wave interaction. Besides, designing of the mode conversion effect, which is rarely discussed, can also affect the mode purity of output power. A...

Analysis of the mode composition of an X-band overmoded O-type Cerenkov high-power microwave oscillator

By combining the Cerenkov-type generator with the cascaded resonators, this paper proposes a Ka-band relativistic backward wave oscillator operating under the guide magnetic field 1.0 T with high power handling capability and high conversion efficiency. It is found that TM02 can be selected as the operation mode in order to increase the power handling capability and provide sufficient coupling with the electron beam. In slow wave structure (SWS), ripples composed of semicircle on top of the rectangle enhance the wave-beam interaction and decrease the intensity of the electric field on the metallic surface. Taking advantage of the resonator cascades, the output power and the conversion efficiency are promoted greatly. The front cascaded resonators efficiently prevent the power generated in SWS from leaking into the diode region, and quicken the startup of the oscillation due to the premodulation of the beam. However, the post cascade slightly postpones the startup because of the further energy extraction from the electron beam. The numerical simulation shows that generation with power 514 MW and efficiency 41% is obtained under the diode voltage 520 kV and current 2.4 kA. And the microwave with the pure frequency spectrum of 29.35 GHz radiates in the pure TM01 mode.

A Ka-band TM02 mode relativistic backward wave oscillator with cascaded resonators

We report an approach to efficiently generate terahertz radiation from a combined periodic structure. The proposed configuration is composed of two metallic slit arrays deliberately designed with different periodic length, slit width and depth. We found that the combination of the two slit arrays could provide special electromagnetic modes, which exhibit nonradiative property above the surface of one slit array and radiative property inside the other one. An electron beam holding proper energy could resonate with those modes to generate strong and directional electromagnetic radiations in the terahertz regime, indicating that the approach has the potential in developing high-performance terahertz radiation sources.

/pdf/terahertz-radiation-from-combined-metallic-slit-arrays-u0svec444t.pdf

Terahertz Radiation from Combined Metallic Slit Arrays.

An oversized transit radiation oscillator is designed to generate high power microwave at X-band. By using a coaxial structure, the power capacity of the device is improved significantly. In order to realize long-pulse operation, the cathode-anode gap, the collector location, and the surface field strength have been emphatically considered in our design. With a 710 kV, 14.5 kA beam guided by a 0.8 T magnetic field, a 2.7 GW microwave at 9.38 GHz has been obtained in the simulation. The power conversion efficiency is 26.2%. The simulation also indicates that the highest axial electric field strength on the surface of electrodynamic structure is only 590 kV/cm, which could be further decreased by increasing the radial dimension of the X-band device.

An oversized X-band transit radiation oscillator

We report on a radial-line relativistic klystron oscillator (RL-RKO), which is physically designed to generate gigawatt-level high power microwaves (HPMs) at Ku-band. The 3π/4 mode of a four-gap buncher is selected to highly modulate the radially propagating intense relativistic electron beam (IREB). A three-gap extractor operating at the π mode is employed to extract the radio-frequency energy efficiently. The Ku-band RL-RKO is investigated experimentally on an intense-current electron beam accelerator. The radially propagating IREB is well focused with an axial-width of 2 mm by a radial magnetic field of 0.4 T. Microwaves with a frequency of 14.86 GHz and a power of 1.5 GW are generated, corresponding to an efficiency of 24%, which indicates a significant advance for the research of radial-line HPM sources.

Experimental demonstration of a Ku-band radial-line relativistic klystron oscillator based on transition radiation

Computer-aided highly efficient electromagnetic particle-in-cell (PIC) (CHIPIC) code, developed at the University of Electronic Science and Technology of China, is a fully computer-aided code which combines modeling, calculations, and analysis with an integrated environment. CHIPIC is specifically designed for efficient modeling and simulation. CHIPIC includes a computer-aided design (CAD) system and a physical kernel. The CAD system is designed based on the common flowchart of PIC simulation to provide efficient modeling and analysis, and the physical kernel is developed to provide fast and accurate calculations. The physical kernel can run on a single processor or in parallel mode. When it runs in parallel mode, the message-pass interface and open specifications for multiprocessing are adopted. The validity of this electromagnetic PIC code is proved by simulating a magnetically insulated transmission-line-oscillator tube.

CHIPIC: An Efficient Code for Electromagnetic PIC Modeling and Simulation

Particle-in-cell (PIC) simulation is the most popular computational modeling of the interaction between charged particles and electromagnetic fields. Lots of PIC codes are written for simulating a wide range of challenging problems, including plasma physics, high power microwave devices, high power lasers, particle accelerators, and so forth. In this research, we design a highly efficient electromagnetic PIC code, called CHIPIC, which is applicable to almost all these problems. This code includes a computer aided design (CAD) system and a physical kernel. Considering the various complex physical models and numerical conditions in PIC simulation, the whole CAD system is divided into six functional modules and three system layers with object-oriented method, of which the six functional modules are basic setting, algorithmic setting, structural modeling, diagnostic setting, simulation control and data processing, and the system layers are user interface layer, data managing layer and computing interface layer. The physical kernel contains a mesh generator, two field solvers, a particle handler, an object manager and a data manager. It provides various algorithms and models to represent structural geometries, material properties, incoming and outgoing waves, particle emission processes and boundary conditions. In addition, powerful filter for numerical noise are also provided. This physical kernel can run on a single processor or in parallel mode. When it runs in parallel mode, the message pass interface (MPI) and open specifications for multiprocessing (OpenMP) is needed. At last, the validity of this electromagnetic PIC code is proved by simulating a magnetically insulated transmission line oscillator (MILO) tube.

CHIPIC: A highly efficient code for electromagnetic PIC modeling and simulation

Simulation of high-power microwave source devices generally uses parallel algorithms to speed up the operation. In recent years, with the upgrade of parallel technology, the parallel efficiency of the particle simulation software has been further improved. Existing MPI-2 parallel technology of particle simulation software CHIPIC realizes the access to the local memory space of other processes through message passing. The new version of the MPI-3 standard provides the shared memory feature, which allows the data to be directly called by each process in the shared memory window, which reduces the information transmission. In this paper, based on the shared memory feature of MPI-3, the electromagnetic particle simulation parallel algorithm and dynamic load balancing algorithm are designed in the particle simulation software. The implementation of the two algorithms can improve the parallel efficiency from different aspects. The RKA and magnetic isolation oscillator high-power microwave devices are used as the test models. The test results show that the electromagnetic particle simulation parallel algorithm based on the shared memory feature of MPI-3 can improve the efficiency of the software by up to 44%. The efficiency of the dynamic load balancing algorithm based on MPI-3 can also be improved by up to 38%.

https://aip.scitation.org/doi/pdf/10.1063/5.0094850

Research on parallel algorithm of high-power microwave devices simulation based on MPI-3

A theoretical model in terms of RF and electric field is proposed for the attenuation and reflection of TEM and TE microwaves through a dielectric with multipactor. The reflected power is much lower than the attenuated power. Also, both ratios of reflected and attenuated powers increase dramatically when the RF decreases to be near the cutoff frequency for TE microwaves. The attenuation ratio can be larger than 10% and smaller than 0.1%, while previously, it is usually considered as a small constant. The phenomena of power attenuation and reflection are also observed directly by electromagnetic particle-in-cell simulations, and simulation results are consistent with theoretical results.

Attenuation and Reflection of TEM and TE Microwaves Through a Dielectric With Multipactor

This article shows that multipactor on a dielectric window exposed to circularly polarized microwave can be suppressed by a proper normal gyromagnetic field, while a normal gyromagnetic field is traditionally considered to strengthen multipactor in a linearly polarized wave. The dc electric field and the deposited power of multipactor in circularly polarized microwave with normal gyromagnetic field are predicted theoretically and validated by particle-in-cell simulations. The results show that the multipactor threshold <inline-formula> <tex-math notation="LaTeX">${E}_{{0}}$ </tex-math></inline-formula> is about <inline-formula> <tex-math notation="LaTeX">$\pi \vert \omega +\omega _{c} \vert ({m}\varepsilon _{\text {low}}/{(}{8}{e}^{{2}}{)})^{1/2} $ </tex-math></inline-formula> for left-handed circularly polarized waves or <inline-formula> <tex-math notation="LaTeX">$\pi \vert \omega -\omega _{c} \vert ({m}\varepsilon _{\text {low}}/{(}{8}{e}^{{2}}{)})^{1/2} $ </tex-math></inline-formula> for right-handed circularly polarized waves, where <inline-formula> <tex-math notation="LaTeX">${E}_{{0}}$ </tex-math></inline-formula> is the amplitude of RF electric field, <inline-formula> <tex-math notation="LaTeX">$\omega $ </tex-math></inline-formula> is the angular frequency of RF electric field, <inline-formula> <tex-math notation="LaTeX">$\omega _{c}$ </tex-math></inline-formula> is the cyclotron angular frequency due to the normal magnetic field <inline-formula> <tex-math notation="LaTeX">${B}$ </tex-math></inline-formula> (<inline-formula> <tex-math notation="LaTeX">$\omega _{c} > 0$ </tex-math></inline-formula> if <inline-formula> <tex-math notation="LaTeX">${B}$ </tex-math></inline-formula> is pointing to the surface, while <inline-formula> <tex-math notation="LaTeX">$\omega _{c} < 0$ </tex-math></inline-formula> if <inline-formula> <tex-math notation="LaTeX">${B}$ </tex-math></inline-formula> is away from the surface), <inline-formula> <tex-math notation="LaTeX">${m}$ </tex-math></inline-formula> is the electron mass, <inline-formula> <tex-math notation="LaTeX">${e}$ </tex-math></inline-formula> is the element charge, and <inline-formula> <tex-math notation="LaTeX">$\varepsilon _{\text {low}}$ </tex-math></inline-formula> is the first crossover energy in the unit of secondary electron yield.

Dagang Liu

Papers

CHIPIC: An Efficient Code for Electromagnetic PIC Modeling and Simulation

CHIPIC: A highly efficient code for electromagnetic PIC modeling and simulation

Research on parallel algorithm of high-power microwave devices simulation based on MPI-3

Attenuation and Reflection of TEM and TE Microwaves Through a Dielectric With Multipactor

Suppression of Circularly Polarized Microwave Dielectric Multipactor by Normal Gyromagnetic Field