Ahmed Elfrgani

Similarity of Properties of Metamaterial Slow-Wave Structures and Metallic Periodic Structures

A study of the evolution of wave dispersion in systems of all-metallic periodic structures with increasing corrugation depth shows a similarity of the properties of waves in metamaterial slow-wave structures (MSWSs) and traditional metallic SWSs used in high-power microwave sources. We show that the main properties of MSWSs, such as the existence of a lowest order negative dispersion wave below cutoff, also appear in ordinary metallic periodic systems with deep corrugations. Furthermore, we find that the appearance of negative dispersion in all-metallic periodic structures with increasing corrugation depth is accompanied by a hybrid mode being identified as the lowest order negative dispersion mode.

Power combiner for high power Cherenkov devices

The output power of two Cherenkov devices is combined using a circular waveguide T-junction. The microwave sources are driven by the same high voltage pulser to prevent phase difference in the outputs. This passive power combiner not only combines the power, but also outputs a Gaussian beam when a TM01-mode is used as the input. An efficient serpentine mode converter has been used as the basis of the power combiner design. A conversion efficiency of 85% with a Gaussian pattern is achieved in simulations and is described in this paper.

Single/dual band relativistic BWO using electron beam magnetic field decompression

For a relativistic backward wave oscillator (BWO) consisting of two cylindrical sections with sinusoidal profiles where one of them is axially symmetric and the other one has a combined left- and right-single-fold spiral corrugation, we demonstrate the possibility of generating the TE11-mode with linear polarization at a single or dual frequencies using magnetic field decompression. Magnetic field decompression, as shown in Fig. 1, can be used to control the radial gap between the electron beam and the slow wave structure and that leads to controlling the microwave frequency at the output. In addition, using a cutoff neck and resonant cavity reflector improves the efficiency of the device. Using particle-in-cell computer simulations we show that generation with frequency 10 GHz is realized when the symmetric section operates as a BWO with the operating TM01-mode and the combined spiral section operates as a Bragg reflector that converts the backward TM01-mode to a forward TE11-mode with linear polarization. Using the same electron beam parameters, generation with frequency 7 GHz is realized when the combined spiral section operates as a BWO with the operating linear polarized TE11-mode followed by amplification in the next section which behaves as a TWT at this frequency. We also present the possibility to change the relation between the durations of generation at different frequencies within a microwave pulse by changing the electron beam current, and to change output frequencies by changing the value of the guide magnetic field near the region of cyclotron absorption.

Relativistic BWO With Linearly Polarized Gaussian Radiation Pattern

An X-band relativistic backward wave oscillator (BWO) is proposed to provide high-power Gaussian radiation that is attractive for many applications. A Bragg reflector with combined single-fold left- and right-spiral corrugations of its wall was applied to the cathode end of the interaction space. Such a reflector converts the operating backward TM01 -mode of the BWO to the forward TE11-mode with linear polarization that is extracted as a Gaussian microwave beam. Frequency tuning over 6% with radiation power 100-350 MW is achieved by varying the magnitude of the guide magnetic field. Computer simulations were used to analyze BWO operation.

Power Combiner for High Power Cerenkov Devices

Two Cherenkov devices are effectively combined using a circular waveguide T-junction in a simulation study. The microwave sources are driven by the same high voltage pulser to prevent phase difference at the output of the power combiner. This passive power combiner not only combines the microwave power, but also outputs a Gaussian beam when a TM01 mode is used as the input. An efficient serpentine mode converter has been used as the basis of the power combiner design where the fields sum up at the common section of the two mode converters. A combined summation/conversion efficiency was ~90% when the output power was more than 1.8 times the input power with a Gaussian output pattern. The combined microwave power was more than 600 MW at 10.2 GHz.

Dual-Band Operation of Relativistic BWO With Linearly Polarized Gaussian Output

We demonstrate the dual-band operation of a relativistic backward wave oscillator (RBWO) whose output is a linearly polarized TE11-mode. The RBWO consists of two cylindrical sections with sinusoidal corrugations where the downstream section is axisymmetric and the upstream section has combined left- and right-single-fold spiral corrugations. We show that generation of 10 GHz is realized when the axisymmetric section operates as a BWO with the operating TM01-mode and the combined spiral section operates as a Bragg reflector that converts the backward TM01-mode to a linearly polarized forward TE11-mode. For the same electron beam parameters, generation of 7 GHz is realized when the combined spiral section operates as a BWO with the operating TE11-mode. We also present the possibility to change the duration of generation at each frequency within a microwave pulse by changing the electron beam current. In addition, one can change the output frequency by changing the values of the guide magnetic field near the region of cyclotron absorption.

Relativistic BWO with Gaussian beam extracted radially using an electromagnetic bandgap medium

Summary form only given. Cerenkov high power microwave sources can be used to generate short, Gaussian microwave pulses that are attractive for many applications. The backward wave oscillator (BWO) is typically known to radiate the TM0n-mode in circular waveguide. The microwave signal is usually extracted axially after reflecting the TM01-mode from the upstream end of the SWS by using either a cutoff-neck or a cavity resonator reflector. In order to produce a Gaussian beam from BWOs, a mode converter is usually used. Even though an asymmetric mode can be generated in these devices, the cutoff-neck reflector is the only means to effectively reflect the wave for radiation. The disadvantage of the cutoff-neck reflector is the proximity to the electron beam and that leads to beam scrape-off and alignment difficulty. In this work, the idea of combining a periodic slow wave structure (SWS) with an electromagnetic band gap (EBG) structure for generating, coupling and extracting a Gaussian radiation pattern is presented. Since SWSs with period ~ λ/2 have been used for a long time to generate high power due to their mechanical strength and resistance to breakdown, it is best to use them for high power microwave generation with added improvements. The EBG structure follows a slot antenna concept where the slot array is cut at the SWS wall to perturb the surface currents. The slots are basically used for coupling between the interaction region and a concentric circular waveguide. After that the generated mode will be radiated axially by using a horn antenna. The EBG structure makes the BWOs more compact and allows more power to be extracted, not only for HE11-modes but also for TM0n-modes. The fully electromagnetic, relativistic particle-in-cell (PIC) code MAGIC and the fully electromagnetic software tool ANSYS-HFSS were used in this study.

X-band tunable relativistic BWO with linearly polarized Gaussian radiation

To provide high power Gaussian radiation that is attractive for many applications an X-band 460 kV relativistic BWO is proposed in which a Bragg reflector with one-fold left-and right-spiral corrugations of its wall was applied in waveguide on the “cathode” end of the interaction space. Such a reflector converts the operating TM01-mode of the BWO to the forward TE11-mode with linear polarization that is extracted as a Gaussian microwave beam. Frequency tuning over 6% with radiation power ranging from 100 - 350 MW is achieved (in computer simulations) by varying the guide magnetic field.

X-band relativistic backward wave oscillator with two-spiral corrugated Bragg reflector

Backward wave oscillators are typically known to radiate in the TM01 mode. A two-spiral corrugated Bragg reflector has been used downstream of the cathode in an X-band relativistic backward wave oscillator (RBWO) to radiate a TEn mode at the output. Simple analytical formulas were used to design the basic parameters of the Bragg reflector, which were later optimized numerically since there is no exact closed form solution for the electromagnetic fields within a periodic waveguide structure or cavity. The fully electromagnetic, fully relativistic particle-in-cell (PIC) code MAGIC was used to simulate the problem. The RBWO was driven by a voltage pulse that has a half sine wave-like shape, 460 kV amplitude, and FWHM duration of 12 ns. A uniform static magnetic field of 1.9 T was applied throughout the simulation volume. With these parameters a microwave power of 330 MW at a frequency of 9.9 GHz in a clean TEn mode pattern was detected at the output.

2D-3D PIC Code Benchmarking/Anchoring Comparisons For a Novel RFQ/RFI LINAC Design

In this study, comparisons are made between several particle dynamics codes (namely CST Particle Studio [CST PS], General Particle Tracker [GPT] and PARMULT) for a specific accelerator system. The structure used for simulations is a novel 200 MHz, 2.5 Mev, CW RFQ/RFI LINAC designed by Ion Linac Systems (ILS) [1]. The structure models and parameters are provided, simulation techniques are explained, and results from all three code families are presented. These results are then compared with each other, identifying similarities and differences. Various parameters for comparison are used, including the transmission efficiency, Q-factor, E-field on-axis, and beam properties. Preliminary anchoring between modeling and simulation performance predictions and experimental measurements are also shown.