High‐Power Microwave Sources

Physical mechanism of pulse shorten in coaxial virtual cathode oscillator (VCO) is analyzed theoretically. Anode plasma is one of the most key factors in causing pulse shorten according to the analytical results. Pulse shorten is suppressed based on controlling of generation and diffusing of anode plasma. GW level experiment has been designed to verify the above conclusion. The effects of process treatment of anode, material and grid dimension on the output characteristic of microwave have been studied. A microwave with 840MW output power and 100 ns pulse width has been obtained with 7% power efficiency and 5.6% energy efficiency through optimization of parameters of the device in experiment. Virtual Cathode Oscillator (VCO) is one of promising high power microwave (HPM) sources with its advantage of simple structure, high output power and without guiding magnetic field. Researches show that pulse shorten is very common in most VCO experiment. The physical mechanism of pulse shorten in VCO is analyzed theoretically in this article. The main physical factors that causing the pulse shorten are studied and methods to suppress pulse shorten are proposed. The proposed methods are verified by GW level VCO experiment with 100 ns pulse width.

Pulse shortening, an effect where the microwave output power from a high-power tube terminates or significantly degrades well before the end of the electron beam pulse, severely limits the energy per pulse and average power capability of many high power microwave (HPM) sources. The cause of pulse shortening varies from device to device, and different causes can simultaneously contribute to the observed power reduction behavior which tends to obscure the underlying mechanisms and possible solutions. In this paper, we show a variety of experimental situations that lead to pulse shortening in HPM sources. The mechanisms of the different pulse shortening triggers are examined in detail in high-vacuum traveling wave tubes (TWT) and plasma-filled backward-wave oscillators (BWO). We find that there are many different causes of pulse shortening such as arcing, mode competition, beam instability, etc. However, the most commonly observed situation that leads to pulse shortening is the combination of sufficiently high power electron beams and poor vacuum conditions that lead to plasma generation. The presence of plasma significantly modifies the beam coupling to the circuit, which can affect the microwave production efficiency on very short time scales. The situations lending to pulse shortening and possible solutions are presented.

Traveling wave tubes (TWTs) and backward wave oscillators (BWOs) are the only solution for high power amplification and RF generation at millimetre waves and THz frequencies. With advances in microfabrication technology, it is now possible to realise these devices with dimensions to support those frequency regimes. Promising applications of millimetre wave TWTs are as high-power amplifiers for wireless communications systems, imaging and satellite communications. BWOs would enable new plasma diagnostic in nuclear fusion capabilities and high-resolution imaging. TWTs are usually constructed using a helical waveguide as slow wave structure (SWS) by which wave energy can propagate. However, fabrication of a helix small enough to support mm-wave is not feasible. Novel SWSs are required suitable to be fabricated with the available processes. The challenge of the simulation of TWTs or BWOs utilising a new SWS is the lack of fast code and the need to rely on powerful, yet slow, 3D electromagnetic software, which are computationally intensive. Both helices and the coupled cavity TWT can be simulated by fast simulation codes using Lagrangian methods. One of the aims of this thesis is to create a fast code for simulating arbitrary three-dimensional SWS, in particular the double corrugated waveguide. The code was validated by comparison with data of a Ka-band TWT available in literature. The second aim is the design of circuits based on double corrugated waveguide SWSs for TWTs and BWO at different frequency to enable different applications. New topologies of couplers were investigated. Extensive 3-dimensional simulations were performed to define dimensions and performance at different frequency bands.

Microwave metamaterials have shown promise in numerous applications, ranging from strip lines and antennas to metamaterial-based electron beam driven devices. In general, metamaterials allow microwave designers to obtain electromagnetic characteristics not typically available in nature. High Power Microwave (HPM) sources have in the past drawn inspiration from work done in the conventional microwave source community. In this article, the use of metamaterials in an HPM application is considered by using an effective medium model to determine the coupling of an electron beam to a metamaterial structure in a geometry similar to that of a dielectric Cerenkov maser. Use of the effective medium model allows for the analysis of a wide range of parameter space, including the “mu-negative,” “epsilon-negative,” and “double negative” regimes of the metamaterial. The physics of such a system are modeled analytically and by utilizing the particle-in-cell code ICEPIC. For this geometry and effective medium representation, optimum coupling of the electron beam to the metamaterial, and thus the optimum microwave or RF production, occurs in the epsilon negative regime of the metamaterial. Given that HPM tubes have been proposed that utilize a metamaterial, this model provides a rapid method of characterizing a source geometry that can be used to quickly understand the basic physics of such an HPM device.

The most promising in the THz range is traveling-wave tubes (TWTs) and backward-wave tubes (BWTs) on a serpentine-curved (zigzag-rolled) rectangular waveguide. They are implemented in the THz range (220 GHz), although their characteristics are far from satisfactory due to the strict restriction on the tape electron beam width, that does not allow reaching the summarizing beam current optimum level. To replace the zigzag convoluted waveguide with the spiraled for the TWT and BWT on a curved rectangular waveguide is the best way to remove the ribbon beam width restriction. In the early TWT and BWT design a waveguide planar spiral was also flat in the upper and lower parts connected by vertical idle (without beam) transitions. Proposed design can be significantly improved both in relation to the electron interaction process with the waveguide field and in relation to the TWT-BWT manufacturing technology if instead of a planar waveguide spiral, a circular one is used. The article proposes the TWT designing a terahertz rectangular waveguide folded as a circular spiral. The design differs from the previously proposed TWT with a planar-spiral waveguide by the improved interaction conditions between the electron beam and the waveguide field, as well as the manufacturing technology simplification for terahertz range. Based on numerical simulation, it is shown that proposed TWT achieves Gн= 42 ÷ 48dB saturation gain in the 220 GHz range with the waveguide turn number n = 40 ÷ 50. The proposed TWT design on a rectangular waveguide folded in a circular spiral is more technologically advanced than the TWT on a planar-spiral waveguide. In the most necessary 220 GHz range the efficiency is very high and can provide the need for amplifiers and generators in this and other ranges. We also note that the TWT on a spirally folded waveguide can operate in the BWT mode and, moreover, simultaneously in the TWT and BWT modes. The latter is possible in modes close to linear one. The TWT magnetic system of the type described above can be implemented in the form of a permanent magnet with pluses on the TWT end parts. The proposed TWT characteristics can be significantly improved by optimizing the waveguide helical winding pitch. Exactly as it is achieved with using the spiral wire deceleration system. The efficiency of such optimized TWT reaches 70% efficiency.

The presence of a controlled amount of background plasma inside microwave tubes can possibly lead to improvement in their characteristics beyond what is available in evacuated devices. In particular, recent results clearly demonstrated that the presence of plasma can significantly increase the bandwidth, efficiency and power handling capabilities of nonrelativistic microwave oscillators and amplifiers and allow operation without a guiding magnetic field. In the present paper, recent scientific advances in this field-both theoretical and experimental-are reviewed with emphasis on basic processes. We review some fundamental physical issues like the formation of hybrid waves in plasma-filled slow wave structures (SWSs) and the role of these modes in improving the beam/wave coupling; the beneficial role of ac space charge in plasma-filled devices and the effect of plasma on electron beam transport (efficiency and bandwidth enhancement due to the presence of the plasma). Also reviewed are recent experimental results on plasma-loaded traveling-wave tube (TWT) amplifiers and backward-wave oscillators (BWO). Plasma-loaded microwave devices have the potential to advance the technological and scientific base of microwave tubes and have an impact on commercial and industrial applications through the development of commercially viable technologies.

: In the earlier phases of the Microwave Tube Built-In Test (MTBIT) program (1999-2001), a new advanced nondestructive testing technique was demonstrated using acoustic emission (AE) for in-situ monitoring of normal and abnormal performance of high- power radar tubes such as magnetrons, traveling wave tubes (TWTs), and klystrons. This report details the next step in transitioning the laboratory MTBIT system into a system that is compatible with condition-based maintenance systems employing the Integrated Condition Assessment System (ICAS) used by the Fleet. This report identifies the interface requirements for the sensors and ICAS, as well as accommodating limitations of the ICAS software, which currently is not configured to accommodate the sensor data rates and data sets required. Ten new electronic circuits were designed, built, and tested to interface the outputs of the current sensor and acoustic emission sensor attached to a magnetron through an OPTO22 digital and analog input/output (I/O) hardware along with its Optomux protocol, and the ICAS software (Version 4.11). The complete computerized system to monitor the in-situ performance of the high-power microwave tube was developed, tested, and reported here. The system could be extended to achieve the same goal with other microwave tubes such as TWTs and klystrons with minor modifications.

High power microwave (HPM) source is attractive in generating gigawatt (GW) class microwaves based on the beam-wave interaction. Generally, HPM source with a high beam-wave conversion efficiency has a higher impedance. To improve the single-tube output power of HPM source, reducing the impedance of the device and increasing its power capacity are necessary. In this paper, a new low-impedance HPM source is proposed and proved to be capable of generating two phase-locked high power microwaves, which makes it promising to realize a higher combined power in a single HPM device.The new low-impedance HPM device consists of a two-cavity TKA (denoting the outer sub-source in the following) and a multiwave Cerenkov generator (referring to the inner sub-source below) inserted in the TKA inner conductor. These two sub-sources are connected in parallel and share a common magnetic field. A dual-concentric annular cathode is used in this microwave source, which is capable of emitting two concentric annular electron beams and driving the internal and external sub-source simultaneously. The advantages of this device are reducing the impedance and improving the injection electric power. When a voltage pulse is applied to the diode, part of microwaves generated in the inner subsource will leak into the outer sub-source (i.e., TKA) through the A-K gap. By amplifying the leakage microwaves, the TKA will be easily locked by the inner sub-source. Considering the fact that the microwave source consists of two sub-sources, the power capacity will also be greatly improved.As a result, particle-in-cell simulation indicates that when the diode voltage is 687 kV and the axial magnetic field is 0.8 T, two microwave beams that have a nearly identical frequency of 9.72 GHz and output powers of 1.20 GW and 2.58 GW respectively, are generated. The corresponding power conversion efficiencies are 28% and 30%, respectively. The frequency difference between these two microwaves fluctuates within 3 MHz and their phase difference is not in excess of 3. When the diode voltage changes from 665 kV to 709 kV, frequency difference between the two sub-sources fluctuates within 3 MHz and their phase difference fluctuation is within 5 in one voltage burst; the phase difference changes 10 in this voltage range. The impedance of this HPM source is as low as 36 .To sum up, the new HPM source proposed in this paper has a lower impedance and higher power capacity. The phase difference between the inner sub-source and the outer sub-source is very stable and favorable for the coherent power combination, which indicates that the new HPM source promises to realize a higher output power in a single-tube device.

Excessive spurious outputs from high-power pulsed magnetrons, klystrons and traveling-wave tubes can cause intolerable interference and deleterious effects in a high-power microwave system. The harmonic output from a klystron may vary appreciably with changes in operating conditions. Harmonic outputs from tubes cannot be eliminated but their radiation can be significantly reduced by using filters. If the parasitic or spurious oscillations are very strong, adverse effects such as amplitude and phase instability of the fundamental frequency output may occur. Some of the spurious outputs may be reduced or eliminated by redesign of the tube or its modulator.

Summary form only given. High power microwave (HPM) with the peak power of 20 MW, pulse width of 14 ns and frequency of 13 GHz generated by a virtual cathode oscillator was irradiated uniformly into a gap with a DC voltage less than the gap-breakdown voltage. The gap consists of two spherical stainless electrodes of 4.1 cm in a diameter. The gap distance is 15 mm. The air pressure was varied from 50 to 760 Torr. Gas discharge luminescence was observed with a high-speed image converter camera. The time history of the applied voltage, the discharge current and the microwave was observed with a digital oscilloscope.

One means of enhancing high power microwave (HPM) tube performance is to improve the cathode technology. Of particular interest are low temperature cathodes that provide high current density electron beams without the problem of plasma closure. Our research focuses on the application of ferroelectric materials as an electron source. The mechanism of ferroelectric emission is initiated by rapidly altering or reversing the polarization of the material with a pulsed electric field. This excitation produces a charge imbalance at the surface of the material. The charge imbalance then accelerates screening charges into the HPM diode. Experiments are conducted using a demountable high vacuum triode with bulk (300 /spl mu/m-1 cm thickness) polar dielectrics. The material compositions investigated include 9/65/36 (Pb,La)(Zr,Ti)O/sub 3/ (PLZT) soft ferroelectrics, 2/95/5 PLZT antiferroelectrics, and hard Pb(Zr,Ti)O/sub 3/ materials. The samples under test are gridded with an evaporated metal film that is patterned with circular apertures. This grid is excited by a solid state driver that erects a field of 15 kV/cm in about 1 /spl mu/s. A DC bias voltage is used to reset the polarization of the material after each shot. Preliminary results indicate that this reset voltage greatly enhances the reliability of the electron emission. A DC voltage (up to 20 kV) applied to the anode accelerates any carriers liberated from the material. The materials have been operated as a repetitively pulsed electron source. Hysteresis measurements are also conducted in situ to determine the dielectric properties of the candidate materials.

Summary form only given. High-power microwaves with the peak power of 20 MW, frequency of 12 GHz and pulse duration of 14 ns, which are produced by a virtual cathode oscillation, are sent to a discharge electrode in a stainless steel vacuum vessel. A DC voltage less than a gap-breakdown voltage is applied to the discharge electrode, consisting of a pair of needle and plane electrodes. The gap separation is 1.7 cm. Time and spatial evolutions of breakdown luminosity are observed by a high-speed image converter camera. Microwave induced breakdowns occur at the pressure range less than 760 Torr for negative polarity and less than 600 Torr for positive polarity. An injection of microwaves at first leads to local luminescence at several spots between the gap. Each luminescence grows mainly toward the positive electrode. When these luminescences link together, the air breakdown occurs when the discharge current between the gaps grows suddenly.

The research of optimum constructions of traveling wave tube (TWT) and backward wave tube (BWT) with a rectilinear cylindrical electron flow in goffered waveguide on the basic wave mode H/sub 11/ of circular waveguide is carried out.

: The objectives of this project are to identify, analyze and eventually pursue opportunities to improve warfighter support and sustainability of high-power microwave tube parts used in fielded weapon systems. Phase I of this effort will result in a roadmap of mini-projects with benefits to high power microwave tube manufacturing, quality, supply, and product design. Upon the conclusion of Phase I, this roadmap and associated mini-projects will be submitted for approval to their associated potential funding sources

Plasma-assisted high power microwave source `PASOTRON' is being developed at a few places internationally to utilize plasma channel transport of the beam through Slow Wave Structure (SWS) to significantly reduce the size and weight in conventional linear high power microwave (HPM) sources by eliminating the need for the applied axial magnetic field [1-2]. In this device, very strong non-linear interaction between electron beam and electromagnetic wave can occur, thus making analytical design very cumbersome. Consequently, a particle simulation code is very much required to make its design simpler. We have made an efforts to study beam wave interaction in plasma filled SWS, which is a backward wave oscillator (BWO). The aim is to develop a particle-in-cell finite-difference-time-domain (PIC-FDTD) code [3] using MATLAB for the simulation of beam-wave interaction in the PASOTRON. The updating equations for electromagnetic fields are formulated using FDTD in cylindrical coordinate system since the SWS geometry is axially symmetric. In order to examine the field configuration in 3D, a field solver is implemented using the BOR-FDTD (Body of revolution FDTD) [4]. The results are being compared with MAGIC [5] tool software to validate the analysis. The results of this analysis will be presented.