High Power Microwave Tubes Research Articles

EM-Thermo-Mechanical Investigations on Pillbox RF Window for 5 kW S-Band Microwave Tubes

This article presents the design of pill box-type RF window working at an operating frequency of 2.998-GHz for 5-kW high-power microwave tubes. Concave, convex, and dome-shaped dielectrics are considered for a study of proposed windows with CVD diamond, Alumina (99.5%), and glass as dielectric materials. The reflection and transmission characteristics of all the proposed windows are investigated. The return and insertion loss for concave CVD diamond are 49.15 dB, 0.026 dB and for concave Alumina (99.5%) are 46.2 dB, 0.018 dB. The relative permittivity of the material and the dielectric thickness affect the reflection characteristics. From the thermal study, the maximum temperatures are observed at the center of the dielectric, causing thermal stress and displacement. The temperature rises when the input power is increased from 5-kW to 50-kW. Compared to CVD diamond and Alumina (99.5%), glass material experiences higher temperature growth. The maximum temperatures at 5-kW average input power for CVD diamond, Alumina (99.5%), and glass are 25.5°C, 25.6°C, and 108°C, respectively. From mechanical analysis, the stress and displacement for concave and dome-shaped dielectrics are less compared to those of convex and conventional. Stress distributed widely at the periphery of the dielectric whereas displacement occurred more at the center. According to EM-thermo-mechanical simulations, the concave CVD diamond is suitable for window design with a 1.6 mm dielectric thickness.

A Repetitive Low Impedance High Power Microwave Driver

A low impedance high power microwave (HPM) driver is designed, which can be used in studying multi-gigawatt HPM devices such as the magnetically insulated transmission line oscillator (MILO), based on a helical pulse forming line (PFL) and the Tesla pulse transformer technology. The co-axial PFL is insulated by ethanol–water mixture, whose dielectric constant can be adjusted; and the helical line increases the output pulse width as well as the impedance to make a better match with the load. By the optimal combination of PFL charging voltage and output switch working voltage, the reliability of the PFL can be improved. The Tesla transformer has partial magnetic cores to increase the coupling coefficient and is connected like an autotransformer to increase the voltage step-up ratio. The primary capacitor of the transformer is charged by a high voltage constant current power supply and discharged by a triggered switch. A transmission line is installed between the PFL and the HPM load, to further increase the load voltage. A ceramic disk vacuum interface is used for improving the vacuum of the HPM tube. The experiments show that the driver can operate at 30 GW peak power, 75 ns pulse width and 5 Hz repetition rate.

Multiphysics and Multipactor Analyses of TE022-Mode High-Power X-Band RF Window

This letter presents the design of an overmoded $X$ -band RF window for use in high-power microwave tubes, such as klystron, magnetron, and linear accelerator (LINAC). The cylindrical section of the window containing dielectric barrier operates in the TE022 mode. Conversion from TE10 mode to TE022 mode is achieved using four rectangular slots made on the narrow walls of the rectangular waveguide sections. The simulation results for the proposed design in CST and HFSS are in good agreement. The return loss, insertion loss, and bandwidth are obtained as 52.76 dB, 0.06 dB, and 41.4 MHz, respectively, at 9.3 GHz. Coupled electromagnetic, thermal, and structural simulations are performed to study the multiphysics performance of the RF window. The multipactor analysis is carried out in SPARK3D to determine the multipactor threshold. The proposed TE022 mode RF window can operate safely at a 20.75 MW of peak power.

Electromagnetic analysis for the side-coupled cavity-type interaction structure for the linear accelerators

The side-coupled cavity (SCC) structure is widely used in Linac, and also finds its application in the high-power microwave tubes. The SCC structure operating under the slow-wave regime, for the π/2 mode in transverse magnetic mode operation, has been electromagnetically analyzed using field-matching technique for the dispersion and interaction impedance characteristics. Furthermore, the effect of different structural parameters on the dispersion and interaction characteristics has been studied. To validate the developed dispersion and interaction relation, the SCC is simulated using commercially available electromagnetic simulation tools. The results obtained through the developed analytical expression are compared with the simulated results, and it is found to be in agreement within 3%. This analysis will help the design engineer to select the approximate accurate parameters before performing the simulation or experimental design. Also, it helps to develop a better understanding about the SCC’s behavior with respect to structural parameter before the simulation.

Developments of Pulsed Electron Beam Sources for High-Power Microwave Applications

High-current pulsed electron beam sources are the core components of high-power microwave systems. In order to meet the requirements of future applications, one needs to improve the performance of electron beam sources in terms of vacuum insulation, beam transportation, and thermal management. In this paper, we report about our recent progress in the development of high-current vacuum electron beam sources. In order to meet the vacuum maintenance requirements of high-power microwave tubes, a high-electric field ceramic vacuum interface is designed and fabricated based on the ceramic metal brazing technique. In our experiments, a stable operation of the ceramic vacuum interface is demonstrated in the 10 Hz repetition mode with withstand voltage of larger than 600 kV and pulse width of about 100 ns. Besides, a cold cathode is developed using SiC nanowires, and an average beam current density of 1.2 kA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> is achieved under the electric field strength of 90 kV/cm. Compared with traditional velvet cathodes, the characteristics of the SiC nanowire cathode, such as macro-electrical stability, emission uniformity, and operating life have been significantly improved. Furthermore, a high-current electron beam collector has been developed for relativistic backward wave oscillator tubes. A spiral flume is designed in the collector to meet the requirement of both high specific energy and low flow rate. It shows that the withstand heat flow density is in the order of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">12</sup> W/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , which is suitable for the long pulse and repetitive operation of the system. These results represent a significant step towards the practical application of long-life high-power microwave systems.

Development of non-shrinkable ceramic composites for use in high-power microwave tubes

Al2O3-CaO-SiC-based ceramic composites with four different compositions were sintered at 1700°C for 3 h in an air furnace. The phase analysis, microstructural characterization, and elemental composition determination of the developed composites were performed by X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), and energy-dispersive X-ray (EDAX) analysis, respectively. The shrinkage, thermal properties, and electrical resistivity of the composites were also studied. The experimental results showed the effects of adding silicon carbide and calcia to alumina on the thermal, electrical, and shrinkage properties of the resultant composites. Among the four investigated ceramic composites, the one composed of 99wt% alumina, 0.5wt% CaO, and 0.5wt% SiC exhibited the best characteristics for use as a potting material in a dispenser cathode of a microwave tube. The material exhibited slight expansion instead of shrinkage during drying or firing. Other properties of the composite powder, such as its thermal properties and electrical resistivity, were comparable to those of a commercial alumina powder.

Research and optimization of a new design of output window for high power microwave tubes

A design of an output window integrated with an output resonator of the high-power klystron is proposed. The results of a theoretical study of the frequency response of an integrated window are presented. Calculation of the electric field in critical areas of the optimized design showed that the losses in the vacuum-tight dielectric are much lower, and the tangential components of the field on its surface, which are the source of breakdowns, are lower or comparable with similar parameters of traditional pillbox windows.

Instrumentation and control system architecture of ECRH SST1

The Electron Cyclotron Resonance Heating (ECRH) system is an important heating system for the reliable start-up of tokamak. The 42GHz and 82.6GHz Gyrotron based ECRH systems are used in tokomaks SST-1 and Aditya to carry out ECRH related experiments. The Gyrotrons are high power microwave tubes used as a source for ECRH systems. The Gyrotrons need to be handled with optimum care right from the installation to its Full parameter control operation. The Gyrotrons are associated with the subsystems like: High voltage power supplies (Beam voltage and anode voltage), dedicated crowbar system, magnet, filament and ion pump power supplies and cooling system. The other subsystems are transmission line, launcher and dummy load. A dedicated VME based data acquisition & control (DAC) system is developed to operate and control the Gyrotron and its associated sub system. For the safe operation of Gyrotron, two level interlocks with fail-safe logic are developed. Slow signals that are operated in scale of millisecond range are programmed through software and hardware interlock in scale of microsecond range are designed and developed indigenously. Water-cooling and the associated interlock are monitored and control by data logger with independent human machine interface.

Structural Investigations of Nanocrystalline Cu-Cr-Mo Alloy Prepared by High-Energy Ball Milling

Cu-Cr-Mo alloy could be a suitable candidate material for collector electrodes in high-power microwave tube devices. An attempt has been made to synthesize ternary Cu-Cr-Mo alloys by mechanical alloying of elemental Cu, Cr, and Mo powders, to extend the solid solubility of Cr and Mo in Cu, using a commercial planetary ball mill. For the first ternary alloy, a mixture of 80 wt.% Cu, 10 wt.% Cr, and 10 wt.% Mo was mechanically milled for 50 h. For the second ternary alloy, a mixture of 50 wt.% Cr and 50 wt.% Mo was mechanically milled for 50 h to obtain nanocrystalline Cr(Mo) alloy, which was later added to Cu powder and milled for 40 h to obtain Cu-20 wt.%Cr(Mo) alloy. Both nanocrystalline Cu-Cr-Mo ternary alloys exhibited crystallite size below 20 nm. It was concluded that, with addition of nanocrystalline Cr(Mo) to Cu, it was possible to extend the solid solubility of Cr and Mo in Cu, which otherwise was not possible by mechanical alloying of elemental powders. The resulting microstructure of the Cu-20 wt.%Cr(Mo) alloy comprised a homogeneous distribution of fine and hard (Cr, Mo) particles in a copper matrix. Furthermore, Cu-20 wt.%Cr(Mo) alloy showed better densification compared with Cu-10 wt.%Cr-10 wt.%Mo alloy.

Operation and control of high power Gyrotrons for ECRH systems in SST-1 and Aditya

The Electron Cyclotron Resonance Heating (ECRH) system is an important heating system for the reliable start-up of tokamak. The 42GHz and 82.6GHz ECRH systems are used in tokamaks SST-1 and Aditya to carry out ECRH related experiments. The Gyrotrons are high power microwave tubes used as a source for ECRH systems. The Gyrotron is a delicate microwave tube, which deliver megawatt level power at very high voltage ∼40–50kV with the current requirement ∼10A–50A. The Gyrotrons are associated with the subsystems like: High voltage power supplies (Beam voltage and anode voltage), dedicated crowbar system, magnet, filament and ion pump power supplies, cooling, interlocks and a dedicated data acquisition & control (DAC) system. There are two levels of interlocks used for the protection of Gyrotron: fast interlocks (arcing, beam over current, dI/dt, anode voltage and anode over current etc.) operate within 10μs and slow interlocks (cooling, filament, silence of Gyrotron, ion pump and magnet currents) operate within 100ms.Two Gyrotrons (42GHz/500kW/500ms and 82.6GHz/200kW/1000s) have been commissioned on dummy load for full parameters. The 42GHz ECRH system has been integrated with SST-1 & Aditya tokamak and various experiments have been carried out related to ECRH assisted breakdown and start-up of tokamak at fundamental and second harmonic. These Gyrotrons are operated with VME based data acquisition and control (DAC) system. The DAC system is capable to acquire 64 digital and 32 analog signals. The system is used to monitor & acquire the data and also used for slow interlocks for the protection of Gyrotron. The data acquired from the system are stored online on VME system and after the shot stored in a file in binary format. The MDSPlus, a set of software tools has been used for data visualization & management for after shot analysis.

Thermal Properties of an Intense Relativistic Electron Beam Collector Under Repetitive Pulse Operation

With the increase in power and miniaturization in a size of repetitively operated high-power microwave (HPM) sources, such as relativistic backward-wave oscillators, the thermal control and cooling methods for a collector have the great effects on efficient operations of these devices. In this paper, the thermal properties of an intense relativistic electron beam collector under repetitive operation were studied. The bombardment area caused by energetic electrons was experimentally quantified, and the pulsed thermal loading for a 700-keV and 7-kA beam current was calculated. With an ANSYS multiphysics package, the typical results of temperature history of the collector under 10–100-Hz operation were presented. The dependence of peak temperatures on forced cooling conditions at different pulse repetition rates was also obtained. Comparison results show that the cooling process is influenced by collector thermophysical parameters, interface thickness, and convective factor of the forced water cooling. Finally, the hydraulic system necessary to achieve appropriate cooling fluid flow was discussed. These results may supply an efficient reference to the optimization and reasonable use of a repetitively operated beam collector in the HPM tubes.

Design and Comparison of Waveguide Windows

The waveguide Windows used in most of the high power microwave Tubes are of different types, namely the rectangular single block window, the multilayer window and the thin disc pillbox window. In this paper, the design of all the three types of windows is carried out in the X-band frequency. The design of the single block window has been carried out with inductive, capacitive and resonant type of irises to find their suitability for window applications. The results obtained from the equivalent circuit have been compared with that of results from the Electromagnetic software HFSS and presented for all the RF windows.

Maintenance of high vacuum level in a compact and lightweight sealed hard-tube magnetically insulated line oscillator system

Magnetically insulated line oscillator (MILO) is compact and lightweight high power microwave tube. To maintain the MILO below 10−2 Pa, we used built-in non evaporable-getter (NEG). In comparison with other pumping technology, the NEG pump operates without power is lightweight and compact. To estimate the vacuum life in static state, we carried out experimental analysis to investigate the outgassing characteristics of the main materials of the MILO in static state. Considering the expected pumping characteristics of NEG pumps in combination with the experimental data, the conclusion is that at room temperature two NEG pumps will maintain high vacuum level in the MILO more than 250 days. Moreover, the pulse desorption characteristics of MILO under high voltage pulse operation (peak voltage 440 kV, pulse width 80 ns) were also measured. At room temperature, the evaluation shows that one NEG pump can maintain the MILO tube in high vacuum level for nearly 8 pulses.

A long-pulse repetitive operation magnetically insulated transmission line oscillator.

The improved magnetically insulated transmission line oscillator (MILO) is a gigawatt-class L-band high power microwave tube. It has allowed us to generate 3.1 GW pulse of 40 ns duration in the single-pulse operation and 500 MW pulse of 25 ns duration in the repetition rate operation. However, because of the severe impedance mismatch, the power conversion efficiency is only about 4% in the repetition rate operation. In order to eliminate the impedance mismatch and obtain repetitive long-pulse high-power microwave (HPM), a series of experiments are carried out and the recent progress is presented in this paper. In the single-pulse operation, when the diode voltage is 466 kV and current is 41.6 kA, the radiated microwave power is above 2.2 GW, the pulse duration is above 102 ns, the microwave frequency is about 1.74 GHz, and the power conversion efficiency is about 11.5%. In the repetition rate operation, under the condition of the diode voltage about 400 kV, beam current about 38 kA, the radiated microwave power is about 1.0 GW, the pulse duration is about 85 ns. Moreover, the radiated microwave power and the pulse duration decline little by little when the shot numbers increase gradually. The experimental results show that the impedance matching is a vital factor for HPM systems and one of the major technical challenges is to improve the cathode for the repetition rate operation MILO.

Electron beam coupling to a metamaterial structure

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.

Performance and modelling of 70kVdc power supply with solid-state crowbar

The experimental activities of tokamak research involve development of high power RF and microwave sources for fusion related heating and current drive applications. High power RF and microwave tubes like Klystron, Gyrotron and Tetrode are in general operated with high voltage DC power supplies. These HVDC power supplies of the order of 70kVdc, must be equipped with necessary arc fault protection in addition to general over current and over voltage protection.The arc fault protection must act within few microseconds to prevent permanent damage to the RF tube, window, etc. When an arc fault is detected, output voltage of the DC power supply is short circuited using a crowbar device (generally Ignitron, Thyratron, thyristor, rail-gap, etc.) that operates in few microseconds. This diverts the fault current from the load to crowbar device, thereby protecting the load. This is necessary as conventional protection in the power supply input takes ∼100ms to switch-off. The crowbar device must be able to take the fault current till the circuit breaker placed at power supply input is switched off.The arc fault protection is tested for its effectiveness by “wire-burn” test. Full power short circuit of ∼1.5MW DC power supply puts enormous stress on the power supply, utility and the crowbar, therefore frequent wire-burn testing is to be avoided.This report presents simulation of wire-burn test using PSIM software. Optimization of the component values without conducting actual wire-burn test could be achieved.

Demonstration of an Acid-Spun Single-Walled Nanotube Fiber Cathode

Field emission dc cold cathodes continue as an important area of research for uses such as electron microscopy, novel X-ray sources, vacuum electronic devices, terahertz sources, and high-power microwave tubes. Each of these applications typically requires high current densities with high-brightness electron beams driven by cathodes exhibiting long lifetime in the presence of deleterious conditions such as ion back bombardment and excessive heating. The Air Force Research Laboratory (AFRL) now investigates cathodes operating in dc mode for use in a terahertz traveling wave tube (TWT). The TWT requires an electron beam of 50 μm in diameter or less, at 10s of kiloelectronvolt energy with energy spreads of less than 10 eV. While AFRL has tested numerous cathodes in this regime, this paper reports on the first demonstration of a dc cathode utilizing a highly aligned carbon nanotube (CNT) rope for the electron emitter. The rope consists of individual single-walled CNTs that have been subjected to a nitrogen-enhanced acid etch and then spun into a rope configuration. Thus, the single rope emitter has an overall diameter of 100 m and a length of 1.5 mm. We report on preliminary results from this cathode, in particular the fabrication of the cathode, the dc cathode test system, and the cathode operation up to a voltage of 5 kV. The cathode operates stably to within 0.6% with a 5-mm anode-cathode gap at 5 keV and 1.0-mA current for hundreds of hours. Finally, we provide estimates of the cathode parameters such as the effective field enhancement factor (βeff) and emitting area (A) through a Fowler-Nordheim plot and comparison of the experimental data with simulations utilizing the particle-in-cell code Improved Concurrent Electromagnetic Particle-in-Cell.

Production of High Power by Using Gyrotron Device for Electron Cyclotron Resonance Heating in Tokamak Reactor

High power microwave tubes are devices which generate or amplify electromagnetic radiation in the frequency range of 0.3GHz to 300GHz (microwave and millimeter wave frequencies) at high peak and high average power level. Among the various kinds of microwave tubes, gyrotrons are the most well known devices which used for amplification and generation of energy at high power levels especially for ECRH which has become a well established heating method for both Tokamaks and Stellarators. The gyrotron operates by transfer of energy from an electron beam to an RF wave, but a fast wave, using relativity, which can reach high frequencies, and working with inhomogeneous fields. The electron gun, immersed in a strong magnetic field, allows to obtaining an electron beam of high density and because these electrons are bunched in phase due to relativistic effects, the radiation is coherent. The wave is generated in a specially designed interaction cavity (resonator) which can sustain oscillations on the desired frequency. In this case we can obtain waves of high frequencies and high powers.

The influence of variable quantity of polarization on the current intensity of the electron emission from La-doped Pb(Zr, Sn, Ti)O3 ferroelectric cathode

Ferroelectric cathodes exhibit huge potentials in high-power microwave tube electron beam source, panel display, and the propeller space navigation, due to their superior properties. The material properties of the ferroelectric cathode have been proved to have a significant influence on electron emission, which is indicated in recent research work. In the course of electron emission, the variation of polarization can be caused by non-shielded surface charge which is induced by high trigger voltage. A certain relationship may be found between polarization variation and current intensity of electron emission. To study the relationship between current intensity of electron emission and polarization variation in ferroelectric cathodes, the samples of lanthanum-doped lead zirconate stannate titanate ferroelectric and antiferroelectric ceramics are prepared by the method of solid state calcinations, and the polarization variations of the material under different voltages are measured in the positive half cycle test of hysteresis loop. The curve of the electron emission current intensity versus the trigger voltage is measured, and then the relationship between electron emission current intensity and polarization variation is investigated. The results show that the electron emission current intensities of the two samples are both directly proportional to the polarization variation.

Development of suitable potting material for dispenser cathodes of a high power microwave tube

Abstract The present study aims to develop suitable advanced potting material for modern high performance dispenser cathodes for high power microwave tube through refinement of the alumina microstructure by using suitable dopant. Calcium oxide was selected as a dopant material and the resultant materials were characterized by X-ray diffraction studies and the microstructure monitored by SEM study and EDX analysis. The shrinkage, thermal and electrical properties of the resultant material was evaluated to establish its suitability to function as an advanced potting material.