High Power Microwave Sources Research Articles

A Compact Modular 5 GW Pulse PFN-Marx Generator for Driving HPM Source

A compact and modular pulse forming network (PFN)-Marx generator with output parameters of 5 GW, 500 kV, and 30 Hz repetition is designed and constructed to produce intense electron beams for the purpose of high-power microwave (HPM) generation in the paper. The PFN-Marx is composed by 22 stages of PFN modules, and each module is formed by three mica capacitors (6 nF/50 kV) connected in parallel. Benefiting from the utilization of mica capacitors with high energy density and a mini-trigger source integrated into the magnetic transformer and the magnetic switch, the compactness of the PFN-Marx system is improved significantly. The structure of the PFN module, the gas switch unit, and the connection between PFN modules and switches are well designed for modular realization. Experimental results show that this generator can deliver electrical pulses with the pulse width of 100 ns and amplitude of 500 kV on a 59-ohm water load at a repetition rate of 30 Hz in burst mode. The PFN-Marx generator is fitted into a cuboid stainless steel case with the length of 80 cm. The ratio of storage energy to volume and the ratio of power to weight of the PFN-Marx generator are calculated to be 6.5 J/L and 90 MW/kg, respectively. Furthermore, utilizing the generator to drive the transit time oscillator (TTO) at a voltage level of 450 kV, a 100 MW microwave pulse with the pulse width of 20 ns is generated.

Analysis of Electromagnetic Pulse Effects Under High-Power Microwave Sources

High-power microwave sources applied to a directed-energy weapon can lead to permanent damage by radiating concentrated energy in a specific direction to disturb or overload electronic equipment. The effect analysis on the target, such as electronics exposed to electromagnetic pulse, should be considered as an important factor in determining the performance of high-power microwave sources and conducting experimental evaluations. In this study, a magnetically insulated transmission line oscillator, one of the representative high-power microwave sources based on vacuum electronics device, was constructed and experimental analysis with respect to electromagnetic pulse effects was performed. The specification of the magnetically insulated transmission line oscillator used in this study corresponded to 3 GW of high-power electromagnetic wave pulses operating at L-band. The power efficiency was approximately 10 – 15%. For effective targeting, a Vlasov antenna that converts TM01 mode to TE11 mode was designed and fabricated. The radiation pattern was confirmed via fluorescent lamps, and to confirm the effect of the directed-energy weapon on the target, an effect analysis was performed using a portable electronic device as a sample. Furthermore, the electric field was measured with a D-dot probe and quantified and compared. This study presents a future blueprint of the value of the directed-energy weapon by predicting the radiant output power of the weapon in the far-field region after it is mounted on a movable ground vehicle or unmanned aerial vehicle.

A Ku-Band Compact Disk-Beam Relativistic Klystron Oscillator Operating at Low Guiding Magnetic Field

The disk-beam relativistic klystron oscillator (DB-RKO) with the radial-line high frequency structure is one of the attractive high power microwave (HPM) sources due to its specific virtues of weak space-charge effect, high power handling capacity and strong electron collection ability. However, such a device generally exhibits a bulky volume in the radial direction, retarding its actual applications in some compact occasions. Besides, it usually saturates slowly because of the insufficient beam-wave coupling strength. For this purpose, a compact DB-RKO with a rapid saturation process is proposed and physically designed in this paper. By compressing the axial width of the electron beam drift tube suitably, the beam-wave coupling strength is enhanced clearly and the saturation process is accelerated significantly. The whole beam-wave interaction structure is shortened with a radial length of about 5 cm. Besides, in order to increase the device compactness further, an improved magnetic-excited method is presented to enhance the magnetic field in the diode area by introducing a pair of soft-magnets. Furthermore, with the guiding of a low magnetic field of 0.4 T, the compact DB-RKO is experimentally demonstrated to be capable of generating Ku-band HPM radiations with a peak power of 810 MW, a power conversion efficiency of 28%, a center frequency of 14.19 GHz and a pulse duration of about 27 ns. The proposed DB-RKO is promising for the package of the permanent magnets which are preliminarily designed in this paper.

P-Conductivity in Co-Implanted and Gyrotron Microwave-Annealed GaN: Optical and Electrical Studies

A key step to the development of future (Al)GaN-based power devices is the ability to form selective-area p-type regions, and a promising method of accomplishing this is through ion implantation of the dominant acceptor dopant, Mg. Ion implantation in GaN induces lattice damage and creates point defects, including vacancies and interstitials. To remove this damage, heal defects, and activate the implanted dopants requires temperatures >1000 °C, beyond the thermodynamic equilibrium stability of GaN [1]. Removal of point defects, especially donor-like nitrogen vacancies (VN) in the material can be accomplished through high temperature annealing and may be aided by co-implantation with N [2]. It is speculated that compensation of implant-induced VN may be enhanced during the annealing process by diffusion of implanted Ni to the corresponding site in the N sublattice, however evidence for such compensation through suppression of VN-related photoluminescence has not been reported in p-type material. Methods of high-temperature nonequilibrium annealing such as symmetric multicycle rapid thermal annealing (SMRTA), microwave annealing, and laser annealing have been reported to achieve p-type material, however activation of Mg-implanted material remains an area of active study.In this study, we report our latest results in obtaining p-type conductivity in implanted GaN by repeated, short-duration thermal cycles using a high-power (kW) gyrotron microwave source and systematic study of Mg implantation and Mg+N co-implantation into GaN and the resulting defect microstructure [3]. We examine the effect of co-implantation to depths of 250 nm and 500 nm, and Mg:N concentration ratios of 1 and 2 with [Mg]=1019 cm-3. Implanted MOCVD-grown u-GaN is capped with AlN and annealed for <60 sec in 1.5–3 sec heating cycles at 1350 °C using a gyrotron under relatively low N2 overpressure of 3 MPa.The optical and electrical properties of the resulting films are characterized by temperature-dependent photoluminescence (PL) and I-V measurements of vertical p-i-n diodes. Highly resistive Mg+N co-implanted and annealed material shows dominant luminescence of the VN-related GL2 band at 2.37 eV and relatively lower-intensity acceptor-related UVL and UVL* bands (at 3.05–3.27 eV). However, implanted/annealed samples showing p-n diode behavior (Vth=3–3.1 V at room temperature) and hole concentrations in the range of 1015–1017 cm-3 show dominant/high UVL/UVL* bands and YL band centered at 2.30 eV. The YL is attributed to unintentionally introduced CN-ON complexes and is commonly observed in GaN grown by MOCVD but is typically not observed in implanted/annealed GaN. Additionally, samples implanted only with Mg and activated under the same annealing conditions show persistent dominance of the GL2 band in this range.This work is funded by the Advanced Research Projects Agency-Energy (ARPA-E), U.S. Department of Energy under the PNDIODES programs directed by Dr. Isik Kizilyalli.[1] G. Alfieri, et al. J. Appl. Phys., 123, 205303 (2018)[2] R. Tanaka et al. Japan. J. Appl. Phys., 59 , SGGD02 (2020)[3] V. Meyers et al. manuscript submitted for publication (May 2020) Figure 1

Design and preliminary experiment of a disk-beam relativistic klystron amplifier for Ku-band long-pulse high power microwave radiation

Spatial coherent combination of multiple high power microwave (HPM) sources is considered as a promising scheme to improve the equivalent radiation power of the HPM system dramatically. Relativistic klystron amplifier (RKA) is one of the most suitable sources for the coherent power combination owing to its specific capabilities of stable microwave frequency and controllable phase. However, the RKAs operating at high frequency-band are severely limited by the problems of the intense space-charge effect and radio frequency breakdown. The radial-line HPM sources driven by the disk-shape electron beam may provide the potential to alleviate this issue due to its attractive features of the weak space-charge effect, the high power handling capacity, and the strong electron collection ability. In this paper, a disk-beam relativistic klystron amplifier (DB-RKA) is proposed and physically designed aiming to generate long-pulse HPM radiation at Ku-band. The physical idea, design principles, and simulation results are presented in detail. In a preliminary experiment, the disk-shape intense electron beam is well focused with an axial-width of 1.2 mm by an improved magnetic-excited method. Furthermore, the DB-RKA is demonstrated to be capable of generating Ku-band HPMs typically with peak power of 320 MW, pulse duration of 100 ns, and gain of 42 dB.

A low-magnetic field high-efficiency high-power microwave source with novel diode structure

The design of compact, lightweight devices has become an important development direction of high-power microwave source devices, with the reduction of the external magnetic field being the leading innovation. By analyzing the transmission equation of the intense relativistic electron beam (IREB), it can be known that magnetic field strength in the diode region has great influence on the transmission quality of the IREB. In this paper, a new structure that introduces a soft magnetic material as the cathode base is proposed. Considering that soft magnetic materials have the capability to concentrate and enhance the magnetic field, the proposed structure leverages this merit to improve the magnetic field strength in the diode area under certain low external magnetic field conditions. Thus, the proposed structure is capable of effectively improving the emission and transmission quality of the IREB without any increase of the external guiding magnetic field. As for the device design, the new structure was introduced into a previously proposed C-band transit time oscillator, and the particle-in-cell simulation was carried out. The results showed that high power microwaves with a power of 2.7 GW are generated with a corresponding frequency of 40% when the electron beam voltage is 550 kV, the beam current is 12 kA, and the guiding magnetic field is 0.3 T.

Field-Line Coupling Method for the Simulation of Gyromagnetic Nonlinear Transmission Line Based on the Maxwell-LLG System

The gyromagnetic nonlinear transmission line (GNLTL) is a compact and solid-state high-power microwave source, which can be treated as a special coaxial transmission line partially filled with the ferrite material. Due to the complex interaction process of the electromagnetic fields with the dynamic damped precessional motion of the magnetization vector in the ferrite, it is of great challenge to theoretically predict the output of the GNLTL. Here, we present a novel field-line coupling simulation method to predict the output waveform of the GNLTL by simultaneously solving the coupled system of the Landau-Lifshitz-Gilbert equation and Maxwell's equations in the 2-D cylindrical coordinate system. In particular, the simulation model is validated by comparing the computation results with the experimental results obtained by Bragg et al. in the HPM experiment. The influences of the GNLTL length and bias magnetic field are analyzed. The proposed computation method can provide a solution for improving the performance of the GNLTL.

A Cerenkov microwave generator with cross-band frequency hopping based on magnetic field tuning

Frequency hopping is one of the most significant technological advancements in high power microwave (HPM) sources. Mechanical alteration is generally adopted in traditional cross-band microwave generation, but it has the detriments of a complex adjustment strategy. In this paper, a cross-band frequency hopping Cerenkov microwave generator with an advantageous adjustment strategy, high beam–wave interaction efficiency, and pure output mode is developed. Two-segment slow wave structures (SWSs), which work in C and X-bands separately, are adopted. Additionally, by utilizing the cyclotron resonance absorption phenomenon reasonably, only the relative band microwave is energized under different magnetic fields, which adequately represses mode competition and realizes cross-band frequency hopping. After the electron beam passes through the first SWS section, the bunching center is consistently at the acceleration or deceleration phase of the subsequent SWS section, which a guarantees high beam–wave interaction efficiency in the two bands. The preliminary results of the particle-in-cell (PIC) simulation are as follows: when the guiding magnetic field is 0.7 T, the microwave output comprising only the X-band is obtained, with an efficiency of 42%; when the guiding magnetic field is 1.5 T, the microwave output comprising only the C-band is obtained, with an efficiency of 30%.

Modern Dielectric Materials for Output Windows of High-Power Microwave and Terahertz Sources

Analysis of dielectric properties of several materials, BN, AlN with BN additive, crystalline quartz, MgAl2O4 spinel, 6H-SiC in the millimeter and submillimeter (terahertz) ranges, is presented. Basing on the experimentally measured dielectric losses of these materials and on their thermal and mechanical properties, we made a calculation to estimate the maximal achievable output power that can be transmitted through disks made of such materials. Prospects for using these materials for gyrotron barrier window production are discussed.

Highly efficient compact gigawatt‐level microwave source using relativistic electrons: radial relativistic magnetron

The relativistic magnetron is one of the most attractive high-power microwave sources, due to its compactness with high efficiency. It can generate hundreds of megawatts of microwave power in the L- and S-bands. However, the efficiency of the magnetron is often limited due to the mode stability. In this Letter, three-dimensional particle-in-cell simulation suggests even further increase of total efficiency of the magnetron can be realised via mode stabilisation using a self-excited extended interaction in a collector region. A six-vaned radial relativistic magnetron with a single radial output port powered by 400 kV, 900 MW with an efficiency of ∼48%. This improvement is attributed to the mode locking of operating π-mode by a power-boosted TE315 mode excited in the extended cavity.

A Frequency-Agile Relativistic Magnetron With Axial Tuning

A frequency-agile relativistic magnetron with axial tuning is presented. To achieve axial tuning, reentrant cavity which has two radial parts connected by a coupling hole had been introduced. Resonant frequency of this cavity configuration can be tuned by adjusting the axial length of each coupling hole without any variation of other parameters. Multi-antenna output technique had been integrated with the reentrant cavity chain to obtain high conversion efficiency in the whole tuning range. Simulation results showed that this tube had a tuning range between 1.23 GHz to 1.70 GHz, corresponding to relative bandwidth of 32% about a center frequency of 1.465 GHz. In this tuning range, the maximum output power is 1.81 GW at 1.605 GHz, corresponding to an efficiency of 54.5%. This axial tuning RM is a promising compact high power microwave source with wide tuning range.

A theory of contact resistance under AC conditions

Electrical contact is an important issue to high power microwave sources, pulsed power systems, field emitters, thin film devices and integrated circuits, interconnects, etc. Contact resistance and the enhanced ohmic heating that results have been treated mostly under steady state (DC) condition. In this paper, we consider the AC contact resistance for a simple geometry, namely, that of two semi-infinite slab conductors of different thicknesses joined at z = 0, with current flowing in the z-direction. The conductivity of the two planar slabs may assume different values. We propose a procedure to accurately calculate the normalized contact resistance under the assumption σ≫ωϵ, where ω is the frequency, σ is the electrical conductivity, and ϵ is the dielectric constant of the material in either channel. We found that in the low frequency limit, the normalized AC contact resistance reduces to the DC case, which was solved exactly by Zhang and Lau. At very high frequency, we found that the normalized contact resistance is proportional to ω, in which case the resistive skin depth becomes the effective channel width, and the physical origin of the contact resistance is identified. The transition between the high and low frequency limits was explored, where, in some cases, the normalized contact resistance may become negative, meaning that the total resistance is less than the total bulk resistance expected from the two current channels. In other cases, the numerical data suggest that the normalized contact resistance is proportional to ω in the transition region. Other issues are addressed.

Compact coaxial slotted‐cavity feed antenna with shaped‐beam pattern for high‐power microwave using high‐order modes and polarisation cancellation techniques

The coaxial slotted-cavity antenna is an easily-manufactured, novel high-power microwave antenna developed specifically for its compact and shaped-beam characteristics. It can directly cascade with the high-power microwave source, and radiate linear-polarisation electromagnetic wave without extra power dividers and mode converters, which obtains a low profile. In this study, a double-layer coaxial cavity operating in two high-order modes is introduced to eliminate the high-order interference modes caused by structural discontinuities, and three shaped beams on the compact aperture can be achieved by the polarisation cancellation technique. Unique proprieties of three shaped beams, namely the peak of gain enhancement beam, the ripple of Gaussian beam and the null depth of conical beam are investigated, all of which can be controlled by the rotation angle of three phase adjusted slot pairs. This study demonstrates the design process of such antenna, and verifies it against simulation and experimental results. The measured return-loss of all three shaped beams are higher than 13 dB, while the corresponding radiation efficiencies are not lower than 98.9%. Furthermore, the estimated minimum power capacity of such antenna exceeds 1.26 GW at 9.375 GHz, making it a highly potential candidate in application towards high-power microwave.

Enhancing electromagnetic radiations by a pre-ablation laser during laser interaction with solid target

Ultra-short untraintense laser interacting with solid targets can produce significant electromagnetic pulses (EMPs), which are strongly pertinent to laser and target parameters. In this study, EMPs' generation due to pulsed laser (30 fs, 6×1019 W/cm2) irradiating aluminum foils are recorded and analyzed. The experimental results indicate a pre-ablation pulse (200 ps, 1×1012 W/cm2) that tends to enhance the electromagnetic emission inside the target chamber and the diagnostic cavity. The largest EMP signal is obtained when the pre-ablation pulse is incident on the target at 100 ps prior to the main laser beam. Meanwhile, the frequencies of EMPs are broadened up to 2 GHz in the diagnostic cavity in the 100 ps delayed case, indicating that EMPs with higher frequencies can be generated if the most energetic electrons are accelerated. The simulating results verify expansion of the pre-plasmas induced by the pre-ablation laser, which is beneficial to lift both the energy and quantity of hot electrons escaping from the target, accounting for the ehancement of EMPs. The resulting conclusions offer a new avenue to generate intense EMPs, which can be potentially used in nondestructive examination, high-power microwave source, and energy transfer.

Development of a long pulsed RF test stand and its applications for performance studies of 1 MW CW klystron

Research and development activities of high power microwave sources for powering RF cavities of Indian Spallation Neutron Source and Accelerator Driven Subcritical Systems are underway at RRCAT. Front end accelerating structures such as Radio Frequency Quadrupoles and Drift Tube Linac demand pulsed RF power up to 1 MW. A 1 MW pulsed RF system based on TH 2089 klystron amplifier at 352.2 MHz with pulse width capability up to 1.5 ms has been developed and tested. A compact 100 kV, 20 A converter type modulator with pulse width capability up to 1.6 ms has been used to energize the klystron of RF test stand. The performance of the klystron in pulsed mode operation has been studied and presented. The variation in the RF output power was measured and it is within ±0.75%. The phase variation of RF output power within the pulse and the pulse to pulse is less than ±2.5°.

Progress in narrowband high-power microwave sources

Even after 50 years of development, narrowband high-power microwave (HPM) source technologies remain the focus of much research due to intense interest in innovative applications of HPMs in fields such as directed energy, space propulsion, and high-power radar. A few decades ago, the main aim of investigations in this field was to enhance the output power of a single HPM source to tens or hundreds of gigawatts, but this goal has proven difficult due to physical limitations. Therefore, recent research into HPM sources has focused on five main targets: phase locking and power combination, high power efficiency, compact sources with a low or no external magnetic field, high pulse energy, and high-power millimeter-wave generation. Progress made in these aspects of narrowband HPM sources over the last decade is analyzed and summarized in this paper. There is no single type of HPM source capable of excellent performance in all five aspects. Specifically, high pulse energy cannot be achieved together with high power efficiency. The physical difficulties of high power generation in the millimeter wave band are discussed. Semiconductor-based HPM sources and metamaterial (MTM) vacuum electron devices (VEDs) are also commented on here. Semiconductor devices have the advantage of smart frequency agility, but they have low power density and high cost. MTM VEDs have the potential to be high power efficiency HPM sources in the low frequency band. Moreover, problems relating to narrowband HPM source lifetime and stability, which are the important determinants of the real-world applicability of these sources, are also discussed.

Competitions among modes in magnetically insulated transmission line oscillator

&lt;sec&gt;As a compact and high power microwave source, the competitions among various modes are prone to appear in the initial stage of the development of the radiated electromagnetic field in a magnetically insulated transmission line oscillator (MILO). If the mode competitions are not controlled effectively, the output characteristics of the MILO may decline in the end. &lt;/sec&gt;&lt;sec&gt;As is well known, the operating mode of MILO is generally designed on the π mode of the TM&lt;sub&gt;00&lt;/sub&gt; mode and the coaxial disk-loaded waveguide is usually adopted as a slow-wave structure for beam-wave interaction in MILO. Therefore from the dispersion relations between the electron beam and the lower order electromagnetic modes(including TM&lt;sub&gt;00&lt;/sub&gt;, TM&lt;sub&gt;01&lt;/sub&gt; and HEM&lt;sub&gt;11&lt;/sub&gt; modes) in the slow-wave structure, the characteristics and possible suppression methods of the three kinds of mode competitions are analyzed simply. The first kind mode competition is between the different axial modes of the fundamental TM&lt;sub&gt;00&lt;/sub&gt; mode. In this case, the electromagnetic field of the competition mode is also axially symmetric and its frequency is slightly lower than that of the π mode. The second is the competition between the TM&lt;sub&gt;00&lt;/sub&gt; and higher order TM&lt;sub&gt;01&lt;/sub&gt; mode. In this case, the competition frequency is rather higher than that of the π mode (TM&lt;sub&gt;00&lt;/sub&gt;). The third is the competitions between the TM&lt;sub&gt;00&lt;/sub&gt; and low order asymmetric HEM&lt;sub&gt;11&lt;/sub&gt; modes. In this case, the competition frequency is slightly higher than that of the main mode. Appropriately choosing the radii of the anode vanes, the number of the anode cavity and the load length of the cathode, the corresponding mode competition intensity can be weakened. &lt;/sec&gt;&lt;sec&gt;Based on the obtained results above and the existing model of the MILO, a compact high output power L-band MILO is proposed. Numerical studies of the mode competitions and output characteristics are carried by using the three dimensional particle-in-cell code. Cold-cavity test shows that in the low frequency range, the easily stimulated electromagnetic modes are the π mode of TM&lt;sub&gt;00&lt;/sub&gt; and HEM&lt;sub&gt;11&lt;/sub&gt; modes with frequencies of 1.61 GHz and 1.77 GHz, respectively. The numerical results of hot-cavity verify that the competition in the initial stage comes mainly from the asymmetric HEM&lt;sub&gt;11&lt;/sub&gt; mode due to the fact that there exists the strut in the output region, the Cartesian coordinates are adopted during the simulation, and totally symmetry cannot be guaranteed. In addition, electron beam emission from the cathode is not ideally even. But stable and high output microwave power is obtained in the end in the L-band MILO by being optimized. The output power and efficiency are 8.1 GW and 18% respectively, and the mode purity reaches about 97%. &lt;/sec&gt;

Microwave source with enhanced radiation power based on orbitron MASERs

We have proposed the design of high power microwave source in a vacuum tube based on the theory of negative mass instability. The proposed source does not operate based on a slow-wave structure but instead makes use of accelerated charged particles. In our structure, an electron emitter injects the charged particles into to the cavity. Emitted particles move around the anode, which leads to electron acceleration and emission of electromagnetic waves with a specific frequency. In this paper, we have enhanced the output power and frequency of microwave source by variation of particles initial energy and the number of anodes incorporating their specific arrangement and applied voltages. Working at room temperature, compactness, wide bandwidth, and tunability of output frequency are among the advantages of the proposed source.

Machine Learning Investigation of the Rising Sun Magnetron Design and Operation

Many emerging applications of high-power microwave sources require operation at new and higher frequencies. Computer modeling is the main tool for the design and optimization of these sources, such as magnetrons. However, when designing a new source, the parameter space to scan can be too large to study with traditional techniques. We apply machine learning (ML) (supervised learning in particular) to search this parameter space automatically. We demonstrate this technique by optimizing the operation of a magnetron at 4 GHz, investigating selected parameters tradeoff and predicting operation space characteristics for the discovered device based on a limited amount of measurements. The combination of ML and optimization techniques that we demonstrate here has wide applicability for virtual prototyping and design of problems with sophisticated computer models and allows us to fundamentally reimagine the role of the human in the design workflow.

Development of Normal Conducting Cavity Magnet for 42 GHz 200 kW Long Pulse Gyrotron

Gyrotrons are high-frequency, high-power microwave sources with potential applications in plasma generation, energy generation, spectroscopy, and security. The Department of Science and Technology initiated the development of a 42 GHz, 200 kW, continuous wave (CW) gyrotron for the Indian Tokamak system through a multi-institutional project. The activity is targeted toward indigenous development of India’s first high-power gyrotron. In gyro-devices, frequency of operation is determined by the cyclotron frequency, and thereby the magnetic field. The development of a high-field intensity normal conducting cavity magnet was an imperative step during the testing and integration of the gyrotron tube assembly to achieve the desired beam transmission with a moderate operating cost. A high and spatially homogeneous magnetic field (1.61 T) is required to be established in the beam–wave interaction region by the cavity magnet. The desired magnetic field, as a function of axial distance from the gun to the collector, is governed by the requirements of gun optics, focusing field in the interaction region, and the adiabatic expansion of the beam into the collector. A water cooled electromagnet has been designed to produce the requisite magnetic field in the cavity region over an axial length of 40 mm with field homogeneity better than ±0.35%. Magnet design was optimized to meet the field requirements in the cavity and cathode gun. Magnetic field simulations were carried out in conjunction with the electrostatic field and space charge analysis. This paper presents finite element computations, design optimization, development and testing of a continuous duty iron core electromagnet for a first harmonic interaction mode (TE03) gyrotron cavity. Magnetic, electrical, thermal, and hydraulic design of the electromagnet is discussed in this paper.