Magnetically Insulated Line Oscillator Research Articles

Performance improvement studies of axially partitioned dual band magnetically insulated line oscillator

In magnetically insulated line oscillator (MILO)s, a proportion of device current is responsible for the self-generated magnetic field that aids electron flow confinement in the anode cathode gap, thereby suitable RF generation. The device currents should not be high due to its direct impact on device efficiency, and electron energy depositions at load can lead to anode plasma formation. Also, the lower currents can facilitate the device to operate at larger voltages and/or for longer operational times. In this article, couple of axially partitioned dual band MILOs (DBMILO) designs that operate at lower device currents in comparison with the existing axially partitioned DBMILOs in the literature is presented. For one design with the radius of the slow wave structure (SWS), i.e., RSWS = 40 mm, the injection of the DC electron beam parameters 505 kV and 49 kA, generated an output RF power of ∼5.8 GW oscillating at 3.71 and 10.16 GHz (S-and X- bands) concurrently with ∼23.8% conversion efficiency. However, the other design (RSWS = 40.6 mm) at beam voltage 505 kV and device current of 47 kA generated an output RF power of >3.85 GW oscillating at 3.5 and 10.5 GHz, concurrently, where the frequencies are harmonically related. Thus, the proposed designs yielded significant device current reduction to ∼47 and ∼49 kA from ∼56 kA (existed), respectively, thereby drop of 16% and 12.5% to its counter designs, respectively. Also, there is substantial reduction in the electron energy depositions, thus the occurrence of anode plasma formations. Moreover, the peak conversion efficiency is enhanced to 23.4% and 16.4%, from 12.8%.

Dual-Frequency, Harmonic, Magnetically Insulated Line Oscillator

Magnetically insulated line oscillators (MILOs) are crossed-field devices which generate a self-induced azimuthal magnetic field via an axial current. This negates the need for external magnets, potentially increasing overall system efficiency at the expense of reduced device efficiency. This article reports the design, simulation, and experimental demonstration of a dual-frequency, harmonic MILO (HMILO), which is composed of two sequential slow wave structures (SWSs) tuned for oscillation at different frequencies, each paired with a set of choke cavities and insulated by the self-generated magnetic field of the common cathode. The two SWSs—designed for operation in <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$L$</tex-math> </inline-formula> -and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$S$</tex-math> </inline-formula> -bands at 1 and 2 GHz, respectively—were tested independently before the HMILO experiments. Results for the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$L$</tex-math> </inline-formula> -band MILO (L-MILO) were reported previously by Packard. In the experiments reported here, the isolated <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$S$</tex-math> </inline-formula> -band MILO (S-MILO) produced <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1.1 \pm 0.7$</tex-math> </inline-formula> MW of output power at <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$2.076 \pm 0.005$</tex-math> </inline-formula> GHz, when supplied with 207 kV and 7.3 kA from the MELBA-C generator. Ultimately, when implementing two separate extractor configurations, the HMILO produced <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$12.7 \pm 7.6$</tex-math> </inline-formula> MW at <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$0.984 \pm 0.013$</tex-math> </inline-formula> GHz and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$3.2 \pm 1.5$</tex-math> </inline-formula> MW at <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$2.074 \pm 0.003$</tex-math> </inline-formula> GHz. These results are compared against simulated performance in the particle-in-cell (PIC) codes CST and improved concurrent electromagnetic PIC (ICEPIC).

Investigation of a Bifrequency Magnetically Insulated Line Oscillator With Ridged-Disk-Loaded Vanes

In this article, a bifrequency magnetically insulated line oscillator (BFMILO) with ridge-disk-loaded vanes has been investigated in order to increase the device efficiency. The influence of ridged slow wave structure (RSWS) on electromagnetic (EM) characteristics of the BFMILO has been studied in the absence of an electron beam with CST Eigenmode solver. Cold (without electron beam) EM simulation demonstrates the impact of the ridge at the disk tip on the resonance frequency, phase velocity, and coupling impedance of the ridged BFMILO (RBFMILO). It shows that the RSWS can significantly enhance the coupling impedance of the RBFMILO, improving overall power efficiency. The CST particle-in-cell (PIC) simulation has been carried out to assess the RF performance of the RBFMILO. For the input dc voltage of 500 kV and the current of 45 kA, the PIC simulation detects a high-power microwave (HPM) with two frequencies of 3.35 and 3.60 GHz, generating a collective peak RF power of 4.2 GW. The peak power conversion efficiency of the device is 18.7%.

Axially Partitioned Dual Band Magnetically Insulated Line Oscillator

In this article, an axially partitioned magnetically insulated line oscillator (MILO) to generate high-power microwave (HPM) with two frequencies in different bands is presented. The electromagnetic characteristics of the dual band MILO (DBMILO) in the absence of an electron beam are investigated to ensure the resonance condition. The RF performance of the device is analyzed through particle-in-cell (PIC) simulation. The proposed DBMILO is designed to operate in <inline-formula> <tex-math notation="LaTeX">$S$ </tex-math></inline-formula>- and <inline-formula> <tex-math notation="LaTeX">$X$ </tex-math></inline-formula>-bands for the input dc voltage and current of 500 kV and 56 kA, respectively. A peak RF power of &#x007E;3.6 GW with peak power conversion efficiency of &#x007E;12.8&#x0025; at 3.4 and 10.1 GHz is detected through PIC simulation. Practical concerns with the HPM source, such as RF breakdown and anode plasma generation at the beam collector in the proposed design, are also explored, along with possible solutions.

Theory, simulation, and experiments on a magnetically insulated line oscillator (MILO) at 10 kA, 240 kV near Hull cutoff condition

The magnetically insulated line oscillator (MILO) is a high power microwave source that has received increased attention recently because it does not require an external magnetic field. Self-magnetic insulation typically requires operation at high currents, ∼50 kA in previous experiments (at ∼10 Ω). This paper reports the first MILO experiment operating at moderate current, less than 10 kA, at a lower voltage of 240 kV, driven by the Michigan Electron Long Beam Accelerator. The viability of this lower current operation was predicted by our recently developed theory on Brillouin flow, which also led to the rigorous derivation, for the first time, of the Buneman–Hartree condition for the cylindrical MILO using both the Brillouin flow and single particle model. The experiments show that more than 90% of shots operate at a magnetic field less than 1.3 times the Hull-cutoff magnetic field, and this magnetic field is significantly lower than the magnetic field required at the Buneman–Hartree condition. These experiments also oscillated at less current than the Hull cutoff condition on over 80% of shots, suggesting that MILOs might operate at a current lower than that expected at exactly Hull cutoff; this peculiar feature was also predicted by the theory. Particle-in-cell simulations from the improved concurrent electromagnetic particle-in-cell (ICEPIC) and CST codes are detailed, which corroborate MILO operation at lower currents than the Hull cutoff condition. The maximum efficiency achieved in these experiments is 1%, at a resonant frequency of 1 GHz. An initial comparison of the newly developed theory against prior MILO experiments is presented.

Multicavity linear transformer driver facility for Z -pinch and high-power microwave research

BLUE is a four-cavity linear transformer driver system in the Plasma, Pulsed Power, and Microwave Laboratory at the University of Michigan. Each BLUE cavity can store up to 2 kJ of electrical energy and provide an open-circuit voltage of 200 kV when using the maximum bipolar charge voltage of $\ifmmode\pm\else\textpm\fi{}100\text{ }\text{ }\mathrm{kV}$. The impedance of each cavity is approximately $0.5\text{ }\text{ }\mathrm{\ensuremath{\Omega}}$, and the rise time of the current pulse into a reasonably matched load is $\ensuremath{\sim}100\text{ }\text{ }\mathrm{ns}$. The system will be used to study high-energy-density $Z$-pinch plasmas and high-power radiation sources. The facility has been equipped to handle intense bursts of x rays, nuclear fusion neutrons, and high-power microwaves (HPM). This paper reports on the construction and performance of the first (prototype) BLUE cavity and its auxiliary systems, as well as preliminary experiments using the cavity to drive resistive loads and a magnetically insulated line oscillator (MILO) HPM device. In these initial single cavity, BLUE-driven MILO experiments, 1.19 GHz microwave oscillations were successfully generated.

Equivalent Circuit Approach for Beam–Wave Interaction Analysis of MILO

The beam–wave interaction analysis in the RF interaction structure of magnetically insulated line oscillator (MILO) is presented. The analysis is carried out using the linearized Vlasov equation by considering the equivalent circuit approach. Taking into account, the beam present dispersion relation is derived to obtain the temporal growth rate and oscillation frequency. In addition, the RF energy stored and power transfer through the slow wave structure during beam–wave interaction process are calculated. Considering the imaginary part of admittance and coupling coefficient of the cavity, the expression for energy stored and power transfer to the load is also derived. The dispersion relation for electron beam interaction with the phase velocity of the oscillating mode, through the slow wave structure, is explained. Furthermore, the derived analytical results temporal growth rate, RF energy, and output power are validated through reported methods (field analysis and particle-in-cell (PIC) simulation). The results are in agreement with the reported results using the field analysis case.

Analysis Design and Simulation of an Axially partitioned Dielectric loaded Bi frequency MILO

In this paper, a bi-frequency magnetically insulated line oscillator (MILO) was proposed and designed. The bi-frequency MILO proposed has two axially partitioned slow-wave interaction structures (SWS) and the second SWS is dielectric-loaded to create the frequency shift in the resonant frequency. The conventional MILO device design methodology was followed along with two SWSs separated by a segregation cavity. The dispersion relation of the dielectric-loaded SWS was calculated using an equivalent circuit approach. Furthermore, the cold analysis was carried out to find the energy stored in the different SWSs to validate the device oscillation frequency. The beam wave interaction behaviour and device RF output performance were investigated through 3D PIC (Particle-in-cell) simulation for typical diode voltage of 550 kV, and current 48 kA, respectively. Simulation results illustrate that the proposed MILO generates RF peak power of ~3.5 GW at frequencies 3.62 GHz and 3.72 GHz. The conversion efficiency of the device was ~13.25%.

HFSS and CST Simulations of a GW-Class MILO

A magnetically insulated line oscillator (MILO) was studied using the simulation software packages high-frequency structure simulator (HFSS) and Computer Simulation Technology-Particle Studio (CST-PS). HFSS was used to find dispersion characteristics of the slow-wave structure and predict resonant-mode frequencies. CST-PS was used to analyze the operation of the device with space charge included. At an applied voltage of 500 kV, CST-PS predicted 1.25-gigawatt (GW) microwave generation at 1.2 GHz while drawing 57 kA. These simulations are consistent with previously published experimental results for a similar MILO. The CST-PS model was used to predict MILO operation as a function of injected voltage, and a separate study was performed to evaluate the effects of an independent, externally applied axial magnetic field. These simulation efforts are in support of upcoming experiments at the University of Michigan, where the MILO investigated in this article will be tested on a new linear transformer driver facility.

MILO Performance Improvement Study—An Equivalent Circuit Approach

In this article, a high-power microwave (HPM) oscillator magnetically insulated line oscillator (MILO) design study has been carried out to improve its performance. Impedances of different subsections of the device have been matched. An equivalent circuit approach, which is simpler and easy to handle, has been adopted for this purpose. First, the input impedance of the tapered inner conductor of the coaxial structure has been matched with the impedance of the choke disks. Then, the extractor inner radius and extractor gap impedance have been optimized at the output section. This impedance matching scheme provides the maximum efficiency of the device with the maximum RF transmission. The optimum position of the stub to match the impedance with the output side has also been analyzed. To validate our concept, initially, an experimentally reported L-band MILO has been simulated using commercial particle-in-cell (PIC) code, which provides an RF output power of 1.38 GW with the efficiency of 7.8%. The PIC simulation results have been found in agreement of ~5% with the experimental values. Furthermore, the PIC simulation with the optimized design parameters has improved the RF output power of 2.5 GW with an efficiency of 14.4%.

Design of a dual-band radiation system for a complex magnetically insulated line oscillator

In this paper, a dual-band radiation system for a complex magnetically insulated line oscillator (MILO) is designed and investigated numerically. The radiation system comprises a coaxial plate-inserted mode converter, a power combiner and a conical horn antenna. The mode converter converts the coaxial TEM mode microwaves (1.775 GHz and 3.175 GHz) which are generated by the complex MILO into the coaxial TE11 mode microwaves, and then the coaxial TE11 mode microwaves are combined by the power combiner in a circular waveguide. Lastly, the microwaves are radiated by a conical horn antenna into the air. The gains of the dual-band radiation system are calculated to be 17.8 dB at 1.775 GHz and 18.9 dB at 3.175 GHz. The 3 dB beam widths are 20.5° in E-plane, 26.4° in H-plane at 1.775 GHz and 20.8° in E-plane, 15.1° in H-plane at 3.175 GHz. The power transmission efficiencies of the dual-band radiation system are 98.5% at 1.775 GHz and 95.7% at 3.175 GHz respectively. The power handling capacities of the dual-band radiation system are 4.2 GW at 1.775 GHz and 4.7 GW at 3.175 GHz, respectively.

Influence of Collector Plasma on the Performance of an L-Band MAC-MILO

The influence of collector plasma has been investigated for an L-band magnetically insulated line oscillator (MILO) with metal array cathode (MAC), which showed obvious performance degradation after 3000 shots in repetitive mode. Positive ion emission model is applied to simulate plasma generation from the collector surface. Different types of positive ions have been applied to simulate different collector materials. The simulation results reveal that the collector plasma has a significant influence on the output characteristics of the original MAC-MILO. The ladder cathode structure is introduced to reduce plasma formation rate on the collector surface. The simulation and experimental results show that the optimized structure reduces the influence of collector plasma successfully.

Simulational Investigation of a High-Efficiency X-Band Magnetically Insulated Line Oscillator**supported by National Natural Science Foundation of China (No. 11075210) and the Special Financial Grant from the China Postdoctoral Science Foundation (No. 201104761)

The magnetically insulated line oscillator (MILO) is a gigawatt-class, coaxial crossed-field microwave tube, which is at present a major hotspot in the field of high-power microwaves (HPM) research. In order to improve the power conversion efficiency and eliminate or at least minimize anode plasma formation in the load region and radio frequency (RF) breakdown in the slow wave structure (SWS) section, an X-band MILO is presented and investigated numerically with KARAT code. The design idea is briefly presented and the simulation results are given and discussed. In the simulation, HPM is generated with peak power of 3.4 GW, maximum electric field of about 1 MV/cm, and peak power conversion efficiency of 14.0%, when the voltage is 559.1 kV and the current is 43.2 kA. The microwave frequency is pure and falls in the X-band of 9.0 GHz. The theoretical investigation and the simulation results are given to prove that the anode plasma formation and the RF breakdown can be effectively avoided or at least minimized, respectively.

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.

Performance Improvement Study of Tapered Magnetically Insulated Line Oscillator Through Impedance Matching

An all metal disk-loaded coaxial tapered waveguiding structure, for its application in a tapered magnetically insulated line oscillator (MILO), as a slow-wave interaction structure, excited in the TM mode, is analyzed for performance improvement study to increase device total efficiency by applying the matching concepts of the impedance for the structure at the input and output sides. Impedance matching is the technique to get maximum efficiency from the device, which is one of the best tools to improve the performance of the device. The novelty of method, which is used for tapered RF interaction structure MILO, is analyzed first time and simulated using impedance matching techniques. Comparison of results for its efficiency between matching and without matching circuit for tapered MILO is presented. The new design is given with high efficiency and good matching condition for its reflection and transmission coefficient. Validation of results for tapered MILO using matching conditions is shown in the close agreement with electromagnetic simulation code MAGIC ~5 %. Author has derived all the new expressions for the device by overcoming the earlier designs and analyzed the effects of the linear parameter properties on the efficiency improvement. The results obtained from the analysis have shown that the efficiency increases up to 12% in total, 11% in conversion, and 20% in extraction for the device, which is 15.5% of total, 21% of conversion, and 73% of extraction efficiencies without matching circuit as well as 27.28% of total efficiency, 32% of conversion efficiency, and 73% of extraction efficiency at 600 kV applied voltage and 35 kA of beam current for the matching circuit implementation for the structure.

Magnetically Insulated Line Oscillator (MILO) Performance Study and its Parameter Optimization

A magnetically insulated line oscillator (MILO), consisting of discs-loaded coaxial cylindrical waveguide as its RF interaction structure, is field analyzed for the beam-wave interaction using linearized-Vlasov equation. We have made a sensitivity analysis and proposed a method to optimize the device structure for its beam parameters, structure parameters, and its subassemblies using field analysis and particle-in-cell (PIC) simulations. In this paper, a MILO device is analyzed for its designed value of 600 kV of dc beam voltage and 35 kA of beam current. The analysis predicts 1.0-GW RF output power, 60-J energy, and ~ 6% of overall efficiency of the device for the typical design parameters. The parametric analysis has been also made, which indicates the sensitivity of the various RF structure parameters on the device performance in terms of RF power output, temporal growth rate, quality factor, and released RF energy. Through optimization, the device parameters are fixed for the device design purposes. Analytically optimized results of the RF interaction structure have been validated using a commercial PIC simulation code, MAGIC, and is found to be in agreement with analytical ~ 5%. The optimized S-band MILO structure provides the RF power output of 1-2 GW and energy 80-100 J. We hope that the present study will be of use to MILO designers in optimizing its structure parameters, keeping the structure losses within the desired limit.

Design Expressions for the Magnetically Insulated Line Oscillator

In this paper, a simple compact magnetically insulated line oscillator (MILO) design methodology, simulation, and optimization are described the MILO device combines the technologies of magnetically insulted electron flow with the slow-wave structure, and the device operates at a wide range of voltage $(0.1~{\rm MV}\leq{\rm V}\leq 1~{\rm MV})$ . The authors present the design methodology for a load-limited MILO device of a desired oscillation frequency, radio-frequency (RF) power, and applied dc voltage. An S-band MILO is designed and simulated using commercial particle-in-cell code MAGIC, for 600-kV dc voltage and 35-kA beam current. The simulated value for RF power is found to be in agreement with the designed value within 5% and an average output power of 1.05 GW at designed 2.725-GHz frequency is obtained.

Electrode-plasma-driven radiation cutoff in long-pulse, high-power microwave devices

The impact of electrode plasma dynamics on the radiation production in a high power microwave device is examined using particle-in-cell simulations. Using the design of a compact 2.4 GHz magnetically insulated line oscillator (MILO) as the basis for numerical simulations, we characterize the time-dependent device power and radiation output over a range of cathode plasma formation rates. These numerical simulations can self-consistently produce radiation characteristics that are similar to measured signals in long pulse duration MILOs. This modeling capability should result in improved assessment of existing high-power microwave devices and lead to new designs for increased radiation pulse durations.

高阻抗磁绝缘线振荡器微波提取结构

The paper presents the potential problem in traditional microwave extractor design in the magnetically insulated line oscillator (MILO). Through high frequency electromagnetic simulation, the dependence of TEM mode transmission efficiency on frequency is obtained at different impedance of the microwave extractor. Simulation results show that the microwave extractor designed in traditional way can not meet the demand of TEM transmission. In order to conquer the transmission difficulty of TEM mode, the paper introduces the scheme using ladder load. Further simulation results indicate that the ladder load conquers the transmission difficulty of TEM mode and improves the adaptability of the microwave extractor to frequency drifting of the MILO.

Beam-wave interaction analysis of a magnetically insulated line oscillator

Magnetically insulated line oscillator (MILO) using a metal disc-loaded coaxial cylindrical waveguide as its RF interaction structure is field analyzed for the beam-wave interaction using the linearized Vlasov equation. The beam present dispersion relation of the device is obtained applying the modal matching technique which is further used to estimate the oscillation frequency, temporal growth rate, output energy, and other device parameters. Further, MILO is simulated using commercial pic code “magic,” and the electron momentum and energy plots are found to be in agreement with those obtained through the present analysis within 5%. Furthermore, the device temporal growth rate reported in the literature for the experimental MILO device is also compared with the present analytical theory and found in agreement ∼5%.