3:30pm - 4:10pmHigh Power Microwaves: 1
High Power Microwave and Pulsed Power Development at the University of Michigan
University of Michigan, United States of America
The Plasma, Pulsed Power, and Microwave Laboratory (PPPML) at the University of Michigan (UM) is home to three large pulsed-power drivers: the Michigan Electron Long Beam Accelerator (MELBA), the Michigan Accelerator for Inductive Z-pinch Experiments (MAIZE), and the Bestowed LTD from Ursa Minor Experiment (BLUE). MELBA is a 7-switch Marx generator with an Abramyan circuit and is capable of generating a 10 kA electron beam at -1 MV for up to 1 µs; this accelerator is currently configured to produce -300 kV and is used for high-power microwave (HPM) research. MAIZE is a 3-m-diameter, single-cavity Linear Transformer Driver (LTD) that supplies a 1 MA, 200 ns pulse for high energy-density physics (HEDP) research. BLUE is the most recent addition to the PPPML, consisting of four 1.25 m diameter LTD cavities which were previously part of Sandia’s 21-cavity Ursa Minor facility. A single cavity of BLUE produces an estimated 150 kA at 100 kV in ~100 ns into a load matched to the driver’s 0.5 Ω impedance. The 4 cavities can be stacked for a total driver impedance of ~ 2 Ω and correspondingly increased matched-load voltage of 400 kV.
Recent HPM developments will be presented, including: a multi-frequency Harmonic Recirculating Planar Magnetron (HRPM) utilizing a dual-frequency (L-band and S-band) slow-wave structure to enable low-Q operation at the MW level; a 5 MW Recirculating Planar Crossed-Field Amplifier (RPCFA) with ~ 9 dB gain at 3 GHz; experimental demonstration of the Recirculating Planar Magnetron with Coaxial-All-Cavity Extraction (RPM-CACE); a moderate current (< 10 kA) Magnetically Insulated Line Oscillator (MILO) operating in L- and S-band; and the implementation of a GW-class MILO on the BLUE LTD.
Pulsed power developments will also be highlighted, particularly the recent improvements to spark-gap switch reliability in MAIZE. UM has tested 3 spark-gap switch designs on MAIZE, and uncovered a number of operating modes that result in inconsistent breakdown and triggering. The lessons learned from these undesirable operating conditions, and the subsequent methods to achieve reliable operation, should benefit the growing population of researchers using and designing LTDs.
Research supported by The Air Force Office of Scientific Research #FA9550-15-1-0097, FA9550-20-1-0409, and FA9550-21-1-0184, Office of Naval Research #N00014-19-1-2262, #N00014-18-1-2499, and #N00014-16-1-2353, NNSA # DE-NA0003764, DEPS Fellowship support to DP, and L3Harris Electron Devices.
4:10pm - 4:30pmHigh Power Microwaves: 2
Modeling Composite Nonlinear Transmission Lines as High-power Microwave Sources
Purdue University, United States of America
Nonlinear transmission lines (NLTLs) can sharpen input pulses and induce output oscillations as high-power microwave (HPM) sources with high pulse repetition rates, frequency agility, durability, and reliability, leading to compact devices with inexpensive construction costs and reduced power consumption . In general, NLTLs use ferroelectric and/or ferromagnetic materials with field-dependent permittivity and/or permeability, respectively; implementing ferromagnetic materials produces microwave oscillations through gyromagnetic precession when biased under an external magnetic field .
In this work, we use COMSOL Multiphysics to model NLTLs containing ferroelectric and/or ferromagnetic composites and compare to experimental results. We have previously measured and simulated the linear effective electromagnetic properties of composites containing various volume loadings of barium strontium titanate (BST) and/or nickel zinc ferrite (NZF), which are nonlinear dielectric and magnetic materials, respectively . We have also measured the nonlinear permeability and nonlinear permittivity of various volume loadings of these materials. These studies demonstrate the tunability of the electromagnetic properties of the composites, which may be used to adjust the RF output from a NLTL.
To reduce computational expense, we model the composite regions in the NLTLs as homogeneous domains. To model the gyromagnetic NLTLs with ferrite composites, we solve the Landau-Lifshitz equation  and treat the gyromagnetic ratio and damping factor as fitting parameters determined by comparison to experiments. The center frequency of the output pulses primarily varies with the gyromagnetic ratio when the bias field, the incident field, saturation magnetization of the applied ferrites, and ferrite filling ratio are fixed . We compare the resulting models to experiments using NLTLs with different compositions of BST and/or NZF driven by different pulse forming lines. We then use the benchmarked model to predict performance with different materials and volume loadings to assess potential future designs. Implications for a complete HPM system design integrating an antenna and pulse forming network will be discussed.
We gratefully acknowledge funding from the Office of Naval Research (Grant No. N00014-18-1-2341).
 A. J. Fairbanks, A. M. Darr, and A. L. Garner, “A review of nonlinear transmission line system design,” IEEE Access, vol. 8, pp. 148606 – 148621, 2020.
 X. Zhu, A. J. Fairbanks, T. D. Crawford, and A. L. Garner, “Modelling effective electromagnetic properties of composites containing barium strontium titanate and/or nickel zinc ferrite inclusions from 1-4 GHz,” Compos. Sci. Technol., vol. 214, 2021, Art. No. 108978.
 I. V. Romanchenko, V. V. Rostov, A. V. Gunin, and V. Y. Konev, “High power microwave beam steering based on gyromagnetic nonlinear transmission lines,” J. Appl. Phys., vol. 117, 2015, Art. no 214907.
4:30pm - 4:50pmHigh Power Microwaves: 3
System Design Considerations for a Nonlinear Transmission Line Used Simultaneously as a Pulse Forming Line and High-Power Microwave Source
Purdue University, United States of America
Nonlinear transmission lines (NLTLs) have been of increasing interest for pulse sharpening and high-power microwave (HPM) generation. Their compact form factor coupled with their inexpensive and rigid design makes them ideal for field implementation where system survivability is a concern.
NLTLs are just one subcomponent in the overall HPM systems structure. Recent efforts have examined using the NLTL simultaneously as both the pulse source and HPM generator by biasing the lines with a DC charging voltage . While this further reduces the spatial footprint of the system, such a design has inherent complications associated with extracting the signal it generates.
This work focuses on full system design considerations when using a NLTL in the PFL format. We manufactured 10-ohm composite based NLTLs that utilize a combination of barium strontium titanate and nickel zinc ferrite encapsulated in PDMS. The output of the NLTL was coupled to a pressurized spark gap switch that closed upon reaching a sufficient charging voltage. An impedance transformer was then designed to taper the impedance to a 50-ohm standard. We demonstrate that the RF output of the NLTL is a strong function of impedance with the RF signal becoming weaker with increasing impedance. This provides additional motivation for the NLTL in the PFL format since the impedance is readily flexible since it does not need to be directly matched to a Pule Forming Network. The ability to enhance RF generation with a lower impedance may permit further reduction in device size by eliminating the need for additional systems, such as a bias magnetic field. Overall implications on system development will be discussed.
1. A. J. Fairbanks, T. D. Crawford, and A. L. Garner, “Nonlinear transmission line implemented as a combined pulsed forming line and high-power microwave source,” Rev. Sci. Instrum., vol. 92, 2021, Art. no. 104702.
4:50pm - 5:10pmHigh Power Microwaves: 4
RF Output Power Detection of the RADAN MG-4 Microwave Generator
Texas Tech University, United States of America
The RADAN series-based MG-4 Microwave Generator is a compact, high power microwave system developed by the Institute of Electrophysics in Ekaterinburg. The system features the RADAN high voltage generator which is a SINUS-series device featuring a Tesla transformer charger and a Blumlein pulse forming line. The MG-4 microwave head is a mm-band relativistic backward wave oscillator (BWO) that operates at 35 GHz with a 5 to 10 MW peak output power and a pulsewidth of 3 nsec. The typical method of output power measurement is done with a cryogenic detector supplied with the system which utilizes a germanium crystal that changes resistivity as microwave radiation is absorbed.
In order to confirm the rf output power level of the MG-4, and because the germanium crystal rf detector was unavailable, a commercially available rf envelope detector was employed. Analog Devices ADL6012 is a broadband envelope detector that operates from 2 GHz to 67 GHz at input powers up to +15 dBm. It also features a 500 MHz envelope bandwidth and 0.6 nsec output risetime capability.
The diagnostic setup features the ADL6012-EVALZ, an evaluation board with the ADL6012 offered by Analog Devices, shielded in a fitted brass box located in the far field (~60 cm) from the microwave output horn. The output mode of the MG-4 is nominally TM01 but a mode convertor allows for a TE11 output mode as well. The surrounding surfaces close to the detector are covered with attenuation foam to limit reflections that could possibly be detected and interfere with measurements. A 20 dBi receiving antenna and four high frequency attenuators are used to reduce the input power to the acceptable input range of the detector. Two equal length coax cables connect the differential outputs from the detector to two channels of a high-speed 1.5 GHz oscilloscope where the positive and negative envelopes of the pulse are captured separately. Based on the peak differential output voltage of the positive and negative signal, the input power of the detector can be determined by the typical performance characteristics curves in the ADL6012 data sheet. Lastly, accounting for the attenuators, antennas, and free space path loss, the peak output power of the MG-4 can be accurately determined. At 60 cm centered from the MG-4, the ADL6012 output a 660 mV differential voltage. Using the typical application curves in the data sheet, this corresponds to a 4.4 dBm input power into the ADL6012. Accounting for the attenuators, receiving antenna, and free space path loss, the transmitted peak power of the MG-4 is 98.17 dBm (6.56 MW). This is in the expected output power range of the MG-4.
5:10pm - 5:30pmHigh Power Microwaves: 5
Compact 60kV High Voltage Capacitor Charger for UWB Electromagnetic Pulse Generator
Chung-Ang University, Korea, Republic of (South Korea)
In this paper, a portable 60kV high-voltage capacitor charger for ultra-wideband electromagnetic pulse generator based on a 24V battery was described. The HVCC should charge storage capacitors up to breakdown voltage(about 55kV) of spark gap switch inside Marx generator of the ultra-wideband electromagnetic pulse generator at 100Hz repetition rate. It should be considered not only the operation specification, but also size and weight for portability, nonetheless current burden on the used components is relatively large because of low input voltage. In addition, there are other difficulties such as the voltage stress of each components and isolation from other parts like grounded case. To satisfy the requirements, a parallel loaded resonant converter which operates as high-efficiency and high-frequency and an output rectifier designed by modifying the basic Cockcroft-Walton voltage multiplier(CWVM) were applied. The parallel loaded resonant converter operating at above resonant frequency was designed with a small value of parallel resonant capacitor to reduce reactive power, crest factor of the resonant current, and conduction loss. Also, proper snubber capacitor design is applied to reduce turn off switching loss. The modified CWVM is composed of two symmetrical CWVMs charged in parallel by a center-tapped transformer and storage capacitors inside the CWVMs are connected in series to the load. With this structure, it can be alleviated the voltage stress and maximum voltage potential. Therefore, it significantly reduces the difficulty of selecting components, design of the high voltage transformer and considering insulation when manufacturing actual system. In addition, it can be operated through very simple sensorless control method, only needs the information of the time to charge the load capacitor once and repetition rate, due to the characteristics of the designed resonant converter, such as a current source characteristic and inherent maximum voltage limit. Between the HVCC and Marx generator, the conductorless high voltage cable is connected to replace the isolation resistor to block a noise generated by UWB EMP, and to limit the excessive charging speed. Finally, the HVCC was actually implemented as small(120*120*245mm) and lightweight(4kg). Various experiments with 3.2nF and 8.4nF load capacitors equivalent to the sum of storage capacitors in the Marx generator were conducted. Also, it is verified that the HVCC can charge to 62kV and be inherently limited due to the characteristic of the designed converter without any control. Finally, it was verified by the experimental result with the actual Marx generator load that the HVCC repeatedly charge over 50kV at 100Hz repetition rate.