David V. Reale

Bias-field controlled phasing and power combination of gyromagnetic nonlinear transmission lines

Gyromagnetic Nonlinear Transmission Lines (NLTLs) generate microwaves through the damped gyromagnetic precession of the magnetic moments in ferrimagnetic material, and are thus utilized as compact, solid-state, frequency agile, high power microwave (HPM) sources. The output frequency of a NLTL can be adjusted by control of the externally applied bias field and incident voltage pulse without physical alteration to the structure of the device. This property provides a frequency tuning capability not seen in many conventional e-beam based HPM sources. The NLTLs developed and tested are mesoband sources capable of generating MW power levels in the L, S, and C bands of the microwave spectrum. For an individual NLTL the output power at a given frequency is determined by several factors including the intrinsic properties of the ferrimagnetic material and the transmission line structure. Hence, if higher power levels are to be achieved, it is necessary to combine the outputs of multiple NLTLs. This can be accomplished in free space using antennas or in a transmission line via a power combiner. Using a bias-field controlled delay, a transient, high voltage, coaxial, three port, power combiner was designed and tested. Experimental results are compared with the results of a transient COMSOL simulation to evaluate combiner performance.

Theoretical performance of a GPS linked Pulsed Ring Down Array

Current research at Texas Tech University is focused on the development of a High-Power Pulsed Ring-Down Source (PRDS) Antenna Array. Previously, a Monte Carlo based analysis was conducted in order to predict the array performance based upon the estimated switching jitter between elements [1]. This analysis showed good performance for jitter times between 0 to 2 periods of the ringing frequency. Therefore, for ringing frequencies up to 500 MHz, jitter times up to 4 nanoseconds can be tolerated. Subsequently, we have shown practical switching solutions capable of the sub-nanosecond switching performance needed for the frequencies of interest [2]. Taking the analysis a step further, we introduce the uncertainty of the absolute position of each antenna element. To implement a randomly distributed array, where the position of elements is not fixed, a method of accurately resolving element positions relative to each other and the target location is required. The use of a variety of GPS technologies and techniques is explored as a method for position and timing resolution. The relative accuracy between elements and the absolute accuracy of each element is discussed. A Monte Carlo based analysis is conducted to predict array performance based upon GPS positional error, GPS timing error, and switch jitter.

Characteristics of a four element gyromagnetic nonlinear transmission line array high power microwave source

In this paper, a solid-state four element array gyromagnetic nonlinear transmission line high power microwave system is presented as well as a detailed description of its subsystems and general output capabilities. This frequency agile S-band source is easily adjusted from 2-4 GHz by way of a DC driven biasing magnetic field and is capable of generating electric fields of 7.8 kV/m at 10 m correlating to 4.2 MW of RF power with pulse repetition frequencies up to 1 kHz. Beam steering of the array at angles of ±16.7° is also demonstrated, and the associated general radiation pattern is detailed.

Material selection of a ferrimagnetic loaded coaxial delay line for phasing gyromagnetic nonlinear transmission lines

Implementing nonlinear transmission line (NLTL) technology in the design of a high power microwave source has the benefits of producing a comparatively small and lightweight solid-state system where the emission frequency is easily tuned. Usually, smaller in physical size, single NLTLs may produce significantly less power than its vacuum based counterparts. However, combining individual NLTL outputs electrically or in free-space is an attractive solution to achieve greater output power. This paper discusses a method for aligning a four element NLTL antenna array with coaxial geometry using easily adjustable temporal delay lines. These delay lines, sometimes referred to as pulse shock lines or pulse sharpening lines, are placed serially in front of the main NLTL line. The propagation velocity in each delay line is set by the voltage amplitude of an incident pulse as well as the magnetic field bias. Each is adjustable although for the system described in this paper, the voltage is held constant while the bias is changed through applying an external DC magnetic field of varying magnitude. Three different ferrimagnetic materials are placed in the temporal delay line to evaluate which yields the greatest range of electrical delay with the least amount of variability from consecutive shots.

Investigation of a stripline transmission line structure for gyromagnetic nonlinear transmission line high power microwave sources

A stripline gyromagnetic nonlinear transmission line (NLTL) was constructed out of yttrium iron garnet ferrite and tested at charge voltages of 35 kV-55 kV with bias fields ranging from 10 kA/m to 20 kA/m. Typically, high power gyromagnetic NLTLs are constructed in a coaxial geometry. While this approach has many advantages, including a uniform transverse electromagnetic (TEM) mode, simple interconnection between components, and the ability to use oil or pressurized gas as an insulator, the coaxial implementation suffers from complexity of construction, especially when using a solid insulator. By moving to a simpler transmission line geometry, NLTLs can be constructed more easily and arrayed on a single substrate. This work represents a first step in exploring the suitability of various transmission line structures, such as microstrips and coplanar waveguides. The resulting high power microwave (HPM) source operates in ultra high frequency (UHF) band with an average bandwidth of 40.1% and peak rf power from 2 MW to 12.7 MW.

Theoretical performance of a mobile GPS linked pulsed ring down array

The development of mobile Pulsed Ring Down Source (PRDS) arrays requires the ability to accurately determine the relative positions of array elements at distances, and in situations, where discrete measurements are not practical. At the frequencies of interest, centimeter level accuracy is required for the array to localize radiated energy at a given target location. Global Positioning System (GPS) devices and techniques are evaluated for the purpose of position acquisition. Previously a Monte Carlo simulation was developed that takes into account the position error, the GPS timing error, and the switch jitter of the element. The error sources are combined and used a metric to evaluate and predict the array performance. Results of the GPS device testing, as well as previous work, are used as the input parameters of the simulation to determine their viability for use in the implementation of PRDS arrays capable of radiating at frequencies of up to 500 MHz.

Synchronization of phased array pulsed ring-down sources using a GPS based timing system

A collaborative effort at Texas Tech University on high power RF transmitters has directly translated to the development of phased array pulsed ring down sources (PRDS). By operating an array of PRDS, peak radiating power on target can theoretically be multiplied by the number of sources. The primary limitation on the application of the array concept is the jitter with which the individual sources can be fired. An ideal jitter of a small fraction of the risetime is required to accurately synchronize the array to steer and preserve the risetime of the radiated pulse. This paper describes in detail the implementation of a GPS based timing system that will synchronize the individual antennas to operate at different geo-locations to function in a coordinated fashion to deliver the peak power of each element to a single position. Theoretical array performance is shown through Monte Carlo simulations, accounting for switch jitter and a range of GPS timing jitter. Each module will include a control unit, low jitter pulser [1], low jitter spark gap, antenna element, as well as a GPS receiver. The location of each module is transmitted to a central controller, which calculates and dictates when each element is fired. Low jitter in the timing of the GPS reference signal is essential in synchronizing each element to deliver the maxim power. Testing using a preliminary setup using GPS technology is conducted with both 1 pps and 100 pps outputs. Jitter results between modules are recorded to ∼10 ns without any correction factors. With the timing and geospatial [2] errors taken into account, the proposed concept will show usable gains of up to several hundred MHz.

High voltage solid dielectric coaxial ferrimagnetic nonlinear transmission line

At the Center for Pulsed Power and Power Electronics, previous coaxial ferrimagnetic Nonlinear Transmission Lines (NLTL) relied solely on pressurized Sulfur Hexafluoride (SF6) as high-voltage insulating dielectric medium [1]. While the use of SF6 provides the necessary electric insulation, there are drawbacks including gas storage and pressure fittings that increase system size and add to the design complexity of the NLTLs themselves. Hence it was deemed necessary to evaluate solid dielectric materials as an alternative. Initial attempts utilized a standard high voltage (HV) epoxy to pot the NLTL assembly. This method was effective at producing magnetic precession in the NLTL; however, there was a reduction in output power due to the high loss tangent of the epoxy. Sylgard 184, commonly used in solar cells, has also found use as an HV potting material. Per datasheet, its loss tangent is an order of magnitude lower compared to standard HV epoxy at 1 kHz. Samples of HV epoxy and Sylgard 184 were tested in a microwave cavity resonator at 3GHz, which yielded their respective loss tangents. The performance of an NLTL potted with Sylgard 184 is compared to that of the HV epoxy NLTL and the earlier SF6 insulated NLTL.

Radiation from SiC PCSS driven gyromagnetic nonlinear transmission line high power microwave source

An all solid-state high power microwave (HPM) source is constructed using a photoconductive semiconductor switch (PCSS) based HV pulse generator to drive a sulfur hexaflouride (SF6) insulated coaxial ferrimagnetic nonlinear transmission line (NLTL) which feeds a TEM horn antenna. The PCSS was fabricated from high purity semi-insulating (HPSI) 4H-SiC and is illuminated with 2 mJ from a frequency tripled Nd:YAG laser at 355 nm with a 7 ns FWHM. Fixed fiber optic delay lines are utilized to generate a burst of four optical pulses from a single solid-state laser source for rep-rate operation. The input to the NLTL is an adjustable pulse from 3 kV to 6 kV with sub-ns rise time and the resulting output of the NLTL is radiation in the L-band to S-band regime with RF power from 100-200 kW depending on charge voltage. A Rexolite® insulated zipper transition, in which the inner conductor of the coax is gradually exposed, is used at the output of the NLTL to transition from a coaxial structure to parallel plate structure in order to feed a free space TEM horn. The zipper transition and feed section of the TEM horn are potted using Sylgard® 184 silicone elastomer to prevent breakdown at the antenna feed and maintain a homogenous dielectric constant for the transition region. Radiated waveforms are presented for several charge voltages and bias conditions.

A high-power transient coaxial power combiner for nonlinear transmission lines

Recent work on Coaxial Ferrimagnetic Nonlinear Transmissions Lines (NLTL) has been focused on developing an array of NLTLs for use as a solid-state High Power Microwave (HPM) source. The pulsed output of an NLTL requires a combiner that can combine transient signals at voltage levels up to 50 kV. Existing combiner designs found in literature require resonant structures to achieve efficient power combination. The presented coaxial combiner is an in-plane structure designed to combine two 50 Ω transmission lines into a single 25 Ω coaxial line output which then uses a logarithmic taper to transition back to 50 Ω. The combiner design was simulated using a transient Finite Element Method (FEM) model in COMSOL Multiphysics® and experimental results are compared with simulation. A 4-way combiner based on an in-plane 2-way design is simulated and the field stresses are examined to determine maximum electric field levels encountered in the structure. This enabled estimating the associated maximum voltage level that the structure is able to support.