Robert E. Myers

Demonstration of an S-band, 600-kW fundamental-mode multiple-beam klystron

We present initial experimental results from the successful operation of a 600-kW peak, fundamental-mode multiple-beam klystron (MBK). The eight-beam device operates at a cathode voltage of /spl sim/45 kV and a total beam current of /spl sim/32 A with an axial guiding magnetic field of 1.8-2.2 kG. In the absence of radio-frequency (RF) drive, the measured beam transmission is in excess of 99%; at a driven frequency of 3.25 GHz, the measured beam transmission at saturation is /spl ges/97%, where the four-cavity circuit generates a peak power of /spl sim/600 kW with an electronic efficiency of 40%. The measured beam transport and RF performance are in excellent agreement with predictions made by the three-dimensional gun/collector code, MICHELLE, and the large-signal klystron code, TESLA. The accuracy of the design codes enabled the achievement of a working device in a single hardware design pass.

Simulations of direct-die-attached microchannel coolers for the thermal management of GaN-on-SiC microwave amplifiers

This paper presents finite-element thermo-mechanical simulation studies of microchannel-based techniques to cool AlGaN/GaN high electron mobility rf transistors grown on SiC substrates. A number of problems are considered, including standard thickness dies on both oxygen-free-high-conductivity (OFHC) copper and AlN microchannel coolers, as well as thinned dies on a hybrid diamond/silicon microchannel cooler. The active device sizes and cooling strategies selected are relevant to X-band (/spl sim/10 GHz) amplifiers dissipating 50-100 W of steady-state waste heat. The effects of die attach materials on device temperature and mechanical stresses are studied. The plastic yielding behaviors of the die attach material and other metallic portions of the package are incorporated into the analysis. The removal of 100 W of steady-state waste heat in an example X-band compatible device is found to be consistent with 140-185/spl deg/C maximum transistor junction temperatures and tolerable mechanical stresses.

Demonstration of a Multikilowatt, Solenoidally Focused Sheet Beam Amplifier at 94 GHz

A technological breakthrough is embodied in the successful demonstration of an extended interaction klystron (EIK) amplifier, which has produced over 7.5 kW of peak output power at W-band (94 GHz). An efficiency of ~17% has been achieved with a depressed collector. The EIK is driven by a 20-kV, 4-A sheet beam in a permanent magnet solenoid, with 99% beam current transmission from gun to collector. Key features that contribute to the success of this device are: tight beam focusing and correspondingly narrow beam tunnel, which are made possible by the solenoidal focusing and which provide high interaction impedance and high gain per unit length and the incorporation of design elements to stabilize the inherently over-moded circuit. Measured performance agrees well with 3-D particle-in-cell simulations.

Characterization of a Ka-band Sheet-Beam Coupled-Cavity Slow-Wave Structure

This paper investigates the properties of a three-slot doubly periodic staggered-ladder sheet-beam coupled-cavity slow-wave structure (SWS) developed at the U.S. Naval Research Laboratory. The structure is overmoded with complicated mode crossings and field structures. The staggered-ladder structure is compared to round-beam structures via full-wave electromagnetic simulations and experimental measurements. We explore the application of this SWS in a traveling-wave tube amplifier.

Linearizability of TWTAs using predistortion techniques

Predistortion linearization is a very effective technique for improving the linearity and efficiency of traveling wave tube (TWT) amplifiers. In this paper, we will study the effectiveness of predistortion techniques for TWT linearization using single-tone, two-tone, and quadrature-amplitude-modulation signals. The results from a series of predistortion linearization experiments for five TWTs covering L-, C-, Ku-, and Ka-bands and including both helix- and coupled-cavity TWTs will be presented. We will demonstrate the additional improvement of fifth-order predistortion linearization over the more commonly used third-order predistortion linearization. To circumvent the complexity and limited availability of a pure fifth-order linearizer, a technique for realizing nonlinear functions of order greater than or equal to five using cascaded third-order nonlinear functions is described. The technique can be used to efficiently generate higher order nonlinearities for predistortion linearization applications. We will demonstrate experimentally that the use of two cascaded third-order functions is comparable to a pure fifth-order implementation in performance.

Demonstration of a Wideband 10-kW Ka-Band Sheet Beam TWT Amplifier

A sheet-beam coupled-cavity traveling wave tube has produced over 10 kW of peak power at a center frequency of 34 GHz, with a 3-dB bandwidth of almost 5 GHz. The power of this amplifier is an order of magnitude higher than state-of-the-art conventional amplifiers of comparable frequency, bandwidth, and operating voltage (<;20 kV). This unprecedented performance is made possible by a unique, Naval Research Laboratory (NRL)-developed sheet electron beam along with a novel slow-wave interaction structure. High-current, low-voltage operation provides high gain per unit length and allows an interaction structure<;5-cm long to be used to achieve the desired gain of 15 dB at saturation. Measured performance agrees well with 3-D particle-in-cell simulations.

Bunch characteristics of an electron beam generated by a diamond secondary emitter amplifier

Electron bunches for high performance free electron lasers are subject to constraints on charge per bunch and pulse shape. A Diamond secondary emitter used in conjunction with a photocathode and drive laser has potential to enable a high brightness, high peak current photoinjector by increasing the effective quantum efficiency of the photocathode. A theoretical characterization of the bunches so produced has been heretofore absent. Using a combination of Monte Carlo and analytical models, the shape of the bunches, their transit time, and emission time constants are determined and shown to be sensitive to the accelerating field in the diamond flake, incident beam profile, doping, and surface conditions. Methods to allow for extension to regimes of technological interest in terms of diamond thickness, external field, and primary pulse shape are given.

Experimental Study and Analysis of an S-Band Multiple-Beam Klystron With 6% Bandwidth

We present experimental results and analyses of an eight-beam five-cavity multiple-beam klystron (MBK) operating at a center frequency of ~3.2 GHz. The device met its performance goals in its first hardware implementation, generating a peak RF output power of 600 kW and a 3-dB bandwidth of ~6%. The circuit was modeled with TESLA, a 2.5-D large-signal klystron/MBK code that was extended to enable simulations of the low- Q multiple-gap cavities used to increase the bandwidth. Details of the model and underlying theory are described, and the simulation results are compared with experimental measurements. The good agreement between the model and the experiment provides a validation for our tools and techniques that will be used in the design of future devices.

High-power millimeter-wave transmitter for the NRL WARLOC radar

High power millimeter wave instrumentation radars have a number of important applications ranging from defense missions to basic scientific studies At the Naval Research Laboratory, a new high power 94 GHz radar named WARLOC has been developed. This radar employs a high power gyro-klystron as the final power amplifier and was developed during 1996-2001. The WARLOC radar has been integrated as a transportable system, using the 100 kW peak, 10 kW average power gyro-klystron amplifier, a low-loss transmission line, a quasioptical duplexer, and a Cassegrain antenna. The transmitter operation and waveguide system is the subject of this paper.

Broadband Microwave and W-Band Characterization of BeO-SiC and AIN-Based Lossy Dielectric Composites for Vacuum Electronics

The complex dielectric permittivity properties of lossy ceramic materials used in vacuum electronics are presented. The studies include broadband room temperature behavior in the 0.1-18 GHz range and in W-Band (75-110 GHz). Variable temperature measurements of selected materials at 94 GHz are also presented