Frank Wood

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.

NRL ubitron amplifier cold test results

Abstract The Naval Research Laboratory ubitron (a low voltage free electron laser) amplifier experiment is nearing operation. The experiment has been designed to explore the potential of the ubitron as an efficient, high power radiation source. The initial experimental configuration consists of a modified SLAC klystron gun, “turnstile” rf input coupler, double taper bifilar helix wiggler, rf output monitor, and an rf calorimeter/load, and has been designed to operate in the collective FEL mode. The results of component cold testing will be presented along with the calculated performance of the amplifier. In particular, a new rf input coupler design has demonstrated reasonable coupling efficiency over the design bandwidth with the ability to control the polarization of the input wave. Detailed comparison of experimental results with a recently developed 3-D ubitron theory will allow an accurate assessment of the ubitron as a millimeter- and submillimeter-wave source.

Sheet electron beam millimeter-wave amplifiers at the Naval Research Laboratory

To meet the need to transmit increasingly massive volumes of data, both the defense and commercial sectors are turning to higher operational frequencies to take advantage of larger signal bandwidths while concurrently requiring increased amplifier power to achieve the necessary signal-to-noise ratios over large transmission distances. In response to these needs, the last decade has seen a leap in performance of a variety of millimeter-wave devices. The Naval Research Laboratory (NRL) is the principal U.S. Department of Defense R&D center focused on the development of the science and technology behind new millimeter-wave high power solid-state and vacuum electronic devices. Selected examples of NRLs research projects are described with an emphasis on high power millimeter-wave vacuum electronic devices.

Development of biased diamond current amplifier

A diamond current amplifier is being developed that can be biased to generate an internal electric field needed for high transport efficiency and emission gain. In the process, a bonded diamond structure has been successfully fabricated that provides mechanical support for the microns-thick diamond film and that allows for the use of processing techniques needed to fabricate the biased device. Emission measurements taken from the hydrogenated surface of a 5.4-micron-thick bonded diamond film reveal a temperature-dependent behavior that suggests the presence of upwards band bending at the surface. Studies are in progress to identify the cause of the band bending at the hydrogenated surface and to develop a better understanding of the surface electronic and material properties that influence the amplifier performance.

Multi-kW sheet beam amplifiers at Ka and W bands

NRLs unique sheet-beam electron gun, which produces a 3.5-4-A, 0.3 × 4 mm electron beam at ~20 kV, has been successfully used to power multi-kW amplifiers at both Ka and W bands. The 94-GHz narrow-band EIK uses a 3-cavity circuit to produce a peak output power of over 7.5 kW, with saturated gain of ~35 dB. The Ka-band amplifier uses a double staggered ladder coupled-cavity circuit to achieve a peak output power >10 kW at 34 GHz, with a 3-dB bandwidth of 5 GHz and saturated gain of ~15 dB. The performance of both amplifiers is in very good agreement with models and simulations.

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, 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.

Multiple-beam klystron development at the Naval Research Laboratory

Multiple-beam klystrons (MBKs) are a vacuum electronic amplifier technology that can provide the high power, low-noise, broadband, compact transmitter technology required to meet the needs of modern surveillance radar systems. We present a review of MBK development at the Naval Research Laboratory and the creation of an accurate, simulation-based MBK design methodology. Examples of specific devices include S-band amplifiers generating up to 670 kW peak power with bandwidths up to 13%. We also present examples of potential radar system applications using MBKs.

High-power MMW sheet beam amplifiers

NRL has developed and demonstrated a unique sheet-beam electron gun, which produces a 3.5-4-A, 0.3 × 4 mm electron beam at ~20 kV. This electron beam has been successfully used to power multi-kW amplifiers at both Ka and W bands. The devices developed thus far employ a permanent magnet solenoidal field of 6 - 8 kG to transport the electron beam through the interaction circuit. While a periodic permanent magnet (PPM) structure would be much more compact, some compromises in performance are required to utilize PPM focusing. We will discuss some of the design considerations and requirements for high-power sheet beam amplifiers and review some key features of the amplifiers demonstrated thus far.

Sub-gap photo-enhanced secondary electron emission from single-crystal CVD diamond

Secondary-electron-emission current measured from high-purity, single-crystal CVD diamond is found to increase when 3.1-eV photons are incident on the hydrogenated surface. Energy spectra indicate that the sub-gap illumination causes the band levels to shift in the bulk, thereby reducing the upwards band bending at the pinned surface.