Proposal Of A High Efficiency Microwave Power Source Using A Field Emission Array

Abstract

Power amplification at microwave frequency and above is one of strategic targets under investigation of vacuum microelectronic devices, because it has possibilities to generate more output power and higher frequency than solid state devices. Most of this work has focussed on a triode structure commonly based on a field emission array (FEA)l). FEA is sufficiently nonlinear in I-V characteristics associated with Fowler-Nordheim equation, which provides highly efficient operation, as expected from B and C class amplifiers. This is an additional advantage in EA based devices. FEA driven by RF power emits a pulsed electron beam with narrow width repeated at the RF signal. When the beam is sufficiently accelerated and coupled with a RF circuit, one can create a high power and high efficiency amplifier having a simple structure without the velocity modulation required in a multicavity klystron. Additionally, highly efficient harmonic amplification up to a higher order becomes possible by coupling with a harmonic RF circuit2). The paper describes the expected operation for the tubes. Figure 1 shows a schematic drawing of the microwave amplifier or frequency multiplier using the FEA. Figure 2 shows a phase distribution of emitted electron current density calculated for a RF-FEA with tip density of 1.6x107A/cm2 by using the WKB approximation under a suitable tip shape. The emitted electrons are well bunched within a narrow range of RF phase, as shown in Fig. 2. Table 1 shows the peak efficiencies and output powers for the fundamental and higher harmonic operations. Here, the cathode diameter of lmm and an accelerating voltage of 30kV are assumed. Extraordinarily high efficiencies of 97%, 90%, 78% and 50% are expected at the fundamental, second, third and even fifth harmonics, respectively, and several tenth kW of output power are extracted from the simple and light weight tube. Operation frequency is commonly limited by the cut-off frequency governed by transconductance and the parasitic capacitance of the FEA. However, there are many method to overcome the frequency limitation. For examples, since FEA is installed in a reentrant cavity at high frequency operation, a field emission may be extracted by RF field added on DC bias or basically no extraction electrode may be required for RF-FEA operation. Furthermore, distributed interaction structure and Bragg resonator structure with or without gate electrode may be considered at millimeter waves and optical waves3).