D.R. Whaley

Application of field emitter arrays to microwave power amplifiers

This paper describes the operation of a field emitter array (FEA) as the electron source of a traveling-wave tube (TWT) amplifier. Issues of beam control and focus at high current density and low magnetic field are addressed as well as issues relating to the inherent high emittance of the FEA beam and cathode protection from ion bombardment. Large signal, nonlinear RF-modulated FEA-TWT interaction simulations show circuit efficiencies that approach 50%, even for minimal bunching of average-to-peak current ratios of 0.7-0.9. Direct radio frequency (RF) modulation at the cathode is predicted to significantly improve linearity in the high-efficiency regime as well as reduce harmonic power levels. An unmodulated C-Band FEA-TWT was built to test the focusing approach as well as the robustness of the emitters in an operating vacuum device. The device uses a 1-mm diameter Spindt emitter with a custom-designed electron gun and helix circuit. The FEA-TWT has operated to date with a maximum current of 91.3 mA and shows 99.5% transmission under both drive and no-drive conditions. Output power of the device is 55.0 W at 1.5 GHz with a saturated gain of 23.4 dB and efficiency of 17%, and performs as predicted by simulation. During all operation, the FEA emission appears extremely stable, with no temporal variations observed at any time.

100 W Operation of a Cold Cathode TWT

Recent demonstration of low-voltage high-transconductance field emitter array operation holds promise for the successful development of reliable cold cathode vacuum electronics device technologies. This paper reports on the experimental studies of implementation of such field emitter arrays as the electron source for a moderate power traveling wave tube (TWT) operating in the C-band frequency regime. The cold cathode TWT has operated for over 150 h at duty factors up to 10%, beam currents up to 121 mA, and RF powers up to 100 W at 5 GHz. High cathode current densities of 15.4 A/cm2 were achieved concurrent with excellent beam control, resulting in 99.4% beam transmission under zero-RF-drive operating conditions and 97.3% transmission at maximum RF output power. The cathode is shown to operate with a 72% reduction in the operating voltage from the previous generation of emitters fabricated by SRI International, bringing the operating voltage for full current operation well below 100 V. Extensive device characterization and life testing has been performed, and interesting variation in cathode performance was observed during the high-duty high-current portion of the testing program. The results presented here represent the highest current, highest power, and highest duty factor ever reported for an RF vacuum device employing a field emission cold cathode electron source.

Experimental demonstration of an emission-gated traveling-wave tube amplifier

This paper reports the results of the development of a traveling-wave tube (TWT) amplifier designed and operated using a high-frequency emission-gated field emitter array (FEA) cold cathode. The TWT was conservatively designed to operate with only 1% cathode current modulation but results show that 30% modulation of the current was achieved in the C-Band frequency range. The emission-gated TWT prototype was operated up to a current of 5 mA and RF output power of 280 mW using a 300-/spl mu/m diameter FEA cathode having 10 000 emitter tips with testing performed in single-pulse mode using 100-/spl mu/s pulses. Excellent beam control was demonstrated under all experimental conditions tested. Simulation shows that, with the same TWT circuit and demonstrated cathode modulation level, a 1-mm diameter cathode would generate /spl sim/60 W of output power in the same frequency band and /spl sim/80 W if the circuit were optimized for the measured level of modulation. Measurements also show that performance of the device does not degrade with frequency up to at least 7.0 GHz, which is the maximum operating frequency of the TWT. Cold measurements of the FEA electron gun alone indicate operation of the cathode up through 20 GHz might be possible. These results represent the first operation of an emission-gated cathode in a TWT and the highest power operation ever recorded in a microwave vacuum device using an emission-gated electron source.

Mode competition and startup in cylindrical cavity gyrotrons using high-order operating modes

The problem of mode competition in cylindrical cavity gyrotrons is considered. The normalized variable equations are used to calculate the oscillation regions of possible operating modes in the energy-velocity-pitch-angle plane. The analysis is self-consistent and includes the effect of changing beam current, pitch angle, and energy during the startup phase. The time evolution of beam parameters during startup is computed for several types of startup methods and used to determine the oscillating cavity modes during startup. Depending on the type of startup chosen, the cavity can be made to oscillate in several modes or in a single chosen operating mode-even for high-order modes where many other possible operating modes exist. Some startup methods are seen to be less favorable than others, allowing for oscillation of unwanted modes and some methods are seen to be more sensitive to small beam/cavity misalignment. The accessibility to the high-efficiency hard-excitation region can also be determined and is seen to depend on the startup scenario. The startup analysis is linear whereas the stability and interaction efficiency computations are fully non-linear. The method is general and can be applied to any operating mode, with the mode competition analysis specifically useful for high-order modes where the spectrum is dense. The analysis of the accessibility to the hard-excitation region is applicable to high- and low-order operating modes. Both q=1 and q=2 longitudinal mode numbers are considered. >

Sixty-percent-efficient miniature C-band vacuum power booster for the microwave power module

Results from a high-efficiency miniaturized C-band vacuum power booster (VPB) development program have resulted in the development of a miniature C-band traveling wave tube (TWT) operating at 32% circuit efficiency and 61% total efficiency at an output power in excess of 170 W at 100% duty factor. When combined with a microwave power module, this VPB results in a module DC-to-RF conversion efficiency of 51%. This high-efficiency power amplifier is particularly well suited for space and airborne applications where small size, light weight, and low thermal dissipation are required. Other parallel development of VPBs operating at lower perveance and higher power has also been pursued and has resulted in nearly identical performance of 32% circuit efficiency and 61% total efficiency at power levels in excess of 240 W at 100% duty factor. A new TWT interaction code, CHRISTINE, is benchmarked with experimental data from several of these VPBs. Simulated performance is seen to accurately predict experimental data.

Design of a linear C-band helix TWT for digital communications experiments using the CHRISTINE suite of large-signal codes

A set of optimization goal functions designed to improve the efficiency and linearity performance of helix traveling-wave tubes (TWT) is described. These goal functions were implemented in the CHRISTINE suite of large-signal helix TWT codes along with a steepest-descent optimization algorithm to automate the process of circuit parameter variation and to facilitate the rapid exploration of alternative TWT designs. We compare the predicted power, efficiency, and linearity of four different helix TWT circuits, each developed according to a different set of optimization criteria. Out of these designs, a single design was selected to be further developed for use in C-band high-data-rate communications experiments. The detailed design of this linearized TWT with a predicted 1-dB small-signal bandwidth of 1.2 GHz, small-signal centerband gain of 35.7 dB (f/sub c/=5.5 GHz), and centerband saturated output power of 52 dBm (158.5 W) is presented.

Practical Design of Emittance Dominated Linear Beams for RF Amplifiers

As the frequency of linear beam RF amplifiers increases, the effect of electron beam emittance starts to play a more important role in the design of real world devices. Accurate analytic models and quantitative predictive capability are of particular value when determining the operating parameter regime of a new device or when performing tradeoffs based on specific design criteria. Several considerations relating to emittance effects in the RF amplifiers are presented, and are seen to play a role for thermionic devices operating at millimeter wave frequencies and above, and for field emitter devices as low as microwave frequencies. Existing theory originally derived for particle beams in high-energy physics research is applied specifically to methods and the parameter regimes found in RF amplifier development. Practical enhancements to some expressions allow for accurate prediction of beam behavior without the need for time consuming numerical analysis and optimization. In this vein, beam emittance effects on RF defocusing in the high power energy extraction sections of RF circuits are also analyzed. The general formulation developed here has been validated with numerical optics simulations across a wide range of beam properties typical of those found in devices operating from the microwave to terahertz frequency regimes.

High average power field emitter cathode and testbed for X/Ku-band cold cathode TWT

A new field emitter geometry that utilizes a dielectric shield between the emitter tip and gate has been optimized to eliminate flashover in the cathode emitter cavities and improve cathode reliability. A high average power testbed has been designed and fabricated to test these cathodes at currents and current densities required for TWT operation in the X/Ku-Band frequency regime. Experimental tests in the water-cooled testbed demonstrated a record 100 hours of CW operation at 100 mA. A 200 mA X/KuBand TWT was designed to integrate these cathodes and to provide RF gain and power across the entire 6 - 18 GHz frequency band. Preliminary results taken at a cathode current of 50 mA show excellent focus of the emittance-dominated electron beam. RF results at 50 mA demonstrated positive gain over the entire frequency band with a maximum of 13.5 dB gain and 10 W output power at 10 GHz and low duty. Cathode testing as well as TWT design and preliminary operation will be described.

Vacuum electronics development at Northrop Grumman [TWTs]

Summary form only given. The past decade has seen revolutionary advancements in traveling wave tube (TWT) performance, with single tube broadband TWTs having a 2-18 GHz bandwidth, and narrowband TWTs having efficiencies near 65%. The development of RF devices using cold cathode field emitter arrays as a replacement for the thermionic cathode is also underway. Millimeter wave TWTs show good performance between 18 and 40 GHz. Several cusp guns have been designed, built and tested, for operation in gyro devices at 35 and 94 GHz. Folded waveguide design and fabrication techniques for 100 GHz operation are also being conducted. Recent results of the above vacuum electronics development programs are presented.

Calculation of DC space-charge fields in a traveling-wave amplifier in the large signal regime

A fully two-dimensional (2-D) dc space charge model has been implemented in a large-signal traveling-wave amplifier code. The simulation algorithm takes an iterative approach by alternately solving the Poisson equation and the beam trajectory equations to converge toward a self-consistent steady-state solution. This approach is similar to that employed in steady-state gun codes. However, it is well known from gun simulations that the iterative algorithm can be slow to converge. We have found the slow convergence is due to a convective numerical instability. To speed up convergence, we implemented and tested stabilization schemes based on mixing one-dimensional and 2-D Poisson potentials during the iteration cycles. These schemes are shown to accelerate convergence considerably. The fully 2-D dc space-charge model permits accurate treatment of the axial dc space-charge field in the computation of the large signal gain and efficiency, taking into account the fast variation of beam parameters along the device axis. Therefore, it can be applied to a mismatched beam with large scalloping motion. The methodology of incorporating dc space charge is general and could be incorporated in other large signal codes.