Performance Of Traveling-wave Tubes Research Articles

A High Efficiency 100-W E-Band Continuous Wave Traveling Wave Tube

A continuous wave (CW) E-band traveling wave tube (TWT) is developed to meet the demand of broadband, high power and high efficiency amplifiers for future wireless communications. Several methods are combined to improve the TWT performance, in particular the efficiency, including: increase of the interaction impedance by setting operation point near the cutoff frequency, use of the folded waveguide (FWG) slow wave structure (SWS) with modified circular bends (MCBs) and the phase velocity jumping (PVJ) technique. A novel dual-mode asymmetry multistage depressed collector is used to significantly improve the recovery efficiency without the return of electrons. The first prototype tube has been assembled and tested. A total efficiency of 43% with an electronic efficiency of 9.8%, output power of 120 W and gain of 41 dB is achieved in 81-86GHz where the beam voltage is 15.8kV and the beam current is 77mA. The tube shows very flat output characteristics in saturation state and provides high linear power and efficiency. The -1dB compression point output is over 50W with the linear (output back off 4dB) total efficiency up to 26.4%.

A Modified Fold Waveguide Slow Wave Structure for W-Band Dual-Beam TWT

Increasing the interaction current capacity and the interaction impedance is crucial to achieving high output power for traveling wave tubes (TWTs) in <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\textit{W}$</tex-math> </inline-formula> -band and terahertz (THz)-band. With the increased distance between the upper and lower metal walls of folded waveguide (FW), the slow wave structure (SWS) named dual-tunnel FW (DTFW) is formed. Benefiting from such a rational design, the electric field in the noninteracting region decreased and naturally formed dual electron beam tunnels. Meanwhile, the operating mode is cutoff at the beam tunnel, the electromagnetic wave in DTFW can be propagated along the path similar to FW and operate in the fundamental mode. Furthermore, compared with FW, due to its larger size, the DTFW has the same structural complexity and less manufacturing difficulty, which is significant for <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\textit{W}$</tex-math> </inline-formula> -band and THz-band. In this article, through the analysis of the high-frequency characteristics of the SWS using FW and DTFW, the physical mechanism of bandwidth and interaction impedance improvement of DTFW-SWS are revealed. Moreover, the particle-in-cell (PIC) simulation results reveal that, compared with a TWT using unprecedentedly excellent FW-SWS, the performance of TWT using DTFW-SWS has been significantly further improved in the aspects of saturated power, gain, and electron efficiency. Therefore, DTFW-SWS is a promising SWS for high-power and wide-bandwidth TWT in <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\textit{W}$</tex-math> </inline-formula> -band and THz-band.

Experiments of Sub-THz Wave Folded Waveguide Traveling-Wave Tube Amplifier

This study showed the possibility of using a sub-terahertz (THz) traveling-wave tube (TWT) via measuring the transmission characteristics and TWT performance of the circuit by applying X-ray LIGA, a micro-fabrication process, to the interaction circuit. The applied circuit type, an E-bend folded waveguide, is a simple structure most suitable for lithography. A total of three applied frequencies were used the W-band, G-band, and 850 GHz. Among the manufactured circuits, the W-band circuit was applied to the TWT, one of the vacuum electronics devices (VEDs). This was done to prove the manufacturing accuracy of the circuit by comparing the nonlinear characteristics of the circuit with the prediction results. Through such testing, the small signal gain was measured as 13 ± 2 dB under the conditions of 13.96-kV and 24.2-mA electron beam energy. The frequency bandwidth was extremely wide, about 9 GHz, and showed similar characteristics to the simulation predictions. The maximum output of the device was obtained up to 1 W or more at 87.12 GHz by slightly increasing the beam current. These characteristic achievements showed the suitability of the TWT for very small circuits fabricated using the X-ray LIGA process, further suggesting the applicability of other sub-THz bands.

Application of transfer matrix method to calculating the effect of multiple internal reflections on the small signal gain ripple of TWT

The low-signal-gain versus frequency slope is often a highly desirable property of traveling wave tube (TWT) used in a communication system. The gain ripple is usually caused by internal reflexions of forward and backward waves in the TWT. Random fabrication error may have a detrimental effect on the performance of TWT. The quartic equation including backward wave models the effect of reflection to analyze the effect of Gain ripple from many small circuit errors in a TWT operating under small-signal condition. We present a transfer matrix method (TMM) to correctly calculate the transmission and reflection of the wave incident respectively from left and right at a single isolated joint. The TMM, which links the input signal to output signal that includes the feedback signal from the reflections at multiple joints to the output end, can calculate the gain ripple of multiple internal reflections. Appling this method to several numerical examples, we look at how small signal gain is affected by a single isolated discontinuity and many small randomly distributed discontinuities. In particular, we investigate the effects of random perturbations of Pierce velocity detuning parameter b and Pierce gain parameter C on the small signal gain at different values of space charge 4QC. The computed result agrees with that from Chernin's model. We find that reflections may significantly increase the statistical effects on the gain. A further conclusion is that the standard deviation of gain, dgain, increases with b gradually, but the ratio of the backward wave power to the forward wave power at x=0 decreases with b when standard deviation of pierce velocity detuning parameter, b, is more than 1.5. In another example, the effects of two discontinuities of pitch distribution and many small random pitch errors on gain ripple are reported for a G-band TWT. We find that larger pitch error and longer distance for the discontinuities may produce a larger ripple in the small-signal-gain versus frequency. Many small discontinuities may produce a large gain ripple, and the gain ripple grows as the level of pitch error increases. These effects of random fabrication errors become increasingly important for very high frequencies, such as 1 THz, at which TWTs are currently being designed and built.

Nonlinear investigation of beam-wave interaction in double-groove loaded folded-waveguide traveling-wave tube

A W-band traveling-wave tube (TWT) with double-groove loaded folded waveguide structure (FWSWS) has been designed and numerically modelled. The nonlinear performance of such a TWT is investigated by a particle-in-cell code MAGIC3D. Simulation results indicate this TWT produces a saturated electromagnetic power of 170.2 W at 90 GHz, corresponding to 36.9 dB gain and 69.6 mm interaction distance. A comparison between the novel folded waveguide traveling-wave tube (FWTWT) and the conventional one is also carried out to verify the effect of groove loading on the large-signal performance of TWT. Within the same working conditions, the double groove-loaded FWTWT could obtain higher saturated output power and gain in a shorter interaction length. The maximum of output power and gain of this novel TWT is 58.6% and 10% higher than those of the conventional FWTWT, while the 3-dB bandwidth of TWT is reduced to 4 GHz. With the additional advantage of ease of fabrication based on micro-electro-mechanical systems (MEMS) technologies, the double-groove loaded FWSWS is suitable for a millimeter-wave TWT with high power capacity and gain.

Latest Advancements in High-Power Millimeter-Wave Helix TWTs

The millimeter-wave spectrum, generally considered to range from 30 to 300 GHz, is one of the most actively used spectrums today, with applications such as high-data-rate terrestrial and space communications, radar, and electronic warfare. The helix traveling-wave tube (TWT) amplifier has long been a critical component for enabling modern millimeter-wave systems. This paper presents an overview of several recently developed high-power millimeter-wave helix TWTs which have extended the helix TWT performance envelope. In particular, the power-frequency-bandwidth product for these devices has increased by a factor of 4 to over 100 depending upon the application. These helix TWTs operate at power levels up to 1-kW peak power or 500-W average or continuous-wave power and over frequencies from 18 to 46 GHz, with efficiencies up to 60%. The helix TWT performance improvements discussed here have not only kept pace with improvements in rival technologies such as GaN solid-state power amplifiers but also pushed helix TWTs into competition with traditionally higher power and higher frequency vacuum electron devices such as extended-interaction klystrons and coupled-cavity TWTs. Indeed, the modern high-power millimeter-wave helix TWT offers system engineers a singularly attractive amplifier technology with an outstanding combination of small size, light weight, high efficiency, good linearity, high reliability, and large bandwidth.

Investigation of fully three-dimensional helical RF field effects on TWT beam/circuit interaction

A fully three-dimensional (3D), time-dependent, helical traveling-wave tube (TWT) interaction model has been developed using the electromagnetic particle-in-cell code MAFIA. The model includes a short section of helical slow-wave circuit with excitation fed by RF input/output couplers, and electron beam contained by periodic permanent magnet focusing. All components of the model are simulated in three dimensions allowing the effects of the fully 3D helical fields on RF circuit/beam interaction to be investigated for the first time. The development of the interaction model is presented, and predicted TWT performance using 2.5D and 3D models is compared to investigate the effect of conventional approximations used in TWT analysis.

Design and performance of traveling-wave tubes using field emitter array cathodes

An X-band miniaturized traveling-wave tube (TWT) using a Spindt field emitter array (FEA) cathode was built. The FEA cathode has 542,000 molybdenum tips in an emission area 1.2 mm in diameter. It also has a trench current-limiter that shows low resistance in normal operation. A saturation output power of 28.2 W, a saturation power gain of 40 dB, and a beam transmission ratio of more than 99.3% have been achieved at a frequency of 11.5 GHz under 3% duty ratio.

Traveling-wave tube devices with nonlinear dielectric elements

The performance of traveling-wave tube (TWT) amplifiers incorporating nonlinear dielectric elements is studied via computer simulation. Two different situations are investigated: (1) the use of nonlinear dielectric elements to reduce intermodulation distortion and (2) the use of voltage-controlled dielectrics to provide rapid modification of the dispersion characteristics of the slow-wave structure. In the first case, the use of dielectrics with negative second-order susceptibilities is studied as a means of reducing phase and intermodulation distortion. Use of these dielectrics along with dynamic velocity taper to reduce amplitude modulation distortion results in marked reduction of predicted intermodulation distortion. In the second case, the goal is to design an amplifying structure whose gain as a function of frequency can be varied electrically. Preliminary design studies show that relatively large changes in the center frequency of the amplification band can be achieved with relatively modest changes in the dielectric constant of helix support structure.

Isotropic graphite multistage depressed collectors-a progress report

A small, isotropic graphite electrode multistage depressed collector (MDC) was designed, fabricated, and evaluated in conjunction with a 500-W, CW (continuous wave), 4.8-to-9.6-GHz TWT (traveling wave tube). The carbon electrode surfaces were used to improve the TWT overall efficiency by minimizing the secondary electron emission losses in the MDC. The design and fabrication of the brazed graphite MDC assembly are described. The TWT and graphite electrode MDC bakeout and processing (outgassing) characteristics were evaluated and found to be comparable to those for TWTs equipped with copper electrode MDCs. The TWT and MDC performance was optimized for broadband CW operation at saturation. The average RF, overall, and MDC efficiencies were 14.9, 46.4, and 83.6%, respectively, across the octave operating band. A 1500-h CW test showed no gas buildup and excellent stability of the electrode surfaces. >

Traveling-wave tube efficiency improvement with textured pyrolytic graphite multistage depressed collector electrodes

The improved efficiency of a multistage depressed collector (MDC) with ion-beam textured pyrolytic graphite electrode surfaces is demonstrated by comparison with the performance of other MDC's tested with the same microwave rf amplifier traveling-wave tube (TWT). Because of the sensitivity of overall TWT efficiency to collector efficiency, significant improvements in TWT performance are possible with the use of textured pyrolytic graphite MDC's.

Effect of Velocity Distribution on Traveling-Wave Tube Gain

The density-function method is applied to the one-dimensional model of the traveling-wave tube to determine the effect of velocity distribution on traveling-wave tube performance. For narrow velocity spreads, a term is introduced into the root equation for the incremental propagation constants which is like the space charge term 4QC and directly added thereto. This term is proportional to the ratio of the variance of the distribution to the square of the gain parameter C. For a typical tube, thermal velocities are found to reduce the gain by less than one part in 104. In a tube employing confined magnetic focusing, and employing a high perveance beam, the velocity spread resulting from space charge lowers the gain by about 1 percent.