A study on helix pitch tolerance impact on TWT small-signal gain

Traveling-wave tubes (TWTs) consist of several sections with different helix pitch to provide the required small-signal gain. An extensive study aiming to improve the comprehension of the effect of the helix pitch manufacturing tolerances on the TWT small-signal gain has been completed. The obtained results demonstrate the relevant impact on the TWT small-signal gain when the helix pitch deviates from its nominal value and allow devising a tuning procedure for the realized tubes.     

Small-signal gain calculations for electrostatically focused klystrons

The small-signal gain of an electrostatically focused klystron (ESFK) will be different from that of a conventional klystron because the drift tube is not dc field free and the RF bunching process is altered. A method for calculating the small-signal gain of an ESFK, based on a space-charge wave analysis of the lens cell, is presented. Calculated and measured values of small-signal gain for several different ESFK's are given.     

Theory of long term gain growth in traveling wave tubes

The properties of the attenuator materials used in traveling wave tubes (TWT) are a major factor in determining the gain of the tube. Carbon is a commonly used attenuator material which is typically deposited on the dielectric rods used to support the helix slow wave structure in TWTs. The deposited carbon layers are susceptible to ion bombardment induced damage during operation and outgassing of the tube, which increases the electrical resistivity of the layer and reduces the rf loss in the tube. The total gain in the tube grows during this process until the bombardment ceases or the layer damage saturates. A simple theory is developed that relates this gain change to the gas evolved in the tube that results in the particle bombardment of the layers during operation. The model accurately describes the observed time dependent behavior of the gain, and the time constant for outgassing the tube is found for several tubes ranging from S to Ka band. The time constant increases significantly for smaller, high frequency tubes with a low conductance and poor pumping speeds. The model shows that stabilization of the gain occurs after several time constants have passed, which may take hundreds to thousands of hours     

Measurement of small-signal parameters in traveling-wave tube theory

An experimental procedure is described which allows the accurate determination of parameters needed to calculate the small-signal gain of traveling-wave tubes using the equivalent transmission-line model of Pierce. A movable electromagnetic probe is used to measure the gain per unit wavelength along the helix and the attenuator and also to measure the initial loss. From these data the gain parameter C' and the space-charge parameter Q'C' are determined simultaneously. The value of C' enables the growth parameter x' to be obtained from the gain per unit wavelength. Excellent agreement with theory has been obtained for the variation of C' and Q'C' with beam-to-helix radius ratio b/a and with perveance, and for the variation of the growth parameter x' with C' and Q'C'. The change of initial loss with helix voltage for various C' and Q'C' has been verified. Finally the growth parameter in the attenuator has been found to be positive, as expected from field theory as well as the equivalent transmission-line model.     

Radial gain profiles in CO2laser discharges

Measurements of the gain of the P(20), 10.6 μ, transition of CO2have been made in a flowing He, N2, CO2amplifier. Both small-signal and saturated gain conditions were investigated as a function of radial position. At low discharge currents, the radial small-signal gain profile followed a J0Bessel function distribution as predicted from the electron density distribution, while at higher currents, the gain was nearly constant across the tube diameter. Further increases in the current produced a lower gain on the tube axis than near the tube wall. This spatial behavior of the small-signal gain with discharge current can be understood following the theoretical models of Gordietz et al. or more recently of Wiegand et al. The theory indicates the small-signal gain behavior is largely explained by the increase of axial gas temperature with discharge current. Gain measurements at signal levels high enough to cause gain saturation indicate the gain is harder to saturate on the tube axis than near the tube wall. An analysis of the experimental data shows that the gain-saturation parameter increases with current density.     

Analysis of Helix Slow Wave Structure for High Efficiency Space TWT

This paper describes the analysis of helix slow-wave structure (SWS) for a high efficiency space traveling wave tube that is carried out using Ansoft HFSS and CST microwave studio, which is a 3D electromagnetic field simulators. Two approaches of simulating the dispersion and impedance characteristics of the helix slow wave structure have been discussed and compared with measured results. The dispersion characteristic gives the information about axial propagation constant (Beta). Which in turn yields the phase velocity at a particular frequency. The dispersion and impedance characteristics can be used in finding the pertinent design parameters of the helix slow-wave structure. Therefore a new trend has been initiated at CEERI to use Ansoft HFSS code to analysis of the helix slow wave structure in its real environment. The analysis of the helix SWS for Ku-band 140W space TWT has been carried out and compared with experimental results, and found is close agreement.     

Cathode voltage adjustment to compensate helix pitch tolerance in TWTs

The correct definition of helix pitch profile in multisection helix travelling-wave tubes is fundamental to obtain the required performance. The impact of tolerances on helix pitch manufacturing is a relevant reason for significant degradation of the small-signal gain. A detailed study analysing the effectiveness of compensating the effect of helix pitch error by a cathode voltage adjustment is proposed.     

Two-stage dielectric-loading for broadbanding a gyro-TWT

A scheme of dielectric-loading for a gyro-traveling wave tube (gyro-TWT) is proposed for a wider bandwidth at a higher gain. The method uses two-section dielectric-loaded cylindrical waveguide as the interaction structure, with the first section overloaded to exhibit a valley between the two peaks in the gain-frequency response, while the other suitably loaded to exhibit a single peak centered at the valley so as to yield an overall wideband response. A bandwidth ~8.8% at a relatively high small-signal gain ~30 dB is predicted     

A synchronous-wave amplifier

The small-signal gain expression for an amplifier consisting of two resonant synchronous-wave couplers is presented and the results of an experimental investigation of such a tube are reported. Some theoretical studies of such an amplifier have been reported earlier but no experimental results have been given. The experimental tube operated at 3.00 kMc and employed a Brillouin flow beam with a dc power of 730 w. The saturation output power was 45 w. In general, the experimental results were in good qualitative agreement with the theory. The discrepancy between the theoretical gain of 13 db and the experimentally observed 8-db gain is attributed to thick-beam effects. Oscillations due to the interaction between the slow space-charge wave and the off-axis longitudinal circuit fields were observed, and it is pointed out that this effect may set some limitations on the gain that can be achieved in the sychronous-wave amplifier.     

Small-signal analysis of the helitron oscillator

In this paper, a small-signal analysis of a microwave oscillator discussed by Watkins and Wada¹is presented. This tube, the helitron, has an electron beam describing the trajectory of a helix between two concentric cylinders. Interaction is with a TEM mode supported on the inner cylinder, and the beam is focused by having a potential difference between the cylinders. This has been termed anE-type tube. TheE-type tube had originally been conceived as a device for exchanging electron potential energy of an electrostatically focused beam for RF energy. In this manner, one would expect to obtain the high efficiencies associated with anM-type tube, without requiring a magnetic field. Watkins and Wada presented experimental results in their paper, and they indicated that the theory that had been developed did not predict the observed behavior. In particular, it was stated that if the propagation constants were those of anM-type tube, the measured starting current would be one-fiftieth of the theoretical starting current. The small-signal analysis of theE-type tube developed in the main body of this paper has yielded two interesting results: 1) The electrons bunch along the direction of rotation, and lose kinetic energy. In this sense theE-type tube behaves similar to theO-type oscillator. Electron motion transverse to the dc path, which is important in theM-type tube, is not important forE-type operation. 2) Space charge forces tend to increase the bunching along the direction of rotation. This results in a negative value for the space-charge parameter, and an attendant reduction in starting current. Growing waves can exist on an electron beam that is electrostatically focused between two conducting cylinders, even without the presence of a circuit field. In this sense theE-type oscillator is similar to theM-type tube.     

Gain stability of traveling wave tubes

The long-term gain stability of traveling wave tubes (TWT's) with helix slow-wave structures is examined. A major variable in the gain of TWT's is the stability of the attenuator material that is placed in the tube to damp oscillations and improve input-to-output isolation. Thin carbon layers are often used for this purpose in TWT's and are deposited onto the helix support rods by several different techniques that produce a variability in the material structure and properties. The carbon layers are also susceptible to physical damage due to the environment in the tube during conditioning and long-term operation. Modification of the electrical conductivity of the layer by energetic particle bombardment and chemical erosion decreases the net RF loss in the tube and causes the gain to increase with time. The presence of impurity gases and rapid conditioning procedures produce gain increases due to the lattice damage of the attenuator material of up to 10 dB in a TWT in the first several hundred hours of operation. Properly designed attenuator loss-patterns and minimization of the gas evolution in the TWT causes these effects to saturate and the gain to stabilize quickly. Techniques to ensure long-term stability of these layers are discussed     

The Key Technology of Efficiency Enhancement for Traveling Wave Tubes in Millimeter Wave

The efficiency enhancement methods for helix and coupled cavity traveling wave tube (TWT) in millimeter wave (MMW) are described. The interaction efficiency has been reached 14.56% by dynamic velocity taper in Ka-band TWT. Efficiency improvement has been carried out by used copper helix for broadband MMW TWT. Owing to a ferruleless coupled cavity TWT used phase-adjusted taper, the interaction efficiency has been increased from 9.6 to 22.6%. Finally, enhancing collector efficiency used multistage depressed collectors is discussed in the paper. These concept and technology of the approach are very useful for tube designer.     

Mirror effects on dynamic response of surface-emitting lasers

The effects of longitudinal wave distribution and mirror structures on the small-signal response of surface-emitting lasers are analyzed for the first time. The analysis is based on an improved dynamic model implemented in the transfer-matrix representation. It is shown that for two structures with the same threshold gain and the same internal structure, high contrast mirrors give rise to a higher relaxation oscillation and modulation bandwidth for the same injection level     

Small-signal analysis of wavelength conversion in semiconductorlaser amplifiers via gain saturation

Wavelength conversion due to gain saturation in a travelling wave semiconductor laser amplifier is analyzed using a small-signal model. An analytic expression is developed showing that the small-signal bandwidth of wavelength conversion is not limited simply to the carrier modulation bandwidth, but also depends on the single-pass gain of the device     

Efficiency Enhancement Of Helix Traveling Wave Tube Based On εMulti-Objective Evolutionary Algorithm

A two-dimensional non-linear helix traveling wave tube (TWT) theory in frequency-domain is described in this article and the theory is integrated with ε Multi-Objective Evolutionary Algorithm(MOEA). The efficiency optimization of a helix TWT with phase taper has been done. The optimization results was verified by a three dimensional nonlinear model. The results show the feasibility and reliability of the optimization algorithm and the two-dimensional nonlinear theory. The integrated program can provide calculable parameters for the design of helix TWT. It can also accelerate development process and reduce test costs.     

Suppression of backward–wave oscillation by filter helix methods

An experimental study has been made of the filter helix properties of a periodically loaded helix, using a special traveling-wave tube. The filter helix is shown to possess filter-like frequency pass bands and stop bands and a phase velocity characteristic such that forward- and backward-wave phase velocities are separated, making the filter helix a useful circuit for traveling-wave amplification at high ka (ratio of helix circumference to free space wavelength) without danger of backward-wave oscillation. Certain difficulties associated with backward-wave oscillation frequency pushing were found to arise in filter helices, but useful gain was obtained at ka greater than 0.5 using filter helix techniques.     

The CW-496 µm methylfluoride laser: Review and theoretical predictions

A rate equation model for an optically pumped submillimeter gas laser by J. R. Tucker has been extended to account for the pump radiation traveling in both directions in the laser tube. Calculations have been carried out for the (v3= 1 → 1, K = 2 → 2, J = 12 → 11) transition in methylfluoride. Results on the small-signal gain, the saturated gain, the pump absorption, and the output power are presented for a number of waveguide diameters with variable pump frequency and pump power. Previous experimental and theoretical work is compared with the theoretical predictions.     

大功率毫米波行波管AM/PM转换特性大信号解析理论的研究

毫米波行波管是未来通信发展的重要方向,由于AM/PM转换是影响毫米波行波管通信的重要非线性特性指标,因此AM/PM转换的研究对实现毫米波行波管线性化具有重要意义．本文基于相位展开的毫米波行波管欧拉大信号注波互作用理论模型,忽略高于二阶的非线性项,建立简化的欧拉大信号注波互作用理论模型．采用逐次逼近法推导其解析解,建立二阶逼近的欧拉大信号解析理论模型．利用欧拉大信号解析理论模型推导AM/PM转换的解析解,并发现AM/PM转换的产生与电子相位直流分量的激励成正比．应用建立的欧拉大信号解析理论和AM/PM转换解析模型对一支Q波段毫米波行波管进行大信号分析,并与拉格朗日理论模型以及传统的欧拉非线性理论模型进行对比．结果表明：欧拉大信号解析解的功率、增益和相移以及AM/PM转换与拉格朗日理论在线性区和中度互作用区十分吻合,饱和增益最大误差小于8.5%．同时,相较于传统的欧拉非线性模型,欧拉大信号解析解对非线性区增益和相移非线性特性的描述更具优势,即不但具有更高的精度,而且表现出传统欧拉非线性模型无法描述的增益压缩和相位失真现象．仿真结果验证了欧拉大信号解析解和AM/PM转换解析模型在线性区到中度互作用区的正确性和有效性．该AM/PM转换解析模型为后续研究毫米波行波管AM/PM转换非线性特性的物理机制与抑制方法奠定了基础．     

The effect of helix loss on noise figure in traveling-wave tubes

Helix loss increases the noise figure of a TWT by altering the amplifying properties of the tube, and also by acting as a source of thermal noise which is amplified by interaction with the beam. Noise power originating in the helix propagates as a wave and undergoes amplification in the same manner as the signal. This noise power adds to the noise figure of the tube by an amount proportional to the attenuation of the helix and to its temperature, and inversely proportional to the tube gain per unit length. The effect of refrigerating the helix has been demonstrated by several experiments in which low-noise TWT's were immersed in liquid nitrogen while monitoring the noise figure. The reduction in noise figure varied from a few tenths to nearly one db. One case of particular interest is that of a WJ-226 low-noise TWT, operating in the frequency band 1.4 to 2.3 Gc, which showed a minimum noise figure of 2.6 db when operating in a room temperature environment. On cooling the tube to liquid nitrogen temperature, a minimum noise figure of 1.7 db was obtained, at a frequency of 1.65 Gc.     

Improved small-signal equivalent circuit model and large-signalstate equations for the MOSFET/MODFET wave equation

A simple non-quasi-static small-signal equivalent circuit model is derived for the ideal MOSFET wave equation under the gradual channel approximation. This equivalent circuit represents each Y-parameter by its DC small-signal value shunted by a (trans) capacitor in series with a charging (trans) resistor. A large-signal model for the intrinsic MOSFET is derived by first implementing this RC topology in the time domain. Modified state equations are then introduced to enforce charge conservation. Transient simulations with this approximate large-signal model yield results that are compared with reported exact numerical analysis for the long channel MOSFET for a wide range of bias conditions. This unified small- and large-signal model applies to both the three- and four-terminal intrinsic MOSFET in the region of the channel where the gradual channel approximation is applicable. A non-quasi-static small-signal equivalent circuit for the velocity-saturated MOSFET wave equation is also reported