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     

Traveling-wave tube limiters

Good broad-band microwave limiters can be built by appropriately modifying conventional traveling-wave tubes. For example, with a single one-watt tube modified suitably the output power could be held constant to within ±½ db over a range of input power of approximately 20 db. By modifying two such tubes in the proper way and using them in cascade the range of input power for limiting could be increased to greater than 45 db, and this accomplished over a 700-mc frequency range centered at S band. This was done with commercially available traveling-wave tubes. All the necessary modifications were made from outside the tube envelope. Experimental results indicate that beam saturation for these tubes when operated conventionally occurs first under the attenuator section. By increasing the gain between the attenuator and the output coupler, the beam saturation can be made to occur first under the output coupler. This fact was of great use in the development of the limiters.     

Calculations of wave propagation on a helix in the attenuation region

It is a well-known fact that the attenuation region of the traveling-wave tube is very important to the performance of the tube, particularly with respect to gain and efficiency of the tube which is relatively short in terms of wavelengths. Characteristics of the attenuator in the passive tube have been measured frequently and variations of attenuation and wave velocity with respect to frequency and thickness of coating are known. In the interests of trying to understand the effect of the attenuator on efficiency, we have attempted to make certain calculations of performance of a theoretical attenuator such as is frequently used on certain spirals and the results of such calculations are presented here.     

螺旋线行波管薄膜衰减器高频衰减特性的研究

利用HFSS仿真软件对螺旋线行波管中衰减器的衰减量分布情况进行了模拟分析。由于直接模拟螺旋线行波管中不同涂碳层厚度下衰减器的衰减分布情况难度大,计算量大,涂碳厚度不易模拟,因此,本文利用等效替换的方法进行了模拟计算,通过对单根夹持杆进行波导测试,获得S21,找到不同S21下的等效体电率或损耗角正切,并将他们对应的这些材料替换到行波管内,计算相应的衰减常数,得出不同位置下的衰减常数。     

Traveling-wave tube efficiency degradation due to power absorbed in an attenuator

Traveling-wave tubes will oscillate if sufficient power is reflected at the input and the output of the tube; that is, if the loop gain is more than unity. As most tubes have large forward gain and matches (or loads) have some reflection, protection against oscillation is usually provided by return or backward loss. In most tubes the loss will be bilateral with the loss in the forward direction both absorbing power and reducing the rate of gain. The power absorbed subtracts from the power output and reduces the efficiency. This report presents the efficiency reduction due to power absorbed in the attenuator.     

Sensitivity analysis of TWT's small signal gain based on the effectof rod shape and dimensions

An extensive study aiming at analyzing the effect of rod shapes and dimensions on the gain of helix traveling wave tubes (TWTs) is performed. The evaluation of tube small-signal gain is obtained by making use of a rigorous field analysis which takes into account the helix tape model and the dielectric inhomogeneous loading conditions. Computing time to perform the analysis is extremely low compared with the time required in the case of a full wave, three-dimensional (3-D) electromagnetic simulator. The accuracy of the simulation approach has been extensively verified in a previous paper. A novel expression for the attenuation constant has been introduced in the model to improve the quality of results. The proposed study allows a better understanding of tube behavior before fabrication highlighting the contribution of the shape, the mechanical tolerances and the ϵ_r variation of the rods to the small-signal gain     

Effect of Attenuator on BWO Start Oscillation Condition in a Helix Millimeter Wave TWT Under Magnetic Focusing

In a practical helix millimeter wave traveling-wave tube (TWT), there are always magnetic focusing system for constraining the electron beam as it passes through the interaction region and attenuator for suppressing the oscillations, including backward-wave oscillation (BWO) and improving the output power. In view of the attenuator and magnetic focusing system, a 2D linear theory is employed to analyze BWO start oscillation condition. Numerical results show that the start oscillation length of the millimeter wave TWT decreases when the start position of the attenuator is close to the input section of the slow wave structure (SWS), and that it increases with the decrease of the attenuation length or the increase of the attenuation quantity. Therefore, in order to predict the BWO accurately, we should take into the attenuator and magnetic focusing system account.     

W波段双带状电子注新型平面慢波结构行波管

提出了一种适用于W波段行波管(TWT)的双注矩形环杆(DBRRB)慢波结构(SWS),该结构具有平面特性,适合于微细加工.在一对T形介质杆的支撑下,RRB SWS适用于双带状电子注工作.利用计算机仿真分析了其高频特性.设计并采用了渐变结构和阶梯波导的宽带输入输出结构.采用粒子(PIC)模拟研究了RRB SWS的热仿真性...     

Traveling-wave tube gain fluctuations with frequency

Periodic traveling-wave tube gain fluctuations with frequency are described in terms of fundamental tube parameters and readily measurable quantities using the simplified one-wave theory of Pierce. An analysis of the feedback existing in a traveling-wave amplifier due to mismatches in the discontinuity regions is presented. Thus, the form, periodicity, and magnitude of these fluctuations and their dependence upon the beam voltage are quantitatively determined. The microwave structure is analyzed from a network point of view and the circuit parameters are defined with mismatches in the lumped center attenuator and at the ends of the slow-wave transmission line. The gain fluctuations are related to the cold circuit parameters and standing wave measurements. Pertinent experimental data, indicating the correlation of gain fluctuations to vswr variations, are included, in addition to an elementary discussion of the distortion effects possible due to such periodic gain variations in a traveling-wave tube.     

Attenuation of Electromagnetic Waves by a Plasma Layer at Atmospheric Pressure

Plasma layers at atmospheric pressure, are good broad band absorbers of electromagnetic radiation. However, to get substantial attenuations, two parameters have to be optimized. These are the plasma number density, and the thickness of the plasma layer. It is found that in order to be an effective attenuator of microwave radiation, a plasma layer has to have a number density in the 10¹³ cm^?3 range, and a thickness equal or larger than the wavelength of the incident wave. However, as the frequency increases, the amount of attenuation tends to reach a limiting value directly proportional to the number density.     

Depressed index substrate tubes to eliminate leaky-mode losses in single-mode fibers

Silica tubing to be used as substrate tubes for MCVD has was demonstrated for a fiber having only one-fourth the deposited clad-merical tubing in that it is both dehydrated and fluorinated during processing to yield a silica which is dry and whose refractive index is lower than that of silica. These tubes are advantageously used to prepare single-mode fibers having fluorinated-silica cladding which controls dispersion and reduces scattering losses. In this case, the refractive index of the deposited cladding can be matched to that of the outer tube thus avoiding leaky-mode losses commonly encountered in depressed-index clad fibers. Using such an approach, low loss (0.28 dB/km at 1.55 μm) was demonstrated for a fiber having only one-fourth the deposited cladding required when a commercial silica tube is used. Such tubes could provide ecomomies to the processing of and the performance of complex cladding single-mode structures and impact multimode fiber designs.     

Qualitative and quantitative characterization of outgassing from SU-8

SU-8 is often used as a structural material in Microsystems. In this work, the outgassing characteristics from such cross-linked SU-8 layers are studied using mass spectrometry and gas-chromatography techniques. With these methods the composition of the released matter can be identified, also the outgassing rate can be quantitatively characterized. A qualitative estimate of outgassing from SU-8 is given for cross-linked layers. The effect of hard-bakes is studied in situ by measuring at typical hard-bake temperatures. These tests indicate that a hard-bake is needed to provide good performance in UHV environments. Using gas-chromatography the outgassing rate from SU-8 is determined. The total outgassing rate is inversely proportional with time which further illustrates the effect of a hard-bake step.     

Low-noise klystron amplifiers

The principles of low-noise guns have been applied to klystron amplifiers with good corroboration of the theory. In the past, many people thought that klystrons had inherently high noise figures, while others advanced the theory that low-noise guns might be used with klystrons as well as with wave tubes. The development to be described here shows that the former impression is not true and verifies that low-noise klystron amplifiers are possible. The most obvious difference between the guns for low-noise klystron amplifiers and those typical of low-noise traveling-wave tubes is the higher beam current which is required for adequate klystron gain. A byproduct of this higher current is a wide dynamic range. In addition to the development of the electrical parameters, a major effort went into klystron construction techniques somewhat peculiar to low-noise klystron amplifiers. The data taken show that alignment of the low-noise gun electrodes with the drift tube, alignment of the beam with the magnetic field, elimination of the collector's secondary electrons from the beam, and cleanliness of the tube are of primary importance in constructing a low-noise klystron amplifier. Several two-cavity, low-noise klystron amplifiers were built for operation in both S-band and C-band. The typical low-level gain was 11.5 db, and the saturated power output was 180 mw. Several tubes exhibited noise figures below 9 db; the lowest value obtained was 6.7. db.     

0.14 THz折叠波导行波管中衰减器的设计

0.14 THz折叠波导行波管是一种宽频带高增益器件,容易产生自激振荡,破坏管子的正常工作。在管内加入衰减器是抑制其振荡的核心技术。本文考虑了衰减材料的配比选择,以及结构形状的匹配,运用三维模拟软件对衰减器的吸收和反射特性进行计算,优化并研制出适合0.14 THz行波管使用的衰减器。进行冷测后,其匹配和吸收特性都满足衰减器需求,为制作0.14 THz行波管放大器奠定了基础。     

0.14 THz双注折叠波导行波管的高频系统设计

随着太赫兹通信技术的发展,对于0.14 THz折叠波导行波管(FWTWT)的研究需求向着更高的功率和更宽的带宽发展。对双注行波管中的双路折叠波导慢波电路进行分析,得到不同参数下的高频特性变化规律。并对双路折叠波导慢波电路的功率分配和功率合成效率进行分析计算,得到功率合成效率96.3%。最后对双路慢波电路、功率分配/合成器和集中衰减器进行建模,并对注波互作用进行计算。在高压15 kV和单注电子的发射电流为40 mA条件下,得到0.14 THz频率下的合成输出功率为56 W,增益为31.4 dB,3 dB带宽为7 GHz。     

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.     

A Versatile Active Attenuator for Wide-Band and Narrow-Band Applications in Ultrasound Pulse-Echo Systems

An active calibrated attenuator which can be used with existing medical ultrasound pulse-echo systems is presented. Accurate signal attenuation over a 122-dB range is performed on signals of frequencies of up to 10 MHz. Wide-band or narrow-band operation is achieved with no insertion loss. The attenuator may also function as a preamplifier with up to 14.8-dB gain.     

Effects of time-delayed amplification in TEA CO2lasers

A novel technique is reported for studying the behavior of the time-dependent gain in a TEA CO2amplifier. The method involves the incorporation of an additional amplifier tube into a laser cavity already containing a laser gain tube. The two tubes are independently operated, but so arranged that they can be fired with a controllable time delay between the discharge current pulses. This system permits effects of the additional gain tube on the lasing properties to be investigated as the time delay is varied. In particular, there is a time delay between the discharge current pulse of the laser tube and the onset of lasing. The variations in this delay produced by the firing of the additional amplifier tube have been investigated. The observed time-delay changes can be related to a simple theory for the time-dependent gain. The analysis of the measurements can be used to determine parameters describing the time-dependent gain. This method has been used to measure decay times of the gain for various gas mixtures. The techniques reported here can also be used to study other time-dependent effects within laser amplifiers.     

HF contamination of 200 mm Al wafers: A parallel angle resolved XPS study

In this work, 650 nm thick Al layers were deposited on 200 mm wafers and intentionally contaminated by exposure to controlled HF airborne concentrations in different humidity conditions. The layers were deposited with an industrial tool, in real clean room conditions. The real dose of HF contaminant was controlled on Al surfaces by LPE-IC (Liquid Phase Extraction in ultra-pure water and analysis by Ionic Chromatography). Several contaminant dose and humidity conditions were investigated and the layers were characterized over time by PAR-XPS parallel angle resolved XPS.Thanks to the full wafer loading capacity of our XPS spectrometer, the XPS measurements were synchronised very quickly after the deposition step. This allowed us to access to the first stage of the oxide/contaminated layer growth that occurs during the storage.Al-F interactions mechanisms were addressed in terms of bounding environment and oxidation/contamination competition.The parallel angle resolved capability of our spectrometer has been used to acquire a great number of angles simultaneously and then to access to accurate and significant depth resolved informations [9].Relative depth plots profiles were then obtained from angle resolved data, giving detailed informations about the depth repartition of the chemical components, as well as their possible relative interactions.Fluorine repartition with depth has been especially studied. The aging in time of the contaminated layer has been investigated too, from the layer deposition/contamination point onto several days. Fluorine atomic percentage has been monitored at several points as well as the evolution in time of the depth profile of the fluorine components of the contaminated layer.     

X射线管的次级电子

本文分析了Ｘ射线管的寄生发光和玻壳的伤的机理，设计了实用的实验，研究了次级电子与管壳内壁电位梯度变化的关系。论述了形成寄生发光和玻壳的的伤的内因和外因，提出了提高质量的对策。