e-Guns and depressed collectors for two-stage free electron lasers

Ane-gun, depressed collector, and guide-field solenoid have been designed and tested for operation at a maximum of 400 kV and 38 A in order to simulate key components of a typical low-voltage free electron laser (FEL). Detailed beam-energy-spread analyses and computer simulations are presented for thee-gun and the drift regions. The results of experiments with the depressed collector show beam-current-recovery efficiencies of over 90 percent fore-beam voltages from 160 to 400 keV. We have also set up and confirmed computer-code predictions that a space-charge depression in the collector (which is essential for efficient collection) forms at a collector-voltage setting of 1 percent of the beam voltage. These results demonstrate the validity of substantially increasing the efficiency of systems by using depressed collectors to recover the bulk of the energy which is left in thee-beam after it leaves the laser resonator. The recovery-efficiency parameter (recovered current/cathode-emission current) is shown to be a function of the relativistic-orbit parameter (B/betagamma) and reaches a maximum of 94 percent. These results imply that FEL system efficiencies can be achieved which are an order of magnitude higher per stage than would be possible without the use of depressed collection.     

Enhancement of overall efficiency of large-orbit gyrotrons

Energy recovery from the spent beam in gyrotrons can enhance their overall efficiency. A technique for converting the rotational motion in the spent beam of a large-orbit gyrotron into axial motion, before leading it into a retarding electrostatic field generated by depressed collectors is proposed. This involves passing the spent beam through a magnetic cusp. A proof-of-principle experiment has been performed which verifies the prediction regarding unwinding of the beam, and its collection at a depressed potential which is higher than would otherwise be possible. The experimental set-up and results are presented. A multi-stage depressed collector for a large-orbit gyrotron has also been designed. Primary electrons representing beamlets of a range of energy values are collected at different depressed potentials after energy sorting. Any turning around of primaries before collection has been eliminated and almost all the back-scattered electrons are re-absorbed either on the same collector or on an adjacent one. A collector efficiency of 70 to 80% has been estimated.     

A ten-stage electrostatic depressed collector for improving klystron efficiency

A ten-stage electrostatic depressed collector, designed with the aid of an analog computer, was tested on a 1-kW CW 750-MHz klystron. Excellent correlation was achieved between computed and measured performance under varying conditions of RF drive. At full RF power output approximately 60 percent of the spent beam energy was recovered by use of the depressed collector. The net power conversion efficiency of the tube was raised from its undepressed value of 54.3 percent to approximately 70.9 percent. At one-half full power output, a collector efficiency of 70 percent was measured. At zero RF power output collector efficiency was 80 percent. To achieve these results it was necessary to install a small focusing coil between the final drift tube and the collector. No spurious oscillations or instabilities were detected when collector depression was employed, nor was electron backstreaming increased significantly. Intentional short circuiting of adjacent collector electrode pairs was shown to cause only minor degradation in collector performance.     

Small-size 81- to 83.5-percent efficient 2- and 4-stage depressed collectors for octave-bandwidth high-performance TWT's

In a joint USAF-NASA Program, Lewis Research Center is carrying out an efficiency improvement program on traveling wave tubes (TWT) for use in electronic counter measures (ECM) by applying multistage depressed collector (MDC) and spent beam refocusing techniques developed at Lewis. In the analytic part of the effort, three-dimensional electron trajectories are computed throughout the TWT. Trajectory computation continues through the spent beam refocuser and the depressed collector. Collector efficiency, collector losses, and overall efficiency are identified and computed. On the experimental side, tube performance is evaluated first without the MDC; then, the spent beam is analyzed for symmetry, circularity, and velocity spread. Finally, the MDC is attached and its performance optimized and evaluated. The three-dimensional theory, for ideal tubes, predicts a MDC-efficiency, at mid-band, of 81 percent for a 2-stage MDC with symmetric, circular, and optimally refocused beams and 85.5 percent for a 4-stage MDC. Experimental results to date have yielded MDC efficiencies of a minimum of 81 and 83 percent for a 2- and 4-stage MDC, respectively, across a one-octave bandwidth of a 4.8 to 9.6 GHz 330-to-550-W TWT.     

A novel electrostatic-focusing depressed collector for linear beam tubes

In a theoretical study, various types of retarding electrostatic fields are compared with respect to their usefulness in a depressed collector. Without space charge, fields having focusing properties lead to higher collector efficiencies ηcollthan defocusing fields. For a special case, the hyperbolic field, ηcollis given as a function of the angular beam spread, beam diameter, and initial beam energy. ηcollincreases with 1) decreasing angular beam spread, 2) increasing ratio of collector size to beam diameter, 3) increasing initial beam energy, and 4) the number of collector stages. Experimentally, a 3-stage collector of the hyperbolic field type was studied in connection with a PPM-focused high-power Dual Mode TWT, having a pulse-up ratio of 8 dB. At 50-, 75,, and 100- percent depression, the tube efficiency was 26 percent in the low-power mode and 34 percent in the high-power mode, corresponding to collector efficiencies of 76 and 71 percent, respectively.     

计算机设计的降压收集极的实验验证

本文提出了利用文献[1]的轴对称多级降压收集极计算模型和通用程序进行设计的方法,以及叙述了用该法设计的收集极在高效率C波段行波管中的使用情况。     

耦合腔行波管电参量对降压收集极特性的影响

该文基于Orprogr软件,在X波段模拟设计出耦合腔行波管的期待电性能,并利用互作用后的电子注为入口条件,优化设计出收集效率大于71.5%、电子注着陆相对均匀的三级降压收集极。在上述条件下,讨论了电子进入收集极层流性(速度比)、电极形状、电极电压以及收集极内电场分布对电子运动的影响,从物理上给出了电子注在收集极表面着陆特点的详细分析。     

Experiments on collector potential depression of T. W. T. 's†

This paper gives the results of experiments aimed at maximizing the efficiency of high power T. W. T. 's with linear slow-wave circuits by depressing the potential of the electron collector. A device for preventing noise deterioration and high current interception on the slow-wave circuit, which usually accompanies the depression of the collector potential, is described. Experimental results are given and from these some approximate relationships are deduced to predict depressed collector performance.     

Why a circuit sever affects traveling-wave tube efficiency

The effect of a circuit sever on traveling-wave amplifier efficiency has been calculated from the large-signal traveling-wave tube equations. Measurements made on high-power traveling-wave amplifiers, which utilize a severed coupled-cavity interaction circuit, have confirmed the calculated results. The results show that a traveling-wave amplifier with C = 0.1 and QC = 0.25 must have 26 db or more growing-wave small-signal gain beyond the sever to obtain the maximum theoretical efficiency. If the growing-wave gain is less than this value, efficiency is reduced; for example, if the gain is 4 db less, the efficiency is reduced by one half. The mechanism which causes this reduction of saturated output power can be understood from the computed electron distribution in phase and velocity. From these phase-velocity diagrams, the degradation in efficiency is seen to be caused by the debunching of the beam by space-charge forces in the region between the sever and the point where the microwave signal builds up to the value it previously reached before the sever. Further study of the computer results shows that the loss in efficiency caused by insufficient gain beyond the sever cannot be made up by depressed collector operations, and that the use of over-voltage to obtain increased efficiency in a severed tube may not be effective because of the resulting reduction in gain that over-voltaging causes.     

Experimental verification of a computational procedure for the design of TWT-refocuser-MDC systems

A comparison of analytical and experimental results is presented for a high performance dual-mode helix TWT, equipped with multistage depressed collectors (MDC), and operated over conditions ranging from saturation to the linear regime. The computations are carried out with advanced multidimensional computer programs which model the electron beam as a series of disks or rings of charge and follow their trajectories from the RF input of the TWT, through the slow-wave structure, through the refocusing system, to their points of impact in the depressed collector. Twr performance, collector efficiency, and collector current distribution are computed and compared with measurements. Very good agreement is obtained between computed and measured TWT performance and collector efficiencies. The analytical techniques were subsequently applied to the design of a smaller MDC of nearly equal efficiency.     

Design of Gateable Transmitter for Acoustic Telemetering Tags

A single-transistor class B oscillator was designed as a gateable transmitter stage for ultrasonic tags. The transmitting efficiency was analyzed by grouping the various losses into two categories: collector circuit losses to determine the collector efficiency; and load circuit losses to determine the load efficiency. The collector efficiency, 58.5 percent for this design, is a function of the internal battery resistance, collector saturation resistance, operating power level, and battery voltage. The load efficiency, 37 percent for this design, includes losses from the transformer, base circuit, and projector. The conditions for maximum power transfer were delineated, and a simple procedure for obtaining an approximate measure of projector efficiency was prepared.     

A depressed collector system for a quasi-optical gyrotron with precisely controlled magnetic flux lines

Design of a depressed collector system for a quasi-optical gyrotron, which had a severe constraint on the maximum allowable radius of the collector region is outlined. The needs for unwinding of spent beam and for energy sorting could be accommodated by precise control of the magnetic field profile, especially in the collector region. Techniques used for defining and obtaining such profiles; and for dovetailing the profile with the collector geometry are discussed. Results on profiles and electron trajectories are presented, which demonstrate the feasibility of the design. From primary electron trajectories a collector efficiency of up to 68% has been calculated for a three collector design.     

D波段行波管三级降压收集极的设计

效率是行波管(TWT)的重要技术指标,为提高某一0.22 THz折叠波导行波管的效率,需设计多级降压收集极。对注波互作用后的电子注信息进行分析,估算收集极效率最高时的电压设置。利用电磁仿真软件对三级降压收集极电极结构和电压设置进行仿真优化,得到效率大于87.5%,回流电流小于0.328 9 mA的轴对称三级降压收集极;在第二电极入口采用斜口结构进行仿真优化,得到回流电流小于0.075 mA的非轴对称三级降压收集极。结果表明,采用斜口结构可以有效降低0.22 THz行波管多级降压收集极的回流电流。     

高效率偏心多级降压收集极的研究

本文以高效率、低返流率为优化目标,对K 波段行波管的多级降压收集极进行设计。通过MTSS 软件对收集极 进行优化,从模拟结果中可知：当电子注工作在动态时,考虑和未考虑二次电子的情况下,收集极的效率分别为82%和 79.15%,电子回流率分别为0.08%和0.39%。此结果满足毫米波行波管设计的要求,相关的实验测试正在进行中。     

Ku波段高效率脉冲行波管的研制

简要介绍了Ku波段高效率脉冲行波管的设计、CAD计算和测试结果。通过对电子枪、聚焦系统、慢波电路、电子注和慢波线高频场大信号互作用的计算、收集极的设计,获得了符合新雷达系统技术指标要求的整管。实测结果为:在Ku波段2 GHz频带范围内,等激励、脉冲输出功率大于2.2 kW、效率大于30%。     

Prebunched beam traveling-wave tube studies

A traveling-wave tube with a prebunched beam is found to have a considerably higher efficiency than the same tube without prebunching. For the particular tube tested the efficiency is increased from 20 to over 35 per cent at a gain of 8. Computer calculations using a discrete disk model give similar results. The beam is bunched tightly in energy at the position of highest efficiency, indicating that very high efficiency could be obtained with a depressed collector. Both current and velocity modulation are required for prebunching, and are obtained by means of a current grid followed by an inductively tuned velocity modulation cavity. The requirements of the current grid are not great so that operation should be possible at frequencies well above the normal operating region of a microwave triode. The operation of the tube is sensitive to the output match, and it appears to be difficult to obtain a good match with the beam on.     

Performance characteristics of an MSDC IOT amplifier

The design and performance of an inductive output tube (IOT) based amplifier incorporating multistage depressed collector (MSDC) technology is summarized in this paper. The amplifier performance goal was to provide an enhanced efficiency rf amplifier for the UHF digital television broadcast market. The operational frequency of the amplifier is 470 to 820 MHz. The design emphasizes simplicity to minimize upgrade requirements for existing digital television transmitter designs and insure a low cost IOT. The enhanced efficiency IOT is based on an existing standard collector IOT capable of providing peak envelope powers of 60 kW. The collector design incorporates three elements with one element serving as a depression stage. The collector design curtails the reflection of spent beam electrons back to the IOT interaction gap and minimizes the generation of secondary electrons at critical collector surface locations. Testing emphasized the 8-VSB modulation scheme adopted by the ATSC for digital television broadcasting. Performance results indicate that the IOT electrical efficiency increased from 34% to 51% with the addition of MSDC collector when the amplifier is operated at 15 kW average rf power using the 8-VSB modulation scheme     

Theory and performance of depressed trochoidal collectors for improving traveling-wave tube efficiency

This paper discusses the theory and performance of depressed trochoidal collectors which are used to improve the efficiency of S-band traveling-wave amplifiers. These crossed-field multistage collectors sort the electrons in the spent beam into velocity classes, and collect each class at an appropriately low potential. The sorting and collecting actions depend on the tendency of electrons to flow to the lower-potential anode in a split-anode cylindrical magnetron. The magnetic field required in the collector region to produce sorting action is evaluated, and the design parameters of the collector structure are determined from an analysis of electron ballistics inside the collector tunnel. In the trochoidal-collector tube, spent electrons are sorted into eight energy levels and collected by eight electrodes biased at different potentials. This tube provides information on the fraction of electrons collected at a voltage equal to, or greater than, the equivalent voltage lost by the electrons in the process of RF interaction, and on the collection voltages required for spent electrons at different levels of RF drive. Complete performance curves for the assembly are included for both an unmodulated beam and an RF driven beam. When the tube produces a power output of 16 watts and a saturated gain of 35 db at 2360 Mc, the collector efficiency is 42 per cent, the over-all efficiency 38 per cent, and the electronic efficiency 16 per cent. These results are achieved without adverse effects on the RF performance as a result of beam instabilities and regenerations caused by space-charge blocking of electron flow or by motion of electrons returning to the helix region.     

A cyclotron-wave rectifier for S-band and X-band

A device for converting microwave power into either dc power or low-frequency ac power has been investigated both theoretically and experimentally. Rotational energy is stored in an electron beam by a Cuccia coupler, then converted to longitudinal energy by interaction with a space-dependent dc magnetic field, and finally recovered as electric energy by a depressed collector. A simple kinematic analysis demonstrates that the Cuccia coupler can convert large amounts of microwave power into electron beam rotation. Limits on the electric field strength and asynchronism between signal frequency and cyclotron frequency are established for both classical and relativistic coupler operation. Efficiency analyses of the process of conversion from orbital energy to dc electric energy, both classical and relativistic, indicate that the efficiency exceeds 95 percent for a particular range of operating conditions. As an ac power supply, the device responds to the modulating frequency of the signal. Theory predicts near-negligible harmonic distortion as well as flatness of frequency response from dc to about 1.0 MHz modulating frequency. Four tubes and a prototype (with "artificial" coupler) were tested experimentally. The first three tubes exhibited efficiencies up to 22 percent, being hindered by reflection of electrons into the coupler. Certain design changes were tested on the prototype, where efficiencies from 36 percent to 75 percent were obtained. Incorporating these design changes into the fourth tube yielded measured efficiencies up to 34 percent, or when corrected to disregard unusually large cavity losses, up to 59 percent. Experimental tests of the tube as an ac converter yielded excellent frequency response and about 20 percent second-harmonic distortion. It is concluded that the theoretical foundation of efficiency predictions has thus far been based on too optimistic assumptions.     

Effects of the collector current fall time on the class E tuned power amplifier

Presents the equations governing the operation of the amplifier when the collector current fall time during the on-to-off transition of the transistor is an appreciable fraction of the signal period. A current-source model of the transistor with a linearly decreasing collector current during the fall time is used. The following basic amplifier parameters are determined: the waveforms of the collector current, the collector-to-emitter voltage, the instantaneous power dissipated in the transistor, the optimum values of the load-network components, the output power, the power-output capability, and the collector efficiency. The decrease of the collector efficiency is e.g. 10 percent at 60/spl deg/ fall time. Experimental results are also given. The author shows the behavior of the class E amplifier at higher operating frequencies and provides more accurate circuit design procedure.