High-power Klystron Amplifier Research Articles

Studies of a Ka-band high power klystron amplifier at INFN-LNF

In the framework of the Compact Light XLS project [1], a Ka-band linearizer with electric field ranging from 100 to 150 MV/m is requested [2, 3, 4]. In order to feed this structure, a proper Ka-band high power klystron amplifier with a high efficiency is needed. This paper reports a possible solution for a klystron amplifier operating on the TM010 mode at 36 GHz, the third harmonic of the 12 GHz linac frequency, with an efficiency of 44% and 10.6 MW radiofrequency output power. We discuss also here the high-power DC gun with the related magnetic focusing system, the RF beam dynamics and finally the multiphysics analysis of a high-power microwave window for a Ka-band klystron providing 16 MW of peak power.

Нелинейные процессы при группировании электронных потоков в мощных клистронах и пределы применимости аналитических и одномерных численных моделей для их анализа.

We present the results of a comparative theoretical analysis of the electron beam bunching in a single-stage klystron amplifier using analytical models, a one-dimensional disk program, and a two-dimensional program. Data on the influence of various one-dimensional and two-dimensional nonlinear effects on the efficiency of electron beam bunching at different values of the space charge parameter and the modulation amplitude are presented. The limits of applicability of analytical and one-dimensional numerical models for electron beam bunching analysis in high-power klystron amplifiers are found.

4.6 GHz lower hybrid wave power control system for EAST.

In order to achieve steady state operation in experimental advanced superconducting tokamak (EAST), a 6 MW/4.6 GHz lower hybrid current drive system with twenty-four 250 kW/4.6 GHz high power klystron amplifiers (model VKC7849A) has been developed. A 4.6 GHz lower hybrid wave (LHW) power control system (4.6 GHz LPCS) with continuous wave mode has been set up, which was used to control the LHW power as needed. This paper introduces the architecture and software design of LPCS system, including microwave preamplifier subsystem, power measurement subsystem, power control computers, and so on. Proportional integral differential (PID) closed-loop control time is less than 100 μs due to the use of high-speed acquisition cards and user-programmable Field Programmable Gate Arrays. So far, high power CW operation and power control experiments with PID feedback mode by plasma parameters (beta, loop voltage, and so on) were performed in detail. Results show that 4.6 GHz LPCS is stable and reliable, suggesting that it meets the needs of EAST.

High-Power RF Testing of 1-MW, 352.2-MHz Continuous Wave Klystron Amplifier

A 1-MW, 352.2-MHz continuous wave (CW) radio frequency (RF) test stand based on TH 2089 klystron amplifier is developed and tested up to its rated power. Klystron amplifier demands stringent performances from its beam bias power supply for its proper operation. A novel crowbar less solid-state modular topology is adopted for -100-kV, 25-A dc beam bias power supply and its achieved performance parameters are presented in this paper. Wire burn test, which is crucial for CW klystron amplifier, was conducted on this power supply to ensure that its stored energy during klystron arcing is less than 20-J. The criterion for the selection of test wire is also presented. A real-time control system is developed for proper sequence of operation of various power supplies as well as for control and monitoring of crucial parameters of this test stand. The testing of this high-power klystron amplifier is quite involved in nature and various tests have been carried out under several operating conditions before it is employed in the high-power RF test stand. This paper studies and analyzes the effect of variations of various parameters on the performance of this high-power klystron amplifier. Variation of output power and RF gain against RF input power as well as variation of RF phase over beam voltage is presented. Variations of RF output power against frequency, beam voltage, beam current, and mod-anode voltage are also presented. Heat run test was conducted on this klystron amplifier for 24 h to ensure its reliability.

High power continuous wave microwave test bench at 4.6 GHz for experimental advanced superconducting tokamak

The lower hybrid current drive (LHCD) is an effective approach for auxiliary heating and non-inductive current drive in the experimental advanced superconducting tokamak. The 6 MW/4.6 GHz LHCD system is being designed and installed with twenty-four 250 KW/4.6 GHz high power klystron amplifiers. The test bench operating at 250 KW/4.6 GHz in continuous wave mode has been set up, which can test and train microwave components for the 6 MW/4.6 GHz LHCD system. In this paper, the system architecture and software of the microwave test bench are presented. Moreover, the test results of these klystrons and microwave units are described here in detail. The long term operation of the test bench and improved performance of all microwave component samples indicated that the related technologies on test bench can be applied in the large scale LHCD systems.

High-frequency devices with weakly relativistic hollow thin-wall electron beams

Slow-wave devices with hollow electron beams and azimuthally symmetric corrugated operating waveguides can be very effective not only for relativistic but also weakly relativistic particle energies. In the weakly relativistic case, the use of hollow beams permits a significant increase in the diameter of the beam channel and, simultaneously, a drastic decrease in the required current density and heat load at the interaction structure wall in comparison with the conventional devices, which basically exploit thin pencil-like beams. Advantages of the hollow beams in the achievement of continuous wave (CW) and long-pulse generation can manifest themselves in a wide range from gigahertz to terahertz frequencies. As an example of the concept, a W-band oscillator (orotron) with kilowatt output power in CW regime is discussed in detail. Modification of the microwave system makes it possible to implement high-power frequency-tunable BWOs, klystron or TWT amplifiers, and many types of hybrid devices.

A combined source of electron bunches and microwave power

In this article, the possibility of using a high power klystron amplifier simultaneously as a microwave power source as usual and an electron bunches source by extracting the spent beam with a magnet and also as an oscillator by feedback is investigated. The purpose of this study is to demonstrate the feasibility of constructing a very compact electron linear accelerator or for other applications of electron bunches. The feasibility of the idea was first examined by computer simulation of the electron motion in a 5 MW klystron and the characteristics of the klystron spent beam. Experimental study was then carried out by installing a radio frequency cavity and a Faraday cage in sequence at the exit end of a bending magnet located at the top of the klystron collector. The energy and current of the chopped spent electron beam can then be measured. By properly choosing the feedback circuit elements, the frequency stability of the klystron in oscillator mode was proved to be good enough for linac operation. According to the results presented in this article, it is evident that an extremely compact linac for research and education with better affordability can be constructed to promote the applications of linacs.

Influence of beam head effects on high-power klystron amplifier

Transient behavior of the cavity excitation caused by beam head effects in high-power klystrons is investigated. Recognizing that the input cavity in a klystron can be excited by the input microwave signal and the head of the electron beam, it is shown that the frequency of the induced cavity voltage Vs driven by the signal is the same as the signal frequency ωs. On the other hand, the frequency of the induced cavity voltage Vh driven by the beam head is the cavity resonance frequency ω0. An expression of the induced voltage Vh is obtained by making use of the Laplace transformation. The induced voltage Vh caused by the beam head is expressed in terms of the current-change profile in time. The magnitude of the induced voltage caused by the beam head is inversely proportional to the beam rise time, thereby decreasing drastically for a large value of the beam rise time. It is also shown that the beam head effects are completely negligible whenever the beam rise time τr satisfies ω0τr=2nπ where n is an integer. The amplitude of the whole cavity voltage oscillates with the beat wave frequency Δω=ω0−ωs. This amplitude oscillation may play an important role in pulsed high-power klystrons.

High power continuous wave microwave system at 3.7 GHz

The lower hybrid current drive (LHCD) system is an important system in superconducting steady state tokamak (SST-1). It is used to drive and maintain the plasma current for 1000 s with a duty cycle of 17%. The LHCD system is being designed to launch 1 MW of radio frequency (rf) power at 3.7 GHz. The rf source is comprised of two high power klystron amplifiers, each capable of delivering 500 kW rf power. In this article, the results obtained during installation and commissioning of these klystrons are presented. Two klystrons (model TH2103D) have been successfully installed and commissioned on dummy loads, delivering ∼200 kW power for more than 1000 s. The maximum output power that could be obtained is limited due to the available direct current (dc) power supply. The test system is comprised of a TH2103D klystron, a low power rf (3.7 GHz/25 W) source, two high power four port circulators, two high power dual directional couplers, two arc detector systems, and two dummy water loads. To avoid rf breakdown in the rf components of the transmission line, the system has been pressurized with dry air to 3 bar. To energize and operate the klystron, a high voltage dc power supply, a magnet power supply, an ion pump power supply, a −65 kV floating anode modulator power supply, and a filament power supply are used. An arc detector unit has been installed to detect and initiate action within a few microseconds to protect the klystron, waveguides, and other rf passive components during arcing. To protect the klystron in the event of an arc, a fast responding (<10 μs), rail gap based pressurized crowbar unit has been used. The entire system is water cooled to avoid excess temperature rise during high power continuous wave operation of the klystron and other rf components. The tube requires initial conditioning. Thereafter, the output rf power is studied as a function of beam parameters such as cathode voltage and beam current.

Millimeter-wave radars for remotely sensing clouds and precipitation

Millimeter-wave radars have been used since the early 1950s to study clouds and precipitation, but until recently these early instruments were limited to simple backscatter power measurements and were plagued by hardware problems. However, development of solid-state millimeter-wave componentry and high-power klystron amplifiers has spurred the evolution of reliable, coherent radars operating up to 95 GHz. In addition, advances in digital signal processing technology have resulted in single-card processors that can simultaneously execute algorithms to compute reflectivity, Doppler, and polarimetric quantities in real time. A review of the current state of the art in millimeter-wave cloud radars is presented, including a discussion of transmitters, antennas, and receiver components. Two radar systems built by the University of Massachusetts are described, including a mobile, dual-frequency (33- and 95-GHz) polarimetric radar, and an airborne 95-GHz polarimetric radar that was recently flown in a cooperative experiment with the University of Wyoming. Spaceborne applications are also discussed, especially the use of satellite-based 95-GHz radars for measuring the vertical distribution of clouds. >

Multiple extraction cavities for high-power klystrons

The design and performance of an X-band high-power klystron amplifier utilizing an output circuit consisting of two uncoupled output cavities are described and compared with 2-D computer simulation results. It is shown that good efficiency, low RF gap gradient, relatively uniform power extraction from each cavity, and freedom from oscillations due to extraneous modes can be achieved. The change in phase relative to the RF input for the two extraction cavities is relatively small within a frequency bandwidth on the order of 1%, when the beam voltage or drive power is varied over a moderate range. For narrowband applications, where the combining of the separate outputs is desired, good combining efficiency is possible. The number of extraction cavities can be extended to obtain still higher peak powers.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

30/20 GHz Band Small Earth Station for ISSDN Experiment

The system design and operational results of a 30/20 GHz band small Earth station for integrated services satellite digital network (ISSDN) experiments are presented. The Earth station has a high efficiency offset Cassegrain antenna with elliptical beam, a high power klystron amplifier with 1 kW output power and an uncooled parametric low noise amplifier with 200 K noise temperature. Experiments were performed using the medium capacity communications satellite for experimental purposes (CS). Performance on the Earth station was high. Effective radiation power (ERP) was greater than 88.1 dBW, and the receiving figure of merit (G/T) was larger than 30.6 dB/K. Good transmission characteristics were obtained through the satellite link for demand assigned time-division multiple access (DA-TDMA) with 20 Mbit/s bit rate.

High Power Klystron Amplifier for Troposcatter Communication System

This paper describes in detail the High Power Klystron Amplifier operating in 2 GHz band developed in HRPU for establishing an operational tropo-communication link between two locations in Northern India.

Transmitters

The evolution of the type, form and scope of radio transmitters is traceable from the nascent technology of not yet a century ago to today's elaborately sophisticated systems, among which are included radar, multichannel microwave, aircraft and marine communications and navigation and commercial broadcast. Apparatus and techniques progressed from the early spark and arc transmitters to alternators of various designs and powers, through the era of the first vacuum-tube transmitters to the later highly efficient vacuum-tube types. More recently, transmitter design has utilized high-power tetrode and klystron amplifiers. Although code signaling was the first practical method of communicating intelligence, interest in voice transmission resulted in concurrent efforts to adapt the then existing transmitters to this purpose. The discovery of ionospheric phenomena permitted advances into areas of transmitter development previously considered impractical for other than short-distance communications. Regulations governing transmitter power, frequency and use were established by government agency as the need became apparent. The regulations have been adjusted to keep pace with the increase in power capabilities and the use of the frequency spectrum as the trend continues toward higher-power transmitters.

Studies in the development of high-power klystrons

A survey is given of the experience gained during the development of high-power klystron amplifiers, with reference to design principles, the application of getter pumps inside sealed-off valves, wide-band tuning mechanism, output windows, influence of secondary emission on tube performance, etc. Also, further experiments on broadbanding of multi-cavity amplifier systems are discussed.

Measuring modulation noise from a high-power c.w. klystron amplifier

The noise spectrum of a high-power klystron amplifier shows that, as well as the r.f. shot noise, the power output contains modulation sidebands caused by variations in gain associated with low-frequency fluctuations.Modulation noise can be caused by any low-frequency fluctuations in the system, e.g. power supply ripple, microphony, or spurious beam oscillations. Apparatus for investigating these effects includes a panoramic spectrum analyser for observing and correlating the r.f. noise associated with the carrier and the video noise fluctuations.A new method of noise detection is described which uses a coaxial diode at high power level instead of the conventional low-power crystal detector. Results show that the coaxial diode has some advantages over the crystal mixer for measuring a.m. noise from a high-power source.The noise phenomena described include effects due to relaxation oscillations in the electron beam, and the variation of noise with cathode temperature.

Development of a 10-kilowatt 1700-2400 megacycle CW klystron amplifier

The paper describes the design of a high-power klystron amplifier which has several unusual featulefi. These are unusually wide-tuning range capabilities, exceptionally high-beam efficiency, unique modulating capabilities for single-side band, high-level modulation, and considerable flexibility in simultaneous bandwidth adjustment.

Recent Developments in High-Power Klystron Amplifiers

The present status of klystron power amplifiers is reviewed by discussing the three types of electron-beam focusing used. The maximum power output, efficiency, gain, bandwidth, temperature compensation, and tuning means of present-day klystrons are briefly discussed. Examples of several typical tubes are given.