Recent research and development has been incredibly successful at advancing the capabilities for vacuum electronic device (VED) sources of powerful terahertz (THz) and near-THz coherent radiation, both CW or average and pulsed. Currently, the VED source portfolio covers over 12 orders of magnitude in power (mW-to-GW) and two orders of magnitude in frequency (from ; 10 THz). Further advances are still possible and anticipated. They will be enabled by improved understanding of fundamental beam-wave interactions, electromagnetic mode competition and mode control, along with research and development of new materials, fabrication methods, cathodes, electron beam alignment and focusing, magnet technologies, THz metrology and advanced, broadband output radiation coupling techniques.

Vacuum Electronic High Power Terahertz Sources

A technological breakthrough is embodied in the successful demonstration of an extended interaction klystron (EIK) amplifier, which has produced over 7.5 kW of peak output power at W-band (94 GHz). An efficiency of ${\sim}{17\%}$ has been achieved with a depressed collector. The EIK is driven by a 20-kV, 4-A sheet beam in a permanent magnet solenoid, with 99% beam current transmission from gun to collector. Key features that contribute to the success of this device are: 1) tight beam focusing and correspondingly narrow beam tunnel, which are made possible by the solenoidal focusing and which provide high interaction impedance and high gain per unit length and 2) the incorporation of design elements to stabilize the inherently over-moded circuit. Measured performance agrees well with 3-D particle-in-cell simulations.

Demonstration of a Multikilowatt, Solenoidally Focused Sheet Beam Amplifier at 94 GHz

Do you design and build vacuum electron devices, or work with the systems that use them? Quickly develop a solid understanding of how these devices work with this authoritative guide, written by an author with over fifty years of experience in the field. Rigorous in its approach, it focuses on the theory and design of commercially significant types of gridded, linear-beam, crossed-field and fast-wave tubes. Essential components such as waveguides, resonators, slow-wave structures, electron guns, beams, magnets and collectors are also covered, as well as the integration and reliable operation of devices in microwave and RF systems. Complex mathematical analysis is kept to a minimum, and Mathcad worksheets supporting the book online aid understanding of key concepts and connect the theory with practice. Including coverage of primary sources and current research trends, this is essential reading for researchers, practitioners and graduate students working on vacuum electron devices.

Microwave and RF Vacuum Electronic Power Sources

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.

Latest Advancements in High-Power Millimeter-Wave Helix TWTs

This is Part I of a two-part report on design and manufacturing methods used at SLAC to produce accelerator klystrons. Chapter 1 begins with the history and applications for klystrons, in both of which Stanford University was extensively involved. The remaining chapters review the theory of klystron operation, derive the principal formulae used in their design, and discuss the assumptions that they involve. These formulae are subsequently used in small-signal calculations of the frequency response of a particular klystron, whose performance is also simulated by two different computer codes. The results of calculations and simulations are compared to the actual performance of the klystron.

/pdf/high-power-klystrons-theory-and-practice-at-the-stanford-4pi6u2u4uw.pdf

High Power Klystrons: Theory and Practice at the Stanford Linear Accelerator CenterPart I

Summary form only given, as follows. Progress to date is reported on the development of a 10 MW, 1300 MHz multiple beam klystron for the TESLA superconducting linear accelerator. A multiple beam klystron will be used because of its ability to produce high rf output powers A moderate electron beam energies. In this device, low cathode current density loading and hence longer cathode life is achieved by the use of off-axis electron beams. Beam bunching occurs in a series of separate fundamental mode klystron cavities whereas drive power input and output power extraction occur in large over-moded cavities for power combination reasons. The essential parameters of the device are 114 kV beam energy, 131 A beam current (21.8 A per beam), 65% efficiency, 50 dB gain and 150 kW of average rf output power.

Multiple beam klystron development for TESLA

Low-voltage, high-perveance sheet beams are attractive for compact, high-power millimeter wave (MMW) amplifiers, because significantly higher beam current and RF power can be accommodated in the larger cross sectional area (as compared to conventional pencil beam devices). Furthermore, solenoidal focusing offers important advantages over the more popular periodic permanent magnet (PPM) focusing because of the higher field (and hence higher current density) that can be achieved with existing permanent magnets and the absence of orbital stability limitations.[1,2] To demonstrate these advantages, a beam stick (gun, magnet, beam tunnel, and collector) is being fabricated to generate and transport a 19.5-kV, 3.5-A sheet electron beam (0.32 mm × 4 mm) in a uniform 8.5-kG magnetic field. This beam is designed to power a 94-GHz extended interaction klystron (EIK) at ∼10-kW peak power. Simulations using MICHELLE [3] show that this beam can be transported with substantially no wall interception through a 0.4-mm-high beam tunnel over distances of 4–5 cm, approximately 4 times the length of the EIK interaction circuit.[4] A fundamental requirement for achieving this stable transport in a uniform magnetic field is avoidance of E×B drift effects (e.g., diocotron instability and beam tilting). We have suppressed these effects by utilizing an essentially planar magnetic field topology and by injecting a very uniform beam into the magnetic field in such a way that the electron guiding centers lie along the midplane and the beam edge is approximately matched to a self-electric-field potential contour in the beam tunnel. Key features of the beam stick and our design methodology will be described.

Low-voltage (≤ 20 kV) sheet beam with solenoidal focusing for w-band extended interaction klystron (EIK)

The extended interaction klystron (EIK) is a compact amplifier that is well-suited to millimeter-wave and sub-millimeter-wave operation, having achieved state-of-the-art performance of 9 W CW at 218 GHz with a tightly focused 12-kV round beam [1] By utilizing an EIK circuit in a sheet-beam topology, our goal is to produce more than an order of magnitude higher power than can be generated with a round beam This talk describes the design of a four-cavity 220-GHz EIK circuit, completed with the code MAGIC-3D based on our sheet-beam EIK cavity topology [2] This circuit includes an input cavity to initiate beam velocity modulation, two synchronously tuned idler cavities to enhance beam modulation, and an output cavity for RF power extraction The input and output circuits are nearly identical with eight gaps each and an external coupling waveguide, which yields a Qext ∼ 300 The idler cavities have seven gaps with no external coupling All cavities operate in the 2p-mode The circuit is driven by a 520 mA, 165 kV sheet beam with an aspect ratio of 19 The beam is transported using a 9 kG solenoidal magnetic field [3, 4] With these beam parameters and the large number of gaps, this circuit configuration effects a very efficient beam modulation Output power of 453 W is achieved in MAGIC-3D with an input power of 25 mW, corresponding to an electronic gain of 416 dB in a circuit length of about 12 cm

220-GHZ sheet-beam extended-interaction klystron design

Summary form only given. Results for the operation of a 1.3 GHz, 10 MW multiple beam klystron which is being developed for the TESLA linear accelerator facility are reported. The long operational lifetime required for this application has resulted in the use of off-axis cathodes in the design. The placement of the individual beam axes on a large bolt circle diameter has allowed for larger cathode areas and hence lower cathode current density loadings. The predicted maximum loading of 2.0 A/cm/sup 2/ for the design should lead to cathode lifetimes in excess of 100,000 hours. In such a device, minimization of the off-axis transverse magnetic fields of the focusing solenoid becomes critical for good beam transmission. Extensive three dimensional magnetostatic simulations were used to design the solenoid which has been manufactured and delivered by Stangenes Industries. Measurements of both the axial and transverse magnetic fields on the beam axes agree well with that simulated using the code Maxwell 3D and are also highly symmetric. Further beam transport simulations using the three dimensional code Michelle and these predicted magnetic fields show a maximum displacement of .050 inches of the beam axis relative to the 1.0 inch diameter tunnel geometric axis along the length of the klystron circuit. This indicates that the achieved transverse field level of the solenoid should result in good beam transport with low beam current interception on the circuit. Currently, the klystron has been completely cold tested, assembled and is being baked out to achieve a good internal vacuum level. Electrical testing of the device is scheduled to commence at the end of January. It is predicted that the device will produce 10 MW of peak RF output power and 150 kW of average power using six 114 kV, 22 A electron beams with an efficiency of 65% a -1 dB bandwidth of 4 MHz and 47 dB of large-signal gain.

Multiple beam klystron operation

Summary form only given. The design and operation of a 1.3 GHz higher-order-mode multiple beam klystron (MBK) that has been developed for the TESLA facility will be described. The klystron contains six off-axis electron beams set on a 26.7 cm bolt circle diameter that interact with a combination of TM02 and TM01 mode cavities to efficiently produce high power RF. The off-axis electron beams are the salient feature of the device which results in a low cathode current density loading and a long product lifetime. Extensive two and three dimensional simulations were conducted to design the novel magnetic focusing and RF circuits. Initial testing proved the basic design concepts and has validated the simulation techniques used. However, an arcing problem developed in one of the windows due to a poor contact in a high RF current carrying joint. This necessitated a repair and re-processing of the klystron. New test results show improved performance. The goals of producing 10 MW of peak power and 150 kW of average power have been achieved, although at a lower efficiency of 59% than had been originally expected. The klystron has also demonstrated stability into a 1.2:1 VSWR mismatch at six equally varying phases over 180 electrical degrees. Approximately 11 MW of power was generated, corresponding to an efficiency of 64.5%, at the mismatch phase corresponding to the highest interaction impedance in the output cavity gaps. Additional refinements to the RF circuit will be implemented in a future klystron to reach the 65% efficiency goal level while maintaining stability into the 1.2:1 mismatch

Design and Operation of an L-Band MBK for TESLA

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