Harmonic and higher order mode mm-wave klystrons

Lead Research Organisation: University of Strathclyde
Department Name: Physics

Abstract

The Klystron is a well-known, high efficiency amplifier, with a simple structure and scalable dimensions. It is typically designed with cylindrical reentrant cavities in the fundamental mode. However as the frequency of the device increases the size of the structure decreases. At mm-wave frequencies this leads to two problems:

1) Manufacturing the complex small scale structures.
2) The gap voltage decreases as the gap gets shorter leading to less gain.

Most mm-wave klystron concepts reported in the literature are simply smaller versions of microwave klystrons. Even if, in principle the dimensions can be scaled according to the frequency increase, the fabrication challenges and the beam characteristic represent a huge obstacle to the realization of a working device when the frequency is higher than 50 GHz. This is consequently true for the frequency range around 95 GHz, which is of great interest for high bandwidth communication, radar and imaging applications.

This proposal is aimed to overcome of the above-mentioned obstacle by the realization of a 95 GHz klystron by two innovative design solutions.

The first solution is to operate the cavity at a higher order mode, chosen with similar Ez field distribution in the gap cross-section as the fundamental mode. The design will adopt reentrant cavities with square or rectangular shape, to be compatible with a photolithographic fabrication technique. The higher mode operation permits the design of the cavities with dimensions larger (at least 3-5 times) than in the case of fundamental mode operation. This eases the technological effort and makes possible a high quality fabrication by mechanical micromachining or by photolithographic processes. Further, the beam tunnel can be larger than in fundamental mode, to support higher beam current. In order to increase the interaction a number of intermediate buncher cavities, spaced all along the drift tube, will be used to increase the beam current modulation.

A separate approach uses a lower frequency input cavity to modulate the beam current. As the beam travels down the drift tube beam harmonics start to form hence a higher order mode output cavity at an integer harmonic frequency of the input cavity can be excited hence acting as a high power frequency multiplier. As the input can be readily available from a lower frequency and hence more cost effective high power microwave source we are able to overcome any moderate gain of the device.
 
Description The ABP group has overcome the challenge of manufacturing the klystron cavities by using a novel 3D printing technique to construct in silver a high order mode re-entrant klystron. The re-entrant cavity geometry is compatible with the 3D printing technique as well as operating in a high order mode resulting in the dimensions of the cavities being 5 times larger than the case if they were to operate at the fundamental mode. This has resulted in a larger beam tunnel which can be used to support a higher beam current. To drive the klystron a novel pseudospark plasma cathode was studied. The results of the research are to be presented to the vacuum electronic academic and industrial community at the International Vacuum Electronics Conference IVEC 2017, 24th to the 26th April 2017, London UK. 3D printing of a corrugated structure was also presented in NIMB refereed journal paper of Phd student Alan Phipps et al.
W-band klystron upconverter driven by pseudospark-sourced electron beam
Dr. Zhang, Liang ; Yin, H. ; Zhao, J. ; He, w. ; Cross, A. W. (Presenting author); Burt, G. ; Lingwood, C. ; Paoloni, C. (Glasgow, United Kingdom)

Professor A.W Cross has presented the 36GHz, 2MW, Gyrotron Klystron Amplifier at 2 H2020 meetings one in Triste, Itally and one in Barcelona, Spain
Exploitation Route The idea of making millimetre wave beam-wave interaction structures has attracted a lot of interest from the rapid design and manufacture community. Engineers such as Dr. Allan Rennie in the Engineering Lancaster Product Development Unit, University of Lancaster are very keen to collaborate with those working on this mini-IPS project to investigate the ultimate potential of this 3D printing in metal technique for the development of new high frequency vacuum electronic devices. For example the 3D printing technique may enable new planar beam-wave interaction structures to be manufactured.

The 2MW, 36GHz gyrotron klystron amplifier is now part of a £3M H2020 project compact Light
Sectors Aerospace, Defence and Marine,Environment,Manufacturing, including Industrial Biotechology,Pharmaceuticals and Medical Biotechnology
URL http://www.ivec2017.org/
 
Description The klystron is a well-known, high efficiency amplifier, with a simple structure and scalable dimensions. It is typically designed with cylindrical re-entrant cavities in the fundamental mode. However as the frequency of the device increases, the size of the structure decreases. At mm-wave frequencies this leads to two problems: (1) Manufacturing the complex small scale structures and (2) the gap voltage decreases as the gap gets shorter leading to less gain. Most mm-wave klystron concepts reported in the literature are simply smaller versions of microwave klystrons. Even if, in principle the dimensions can be scaled according to the frequency increase, the fabrication challenges and the beam characteristic represent an obstacle to the realization of a working device when the frequency is higher than 50 GHz. To reduce the cost of a mm-wave klystron operating at 100GHz the input cavity was designed and constructed to modulate the beam current at a lower frequency for as the beam travels down the drift tube beam harmonics start to form and hence a higher order mode output cavity at an integer of harmonic frequency of the input cavity can be excited acting as a high power frequency multiplier. A readily available cheap microwave source at 35GHz was assembled and tested to drive the input cavity. A 100GHz mm-wave klystron has been designed and constructed including: the thermionic cathode electron beam source and vacuum system; guide solenoid; the 3 cavity mm-wave klystron consisting of a low frequency (35GHz) re-entrant input cavity, a re-entrant intermediate cavity and a high order mode re-entrant output cavity (100GHz) was manufactured using a novel 3D printing of metal technique. Millimetre wave measurements of the klystron were carried out using a Ka-band (26.5GHz to 40GHz) and a W-band (75GHz to 100GHz) Vector Network Analyser with the measured performance of the cavities agreeing well with numerical simulations (CST Microwave Studio). To ensure the millimetre wave klystron is able to be developed for applications, follow on mini-IPS support is being pursued to allow high power testing of the 100GHz klystron due to the relevance of this source to UK industry for high frequency imaging applications as no intermediate power low cost W-band sources are currently available. Understanding of the klystron has enabled a 2MW, 36GGHz gyrotron klystron to be designed and modelled for a 3rd harmonic linearisation cavity for a compact X-ray Free Electron Laser to be developed supported by H2020 project CompactLight
First Year Of Impact 2018
Sector Healthcare,Manufacturing, including Industrial Biotechology,Pharmaceuticals and Medical Biotechnology
Impact Types Societal,Economic
 
Description Small diameter, high current density electrons beams forr commercial THz radiation sources
Amount £10,379 (GBP)
Organisation Cockcroft Institute 
Sector Academic/University
Country United Kingdom
Start 02/2016 
End 01/2017
 
Title Metal 3D printing 
Description A Klystron multiplier and millimetre wave components for the gyro-TWA were manufactured using 3D printing of silver with performance of components measured using a Vector Network Analyser (VNA). 
Type Of Material Improvements to research infrastructure 
Year Produced 2013 
Provided To Others? Yes  
Impact A new manufacturing technique based on the 3D printing of metal was devised for the construction of millimetre wave components with the performance measured for the first time using a Vector Network Analyser 
 
Title CST Particel Studio model of a klysatron multiplier 
Description CST Particle studio model of a klystron multiplier was developed 
Type Of Material Computer model/algorithm 
Year Produced 2016 
Provided To Others? Yes  
Impact A klystron multiplier where the input frequency is 35GHz and the output frequency is 105GHz has been developed with a joint paper with the depatment of engineering university of Lancaster submitted for oral presentation at IVEC 2017, London, UK. 
URL http://www.ivec2017.org/
 
Title Particle-in-Cell computation modelling techniques 
Description Two different 3D Particle-in-Cell codes MAGIC and CST Particle Studio have been used to model the gyro-TWA with the output of the different codes compared to each other as well as experimental measurement. 
Type Of Material Computer model/algorithm 
Provided To Others? No  
Impact The computational modelling capability of the group was noticed by UK company e2v Ltd who realised the benefit of Strathclyde working more closely with e2v Technologies Uk Ltd on computational modelling their propriety high power microwave sources. 
 
Description Corvotech Ltd 
Organisation Corvotech
Country United Kingdom 
Sector Private 
PI Contribution The University of Strathclyde devised a new method to manufacture a mm-wave klystron multiplier using the 3D printing of silver. The 3D printing technique has the potential to manufacture annular and planar structures for millimeter and sub-millimetre wave cylindrical and planar, respectively Backward Wave Oscillators or Extended Interaction Oscillators.Corvtech also provided engineering expertise on the brazing of ceramics into a pillbox structure for the input window for a 90GHz to 100GHz Gyro-TWA.
Collaborator Contribution Corvotech Ltd provided advice on low temperature brazes that would be needed to ensure the 3D printed structure was vacuum tight, a necessary requirement for a vacuum electronic device as well as the brazing of a think ceramic in a W-band pillbox structure. Collaborators at the University of Lancaster provided the klystron multiplier design.
Impact A new manufacturing technique for the construction of a millimetre wave klystron using the 3D printing of metal was investigated for the first time by the Strathclyde team.
Start Year 2013
 
Description Visit to primary schools 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Schools
Results and Impact The staff working on the project helped organise and participate in the University Research Day event held at the University of Strathclyde by serving on the organising committee running this public engagement event with local primary schools. As part of this event staff were involved in public outreach at a local primary school. This involved visiting the school and giving talks on millimetre wave research. A poster making competition relating to the research group's interests was held. The school was visited on 3 occasions with staff interacting directly with the class of school pupils on how best to prepare a poster for presentation at the University of Strathclyde.

After the poster competition many of the primary school children expressed a renewed interest in studying science.
Year(s) Of Engagement Activity 2013
URL http://www.strath.ac.uk/researchday/schoolsinvolved/