THz backward wave oscillator for plasma diagnostics in nuclear fusion

Lead Research Organisation: University of Leeds
Department Name: Electronic and Electrical Engineering

Abstract

Terahertz technology and nuclear fusion are two fascinating scientific fields of strategic importance for the scientific progress and a sustainable future. The technological challenges are formidable and require a joint effort at global level.
The Lancaster University leads an ambitious project in collaboration with the University of Leeds and two international partners of the calibre of University of California Davis, US, and Beijing Vacuum Electronics Research Institute, China, to solve the lack of compact, affordable and powerful THz sources required to foster a breakthrough in the understanding of the mechanisms of nuclear fusion and to open new frontiers in many outstanding applications at THz frequency, presently limited only at laboratory level.

Nuclear fusion is unanimously considered as a limitless and clean source of energy of the future. The UK strongly supports national fusion programs as MAST at the Culham Center for Fusion Energy (CCFE) and the ITER project for the first commercial fusion reactor.
Cancer early diagnosis or burn diagnosis, imaging for non destructive quality inspection, food quality analysis, detection of dangerous or illegal substances, high sensitivity receiver for space explorations (about 97% of the space radiation is at THz frequency), wireless communications with the same data rate as multigigabit optical fibres, art conservation and many others are only some of the numerous outstanding applications of THz radiation. Further, the very low energy level (1/100000 in comparison to X-rays) of the THz radiation will not raise the same health concerns as X-rays, making its use acceptable to the general public.
The nuclear fusion process requires extremely high temperatures (more than 100 million degree) for the fuel, a hot plasma, that has to be confined by a proper magnetic field. Unfortunately, due to perturbation causes, the plasma suffers from undesired turbulence that, if too intense, can lead up to the blocking of the fusion reaction. Measurement of plasma turbulence based on THz frequencies is of fundamental importance to define methodologies to strongly reduce the phenomenon.
A team at University of California Davis (UC Davis) led by Prof. Neville Luhmann is realising a novel advanced plasma turbulence diagnostic system based on high-k collective Thomson scattering at THz frequencies to be tested at the National Spherical Torus Experiment (NSTX) at Princeton Plasma Physics Laboratory (PPPL) and of interest to the MAST experiment in UK. The new system will require compact, affordable and powerful (above 100 mW) THz sources. The conventional electronic and photonic approaches fail to provide devices with adequate power and such sources, where available, are very narrow band, weak and expensive.
The recent advances in microfabrication processes have opened new routes in realising micro vacuum electron devices to generate high power at THz frequencies. However, the technological challenges of affordable THz vacuum sources remain formidable.
Lancaster University will lead this project for the realisation of the first compact, powerful, affordable 0.346 THz backward wave oscillator vacuum tube, supported by the outstanding technological facilities at Leeds University, UC Davies and BVERI, and will establish a new low cost fabrication process for fast prototyping assisted design and fabrication of metal microstructures for THz vacuum electron devices in the UK.
This project represents a unique opportunity for UK academia to have a central role in the advancement of the knowledge in two fundamental scientific fields such as THz vacuum electronics and nuclear fusion.
This research is the first step of a long-term joint strategy to develop a new family of compact, low cost THz sources to open new perspective in the THz science in the UK.
 
Description Over the past several months, the SU-8 procedure has been optimised, and is currently yielding uniform and repeatable results. Several test structures have been fabricated and inspected under reflected light optical microscope and SEM. These test structures were made using four different pillar profiles, in order to investigate their effect on BWO performance. It was found that the pillars have sufficiently uniform height across the entire wafer, in addition to well-defined vertical sidewalls. Following this confirmation, the structures' metallisation is currently under way, and they will be ready for measurement once the waveguide housing block is completed.
Parallel to the SU-8 optimisation procedure, work has been going on the fabrication of the waveguide housing block, using the facilities available at the EPSRC National Facility for Innovative Robotic Systems. Unfortunately, this has taken a little longer than expected, however the project is still on track with respect to the timeframes specified in the original Case for Support.
Furthermore, alternative ways of making these SWS have been investigated, and a promising new way procedure has been identified, with trials expected to begin within a couple of weeks' time.
A key finding is that although the proposed structure has been successfully fabricated, the current / power leakage in the several test structures measured to date is believed to be compromising the performance and measurements. It is clear that a redesign to include an integrated microfabricated enclosure is necessary to guarantee success for the underlying structure and overall component. Work is underway in this regard to confirm feasibility but there may be insufficient time to confirm full success due to the termination of funding.
Exploitation Route Successful conclusion of this project will be a collaboration between Lancaster University, University of Leeds, UC Davis, and Beijing Vacuum Electronics Research Institute, with characterisation and sharing of the parts realized. The aim of the project was to develop repeatable, reliable, and cheap THz vacuum sources, for the purposes of investigating plasma turbulence in nuclear fusion research and if successful then the next phase would be to follow on with a technology readiness improvement application.. A secondary goal is to address the lack of compact, affordable, and powerful sources at THz frequencies for remote and stand-off sensing, and high speed / novel chemical processing .
Crossover uses would be in the microfluidic areas and especially lab-on-a-chip type applications.
Sectors Agriculture, Food and Drink,Chemicals,Electronics,Energy,Environment,Healthcare,Manufacturing, including Industrial Biotechology,Pharmaceuticals and Medical Biotechnology,Security and Diplomacy,Transport

 
Title Micromachined THz backward wave oscillator for plasma diagnostic in nuclear fusion 
Description This project is a collaboration between Lancaster University, University of Leeds, UC Davis, and Beijing Vacuum Electronics Research Institute. The aim of the project is to develop repeatable, reliable, and cheap THz vacuum sources, for the purposes of investigating plasma turbulence in nuclear fusion research. A secondary goal is to address the lack of compact, affordable, and powerful sources at THz frequencies. The contribution of the team at the University of Leeds to this project will be mainly in the rapid and high-quality fabrication of Slow Waveguide Structures (SWS), which are the crucial part of a Backward Wave Oscillator (BWO). Other contributions will include the manufacture of the waveguide housing block for these SWS, and the subsequent measurement of their parameters. In order to satisfy the requirement for quick turnaround from design to implementation of SWS, a combination of conventional photolithographic and metallisation procedures are performed in the University of Leeds' Wolfson Nanotechnology Cleanroom. The structures, which consist of rows of pillars with specific height and different shape profile, are defined using thick layers of SU-8, a negative photoresist, and are subsequently coated with gold via initial sputtering and secondary plating. The main challenge so far has been optimising the parameters of resist deposition on a host silicon wafer, such as spin speed, bake, exposure, and development times, as well as subtype of SU-8; so that an uniform layer across the wafer is achieved. Furthermore, the thickness of said layer has to be within certain margin of the pillar height design value, as it has direct effect on the overall performance of the BWO. Over the past several months, the SU-8 procedure has been optimised, and is currently yielding uniform and repeatable results. Several test structures have been fabricated and inspected under reflected light optical microscope and SEM. These test structures were made using four different pillar profiles, in order to investigate their effect on BWO performance. It was found that the pillars have sufficiently uniform height across the entire wafer, in addition to well-defined vertical sidewalls. Following this confirmation, the structures' metallisation is currently under way, and they will be ready for measurement once the waveguide housing block is completed. Parallel to the SU-8 optimisation procedure, work has been going on the fabrication of the waveguide housing block, using the facilities available at the EPSRC National Facility for Innovative Robotic Systems. Unfortunately, this has taken a little longer than expected, however the project is still on track with respect to the timeframes specified in the original Case for Support. Furthermore, alternative ways of making these SWS have been investigated, and a promising new way procedure has been identified, with trials expected to begin within a couple of weeks' time. 
Type Of Material Improvements to research infrastructure 
Year Produced 2016 
Provided To Others? Yes  
Impact Impacts have yet to be fully proven but the goal is rapid low cost micro-machined BWO cavities, which at present (using conventional manufacturing methods) are extremely costly (£10's of thousands) and take several weeks manufacture on an individual basis). These BWO cavities are short lived 150-300hrs by their nature (high current density and electron bombardment) and thus require frequent replacement and the goal is to provide low cost "light-bulb" type replacement to help improve the study and probing mechanisms (remote imaging) of high energy plasma dynamics as harnessed in nuclear fusion research instruments. 
 
Title Thick negative resist patterning to form high aspect ratio (1:10) pillars for a new form of backward wave oscillator realisation 
Description Repeatable patterning and conductive coating of high aspect ration pillar structures (20um x 170um) suited to producing rapid, potentially low-cost high-quality Slow Wave Structures (SWS), which are the crucial part of a Backward Wave Oscillator (BWO) which are themselves an essential part of any high power millimetre wave source for uses in radar, remote imaging and spectroscopy. Measurement methodologies on same at frequencies up to 300GHz as well as the machining of test fixtures for this range of frequencies is another outcome of this work. 
Type Of Material Improvements to research infrastructure 
Year Produced 2016 
Provided To Others? Yes  
Impact On-going, with publications in preparations. 
 
Description THz backward wave oscillator for plasma diagnostic in nuclear fusion: - collaboration Lancaster 
Organisation Lancaster University
Department Department of Engineering
Country United Kingdom 
Sector Academic/University 
PI Contribution The contribution of the team at the University of Leeds to this project will be mainly in the rapid and high-quality fabrication of Slow Waveguide Structures (SWS), which are the crucial part of a Backward Wave Oscillator (BWO). Other contributions will include the manufacture of the waveguide housing block for these SWS, and the subsequent measurement of their parameters. In order to satisfy the requirement for quick turnaround from design to implementation of SWS, a combination of conventional photolithographic and metallisation procedures are performed in the University of Leeds' Wolfson Nanotechnology Cleanroom. The structures, which consist of rows of pillars with specific height and different shape profile, are defined using thick layers of SU-8, a negative photoresist, and are subsequently coated with gold via initial sputtering and secondary plating.
Collaborator Contribution BWO cavity design expertise and the potential of wider collaborations with groups in the US and China.
Impact Professor Paoloni is successfully continuing with this avenue of research in the form of an EC funded network project: H2020 : ULTRAWAVE : Ultra capacity wireless layer beyond 100 GHz based on millimeter wave Traveling Wave Tubes. Leeds is no longer formally part of this work for either micro-fabrication or measurements that they contributed to the previous project, but there are some legacy measurements remaining from the previous join work. The component characterisation at sub-millimetre wave frequencies undertaken at Leeds has proven to be extremely challenging, as was expected, and the results to dat have been inconclusive due to micro-machined component assembly and suspected guided energy leakage as vindicated via both high frequency VNA measurements and FE-EM modelling. . 1/09/17 ? Professor Claudio Paoloni (Principal Investigator) , Dr Rosa Letizia (Team Member) at Lancaster: The ULTRAWAVE project is aimed at developing a high capacity backhaul that enables 5G cell densification by exploiting bands beyond 100 GHz. New travelling wave tubes delivering high power will allow the creation of an ultra capacity layer providing more than 100 Gbps per kilometer square in Point to Multi point at D-band (141 - 174.8 GHz) fed by novel G-band (300 GHz) Point to Point high capacity links. The ULTRAWAVE system is empowered by the convergence of three main technologies: vacuum electronics, solid-state electronics and photonics. This ULTRAWAVE layer will enable backhaul of hundreds of small and pico cells, no matter the density, opening scenarios for new network paradigms aiming at a full 5G implementation. Project website www.ultrawave2020.eu
Start Year 2015