Gyrotron Travelling Wave Amplifier for high field Electron Paramagnetic Resonance and high frequency Dynamic Nuclear Polarisation

The aim of this program is to develop a stable, high power, wideband vacuum tube amplifier and sub-millimetre wave solid state driver, and demonstrate their use for a wide range of applications that cover the scientific spectrum from biochemistry through physics, electrical engineering and energy studies. In the proposed gyrotron amplifer, electrons are made to travel along a helical path in a magnetic field. The design of the device is such that the electrons' kinetic energy is transferred to electromagnetic radiation confined to the same region, thereby amplifying the latter. We will investigate a novel concept which uses a 5-fold helical corrugation on the inside surface of a 'cylindrical' waveguide to radically modify the electromagnetic wave dispersion giving eigenmodes with finite, constant group velocity in the region of near infinite phase velocity. This novel dispersion opens up for the first time the potential for a high power (100 W), broadband (~10 %), high gain (>40 dB) gyrotron amplifier in the 360 to 395 GHz frequency range.

We have performed a preliminary experiment at W-band, 92 to 98 GHz, frequencies and will build on our lead to create an amplifier in the 360 to 395 GHz frequency range based on the best understanding of this new concept and perform precision measurements of its gain, bandwidth, efficiency and stability against oscillations. This development would represent a generic technology with major commercial and scientific applications. These include improving NMR sensitivity through Dynamic Nuclear Polarisation techniques, high field pulse Electron Paramagnetic Resonance spectroscopy, materials processing, fusion diagnostics and long range, high bandwidth, line of sight communications. The proposal is a collaboration between two of the UK's leading millimetre wave groups: the Atoms, Beams and Plasmas Group at the University of Strathclyde and the Millimetre Wave Technology Group at the STFC Rutherford Appleton Laboratory (RAL). Collectively these groups have decades of experience and strong international reputations in the development of high power mm-wave sources, instrumentation and components, with a strong track record in commercialisation, links with industry and delivering on project objectives. The proposed work is in a core area that is likely to lead to UK leadership in advanced high frequency, high power millimetre wave amplifiers and solid state sources that will impact on studies on high field EPR/DNP spectroscopy in years to come.

In the course of the work, Strathclyde staff will design the gyrotron amplifier and pass the designs to RAL for precision manufacture of the fine features, which will be cut into a helix with the diameter of the pencil lead using state of the art computer controlled milling machines. After preliminary testing of their waveguiding properties at RAL, the components will be passed to Strathclyde for final testing and integration with the custom built electron source and vacuum enclosure. RAL will also design and build the frequency multiplier cascade required to drive the novel amplifier: they will use new configurations of their GaAs Schottky diodes to make these components. The frequency coverage, gain and maximum output power of the new amplifiers are such that world class sources are needed to provide the high input powers necessary to exploit fully the amplifier properties.

Pulse Electron Paramagnetic Resonance (EPR) experiments are used to characterise paramagnetic (unpaired electron spin) centres, which often play a central role in determining a materials electrical, magnetic, optical, catalytic and mechanical properties. Transient radicals (paramagnetic centres) are central to understanding electron transfer processes, such as those used in photosynthesis in biochemistry, and techniques based on site directed spin labelling techniques are currently becoming the method of choice for characterising many types of biomolecular processes and interactions. In moderately high magnetic fields, the resonant frequencies of electron spins are in the mmw and sub-mmw range and can hugely benefit from technologies that provide high power amplification over high bandwidths, due to improved absolute and concentration sensitivity, higher electron polarisation, improved spectral resolution and orientational selectivity. High frequency pulse EPR is also key to next generation Dynamic Nuclear Polarisation (DNP) techniques that have recently dramatically improved the sensitivity of NMR (by orders of magnitude) for many types of sample. DNP has also been used to produce highly polarised samples (metabolites) for enhanced Magnetic Resonance Imaging that hold great promise for the detection and monitoring of cancer. Both NMR and MRI represent $Billion industries and so DNP is currently a very active field of research in the magnetic resonance community. There is a strong requirement for the development of high peak power pulse sources at modest average power and bandwidth at 395GHz for pulse DNP. Both Strathclyde and RAL have strong track records in engagement with industry, commercialisation of technology and delivering on both Research Council and commercial research or equipment development contracts. Strathclyde is presently working on several contracts directly placed by UK industry to provide novel solutions for identified future markets. RAL has undertaken many tens of commercial contracts and commercialised the mm-wave instrumentation. We plan to pursue commercialisation opportunities as they arise.