Novel gyro-TWA amplifier for high power mm-wave radar remote sensing

"Modelling the global climate accurately, and developing tools which can predict the weather more reliably, is of fundamental importance us all. To improve the quality of atmospheric models we need increasingly widespread and more sensitive measurements of atmospheric constituents. In particular, clouds play an enormous role in the earth's atmospheric processes but currently they are still relatively poorly understood, partly due to a lack of measured data, and this lack of data means that atmospheric computer simulations are of limited validity. As global warming takes effect, this can result in more moisture in the atmosphere, increasing the frequency of extreme weather events. Thus, improving our ability to measure clouds is an important goal for climate researchers. Radars which operate with millimetre wavelengths are ideally placed to measure clouds, ice particles, aerosols and volcanic ash since their operating wavelength is appropriate to the scale of these atmospheric constituents. However, current millimetre wave cloud profiling radars, which are usually ground based and use narrow frequency band high power pulse amplifiers, have limited ability to detect the most tenuous ensembles of very fine particles, especially at very high altitudes, where their interaction with solar radiation is highly significant. Furthermore, the limited sensitivity of earlier generations of cloud profiling radars tended to mean they measured slowly and only looked in a single direction, usually vertically upwards. This limited view of clouds then fails to capture their true three dimensionality and dynamic behaviour. The next generation of cloud profiling radars will scan their beam around in space to reveal cloud structure and record the temporal evolution of cloud masses, but this requires increased transmit power.

The aim of our project is to demonstrate a new class of high power, wideband millimetre wave amplifier, called a gyro-TWA, which offers a ten-fold increase in available bandwidth and a five-fold increase in available peak power over the amplifiers used in current cloud profiling radars. This will lead to greater radar sensitivity, enabling measurement of smaller or more tenuous particulates, with finer resolution, at longer ranges or in a shorter timescale. The technology also has the potential to be applied to the ground based mapping of space debris, a major consideration for all orbiting systems including environmental monitoring satellites. The proposal is a collaboration between two major millimetre wave groups at the University of Strathclyde and the University of St Andrews who collectively have decades of experience and vibrant international reputations in the development of high power millimetre wave sources, radars, instrumentation and components, plus a strong track record in commercialisation, industrial collaboration, and delivering on project objectives. The gyro-TWA represents a core technology that is likely to lead to UK leadership in the field of high power millimetre wave radar."

"STFC CLASP project ST/K006673/1 is developing a new class of high power, wideband millimetre wave amplifier, called a Gyrotron Travelling Wave Amplifier (gyro-TWA) which offers a ten-fold increase in available bandwidth and a five-fold increase in available peak power over current amplifiers. The novel gyro-TWA is being incorporated in a millimetre wave radar that could be used to observe and monitor clouds, aerosols and precipitation which is critical to understanding the earth's atmosphere and a vital prerequisite for the validation of global climate models. Millimetre wave radars are uniquely placed to serve as atmospheric remote sensing tools since their wavelength is appropriate to the size of the particles being probed. Greater sensitivity will be achieved with the use of a gyro-TWA in a mm-wave radar as compared to extended interaction klystron amplifiers currently used in cloud profiling radar systems. The use of the gyro-TWA will enable measurement of tenuous particulates, with finer resolution, at longer ranges or in a shorter timescale.

The gyro-TWA thermionic cathode electron beam source based on a Cusp magnetic field profile has been designed, constructed and tested. Due to the knowledge and expertise of Dr. Wenlong He and his research team Strathclyde is the only research group in the world to have managed to successfully incorporate a cusp electron gun in a high power gyro-TWA operating at ~ 90 to 100GHz. The helically corrugated waveguide (HCW) interaction region which enables high power (5kW) broadband coupling between the electron beam and the millimetre waves to take place has been designed and constructed with the wave dispersion of the HCW measured using a Vector Network Analyser. All the amplifier components such as a broadband pillbox input coupler and three layer output window have been installed and tested. A fibre optic controlled pulsed high voltage modulator has been developed consisting of an inverting doubled Blumlein network, switched with a thyratron (CX1935X) and charged by an ALE 802L capacitor charging unit with a constant charge rate of 9000 J/s. Output voltage pulses of magnitude 40kV and pulse duration of 200ns at a Pulse Repetition Frequency of 6kHz required for the cloud profiling millimetre wave radar has been successfully tested. The millimetre wave radar transceiver and data acquisition subsystem for the radar experiments has been time correlated with the fibre optic controlled high voltage modulator. The 90GHz to 100GHz gyro-TWA has been assembled and is under vacuum. Electron beam and millimetre wave measurements are currently taking place.

The successful use of the gyro-TWA in millimetre wave radar will provide a compelling demonstration of the potential of this technology, which has worldwide market potential not only in the cloud profiling field, but also in other radar applications, THz imaging, wireless communications and magnetic resonance spectroscopy. The importance of this project is significant in terms of relevance to: global challenges in environment, healthcare and security."