First Observations of G-Band Radar Doppler Spectra

The first Doppler spectra ever acquired by an atmospheric radar at 200 GHz (G-band) are presented. The observations were taken during a light precipitation event in May (rain rates <2 mm hr−1) at Chilbolton Observatory, UK, with coincident Ka-band and W-band Doppler radar measurements. The collected rain spectra agree with Mie theory predictions: at G-band they show significant reductions in the spectral power return—as compared to theoretical Rayleigh scattering—corresponding to high Doppler velocities (i.e., large raindrops) with the presence of multiple peaks and “Mie notches” in correspondence to the maxima and minima of the raindrop backscattering cross sections. The first two G-band Mie troughs correspond to smaller velocities/sizes than the first W-band Mie notch. These features offered by G-band radars pave the way toward applying, in rain, Mie notch vertical wind retrievals and multifrequency drop size distribution microphysical retrievals to smaller rain rates and smaller characteristic sizes than ever before

EMP control and characterisation on high-power laser systems

Giant electromagnetic pulses (EMP) generated during the interaction of high-power lasers with solid targets can seriously degrade electrical mea- surements and equipment. EMP emission is caused by the acceleration of hot electrons in- side the target, which produce radiation across a wide band from DC to terahertz frequencies. Improved understanding and control of EMP is vital as we enter a new era of high repetition rate, high intensity lasers (e.g. ELI, the Extreme Light Infrastructure). We present recent data from the VULCAN laser facility that demonstrates how EMP can be readily and effectively reduced. Characterisation of the EMP was achieved using B-dot and D-dot probes that took measurements for a range of different target and laser parame- ters. We demonstrate that target stalk geometry, material composition and foil surface area can all play a significant role in the reduction of EMP. A combination of electromagnetic wave and 3D particle-in-cell simulations are used to inform our conclusions about the effects of stalk geometry on EMP, providing an opportunity for comparison with existing charge separation models

EMP control and characterization on high-power laser systems

Giant electromagnetic pulses (EMP) generated during the interaction of high-power lasers with solid targets can seriously degrade electrical measurements and equipment. EMP emission is caused by the acceleration of hot electrons inside the target, which produce radiation across a wide band from DC to terahertz frequencies. Improved understanding and control of EMP is vital as we enter a new era of high repetition rate, high intensity lasers (e.g. the Extreme Light Infrastructure). We present recent data from the VULCAN laser facility that demonstrates how EMP can be readily and effectively reduced. Characterization of the EMP was achieved using B-dot and D-dot probes that took measurements for a range of different target and laser parameters. We demonstrate that target stalk geometry, material composition, geodesic path length and foil surface area can all play a significant role in the reduction of EMP. A combination of electromagnetic wave and 3D particle-in-cell simulations is used to inform our conclusions about the effects of stalk geometry on EMP, providing an opportunity for comparison with existing charge separation models

A high-k mm-wave scattering diagnostic for measuring binormal wavenumber electron scale turbulence on MAST-U

We describe a high-k mm-wave scattering diagnostic for measuring binormal wavenumber electron scale turbulence on the MAST-U spherical tokamak. Gaussian wave optics and beam-tracing calculations are presented that show the predicted spatial and wavenumber resolution of the diagnostic along with the sensitivity of measurement. The proposed system will operate at a frequency of 376GHz and will facilitate adjustable localization of the turbulence measurement from the magnetic axis out to the plasma edge

Study of backward terahertz radiation from intense picosecond laser-solid interactions using a multichannel calorimeter system

A multichannel calorimeter system is designed and constructed which is capable of delivering single-shot and broad-band spectral measurement of terahertz (THz) radiation generated in intense laser-plasma interactions. The generation mechanism of backward THz radiation (BTR) is studied by using the multichannel calorimeter system in an intense picosecond laser-solid interaction experiment. The dependence of the BTR energy and spectrum on laser energy, target thickness and pre-plasma scale length is obtained. These results indicate that coherent transition radiation is responsible for the low-frequency component ( 3 THz) of BTR

The ESA Earth Explorer 10 Candidate Mission LOCUS

We present the ESA Earth Explorer candidate mission LOCUS. LOCUS is under evaluation for Phase-0 Study in the current 10th ESA Earth Explorer Call (EE10). It is a UK mission proposal for an upper atmospheric research satellite that uses disruptive receiver technology to make novel atmospheric measurements. At the core of the LOCUS instrument is a heterodyne Schottky receiver. Such receivers have long been used very successfully for satellite Earth Observation in the millimetre- and submillimetre-wave range. But the desire to extend the observation frequencies into the THz range has been met with fundamental technological difficulties, namely the lack of high-power Local Oscillator (LO) sources to pump the frequency down-conversion process (i.e., frequency mixing) at THz frequencies. This is known as the “THz-Gap”. The development of novel Quantum Cascade Laser (QCL) local oscillators in the UK would make it possible, for the first time, to build THz and supra-THz heterodyne remote sensing instrument in a very compact, low power implementation, with very moderate cooling requirements (2–3 W heat-lift at ~70 K). This combination of novel technologies is ideally suited to bring down the cost of potential space-borne deployment. The CEOI has played a major role in the past to develop THz Schottky receivers at RAL Space, QCL devices at the University of Leeds, miniature space-coolers at STFC Technology, and high-resolution, wide-band digital spectrometers at STAR-Dundee. The scientific motivation that drive this UK technology development is captured in the LOCUS missions: To measure the composition of atomic oxygen (O) in the Mesosphere – Lower Thermosphere (MLT). O is the main component of the MLT, but because it can only be measured remotely at two distinct THz frequencies (4.7 & 2.0 THz), its abundance, and particularly its global and temporal variability is still largely unknown

The 2017 Terahertz Science and Technology Roadmap

Science and technologies based on terahertz frequency electromagnetic radiation (100GHz-30THz) have developed rapidly over the last 30 years. For most of the 20th century, terahertz radiation, then referred to as sub-millimeter wave or far-infrared radiation, was mainly utilized by astronomers and some spectroscopists. Following the development of laser based terahertz time-domain spectroscopy in the 1980s and 1990s the field of THz science and technology expanded rapidly, to the extent that it now touches many areas from fundamental science to “real world” applications. For example THz radiation is being used to optimize materials for new solar cells, and may also be a key technology for the next generation of airport security scanners. While the field was emerging it was possible to keep track of all new developments, however now the field has grown so much that it is increasingly difficult to follow the diverse range of new discoveries and applications that are appearing. At this point in time, when the field of THz science and technology is moving from an emerging to a more established and interdisciplinary field, it is apt to present a roadmap to help identify the breadth and future directions of the field. The aim of this roadmap is to present a snapshot of the present state of THz science and technology in 2016, and provide an opinion on the challenges and opportunities that the future holds. To be able to achieve this aim, we have invited a group of international experts to write 17 sections that cover most of the key areas of THz Science and Technology. We hope that The 2016 Roadmap on THz Science and Technology will prove to be a useful resource by providing a wide ranging introduction to the capabilities of THz radiation for those outside or just entering the field as well as providing perspective and breadth for those who are well established. We also feel that this review should serve as a useful guide for government and funding agencies

Temporal, spatial, and structural patterns of adult trembling aspen and white spruce mortality in Quebec's boreal forest

Temporal, spatial, and structural patterns of adult trembling aspen (Populus tremuloides Michx.) and white spruce (Picea glauca (Moench) Voss) mortality were studied in intact 150-year-old stands in the southwestern boreal forest of Quebec. For both species, mortality decreases (number of dead trees/total number of trees) with distance from the lake edge until 100-150 m, from which point it slightly increases. Strong peaks in mortality were found for 40- to 60-year-old aspen mainly between 1974 and 1992. Such mortality in relatively young aspen is likely related to competition for light from the dominant canopy trees. Also, the recruitment of this young aspen cohort is presumably the result of a stand breakup that occurred when the initial aspen-dominated stand was between 90 and 110 years old. For spruce, strong peaks in mortality were found in 110- to 150-year-old trees and they occurred mainly after 1980. No clear explanation could be found for these peaks, but we suggest that they may be related to senescence or weakening of the trees following the last spruce budworm outbreak. Suppressed and codominant aspen had a much higher mortality ratio than spruce in the same height class, while more surprisingly, no difference in mortality rate was found between dominant trees of the two species. Most spruce trees were found as standing dead, which leads us to reject the hypothesis that windthrow is an important cause of mortality for spruce in our forests