Innovations in terrestrial transportation technologies, such as cars, trains, and aircraft, have driven historical technologies and industries so far; now, a similar breakthrough is occurring in space thanks to electric propulsion technology.

Electric propulsion is a technique utilizing electromagnetic fields to accelerate a propellant and to generate thrust that propels a spacecraft. Space agencies have pioneered electric propulsion technology as the future of space exploration.

Already, several space missions have successfully been completed using electric propulsion devices, such as gridded ion thrusters and Hall thrusters. Solar power is converted into thrust energy when the propellant becomes ionized, i.e., a plasma, and gets accelerated by electromagnetic fields. Yet, the electrodes necessary for these devices limit their lifetime, since they get exposed to and damaged by the plasma, especially at a high-power level.

To circumvent this, scientists have turned to electrodeless plasma thrusters. One such technology harnesses radio frequency (rf) to generate plasma. An antenna emits radio waves into a cylindrical chamber to create plasma, where a magnetic nozzle channels and accelerates the plasma to generate thrust. MN rf plasma thrusters, or helicon thrusters as they are sometimes known, offer simplicity, operational flexibility, and a potentially high thrust-to-power ratio.

But the development of MN rf plasma thrusters has been stymied by the conversion efficiency of the rf power to thrust energy. Early experiments generated single digit conversion rates, but more recent studies have reached a modest outcome of 20%.

In a recent study, Professor Kazunori Takahashi, from Tohoku University’s Department of Electrical Engineering, has achieved a 30% conversion efficiency.

Whilst mature electric propulsion devices often use xenon gas, which is expensive and difficult to supply in sufficient quantities, the current 30% efficiency was obtained with argon propellant. This indicates that a MN rf plasma thruster would reduce the cost and the resource load from the Earth.

“Applying a cusp-type magnetic field inhibited the energy loss that generally occurs to the plasma source wall,” Takahashi said. “The breakthrough opens the door to advances in high-power space transportation technology.”

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Materials provided by Tohoku University. Note: Content may be edited for style and length.

Ordinary solid matter consists of atoms organized in a crystal lattice. The chemical character of the atoms and lattice symmetry define the properties of the matter, for instance, whether it is a metal, a semiconductor or and electric insulator. The lattice symmetry may be changed by ambient conditions such as temperature or high pressure, which can induce structural transitions and transform even an electric insulator to an electric conductor, that is, a metal.

Larger identical entities such as nanoparticles or atomically precise metal nanoclusters can also organize into a crystal lattice, to form so called meta-materials. However, information on how to engineer the growth of such materials from their building blocks has been scarce since the crystal growth is a typical self-assembling process.

Now, first insights into engineering crystal growth by atomically precise metal nanoclusters have been achieved in a study performed by researchers in Singapore, Saudi Arabia and Finland. They synthesized metal clusters consisting of only 25 gold atoms, one nanometer in diameter. These clusters are soluble in water due to the ligand molecules that protect the gold. This cluster material is known to self-assemble into well-defined close packed single crystals when the water solvent is evaporated. However, the researcher found a novel concept to regulate the crystal growth by adding tetra-alkyl-ammonium molecular ions in the solvent. These ions affect the surface chemistry of the gold clusters, and their size and concentration were observed to have an impact on the size, shape, and morphology of the formed crystals. Remarkably, high-resolution electron microscopy images of some of the crystals revealed that they consist of polymeric chains of clusters with four-gold-atom interparticle links. The demonstrated surface chemistry opens now new ways to engineer metal cluster -based meta-materials for investigations of their electronic and optical properties.

The cluster materials were synthesized in the National University of Singapore, the electron microscopy imaging was done at the King Abdullah University of Science and Technology in Saud Arabia, and the theoretical modelling was done at the University of Jyvaskyla, Finland.

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Materials provided by University of Jyväskylä – Jyväskylän yliopisto. Note: Content may be edited for style and length.

The research team led by Vlad Tereshenko and Oskar Aszmann from the Clinical Laboratory for Bionic Limb Reconstruction at MedUni Vienna’s Department of Plastic, Reconstructive and Aesthetic Surgery discovered this facet of the interaction between nerves and muscles — which was previously unknown to science — in the course of its preclinical research on facial nerves and muscles. After a nerve has been injured or severed, it is no longer able to control the motor function of the facial muscles, resulting in facial paralysis in the animal model.

In some cases, the scientists observed spontaneous recovery of muscle function days or weeks after the nerve lesion. Using novel, complex techniques, they were able to establish that the autonomic nervous system takes over the function of the injured nerve, as it were. “Until now, we were unaware that the autonomic nervous system can control muscle motor function with nerve impulses. As we have seen in our experiments, the parasympathetic nerve fibres form new functional neuromuscular synapses to do this. At the same time, the patterns of the muscle fibres are modified and, hence, the physiological properties of the autonomously reinnervated muscles are changed,” explains first author Vlad Tereshenko, outlining the key findings from the study.

Potential actor in nerve reconstruction

Following injuries or certain diseases, nerves can temporarily or permanently lose their ability to provide motor control to muscles. Well-established therapeutic concepts such as the relocation of nerves or nerve transplants are now available to remedy the resultant motor deficits. However, clinical outcomes may be affected by several factors, such as the slow rate of nerve regeneration or the lack of donor nerves. “By identifying this previously unknown ability of the autonomic nervous system, we have discovered a new potential actor in nerve reconstruction. The results of our study can therefore help to improve existing therapeutic measures and to develop new ones,” says Vlad Tereshenko, looking into the future. Follow-up studies are expected to deepen our knowledge of this new facet of the neuromuscular system. One of the questions to be addressed is whether and how autonomic nerve fibres can be surgically relocated in order to restore muscle function on a temporary or permanent basis.

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Materials provided by Medical University of Vienna. Note: Content may be edited for style and length.

Ultrafast laser technology has enabled a trove of methods for precision measurements. These include in particular a broad class of pulsed-laser experiments in which a sample is excited and, after a variable amount of time, the response is measured. In such studies, the delay between the two pulses should typically cover the range from femtoseconds to nanoseconds. In practice, scanning the delay time over a range that broad in a repeatable and precise manner is a significant challenge. A team of researchers in the group of Prof. Ursula Keller in the Department of Physics at ETH Zurich, with main contributions from Dr. Justinas Pupeikis, Dr. Benjamin Willenberg and Dr. Christopher Phillips, has now taken a major step towards a solution that has the potential to be a game changer for a wide range of practical applications. Writing in Optica, they recently introduced and demonstrated a versatile laser design that offers both outstanding specifications and a low-complexity setup that runs stably over many hours.

The long path to long delays

The conceptually simplest solution to scanning optical delays is based on a laser whose output is split into two pulses. While one of them takes a fixed route to the target, the optical path for the second pulse is varied with linearly displacing mirrors. The longer the path between mirrors, the later the laser pulse arrives at the target and the longer is the delay relative to the first pulse. The problem, however, is that light travels at famously high speed, covering some 0.3 metres per nanosecond (in air). For mechanical delay lines this means that scanning to delays up to several nanoseconds requires large devices with intricate and typically slow mechanical constructions.

An elegant way to avoid complex constructions of that kind is to use a pair of ultrashort pulse lasers that emit trains of pulses, each at slightly different repetition rates. If, say, the first pulses emerging from each of the lasers are perfectly synchronized, then the second pair has a delay between the pulses that corresponds to the difference in repetition times of the two lasers. The next pair of pulses has twice that delay between them, and so on. In this manner, a perfectly linear and fast scan of optical delays without moving parts is possible — at least in theory. The most refined type of a laser system generating two such pulse trains is known as a dual comb, in reference to the spectral structure of the output consisting of a pair of optical frequency combs.

Whereas the promise of the dual-comb approach has long been clear, progress towards applications was hindered by challenges related to designing a readily deployable laser system that provides two simultaneously operating combs of the required quality and with high relative stability. Now, Pupeikis et al. made a breakthrough towards such a practical laser, and the key is a new way to generate the two frequency combs in one and the same laser cavity.

Two from one

The task the researchers had at hand was to construct a laser source that consist of two coherent optical pulse trains that are basically identical in all properties except from that all-important difference in repetition rate. A natural route to achieve this is to create the two combs in the same laser cavity. Various approaches for realizing such laser-cavity multiplexing have been introduced in the past. But these typically require that additional components are placed inside the cavity. This introduces losses and different dispersion characteristics for the two combs, among other issues. The ETH physicists have overcome these issues while still ensuring that the two combs share all of the components inside the cavity.

They achieved this by inserting into the cavity a ‘biprism’, a device with two separate angles on the surface from which light is reflected. The biprism splits the cavity mode into two parts, and the researchers show that by suitable design of the optical cavity the two combs can be spatially separated on the active intracavity components while still taking a very similar path otherwise. ‘Active components’ refers here to the gain medium, where lasing is induced, and to the so-called SESAM (semiconductor saturable absorber mirror) element, which enables mode-locking and pulse generation. The spatial separation of the modes at these stages means that two combs with distinct spacing can be generated, while most other properties are essentially duplicated. In particular, the two combs have highly correlated timing noise. That is, while imperfections in the temporal comb structure are unavoidably present, they are almost the same for the two combs, making it possible to deal with such noise.

A gate to practical applications

An outstanding feature of the novel single-cavity architecture now introduced is that it does not require compromises in laser design. Instead, cavity architectures that are optimal for single-comb operation can be readily adapted for dual-comb use. With that, the new design also represents a major simplification relative to commercial products and opens up a path for the production and deployment of this new class of ultrafast laser sources.

The benchmarks achieved in the first demonstrations are highly encouraging. The researchers scanned an optical delay of 12.5 ns (equivalent to a distance of 3.75 m in air) with 2-fs precision (which is less than a micrometre in physical distance) at rates of up to 500 Hz and with record-high stability for a single-cavity dual-comb laser. The obtained performance — including the high power of more than 2.4 W for each comb, the short pulse durations of less than 140 fs, and the demonstrated coupling to an optical parametric oscillator (OPO) for converting the light into a different wavelength regime — underline the practical potential of the approach for a wide spectrum of measurements, from precision optical ranging (the optical measurement of absolute distance) to high-resolution absorption spectroscopy and nonlinear spectroscopy for sampling ultrafast phenomena.

The research team developed a model of the gravitational field of the galaxy that acted as a lens, and that way it was possible to determine that the light from these three images travelled along three different paths, differing in distance by a few days. This accounts for the three colours obtained in the images, because a variation in the colour emitted takes place as the gas in the supernova expands and cools. The higher the temperature is, the bluer the light emitted will be, and as the temperature falls, the light emitted tends towards red. So the blue image is a photograph of the supernova a few hours after the stellar explosion, while the green and red images correspond to 2 and 8 days, respectively, after the explosion.

This information enabled the radius of the star that exploded to be determined; it was a red super-giant with a radius equal to 500 times that of the Sun, and exploded 11.5 billion years ago, long before the Earth was born, specifically at the moment when our Galaxy is thought to have formed. The images of this supernova captured by the Hubble Space Telescope are highly magnified by the gravitational field of a nearby galaxy that acts as a lens and allows us to see much further in distance and in time than all local supernovae in nearby galaxies.

The study of the explosions of these red super-giant stars tallies with the current understanding of how the heavier atomic elements were created inside stars and during supernova explosions: elements forged inside stars are released in these supernova explosions to become the next generation of gas and material from which solar systems and life as we know it are created. Without these explosions, the gas in today’s galaxies would only include the hydrogen and helium that formed during the Big Bang and would not support complex life that requires other heavier chemical elements. Moreover, this supernova observed through a gravitational lens demonstrates that an event taking place in the distant Universe can be witnessed several times, so in principle we could focus our instruments in advance to get a detailed view of the eruption of a star turning into a supernova.

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Materials provided by University of the Basque Country. Note: Content may be edited for style and length.