Krypton Hall effect thruster for spacecraft propulsion

 Electric propulsion (EP) is the future of astronautics. It can already compete successfully with chemical thrusters, especially for attitude control, orbit transfer and/or orbital station-keeping as well as for the main propulsion system for deep space missions. However, xenon, the propellant of choice in most EP devices, has a substantial drawback: its cost is very high. On the basis of the experience with plasma jet accelerators, a team of scientists and engineers from the Institute of Plasma Physics and Laser Microfusion in Warsaw has designed the Hall effect thruster optimised to work with krypton, a much more affordable noble gas.

Chemical propulsion is invaluable for the launch of payloads into space. The thrust, generated exclusively from the energy released by combustion of the propellants, is very large, but limited to durations of the order of seconds or minutes. In space, however, where atmospheric drag is negligible, technologies delivering much lower thrust over significantly longer durations (months or even years) have proven much more efficient. The leading low-thrust technology is plasma propulsion, where xenon is the preferred working gas. In the Institute of Plasma Physics and Laser Microfusion (IPPLM) in Warsaw, a Hall effect thruster has been designed to work with krypton, a noble gas ten times cheaper than xenon.

The Hall effect thruster is one of several existing electric propulsion technologies. In use since the 1970s in unmanned space flights, it has made it possible to manoeuvre precisely and correct satellite orbits. Lately, devices of this type have increasingly been used as the main propulsion system for deep space missions.

Hall effect thrusters convert the propellant into a plasma and produce thrust using an external electrical power source, most typically solar panels. Plasma particles (ions and electrons) are electrically charged and can thus be accelerated by an electric field to high velocities, of the order of 15-30 km/s as is the case with Hall thrusters (in contrast, expelled gases do not reach more than 4 km/s with chemical propulsion). Plasma propulsion produces a low thrust (from a few to 1000 mN depending on available power) but can operate over long durations and ultimately increase the velocity of the spacecraft by several kilometres per second.

"Plasma jet accelerators have been studied for many years in IPPLM. Building on this experience, our team has started, in May 2008, the development of a plasma Hall effect thruster using krypton as a propellant," said Dr Jacek Kurzyna, the person responsible for the project.

The propellant used in the vast majority of Hall effect thrusters is xenon, a very rare and therefore expensive noble gas. Krypton, another noble gas, is up to ten times less expensive. Although a slightly higher energy is necessary to produce krypton ions, they are lighter than xenon ions and accordingly require lower acceleration voltages to achieve the same velocity. "From the very beginning, our thruster has been developed and optimised to operate with krypton. We had to design properly the magnetic field configuration and the appropriate magnetic circuit. Some elements had to be constructed in such a way that they can withstand increased heat loads," explains Dariusz Daniłko, a PhD student from IPPLM.

The new thruster is medium-power, continuous-thrust propulsion device. Weighing less than 5 kg, it operates at a power of about half a kilowatt. "The SMART-1 lunar space probe sent by the European Space Agency (ESA) had a xenon thruster with power below 2 kW. It accelerated the vehicle by 3,6 km/s. Our thruster could therefore prove suitable as a main propulsion system in small spacecrafts," says Dr Serge Barral from IPPLM.

The newly built Hall effect thruster is a prototype device ready to be tested in vacuum conditions. "If the outcome of the tests is positive, optimization of the device and a round of assessment tests will follow. The project, submitted to the second PECS call (Plan for European Cooperating State, an agreement concluded between Poland and ESA), has been recommended for funding. If funding is confirmed, this project will mark the beginning of the qualification process," explains Dr Kurzyna.

The research on krypton Hall effect thrusters is expected to find applications beyond the field of astronautics. Plasma accelerators are routinely used in many technological processes, inter alia, for surface cleaning by plasma sputtering or etching, surface modification and thin film (e.g. diamond-like carbon) deposition. The team of scientists from IPPLM has suggested, in particular, a deposition process of thin oxide layers for photovoltaic solar panels based on the Hall thruster technology.

Story Source:

The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by Institute of Plasma Physics and Laser Microfusion, via AlphaGalileo.

Technique to control light from nanoparticles

A nanoscale game of "now you see it, now you don't" may contribute to the creation of metamaterials with useful optical properties that can be actively controlled, according to scientists at Rice University.

A Rice laboratory led by chemist Stephan Link has discovered a way to use liquid crystals to control light scattered from gold nanorods. The researchers use voltage to sensitively manipulate the alignment of liquid crystal molecules that alternately block and reveal light from the particles; the gold nanorods collect and retransmit light in a specific direction.

The research was reported in the American Chemical Society journal Nano Letters.

It seems simple, but Link said the technique took two years to refine to the point where light from the nanoparticles could be completely controlled.

"The key to our approach is the in-plane rotation of liquid crystal molecules covering individual gold nanorods that act as optical antennas," said Link, an assistant professor of chemistry and electrical and computer engineering. "Learning how our devices work was exciting and has provided us with many ideas of how to manipulate light at the nanoscale."

Link said the device is actually a super half wave plate, a refined version of a standard device that alters the polarization of light.

With the new device, the team expects to be able to control light from any nanostructure that scatters, absorbs or emits light, even quantum dots or carbon nanotubes. "The light only has to be polarized for this to work," said Link, who studies the plasmonic properties of nanoparticles and recently authored a perspective on his group's recent research in plasmonics for the Journal of Physical Chemistry Letters.

In polarized light, like sunlight reflecting off water, the light's waves are aligned in a particular plane. By changing the direction of their alignment, liquid crystals can tunably block or filter light.

The Rice team used gold nanorods as their polarized light source. The rods act as optical antennas; when illuminated, their surface plasmons re-emit light in a specific direction.

In their experiment, the team placed randomly deposited nanorods in an array of alternating electrodes on a glass slide; they added a liquid crystal bath and a cover slip. A polyimide coating on the top cover slip forced the liquid crystals to orient themselves parallel with the electrodes.

Liquid crystals in this homogenous phase blocked light from nanorods turned one way, while letting light from nanorods pointed another way pass through a polarizer to the detector.

What happened then was remarkable. When the team applied as little as four volts to the electrodes, liquid crystals floating in the vicinity of the nanorods aligned themselves with the electric field between the electrodes while crystals above the electrodes, still under the influence of the cover slip coating, stayed put.

The new configuration of the crystals -- called a twisted nematic phase -- acted like a shutter that switched the nanorods' signals like a traffic light.

"We don't think this effect depends on the gold nanorods," Link said. "We could have other nano objects that react with light in a polarized way, and then we could modulate their intensity. It becomes a tunable polarizer."

Critical to the experiment's success was the gap -- in the neighborhood of 14 microns -- between the top of the electrodes and the bottom of the cover slip. "The thickness of this gap determines the amount of rotation," Link said. "Because we created the twisted nematic in-plane and have a certain thickness, we always get 90-degree rotation. That's what makes it a super half wave plate."

The research was funded by the Robert A. Welch Foundation, the Office of Naval Research, the American Chemical Society Petroleum Research Fund and a 3M Nontenured Faculty Grant.

The above story is reprinted (with editorial adaptations) from materials provided by Rice University, via EurekAlert!, a service of AAAS.

Journal Reference:

Saumyakanti Khatua, Wei-Shun Chang, Pattanawit Swanglap, Jana Olson, Stephan Link. Active Modulation of Nanorod Plasmons. Nano Letters, 2011; 11 (9): 3797 DOI: 10.1021/nl201876r

Scientists and engineers create the 'perfect plastic'

 Researchers at the University of Leeds and Durham University have solved a long-standing problem that could revolutionize the way new plastics are developed. The breakthrough will allow experts to create the 'perfect plastic' with specific uses and properties by using a high-tech 'recipe book.' It will also increase our ability to recycle plastics. The research paper is published in the journal Science on September 29.

The paper's authors form part of the Microscale Polymer Processing project, a collaboration between academics and industry experts which has spent 10 years exploring how to better build giant 'macromolecules.' These long tangled molecules are the basic components of plastics and dictate their properties during the melting, flowing and forming processes in plastics production.

Low-density polyethylenes (LDPEs) are used in trays and containers, lightweight car parts, recyclable packaging and electrical goods. Up until now, industry developed a plastic then found a use for it, or tried hundreds of different "recipes" to see which worked. This method could save the manufacturing industry time, energy and money.

The mathematical models used put together two pieces of computer code. The first predicts how polymers will flow based on the connections between the string-like molecules they are made from. A second piece of code predicts the shapes that these molecules will take when they are created at a chemical level. These models were enhanced by experiments on carefully synthesised 'perfect polymers' created in labs of the Microscale Polymer Processing project.

Dr. Daniel Read, from the School of Mathematics, University of Leeds, who led the research, said, "Plastics are used by everybody, every day, but until now their production has been effectively guesswork. This breakthrough means that new plastics can be created more efficiently and with a specific use in mind, with benefits to industry and the environment."

Professor Tom McLeish, formerly of the University of Leeds, now Pro-Vice Chancellor for Research at Durham University leads the Microscale Polymer Processing project. He said, "After years of trying different chemical recipes and finding only a very few provide useable products, this new science provides industry with a toolkit to bring new materials to market faster and more efficiently."

Professor McLeish added that as plastics production moves from oil-based materials to sustainable and renewable materials, the "trial and error" phase in developing new plastics could now be by-passed. He said, "By changing two or three numbers in the computer code, we can adapt all the predictions for new bio-polymer sources."

"This is a wonderful outcome of years of work by this extraordinary team. It's a testimony to the strong collaborative ethos of the UK research groups and global companies involved," he added.

Dr. Ian Robinson of Lucite International, one of the industrial participants in the wider project said, "The insights offered by this approach are comparable to cracking a plastics 'DNA.'"

The model was developed by Dr. Daniel Read, School of Mathematics, University of Leeds, Dr. Chinmay Das of the School of Physics & Astronomy, University of Leeds and Professor Tom McLeish, Department of Physics, Durham University. Their predictions were compared to the results of polymer analysis by Dr. Dietmar Auhl, at the time a physicist at Leeds.

The research was carried out at the University of Leeds, Durham University, LyondellBasell and Dow Chemical and was funded by the Engineering and Physical Sciences Research Council and the European Union.

The Microscale Polymer Processing collaboration includes researchers from the universities of Durham, Bradford, Cambridge, Leeds, Nottingham, Oxford, Reading, Sheffield and University College London alongside their industry counterparts from Lucite International, Ineos, LyondellBasell, BASF, Dow Chemical, DSM, and Mitsubishi.

Story Source:

The above story is reprinted (with editorial adaptations ) from materials provided by University of Leeds, via EurekAlert!, a service of AAAS.

Journal Reference:

D. J. Read, D. Auhl, C. Das, J. den Doelder, M. Kapnistos, I. Vittorias, T. C. B. McLeish. Linking Models of Polymerization and Dynamics to Predict Branched Polymer Structure and Flow. Science, 2011; 333 (6051): 1871 DOI: 10.1126/science.1207060

New method cleans up textile industry's most dangerous chemicals

Textile dying is one of the most environmentally hazardous aspects of the textile industry. During dying, harmful chemicals that are difficult to break down are released, all too often into rivers and agricultural land. However, Maria Jonstrup, a doctoral student in Biotechnology at Lund University, has developed a new, environmentally friendly purification process which leaves only clean water. The findings are presented in Maria Jonstrup's thesis.

The research is so far only research, and has therefore only been tested in the laboratory, but Maria Jonstrup is optimistic about its future potential.

"In the long term it should be possible for textile factories in India, China and Bangladesh to use the technique. If it works on a laboratory scale it is quite likely that it will also work in a real-life situation", she says.

While working on her thesis, she has conducted experiments with both fungal enzymes and bacteria from the drains at textile industry and municipal wastewater treatment plants. However, it was only when she combined two different types of purification process, one biological and one chemical, that the breakthrough came.

"First, microorganisms break down the dyes in a reactor. This biological step is the most important. However, to be certain that the water is completely purified, I also use some chemicals. Small amounts of iron and hydrogen peroxide in combination with UV light break down even the most difficult structures", she explains.

A combination of both biological and chemical purification is already used in some places, but these methods are rarely effective, which means that large quantities of hazardous chemicals are released.

Soon, two Master's students will be taking over the baton. They will spend a year testing the technique in larger volumes of water, which more closely reflect the conditions in real textile factories. Their challenge will be to study how the UV light in the chemical stage could be replaced with normal sunlight. Maria Jonstrup will be their supervisor. After that it is hoped that the technique will be tested 'live', in a real factory.

"Through contacts with the Swedish clothing company Indiska Magasinet and their suppliers, we have already taken samples and performed tests at a factory in India. Because clothing manufacture has received quite a bad reputation over recent years, it can otherwise be quite difficult to gain access to the factories", she explains.

One obstacle on the path to implementation is legislation. The law only stipulates that the water is to be clean. This has made it legally permissible to filter out large amounts of environmentally hazardous mud and dump it on agricultural land and elsewhere – since the water itself is clean!

"But sometimes factories don't bother to clean the water at all and only do it when the inspectors come round", she says.

Provided by Lund University