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Big science and China hold future promise

This special Physics World supplement looks at four such “big science” projects that have extraordinary vacuum requirements. CERN’s Large Hadron Collider (p17) will be the world’s largest vacuum system, while the Laser Interferometer Space Antenna will require a vacuum system that can survive the traumas of space flight (p14). The vacuum pumps on the International Thermonuclear Experimental Reactor must remove large quantities of radioactive tritium (p7), while contamination with a few specks of dust could disrupt the superconducting cavities of the European X-ray Free-electron Laser (p11). While these projects all have their unique problems, they pose a common set of challenges to the vacuum scientist, as outlined by Joe Herbert on p5.

The past decade has seen extraordinary growth in the use of vacuum technologies in China. On p19, Jiangou Hou, president of the Chinese Vacuum Society, explains how the organization is planning for continued growth and gives his predictions for the future. China’s burgeoning hi-tech sector is also becoming a major market for vacuum equipment, and this is attracting an increasing number of foreign vacuum-equipment suppliers to China (p22).

Download a full digital version of the Vacuum Challenges and Solutions supplement here (PDF, 5MB).

Thoughts of an astronomy master

You graduated in maths from Cambridge University. What inspired you to go into astrophysics?

It was Fred Hoyle’s book Frontiers of Astronomy, which I bought while I was still an undergraduate. He presented this wonderful series of unsolved problems and speculated on a solution to each. It seemed a very lively field. So after graduating I did a one-year Masters course in maths and made sure I took all the astrophysics options.

Did the discovery of the cosmic microwave background in 1965 influence you too?

Absolutely. The year before, I had just started my PhD in cosmology with Bill McCrea, who was very interested in the steady-state theory – the idea that the universe is unchanging and that new matter is created continuously to compensate for the expansion of the universe. The discovery of the cosmic microwave background (CMB) was a bit of a blow to the steady-state idea because it seemed to be evidence of a hot Big Bang. But the results didn’t immediately rule out the steady-state theory. At first the measurements of the CMB were only at one or two wavelengths, so it wasn’t clear whether the results really showed true black-body radiation, which was integral to the Big Bang theory. So some of my early work investigated whether the steady-state theory really could be ruled out.

NASA’s WMAP satellite has allowed us study the CMB in great detail. What do you hope the Planck Surveyor will reveal about the CMB when it launches next year?

The Planck Surveyor will map the CMB to even higher resolution than the WMAP satellite, so its results should allow us to measure cosmological parameters like the density of the universe or the Hubble constant to an accuracy of 1%. At this level we should really feel confident that we know what kind of universe we live in. It is also possible that it could detect the imprint of primordial gravitational waves from the very early universe. We have no idea what the levels of these waves would be – they’d have to be pretty strong to be detected.

The bit I’m working on, however, is the all-sky survey of point sources at sub-millimetre wavelengths. This will help us to understand how galaxies formed in the first place, because we will be able to observe some of the most violent star-forming galaxies at very early times.

Some astronomers have claimed to see a pattern in the CMB that they call the “axis of evil”, suggesting the hot and cold patches in the CMB are not randomly distributed. Do you think data from the Planck Surveyor will rule out these claims?

It will certainly help clarify the situation. The “axis of evil” is an effect over the whole sky, so it might suggest large-scale anisotropies. This could mean that the universe is not infinite but cellular – a finite region that is endlessly repeated. But it is more likely that the pattern is a systematic error in our measurements.

While there may be primitive life on extrasolar planets, I do not think there is a profusion of extraterrestrial intelligence

What do you think is currently the biggest question in astrophysics?

Obviously there is a big issue with dark energy, but I’m not sure whether we’re going to succeed in solving that problem. I think it might just be something terribly basic that was laid down in the Big Bang that makes gravity pull locally and push on larger scales – basically Einstein’s theory of gravity with a cosmological constant.

In the next 10 to 20 years, however, the main thrust of astrophysics may shift away from cosmology towards planetary systems such as exoplanets.

Do you think that the search for exoplanets will reveal extraterrestrial life?

Personally I think that while there may be primitive life on extrasolar planets, I do not think there is a profusion of extraterrestrial intelligence. Given that there has been such an immense time period during which other civilizations could have existed, the fact that “they” don’t seem to be signalling to us and that they’re not visiting us suggests to me that for some reason they are not common.

There are two explanations for this. One is that it is very difficult to make that first step towards any kind of intelligent organism that would be capable of communicating with us. In the case of the Earth, we don’t know how we got from organic molecules to self-replicating plants and animals – it may have been the most amazing fluke that will never be repeated.

The other explanation is that as soon life gets intelligent enough it will soon destroy itself with nuclear weapons. But I don’t think that this idea works. Not all life would be as war-like as us – if the same experiment is repeated many times, it’s bound to last longer on some occasions than others.

You recently criticized NASA boss Michael Griffin for his dismissive comments on the need to tackle climate change. Do you think engaging climate change should be scientists’ number-one priority?

I do. Michael Griffin’s comments were unacceptable. He’s allowed to have his own reservations, but it was wrong of him to voice them to the media since he represents not only US space scientists but also those international partners like ourselves who collaborate with NASA. Space scientists have played an important part in identifying the problem and in convincing politicians that they need to act now.

Big science needs vacuum innovation

The next generation of big-science experiments, such as CERN’s Large Hadron Collider (LHC) and the International Thermonuclear Experimental Reactor (ITER), will rely on vacuum systems that push the limits of today’s technology in terms of volume and pumping capacity. Others, such as the European X-ray Free Electron Laser (XFEL), will require extremely clean vacuum conditions, while the Laser Interferometer Space Antenna (LISA) will involve launching vacuum systems into space.

The key challenge for a vacuum scientist working on such experiments is how to provide a suitable vacuum environment at reasonable cost. How this is achieved can differ from project to project. This supplement looks at the specific challenges associated with each of these four projects.

While these experiments are all very different, they pose a similar set of challenges to the vacuum scientist. These involve understanding the vacuum requirements of the experiment and how its mechanical design will affect the vacuum system; dealing with the various uncertainties in the operating parameters of vacuum equipment; manufacturing and processing the vacuum components; and finding enough suitably trained people to build and operate the system.

Many early challenges arise because the designers of the experiment – the physicists who will ultimately use the facility as a research tool – may not know what is required of its vacuum systems. Researchers tend to demand the best possible vacuum, even though it may not be necessary. Overspecification leads to unnecessary expenditure, which can be avoided if the vacuum scientist has a good initial understanding of how the experiment will be influenced by the vacuum conditions. Even if the vacuum requirements are well defined, it can be very difficult to calculate to any degree of accuracy the pressure that can be expected at the important positions in a large vacuum system. Many uncertainties must be taken into account and even a small design change can make a large difference to the vacuum conditions. As a result a design will often go through several iterations before a satisfactory vacuum is achieved.

The vacuum scientist must also understand the demands and limitations that will be imposed on the vacuum system by the physical design of the experiment. For example, exotic materials may have been chosen for their mechanical, thermal or electrical properties, even though the vacuum characteristics of the materials are not well known. Measuring the thermal outgassing properties of such materials and finding processing and cleaning recipes to reduce outgassing to a satisfactory level can take a great deal of time. Outgassing is a significant challenge for those building the ITER, for example. The vacuum system for this project must handle gas loads from many thousands of components that are manufactured from specialized materials that have to cope with the harsh envir­onment inside a fusion device.

Particle accelerators, such as the LHC and the XFEL, are usually housed in tunnels where there is little space for vacuum pumps. In addition, the huge lengths of many modern accelerators (nearly 27 km in the case of the LHC) can put severe restrictions on pumping speeds. In the LHC this problem is alleviated by non-evaporable getter (NEG) coatings, which turn the walls of the vacuum chambers into pumps.

Another challenge is the vacuum equipment itself, the performance of which is often not very well defined. For example, the pumping speeds of UHV pumps may only be known for a few gas species. To complicate matters further, similar pumps may achieve different speeds, depending on how an individual pump has been used in the past. Vacuum gauges can also have a major effect on the vacuum – especially at very low pressures – and their stability often leaves something to be desired. It may be that the special requirements of a large facility cannot be met by commercial equipment and new systems must be developed. This usually involves extending existing technologies rather than having to develop something from scratch.

Once the design has been specified, the components must be manufactured and prepared, often using materials and methods at the boundaries, or even beyond the capabilities, of the vacuum industry. Quality assurance is a major challenge, and the project often has to provide the necessary protocols, personnel and equipment. Needless to say, big projects require a lot of detailed design, manufacturing and processing capacity, plus careful record keeping. The final challenge is that of finding enough suitably trained and experienced people who really understand vacuum. It’s not a black art, but it’s not an exact science either.

Download a full digital version of the Vacuum Challenges and Solutions supplement here (PDF, 5MB).

Suppliers can reap big benefits from China’s hi-tech boom

The boom in China’s hi-tech manufactur­ing has made the country a major market for vacuum technology. While challenges abound for foreign suppliers, the rewards can be great for those that can master the Chinese way of doing business.

China has the fourth largest economy in the world (in terms of gross domestic product) and has enjoyed an annual growth of about 9% throughout the past decade – more than double the global average. The country is home to a booming hi-tech sector, including a massive semiconductor industry, which is a big end user of vacuum technology.

The main opportunities for foreign suppliers are in the high-end vacuum market, which serves China’s semiconductor, metallurgical, solar-cell, medical, optical and other hi-tech industries. This high-end equipment is still largely made by US, European and Japanese firms, according to Kuno Herrmann, sales and marketing manager for Pfeiffer Vacuum. He estimates that this market is worth some €80 m per annum.

Although foreign companies have been selling in China for decades, the vacuum market took off about five or six years ago, says Frédéric della Faille, vice-president of sales at Alcatel Vacuum Technology. As a result, most leading vacuum suppliers have responded by opening sales offices and service centres there.

“Having a ‘presence’ is very, very important,” said Ting Zhang, founder and CEO of China Business Solutions Ltd, a UK-based business consultancy. A Chinese presence allows a firm to build trust by being closer to potential customers, and a local office can deliver more and better training and support to customers. “Before, there was very little need for this kind of upkeep,” said della Faille, because many vacuum users in China simply threw out their pumps after a few hundred hours and just bought new equipment.

Indeed, convincing buyers to invest in higher-quality vacuum equipment that will last has been a key challenge. Many Chinese customers look for the lowest bid, even though the higher initial costs of a well designed system are usually offset in the long run by a longer lifetime and significantly more efficient operation. “We have made this argument successfully in other parts of the world,” said Mark Fitch of Vergason Technology, but “it often falls on deaf ears in China”.

Understanding and adhering to industrial regulations in China can also be a major challenge for foreign vacuum suppliers. Local interpretations of the law can vary significantly throughout the country, so a firm must develop good working relationships with people in administrative positions. “The most popular buzz word for foreign business people in China is ‘guanxi’, which means ‘connections’,” says Zhang. Oerlikon Leybold Vacuum got guanxi by retaining a business consulting firm that had been working in China for some time. “You need to know someone there who understands the regulations and knows how to haggle,” explained Christina Steigler of Oerlikon Leybold.

Another way of dealing with the intricacies of the Chinese system is to hire locally. Although there is a large pool of skilled workers in China, Steigler’s experience is that few of them are able to speak English. Also, once a qualified Chinese employee has been identified, retaining their services can be difficult because employee turnover rates are often as high as 30%, according to Zhang.

After establishing a presence in China, the next step for a company is to start a manufacturing operation there. While this could be done through joint ventures with established Chinese businesses, many firms have legitimate concerns that their intellectual property will not be respected.

To minimize the risk of patent infringement, Oerlikon Leybold has been operating its own manufacturing site in China since 1997. According to Steigler, the facility has lower production costs than plants in the West and can avoid certain import fees. In addition, she believes that the “made in China” mark is a selling point with local companies, although it is less important for multinational customers.

Making it in China is not easy, and some vacuum companies have already failed. The lack of a suitable infrastructure for supporting foreign manufacturers is still a problem, according to Herrman. However, the overall situation for manufacturers is improving in China, says Zhang, with many regulations being updated, a growing awareness about intellectual property rights and a greater number of young people learning English.

Despite the problems, Herrmann believes that the high-end vacuum will enjoy an annual growth rate of at least 10% in the foreseeable future. He is confident that more foreign companies will start production in China. “It’s only a question of when,” he said.

Download a full digital version of the Vacuum Challenges and Solutions supplement here (PDF, 5MB).

Perfect lens could reverse Casimir force

The mysterious attraction between two neutral, conducting surfaces in a vacuum was first described in 1948 by Henrik Casimir and cannot be explained by classical physics. Instead it is a purely quantum effect involving the zero-point oscillations of the electromagnetic field surrounding the surfaces. These fluctuations exert a “radiation pressure” on the surfaces and the overall force is weaker in the gap between the surfaces than elsewhere, drawing the surfaces together. Tiny though it is, the Casimir effect becomes significant at distances of micrometres or less and actually causes parts in nano- and micro-electromechanical systems (NEMS and MEMS) to stick together.

Now, Leonhardt and Philbin have calculated that the Casimir force between two conducting plates can turn from being attractive to repulsive if a “perfect” lens is sandwiched between them. A perfect lens can focus an image with a resolution that is not restricted by the wavelength of light. Such a lens could be made from a metamaterial made of artificial structures that are engineered to have negative index of refraction — which means that the metamaterial bends light in the opposite direction to an ordinary material.

According to the researchers, the negative-index metamaterial is able to modify the zero-point oscillations in the gap between the surfaces, reversing the direction of the Casimir force. Indeed, the researchers believe that this repulsive force is strong enough to levitate an aluminium mirror that is 500nm thick, causing it to hover above a perfect lens placed over a conducting plate.

Since the Casimir force acts on the length scale of nanomachines, manipulating it could be important for future applications of nanotechnology. “In the nano-world, the Casimir force is the ultimate cause of friction,” Leonhardt told physicsworld.com. “Our result means we could now envision frictionless machines or novel micromotors.”

While physicists have had some success creating perfect lenses from negative-index metamaterials, the technology is still in its infancy. “The work points towards new applications of left-handed materials that are not strictly optical,” says Federico Capasso of Harvard University, who studies the effect of the Casimir force on MEMS. “However, the materials are not easy to make so the concept may take a few years to realise.”

Microscope unravels the intricacies of protein folding

Proteins – the building blocks of life – consist of a long chain of molecules called amino acids folded into a 3D shape. An atomic force microscope (AFM) can be used to study this folding by attaching one end of a protein to a substrate and the other end to the AFM’s cantilever. As the protein is stretched, the cantilever is oscillated and the force restoring the protein and cantilever back into equilibrium is measured.

In theory, monitoring this non-equilibrium force should provide information about the many intermediate equilibrium energy states that the protein goes through on its way to being fully extended. In practice, however, interpreting the data has proved controversial, and until now researchers have only had a clear understanding of the equilibrium states at the beginning and end of the folding process.

Now Ching-Hwa Kiang and colleagues from Rice University have improved the AFM technique to determine the intermediate states. To do this, they built a computer program based on an equation formulated by University of Maryland physicist Chris Jarzynski a decade ago.

Although scientists had believed “Jarzynski’s equality” could be used to obtain equilibrium information from non-equilibrium measurements, none had been able to apply it successfully. “Through numerous discussions with Jarzynski, we had a thorough understanding of where and how the theory applies,” Kiang explained.

The Rice group proved their technique works using section of “titin”, the largest known protein and the one that constitutes elastic muscle in the heart, and managed to map eight individual energy states as they used an AFM to unfold it. This, they say, paves the way for investigating how environmental changes such as temperature affect protein folding.

Atoms swap spins

Optical lattices use criss-crossing laser beams to create a matrix of potential wells that can each trap one or more atoms. They are one of several experimental systems that could be used to create practical quantum computers, which exploit the ability of quantum systems to exist in two states at the same time. Rather than use bits, which are either 1 or 0, quantum computers use qubits, which can be in a superposition of both 1 and 0 simultaneously. The idea is that if a quantum computer has N such qubits, these can then be combined or “entangled” to represent 2N values at the same time. By processing each of these values simultaneously, a quantum computer could, in principle, operate exponentially faster than its classical counterpart.

A SWAP gate exchanges the state of two qubits — the spin state of two atoms in an optical lattice, for example. If one atom starts in spin state 1 and the other in 0 (1-0), they end up in 0 and 1 respectively (0-1). What is more interesting to those trying to build quantum computers is the half-SWAP gate, whereby the process is stopped halfway when the state of each individual atom is simultaneously 1 and 0 — and the atoms are entangled. Then, the two atoms could, in principle be physically separated while still entangled.

Now Trey Porto and colleagues at NIST and the University of Maryland in the US, have created at SWAP gate in an optical lattice. They began with two overlapping lattices that were offset slightly in space from one another. In both lattices, each well was occupied by one atom and a radio signal was used to set the spins of all the atoms in one lattice to 1 and all the atoms in the other lattice to 0.

The researchers then carefully adjusted the laser beams to merge the two lattices into one lattice in which two atoms occupy one well. When the two atoms are in the same well, quantum mechanics dictates that the overall quantum state of the two atoms must have a specific symmetry and this restriction causes the system to oscillate between two spin states: 0-1 and 1-0.

By switching off the trapping lasers and applying a magnetic field gradient to the ensemble, the researchers were able to measure the spin state of the atoms at different points in time and confirm that they were oscillating between the two spin states with a period of about 0.4 ms

The team also performed the experiment with both atoms in the same initial states (0-0 and 1-1) and saw no oscillations. Porto told physicsworld.com that this was particularly challenging to achieve because these states are more likely to be destroyed by noise than 0-1 and 1-0.

This is not the first time that a SWAP gate has been demonstrated – in 2005 physicists at Harvard University swapped spin states between electrons confined to two quantum dots. However, the Harvard experiment could only swap 0-1 and 1-0 states, and not 0-0 and 1-1. Although the latter two swaps seem trivial, any practical gate must be able to handle these states.

Porto accepts that the NIST team have also fallen short of performing a complete half swap, because they did not separate the atoms in the entangled state, something that the team are now working on.

Another key challenge facing Porto and others who are trying to build quantum computers based on optical lattices is how to read and write information from individual atoms and manipulate individual wells. This was not done in this experiment — instead the measurements were made on the entire ensemble of atoms and all the wells were controlled in unison.

Graphene oxide weaved into ‘paper’

First isolated in 2004, graphene is a one-atom-thick sheet of graphite that, aside from having unique electronic properties, is very strong. But as of yet there is no way of producing it in large quantities, which has limited its potential as a building block for new types of specialist materials.

Now, however, a group from Northwestern University in the US including Rodney Ruoff have discovered that large quantities of oxidized graphene can be weaved together to create a new type of “paper” that is stiffer and stronger than other thin materials.

“My dream has been to disassemble graphite into individual sheets, and then reassemble those sheets in different ways,” Ruoff told physicsworld.com. To do this his group begins by oxidizing graphite to make graphite oxide, which leaves roughly half the carbon atoms with an attached oxygen atom. When graphite oxide is mixed into water, these oxygen atoms repel water molecules, forcing the individual layers – graphene oxide – to disperse or “exfoliate”. The researchers filter this exfoliated mixture through a membrane, which collects the layers in such an arrangement to produce graphene oxide paper.

Normal graphite has a delicate structure, needing only a small lateral force to break apart its regularly-stacked layers. Conversely, the layers in graphene oxide paper interweave with one another and wrinkle on larger scales. This allows load to be distributed across the structure, making it stronger than graphite foil and “bucky paper”, which is made from carbon nanotubes. In fact, Ruoff claims, the only material stronger could be diamond.

Graphene oxide paper
Many layers

The interwoven structure also lets individual layers shift over each other, so that the collective layers become pliable. But most importantly the paper can be chemically tuned by altering the amount of oxygen on the layers. Reducing the oxygen content, for example, would take it from being an electrical insulator to a good conductor. Moreover, the paper could be infused with polymers, ceramics or metals, to make composite materials that outperform their pure counterparts.

This wide array of properties could mean applications as diverse as membranes with controlled permeability to supercapacitors for energy storage.

Artificial swimmers have no moving parts

Making micrometre-sized objects swim is no easy task because over very short distances, water behaves like a very viscous fluid such as honey. Some bacteria manage to swim by using highly specialized undulating whips called flagella – and while some progress has been made in creating artificial flagella, they have proved very difficult to mimic in a tiny machine.

In 2005 Ramin Golestanian, a theoretical physicist at the University of Sheffield in the UK and colleagues proposed a much simpler way of propelling tiny objects that uses no moving parts. Now a team led by fellow Sheffield physicist Richard Jones has created such a propulsion system for making particles swim in a solution of water and hydrogen peroxide.

The team used polystyrene balls that were about 1.6µm in diameter and had one side coated with platinum — a catalyst that boosts the rate at which hydrogen peroxide is converted into oxygen and water. This reaction decreases the concentration of hydrogen peroxide in the region near the platinum-coated side of the sphere, causing water to flow away from the region in order to maintain equilibrium. This flowing water pushes the object in a specific direction relative to the coating– if the platinum is on the right hand side of the sphere, for example, the sphere would move to the left.

By looking at the system with an optical microscope, the researchers saw that the balls could reach speeds of 5µm/s — which is not far off the 10µm/s observed in similarly-sized bacteria. According to Golestanian, the propulsion technique could be adapted to work in other liquids including blood, which could someday allow micromachines to swim within the body to deliver drugs to specific locations.

However like bacteria, the swimmers also have to contend with another consequence of being very small –- being knocked off course by random collisions with water molecules in a process called Brownian motion. Indeed, after a few seconds of motion in a specific direction, the Sheffield swimmers followed completely random paths.

Golestanian told physicsworld.com that thermodynamics makes it impossible to design a tiny object that would be able to avoid Brownian motion on its own and travel in a straight line. Instead, he believes that the objects could be guided externally – for example, if a magnetic dipole could be placed in the object, it could be steered using a magnetic field.

Tiny magnets help drugs reach the spot

Many pulmonary diseases, such as asthma, cystic fibrosis and lung cancer, need drugs to be inhaled so that they can reach the affected area. To do this, patients have to gasp on an inhaler that emits the particulate drugs into the windpipe.

But the effectiveness of these inhalers is not great: typically only 4% of the drug makes it through the windpipe, forcing doctors to administer higher doses, which can exacerbate unwanted side effects.

A better way, according to Carsten Rudolph at Ludwig-Maximilians University in Munich and co-workers from elsewhere in Germany, is to mix the drugs with iron-oxide magnetic nanoparticles and microdroplets of water, or so-called “nanomagnetosols”. These nanomagnetosols can then be guided directly to problem areas using a magnetic field. The idea is not new, but Rudolph’s group show for the first time that it can be performed in a real organism – in this case, a mouse.

The researchers began by creating a computer simulation of a mouse’s airways where the windpipe forks into two bronchi, taking into account air flow rates measured in previous physiological studies. Assuming they were to use nanomagnetosol droplets with an average diameter of 3.5 µm, they predicted that they could use a magnetic probe placed close to a bronchus to get up to 16% coverage of the microdroplets.

Rudolph’s group tested their prediction by opening up the chest of a mouse, and placed a specially designed magnetic tip probe with a high flux gradient of 100 Tm-1 next to one of the lungs. When they squirted the nanomagnetosol droplets into the mouse’s airways, they found that the lung next to the probe received eight times more drug coverage than the one without. Upon placing the probe on another mouse with its chest intact, the benefit was reduced, with just two and a half times more coverage.

Performing the same feat in humans will not be so straightforward, however. Human lungs are much larger and more intricate, so it will be difficult to guide the nanomagnetosol droplets with the same accuracy. Moreover, a much more powerful magnetic probe will be required to overcome the additional distance between the probe and inner lung.

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