Particle accelerators are the superstars of science. Researchers have been using them to decipher the structure of matter, examine the fundamental forces of nature, and recreate the birth of the universe. Not all particle accelerators are used for fundamental research, however. In fact, most of the world’s more than 30,000 accelerators are operated by medical technicians and industrial engineers. The applications involved are varied and include radiation treatment for cancer patients, the creation of radio isotopes for therapeutic purposes and imaging procedures, the sterilization of food and medical equipment, and the alteration of plastics’ attributes. In other words, these seemingly exotic devices actually play a major role in everyday life – which makes it all the more important to reduce their production and operating costs. Researchers at Siemens Corporate Technology (CT) are attempting to achieve such cost reductions in cooperation with scientists from four institutions. Two of them are situated in Russia: the Budker Institute of Nuclear Physics in Novosibirsk and the Institute of Theoretical and Experimental Physics in Moscow. The others are German institutions: the GSI Helmholtz Center for Heavy Ion Research in Darmstadt, and Goethe Universität Frankfurt am Main. Within the framework of a strategic partnership between Siemens and the new Skolkovo Institute of Science and Technology, which is located near Moscow, project participants are developing a powerful high-frequency generator. Based on new silicon carbide transistors, the generator will make it possible to produce extremely compact, robust, and energy-efficient particle accelerators. Cooperation with Russian scientists in this field makes sense because Russia has gained experience with many particle accelerators over the last few decades. In addition, ideas from the country’s scientists have repeatedly contributed to the further development of the technology. The current joint development work focuses on so-called accelerator “drive systems.” Particle accelerators generate electric fields that exert powerful forces on charged particles such as electrons, protons, and ions, causing them to accelerate to high speeds. The simplest acceleration technique involves producing direct current voltage between two electrodes; the resulting electric field then propels the particles. “This type of electrostatic accelerator soon reaches its limits because any voltage in excess of ten million volts causes electric flashovers,” says Prof. Oliver Heid, a medical systems specialist and Siemens Top Innovator. Heid also initiated the accelerator project and serves as a scientific advisor on the Skolkovo Board. “To get around this problem, researchers have developed alternatives that use alternating voltage.” The basic idea here is that rather than having particles pass through very high voltages once, they should instead be sent through a series of weaker electric fields and gradually collect more and more energy. However, the problem is that when a particle is exposed to AC voltage, it normally doesn’t accelerate but instead flies back and forth. Scientists therefore designed their unit in such a way that the particles always only “see” a half-wave as they pass through the individual accelerator segments. This half-wave gives them a strong push in the same direction each time. It’s almost as if they were surfing on an acceleration wave. Achieving this feat requires extremely powerful AC voltages of the highest possible frequency because such frequencies ensure the accelerators work especially efficiently. This in turn presents a huge challenge for the accelerator’s electronic systems. “The vacuum tubes previously used here – for example, triodes and klystrons – have a maximum efficiency of only 60 percent,” says Heid, who is also a Visiting Professor in the Physics Department at Oxford University in England. “In addition, supplying the high voltages they need is a complex process. That’s why we’ve been working on an alternative in the form of a high-frequency amplifier with semiconductor elements since 2008. We’ve managed an efficiency of more than 70 percent with this setup. That achievement plus a less complex power supply system has halved the energy costs for a particle accelerator.” The semiconductor amplifier also costs around half as much as its conventional counterpart. Silicon Carbide Pioneer. At the heart of the new solid state direct drive technology are transistors made of silicon carbide (SiC). Electrons can move inside this semiconductor material much more freely than in conventional silicon. As a result, SiC transistors can operate at frequencies up to ten times higher – in the several hundred megahertz range (MHz). The term “microwave” is used for frequencies of 300 MHz and above. “The SiC transistors are also better conductors of heat, which means they can withstand higher output levels,” says Heid. “Siemens was one of the pioneers of silicon carbide transistor technology, which is now on the verge of triggering a revolution in electronics.” The agile components are also tiny. For example, a SiC transistor with an output of five kilowatts has a surface area of only six square millimeters. A vacuum tube with a similar output would have a volume of around ten liters. In order to achieve even greater output, researchers combined eight transistors on one board and were then able to generate 25 kilowatts at 324 MHz. As impressive as this may seem, it’s still not enough for a particle accelerator, which requires output in the megawatt range. The researchers have therefore now mounted several of their circuit boards onto a cylinder-shaped component; the entire unit looks like a green-copper colored turbine rotor as a result. Each mounted module contributes to the power collected via a copper ring, with the resulting output totaling 160 kilowatts. The next step is to combine 128 circuit boards with an output of 25 kilowatts each, which would result in around three megawatts of power at 324 MHz – and put SiC technology firmly within the output range of scientific particle accelerators. Unlike these units, which need to generate several megawatts of power, the accelerators used in medical and technical applications only require ten to 100 kilowatts. Along with the relatively low level of complexity involved, this linking of many small amplifiers also offers another major benefit: if one of the amplifiers fails, the accelerator will still continue to operate. By contrast, a conventional unit will shut down completely if one vacuum tube fails. “The problems associated with the tubes and their power supply are among the most frequent causes of particle accelerator shutdowns,” says Heid. “Our solution, on the other hand, makes it possible to replace a defective part even as the accelerator continues to operate – and users don’t notice a thing.” Scalable Output. The new technology is helping Siemens researchers and their Russian and German partners to introduce standards that will lower the cost of drive systems for particle accelerators. “We want to separate the generation of high frequencies from the actual design of the accelerators,” Heid says. “That’s why we’re developing a whole range of standardized electronic control cabinets to house our new amplifiers in Skolkovo. These cabinets can be combined in any desired way. In other words, there are no limits to the outputs we can achieve.” The high-frequency AC voltage can then be transmitted via thick cables with a diameter of 30 centimeters to all types of accelerators – in much the same way as a stereo amplifier can send its signal to speakers built by any manufacturer. The first control cabinet prototype is scheduled to be completed before the year is out. This development could be of interest in the future to scientific facilities such as the European Organization for Nuclear Research (CERN). It could also attract the attention of companies interested in separating oil from oil sands using microwaves, or firms that produce particle accelerators for medical applications. “Our technology also opens up completely new possibilities for everyday applications,” Heid says. “For example, a small version of our high-frequency generator could be installed in home microwave ovens. Here it would produce the required output of several hundred watts at 2.4 GHz more effectively than today’s magnetrons. What’s more, it would also take up a lot less space.” Christian Buck
Posted on: Sun, 16 Jun 2013 23:08:21 +0000
Trending Topics
Recently Viewed Topics
© 2015