The pulse of progress

Large, sharp pulses of electricity have multiple applications – and that list is expanding, as Tom Shelley reports Massive, but very brief, amounts of electrical energy can be delivered involving thousands of joules, produced by millions of volts unleashed during tens of nanoseconds. It is heady stuff and is attracting huge interest. Previously only of significance to the defence and scientific research communities, it is an area of technology now being recognised as vitally important to delivering controlled amounts of fundamental particles and ions for a range of applications, such as new cancer treatments. But it has a range of other applications: probing the fundamentals of material structures used in engineering and medicine; allowing pulsed laser systems in materials processing to be made cheaper and larger; sterilising food; replacing explosives in a number of applications; offering novel ways of forming materials; and even opening the door to fusion power. The basic technique is still to store up energy in capacitors and let it go as quickly as possible. However, if the energy pulse is to be large and short, there are all kinds of complications, in terms of minimising inductances in cables and transmission lines, and having switching devices that close sufficiently quickly. With this in mind, Stephen Griffiths from the Daresbury Laboratory has been offering insights into the new Emma (Electron Model with Many Applications), a small 16.6m circumference accelerator. Speaking at a symposium on ‘Pulsed Power’ at the Rutherford Appleton Laboratory, organised by the Institution of Engineering and Technology, Griffiths explained that Emma is to be the precursor to Pamela (Particle Accelerator for Medical Applications), which will probably involve a machine with three concentric rings. The first stage will take low energy particles from a cyclotron, or radio frequency quadrupole, and accelerate them up to an intermediate energy. A second ring would accelerate protons (hydrogen nuclei) up to the 250MeV needed for a complete proton therapy system. The final ring would bring carbon nuclei up to the 400MeV necessary for clinical use. Particle therapy, as opposed to X-rays, is a cancer treatment that works by damaging the DNA of cancerous cells, while minimising the impact on surrounding tissue. X-ray photons lose a significant amount of energy before they reach a tumour. However, if a beam of charged particles enters the human body, it deposits most of its energy at a depth that, according to Griffiths, depends precisely on the energy of the particles – enabling the delivery of higher doses to the tumour, without causing excessive collateral damage. At present, hadron therapy is only available in the UK at the Clatterbridge Centre for Oncology, but the goal is to make it more widely available. What Emma, Pamela and other accelerators depend on heavily – as do ring accelerators in general – is pulsed power to deliver particles into their rings and to power magnetic ‘kickers’ to send them out again. These require large, and very short, current pulses that are today almost entirely produced by thyratrons – partially gas-filled tube devices capable of switching up to tens of thousands of amps, with rise times measured in nanoseconds. E2V has been making these for many years and, as a sponsor of the event, had a number on show. It was noticeable that none of these was encased in traditional glass tubes. All were made up of alumina and plated copper rings. Significantly, their design has to accommodate differential thermal expansion coefficients, says E2V’s electronic systems manager Ray Rush. “It’s a problem which we solve by clever engineering design,” he says. “Our seals have to maintain a hard vacuum for 10 to 15 years.” The company has its own proprietary technology to produce sufficiently good seals, maintaining a symbiotic relationship with its ceramic supplier, in which E2V also metallises its ceramics for other customers. High current, fast-switching solid state devices are under development and at some point, states Mark Sanders, team leader at AWE (the Atomic Weapons Establishment), “are going to move into the area where we can make use of them”. Adriaan Wellemen, from ABB Semiconductors in Switzerland, described a 6.5kV, 1500A, 10 microsecond, 6kA per microsecond, 1300Hz solid state switch that uses three Insulated Gate Controlled Thyristors (IGCTs) in series. IGCTs are based on GTOs – Gate Turn Off devices – widely used in electric traction controllers, although they are now being replaced by IGBTs, (Insulated Gate Bipolar Transistors). High current solid state switches last longer than thyratrons, although they do not last forever, despite what some might think. Wellemen believes that, in the operating envelope that ABB’s devices cover, “system cost is in favour of the solid state version after about three to four years of operational use”. One of the large-scale uses of pulse power is in medical X-ray systems, especially for cancer treatment, but none is on the scale of those used by AWE to study the exploding components of nuclear weapons, especially now that the Comprehensive Test Ban Treaty prevents the testing of complete nuclear devices. Jets of metal have to be observed travelling at 1km/s and, during the last 40 years, the laboratory has developed a suite of X-ray machines that can produce hundreds of Roentgens at 1m in pulses that last only 80ns. In the case of their Mogul machines, a 40kA electron beam is accelerated to 10MeV, which charges two 8.5m long Blumlein Pulse Forming Line capacitors. The 10MV 80ns pulse then reaches the X-ray tube, which is made up of a number of acrylic rings, separated by aluminium disks. Through the centre of the tube runs a steel stalk, connected to a diode. The electric field distributed along the tube causes electrons to flow down the stalk towards the diode. Within the diode itself, the electrons are directed on to a tantalum target, which emits the X-rays. The existing facilities are to be supplemented by a new facility called ‘Hydrus’, which will eventually allow five X-ray views to be obtained at once. This is something that might also prove useful in civilian applications, such as studying explosive forming, cladding and welding, and hydrodynamic forming. Pulsed power food sterilisation and rock fracturing, for example, are two civilian applications that have been extensively researched, but not yet implemented commercially – not because they do not work, but because of the present costs and inefficiencies in delivering energy in this way. Drilling and demolition achieved by passing sudden large currents between a pair of electrodes has been found to work well, even on granite, in research conducted at Kumamoto University in Japan. Also, a pulsed power system fuel sterilisation system installed in Ohio State University’s department of food technology has shown itself able to sterilise fruit juices by injecting 60kV 600A pulses at 75kW average power into four chambers. Invented by Diversified Technologies of Bedford, Massachusetts – which calls this pulsed electric field processing – the process is said to preserve the fresh juice taste usually destroyed by heat sterilising. In addition, pulses of magnetic force have been extensively researched, particularly in Russia, as a way of fast forming metal objects. And massive pulses of energy underlie a number of the technologies being researched to fuse heavy hydrogen atoms together to yield large amounts of what Tom Todd, chief engineer of UKAEA Culham, describes as “fairly clean energy”. One company enjoying commercial success is Phoenix Science and Technology, based in Chelmsford, Massachusetts. As well as making big sparkers, it has also invented and patented its ‘surface discharge’ ultra violet lamps. These produce, it says, a pulsed electric discharge that has 80% higher efficiency and 30 times greater light intensity than conventional pulsed lamps. Powerful UV lamps are used in water sterilisation, integrated circuit lithography and paint stripping, among other applications. The sparker has also shown itself to be effective in persuading zebra mussels to part company with ships’ hulls. And pulsed lasers, particularly excimer lasers, have come to be taken for granted in leading-edge manufacturing industries. Pointers * Pulsed power is already used for cancer treatment, flash X-rays, seismic survey work, water sterilisation, paint stripping and numerous pulsed laser applications * Applications including rock fracturing and small particle accelerators for improved cancer treatment are under development * Very high voltage and high power applications still require the use of gas filled tubes – thyratrons – but solid state devices are being developed to handle increasing powers and voltages. Costs can be expected to keep falling and efficiencies enhanced