Superconducting cavity special

Transcription

1 Volume 1 Quarter 4 December 2007 uarterly The ILC a proposed particle accelerator International Linear Collider News Feature: Pushing the performance yield Feature: XFEL: A source of light and experience Profile: The superconducting radiofrequency cavity In the news: ILC-HiGrade. Materials science. Cavity production. US cavity achievement. Superconducting cavity special The ILC will advance superconducting radiofrequency technology. Photo: Fermilab

2 Director s corner 2 SCRF A forward-looking technology Superconducting radiofrequency is the technology of choice for the ILC and many other new projects too. In this issue, we present R&D developments on superconducting radiofrequency, or SCRF, technology for the International Linear Collider. The history of the Global Design Effort began when the crucial decision to pursue cold SCRF technology for the ILC occurred in August I chaired the International Technology Recommendation Panel that submitted its recommendation to the International Linear Collider Steering Committee and the International Committee for Future Accelerators. GDE Director Barry Barish admires a superconducting cavity. Photo: Fermilab Shortly thereafter, these international committees created the GDE to produce a design for the ILC. The main vehicle for SCRF technology is the cavity, a hollow structure that drives particles to higher energies. For the ILC, we will use roughly 16,000 metre-long nine-cell 1.3 GHz (gigahertz) niobium cavities. We will need to develop enough industrial capability in each region of the world to supply these cavities for the ILC, wherever it is built. In addition to preparing for mass production, we established an aggressive performance goal of achieving an accelerating gradient of 35 megavolts per metre (MV/m) in each nine-cell cavity. The quest for high-gradient and affordable SCRF technology for high-energy physics has revolutionised accelerator applications. In addition to the recently completed Spallation Neutron Source (SNS) in Oak Ridge, Tennessee and the European X-Ray Free Electron Laser (XFEL) soon under construction in Hamburg, Germany, many new projects utilising SCRF technology are underway, including energy recovery linacs. For the majority of new accelerator-based projects, SCRF has become the technology of choice. Want to follow ILC progress more closely? Subscribe to ILC NewsLine, a weekly online newsletter. NewsLine Q: Quarterly News of the International Linear Collider 2007 uarterly All rights reserved uarterly PO Box 500 MS 206 Batavia, Illinois USA telephone fax communicators@linearcollider.org Managing Editors Elizabeth Clements Fermilab, USA Perrine Royole-Degieux CNRS/IN2P3, France Rika Takahashi KEK, Japan Barbara Warmbein DESY, Germany Publisher ILC Global Design Effort Print Design & Production Sandbox Studio, Chicago, Illinois

3 Feature story 3 Pushing the performance yield ILC scientists focus on a high accelerating gradient for superconducting cavities. The International Linear Collider will accelerate electrons and their opposites, positrons, to high energies of 500 billion electron-volts (GeV). To go from zero to 500 GeV in much, much less than 60 seconds, the ILC will use roughly 16,000 superconducting cavities. Each cavity must accelerate the particles to the highest possible energy over the shortest possible distance. The accelerating gradient is a measure of how much an accelerator can increase the energy of a particle over a certain stretch, typically given in volts per metre. The higher this gradient, the shorter, and hence cheaper, the ILC can be made. Basic physics defines an upper limit for superconducting cavities, and ILC scientists are trying to push these cavities as close to this limit as possible. Fifteen years ago the highest gradient achieved was about five million volts per metre. With intense R&D this has increased dramatically, and the target gradient for each ILC cavity is now 35 million volts per metre (MV/m). So far ILC scientists have achieved the target gradient goal in roughly a dozen cavities. We have proven that the technology works, says Cornell University s Hasan Padamsee. Now we need to improve the yield. In some cases, ILC cavities have actually exceeded the target goal and reached a gradient of 40 MV/m. Consistently reaching a gradient of 35 MV/m, however, in a large number of cavities remains a problem. Padamsee explains that different phenomena limit the ILC cavities, but scientists identified one of the main culprits as field emission. For a superconducting cavity, the tiniest speck of dust or dirt poses a threat to its performance. When charge flows through the cavity, electrons sometimes get more easily knocked out from the metal in the presence of the dust. These emitted electrons rob energy from the cavity. Hence the cavity cannot accelerate the main beam of particles as efficiently as it should. This is field emission. Acceleration an artist s impression. Image: DESY We have proven that the technology works. Now we need to improve the yield. A by-product of electropolishing, one of the surface treatments for cavities, appears to aggravate field emission. As a result of R&D studies, ILC scientists found that particles of sulphur are left behind on the cavity s surface after electropolishing. To solve the problem, scientists are improving the electropolishing parameters and implementing innovative rinsing techniques. So far they found that rinsing the cavity with simple soap and water helps. Soaking the cavity in ethanol to dissolve the sulphur particles is another big help. A short cycle of final electropolishing with fresh acid also shows promise. We have made a lot of progress because we now understand one of the leading causes of the cavity limitations, says Padamsee. We now have possible cures and those will lead to many improvements.

4 Feature story 4 XFEL: A source of light and experience Construction of the European X-Ray Free Electron Laser to start in 2008 Photon scientists working on the European X-Ray Free Electron Laser at DESY in Hamburg and around the world do not like it when scientists from the International Linear Collider community call the XFEL a 10-percent prototype of the ILC. Understandably so its science goals, which include impressive plans to probe the structure of matter and explore the minute details of chemical processes, are more than just a demonstration of technology. Nevertheless the light source project is a 3.4-kilometre complex that uses superconducting radiofrequency technology in its linear accelerator just like the planned 31-kilometre ILC. The cavities used in the accelerator of the XFEL, like those for the ILC, are based on the TESLA design. Originally, the European XFEL was planned as an integral part of an electron-positron collider project called TESLA, an international effort led by DESY and planned for the Hamburg region. Early in 2003, the German government approved the XFEL as an international project but postponed a decision on the linear collider part. It recommended that the international community interested in linear collider physics join forces and decide together on the accelerating technology for a common future project: the ILC. XFEL and FLASH The DESY facility FLASH is a smaller version of the future European X-ray laser XFEL. Both light sources generate X-ray radiation, which differs essentially in its wavelength. XFEL Construction start: 2008 Planned commissioning: 2013 Length: 3.4 kilometres Max electron energy: 20 billion electronvolts (GeV) X-Ray wavelength: 0.1 nanometres Technology: Superconducting Radiofrequency Cost: Roughly 1 billion Euros, funded up to 60 percent by Germany and 40 percent by the European partners Governance: As of 2008: XFEL Limited Liability Company The details of the two machines are not completely identical. For example, the XFEL has chosen a lower gradient. Its modules will hang from the tunnel ceiling rather than stand on the ground, and the XFEL accelerator will be accommodated in a single tunnel. But cavities, modules and the high frequency system will be more or less the same for the two machines. The experience of building the XFEL will serve as a test run for building the ILC: cooperation with industry for accelerator parts that need to be mass-produced is already starting. Just recently two companies assembled their first cryomodule (for FLASH see info box) as part of an industrial study for building the XFEL. The European partners will make in-kind contributions to the machine, a process foreseen for the ILC as well. First contracts are supposed to be placed in spring 2008 construction starts a few months afterwards. There is also close contact between machine physicists working around the world on accelerator technology for the ILC and the XFEL. XFEL and ILC can learn a lot from each other, said Reinhard Brinkmann, Director of the DESY accelerator division. Artist s impression of the future main XFEL building showing the underground experimental stations. Image: DESY

5 NewsLine profile 5 The superconducting radiofrequency cavity Curvy, shiny and full of power: cavities are the stars of the ILC Portrait of a cavity. Photo: DESY Nine smooth cells, polished in all possible ways. Made of the purest niobium. Not a speck of dust or the slightest difference in shape. Superconducting when supercold, and even the last curve filled to the rim with power: Superconducting cavities are the core, the heartbeat of the International Linear Collider. The 1-metre metal beauties are responsible for powering up the electrons and positrons to the energies at which they are supposed to collide, and companies and institutes around the world will build at least 16,000 of them each one exactly like the other. There is little tolerance in the quality, because any blemish could mean the loss of their superconductivity. When cooled to almost absolute zero, niobium becomes superconducting, which means that the cavity can sustain very high electromagnetic radiofrequency fields. Charged particles need an electromagnetic field to pull them along the higher the field, the better they accelerate. The rate at which they accelerate is called gradient (see Pushing the performance yield, page 3). The field is sent into the cavity by a voltage generator and tuned to the correct frequency. The energy is then transferred to the charged particle beam, composed of a train of bunches of electrons and positrons. To ensure that the gradient is as high as possible so that the particles get the high energy needed for the planned collisions, the cavities are welded, polished, high-pressurerinsed and tested to the best performance. Researchers are constantly looking for ways to improve the treatment to get even better cavities, but also to minimise time and cost for the time when large-scale production starts. Improvement possibilities start in the material. Instead of regular niobium, teams are testing cavities made out of large grains of the metal to eliminate grain boundaries that could degrade the superconductivity. Other people experiment with the way the cavity gets its shape, using water instead of welds. A treatment called electropolishing looks like a promising candidate for future standard procedures as it produces very smooth surfaces. So far, not one production method or treatment procedure has emerged as the best. Tests in all three regions of the ILC over the next few years will compare the performance, reliability and gradient of the different cavities. By 2010, the cavity experts will most likely be able to make a recommendation for the optimal methods to use for the collider. DESY accelerator physicist Lutz Lilje inspecting a TTF cavity. Photo: Christian Schmid, DESY

6 6 In the news Five million Euros to fund Europe s ILC-HiGrade proposal The European Commission has accepted to fund the ILC-HiGrade, or International Linear Collider and High Gradient Superconducting RF-Cavities, proposal within its Seventh Framework Programme (FP7) with five million Euros over the next four years. Under this contract, at least 24 superconducting cavities will be created to demonstrate the gradient feasibility for the ILC. Other objectives are the development of a possible organisation and governance for the ILC in Europe and measures to prepare for the actual construction of the machine, including a detailed siting study for Europe. Six institutions have come together for the project: DESY, CEA/Dapnia, CERN, CNRS/IN2P3, INFN and Oxford University. All of these have already been involved in research and development for the ILC for many years. The consortium also makes optimum use of the existing infrastructures in Europe, including the test infrastructures in place at DESY for the future European X-Ray Free Electron Laser (XFEL) and a high-tech laboratory at Saclay and Orsay. The ILC is one of the projects identified by the ESFRI (European Strategy Forum on Research Infrastructures) roadmap as an important project for the future of science in Europe. ILC challenges materials science Physicists and engineers already know most of the empirical recipes to build very good accelerating cavities: highly pure niobium is essential, welding needs to be carefully controlled and surfaces undergo advanced cleaning and annealing procedures. But, as for every complex system, a lot of phenomena remain unexplained. The theoretical limits of RF superconductivity are not well known, and engineers also meet practical limitations to reach high gradients. Accelerator experts work very closely with material scientists to understand cavity properties better. Everything happens in a thin slice of less than 100 nanometres, says Claire Antoine, materials physicist working at Dapnia in Saclay, France. In superconducting niobium, the electric current stays at the surface. That is why each cavity has to be meticulously polished and cleaned. So far, electropolishing followed by baking seems to be a very promising technique which reaches very high gradients but unfortunately not every time. A stabilised production will be essential for the numerous ILC cavities. Materials physicists can help understand what the optimal experimental conditions should be. Another example is the delicate problem of the mechanical properties of niobium. How do you achieve the two contradictory requirements of an easily formable niobium which can also sustain high vacuum conditions? Material scientists say that only a fully recrystallised niobium satisfies these conditions. R&D for superconducting cavities requires such fine analysis techniques as like surface synchrotron radiations, X-rays diffractions, optical and electronic microscopy, electrochemistry. There are so many applied fields of physics related to cavity studies that we definitely need to work with these experts to benefit from their knowledge, says Antoine. Cuts of niobium sheets: only well recrystallised niobium (top) can successfully be formed to a cavity shape, while the bottom type of sheets tends to tear upon forming. Photos: CEA

7 7 Cranking out cavities The International Linear Collider will require 16,000 superconducting radiofrequency cavities. Each one should reach an accelerating gradient of 35 megavolts per metre in the qualifying test to ensure an overall operational gradient of 31.5 MV/m. Cavity production is increasing around the world for ILC R&D, making it possible to one day achieve these goals. In the past year, industries and laboratories in the Americas, Asia and Europe produced more than 50 superconducting cavities that will contribute to ILC R&D. In Asia, KEK made six ILC cavities and eight alternative, ICHIRO-style cavities. In Europe, DESY received 30 Superconducting cavity production is increasing around the world. Photo: KEK cavities from ACCEL and Zanon, both European companies. (This is on top of more than 100 cavities that DESY received from European industry over the past 10 years for the TESLA Test Facility, several of which are installed in the FLASH accelerator.) In the United States, Jefferson Laboratory produced four cavities and Advanced Energy Systems, a company in New York, produced another four. By the end of 2007, the US will receive nine cavities from ACCEL and another six cavities from AES. Over the course of the next two years, the three regions plan to produce roughly 100 more cavities in an effort toward proving 35 MV/m performance and qualifying future vendors for the ILC. US-manufactured cavity achieves high gradient Damon Bice, a Fermilab technician studying at JLab for the year, disassembles the AES2 cavity from the test stand for another round of electropolishing and vertical tests. Photo: JLab On 21 November, a superconducting cavity manufactured by Advanced Energy Systems in Medford, NY, reached a high gradient of 32.6 megavolts per metre (MV/m) at Jefferson Laboratory. This is the first US-built ILC nine-cell cavity to reach a gradient close to the ILC specification, said JLab s Rongli Geng. JLab scientists are hopeful that the cavity, dubbed AES2, will reach an even higher gradient after further processing. The AES2 gradient is significant because it demonstrates the increasing capabilities of US cavity vendors. In comparison to manufacturers in Europe and Asia, US vendors are considered new to the game and reaching a gradient of 32.6 MV/m shows significant progress. This is a very encouraging result, said Fermilab s Shekhar Mishra. It shows that a US vendor can produce an ILC-quality cavity. Fermilab purchased the first four US manufactured ILC nine-cell cavities, and JLab started processing and testing them in April Using the facilities at their Superconducting Radio-Frequency Institute, JLab performed the current standard ILC electropolishing process along with the lab s developed final cleaning and assembly steps. At first the AES2 cavity only reached a gradient of 19.6 MV/m. JLab electropolished the cavity again. Geng explained that the gradient of AES2 increased as they removed surface material. After the fourth light electropolishing, AES2 finally reached its record gradient of 32.6 MV/m a vast improvement from the initial result. Scientists suspect that the niobium material used for this cavity may have had defects, which is why the removal of surface material would improve its performance. If this is true, further performance improvement is anticipated, said Geng. In addition to AES2, JLab is testing the AES4 cavity now. Initial tests indicate that it too will have a high gradient.

8 Snapshot gallery 8 1 A technician makes adjustments on a cryomodule. Photo: Fermilab 2 Cavity controls at KEK. Photo: Nobu Toge, KEK 3 Former ILCSC Chair Shin-Ichi Kurokawa welcomes Research Director Sakue Yamada and Enzo Iarocci, the new ILCSC chair. Photo: Fermilab 4 DESY and Fermilab collaborated to build the first US cryomodule. Photo: Fermilab 5 Cavity or cake? The German speciality, Baumkuchen. 6 If electrons had eyes... Photo: Nobu Toge, KEK 7 DESY s Rolf Heuer will become the new Director General at CERN. Photo: DESY 8 Cavity lookalike on top of a shrine in Japan. Photo: Barbara Warmbein 9 Chinese lamp in a traditional Beijing courtyard hotel. Photo: Neil Calder Architecture, decorations and even baked goods superconducting cavities show up everywhere!