Supraconductivité appliquée au KEK

Deux pionniers de la physique – Hiromi Hirabayashi et Yuzo Kojima – ont joué un rôle majeur dans le démarrage de la recherche et développement sur les aimants supraconducteurs et la supraconductivité RF, respectivement, au Laboraboire japonais de recherche sur les accélérateurs de haute énergie (KEK). L’application de la supraconductivité aussi bien dans les accélérateurs de particules que dans les détecteurs de particules a depuis progressé au même rythme que le programme de recherche du KEK. Akira Yamamoto évoque les principales réalisations, y compris le développement de supraconducteurs stabilisés à l’aluminium pour les aimants des détecteurs de particules et les cavités RF « en crabe » qui ont permis au collisionneur KEKB d’atteindre un record mondial de luminosité.

The Japanese High-Energy Accelerator Research Organization, or KEK, was established (originally as the National Laboratory for High Energy Physics) in Tsukuba in 1971, around the same time that superconductivity – discovered 60 years earlier – was just beginning to find large-scale applications in physics. The laboratory became involved in superconducting technology almost from the start and KEK has continued to push frontiers in the field as its research programme has evolved. Two pioneering scientists, the late Hiromi Hirabayashi and Yuzo Kojima, deserve particular mention for their leading roles in starting research and development at KEK in the mid-1970s – on superconducting magnets and RF superconductivity for accelerator science, respectively.

Superconducting-magnet technology was first put to practical use at KEK in a secondary-particle beamline at the 12 GeV proton synchrotron. Two cosθ dipole magnets and one superconducting septum magnet formed major components in the beamline, while a large-aperture "window-frame" superconducting spectrometer-magnet was built for one of the physics experiments. Hirabayashi not only took the lead in this milestone project, he also used it to train the next generation of magnet scientists and engineers. They would take forward the various superconducting-magnet projects that were subsequently carried out at KEK and in collaborative international programmes, including R&D on the Superconducting Super Collider in the US and LHC project at CERN.

Frontier projects with superconducting magnets

The frontier project for the 1980s was an electron–positron collider, TRISTAN, which had a maximum beam energy of 30 GeV and operated between 1987 and 1995. KEK successfully developed large-aperture insertion-quadrupole magnets for the four interaction regions, to bring high-brightness beams into collision in the physics experiments.

Following on from TRISTAN, KEK constructed the accelerator for the B-factory, KEKB – an energy-asymmetric electron–positron collider with two rings handling 3.5 GeV positrons and 8 GeV electrons – built in the TRISTAN tunnel. Superconducting interaction-region quadrupole (IRQ) magnets were again developed. Based on a sophisticated coil design, with corrector-coils in additional coil layers, they were very closely integrated with the collider detector, BELLE (figure 1). The IRQs contributed to the highest beam luminosity ever achieved, as described later, enabling the KEKB accelerator and the BELLE experiment to help in establishing the Kobayashi-Maskawa theory for which the Nobel prize was awarded in 2008 (CERN Courier November 2008 p6). A further sophisticated multiple-magnet system is now being developed for the interaction region at Super-KEKB, the upgraded B-factory, which was approved in 2010.

The experience acquired in these projects was to allow KEK to make important contributions to the LHC, in particular in a fundamental study of high-field dipoles to reach 10 T and in the construction of insertion quadrupoles with a design field gradient of 215 T/m at a coil aperture of 70 mm (figure 2). The quadrupole magnets were developed and supplied in collaboration with Fermilab.

More recently, KEK developed a primary proton-transport line at the Japan Proton Accelerator Research Complex (J-PARC) in Tokai, in a collaboration between KEK and the Japan Atomic Energy Agency (JAEA). To create and direct a neutrino beam towards the Kamioka neutrino observatory nearly 300 km away, an internally extracted proton beam from J-PARC has to bend through around 90°, with a much smaller bending radius than that of the main-ring accelerator. This requirement has been achieved using a series of uniquely fashioned superconducting magnets with combined-function coils having dipole and quadrupole field components within a single-layer coil (figure 3). The experience accumulated in the earlier projects contributed to achieving this distinctive superconducting-magnet design, which also involved important co-operation with Brookhaven National Laboratory. At J-PARC superconductivity has taken an essential role in providing high-intensity pulsed muon beams in the meson-science laboratory, as well as the superconducting solenoid beamlines for muon science and a superconducting magnetic spectrometer for particle physics.

For the future, KEK intends to contribute to upgrade programmes for the LHC, to the application of advanced high-field superconductors in co-operation with the National Institute of Materials Science and to high-temperature superconductors in co-operation with other laboratories and industry. Fundamental research on the effect of stress-strain on superconductor performance is crucially important for high-field superconducting magnets. Experimental studies of structural and stress analysis are in progress using neutron-diffraction techniques at the J-PARC neutron-beam facility in co-operation with JAEA.

KEK has also applied superconducting-magnet technology to particle-detector magnets. The TRISTAN collider’s three major particle detectors – TOPAZ, VENUS and AMY – and the BELLE detector at KEKB were based on superconducting solenoid magnets to provide the magnetic fields for momentum analysis in particle spectroscopy. In particular, these involved a great deal of development work on aluminium-stabilized superconductor technology.

The key feature of this technology is that it allows the maximum magnetic field for the minimum material – an important step in matching the physicists’ dream of having only a magnetic field, without additional material, in an experiment. It therefore leads to the possibility of "thin-walled" superconducting coils that are in effect transparent to particles passing through. The use of aluminium stabilizer instead of ordinary copper stabilizer allows for low density and low resistance but requires sufficiently high strength. It has become a fundamental technology in the construction of magnets for large-scale particle detectors, including – most recently – the magnet systems of the ATLAS and CMS experiments at the LHC. KEK provided the ATLAS central solenoid magnet, which had the extremely demanding requirement that it should be installed in a common cryostat with the liquid-argon calorimeter system in addition to employing the advanced high-strength aluminium-stabilized superconductor technology to meet the physics requirement for the magnetic field to be as transparent as possible (CERN Courier September 2006 p5).

KEK has also applied this technology in a variety of global collaborations, including the muon g-2 parameter measurement experiment (E-821) at Brookhaven National Laboratory and the WASA experiment at Uppsala University (now transferred to the Cooler Synchrotron (COSY) ring at the Forschungszentrum Jülich). A more extreme application is in the field of astroparticle physics. The Balloon-borne Experiment with a Superconducting Spectrometer (BESS) has successfully flown twice over Antarctica to search for primordial antiparticles in the universe, in collaboration with NASA in the framework of Japan–US co-operation in space science.

Superconducting acceleration

Turning now to RF superconductivity, TRISTAN was the first high-energy particle accelerator in the world to use superconducting RF cavities as the main acceleration components with a frequency of 500 MHz in the routine operation of the accelerator (figure 4). This is where Kojima took the lead and established a milestone by using superconducting RF to provide a high continuous-wave accelerating gradient in storage rings. He also trained many next-generation scientists in RF superconductivity, who have since extended the application in a variety of subsequent projects and global collaborations.

The technology pioneered at TRISTAN was extended for the KEKB accelerator, which was commissioned in 1998 with superconducting RF cavities as a major accelerating component. Eight single-cell cavities with sufficiently damped higher-order modes (HOM) accelerated the electron beam of 1.4 A, delivering the RF power of 350 kW per cavity. This technology was also applied to the Beijing Electron–Positron Collider II in co-operation with the Institute for High-Energy Physics in Beijing. Furthermore, collaboration with the National Synchrotron Radiation Research Center is under way to apply superconducting RF technology to its new synchrotron-light source, the Taiwan Photon Source. At the same time, a unique superconducting RF cavity, called the "crab cavity", was successfully developed as a key component to maximize the peak luminosity of KEKB (figure 5). It was designed to reach the optimum beam-interaction efficiency by tilting the beam and then compensating the crossing angles (CERN Courier September 2007 p8). Once installed at KEKB, the crab cavity contributed to the facility’s world-record luminosity of 2.11 × 1034 cm2s–1 achieved in 2009. KEKB shut down in June 2010 to be upgraded to Super KEKB, so as to allow operation with a peak luminosity of 8 × 1035 cm–2s–1.

Looking to future applications of RF superconductivity in accelerator science, KEK is now undertaking research and development in two major directions. Energy-recovery linacs (ERLs), which in effect recycle energy from the beam, will inevitably be required for efficient acceleration, especially in applications of intense electron beams and in photon science. KEK is building a compact ERL facility as a prototype for a potential future ERL accelerator.

Aiming towards the high-energy frontier, research and development for the International Linear Collider (ILC) is being carried out in a global co-operation led by the Global Design Effort (GDE) (CERN Courier September 2011 p9). The design, based on RF superconductivity, foresees more than 16,000 superconducting 9-cell 1.3 GHz cavities in series, operating with an average field gradient of 31.5 MV/m, to achieve a linear electron–positron collider based on two 250 GeV linear accelerators.

KEK is contributing to developing the advanced superconducting RF cavity technology for the ILC within the global collaboration. There has been successful progress towards demonstrating a field gradient of more than 40 MV/M in 9-cell cavities, based on accumulated long-term fundamental research and development. In a unique global effort, KEK has hosted a cavity-string test (the so-called S1-Global) with a cavity-string and a cryomodule system jointly contributed by DESY, Fermilab, INFN, SLAC and KEK (figure 6). The test facility has demonstrated how international collaboration can be possible in providing a plug-compatible cavity-string assembly, which would inevitably be required in constructing the ILC accelerator.

Applied superconductivity has been an essential and fundamental technology in all of the major experimental facilities for accelerator science and for physics programmes that have and will be carried out at KEK, as well as for international co-operation programmes, including the LHC and the ILC. The hope is that KEK will continue both to play an important role in contributing to advanced technology and to be a centre of excellence in applied superconductivity for fundamental physics and accelerator science.