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Successful High-Volume Production of Ultrahigh-Performance Magnetic Cores at J-PARC:
Toward higher intensity acceleration as a sign of resurrection from the Great East Japan Earthquake of 2011

Aug. 3, 2011

J-PARC Center

The Japan Proton Accelerator Research Complex (J-PARC),*1 Tokai-mura, Ibaraki, resumed the development and production of large ultrahigh-performance metal magnetic cores*2 for radio-frequency (RF) accelerating cavities,*3 which once had halted its operation due to the damage caused by the Great East Japan Earthquake of 2011, has succeeded the high-volume production of the cores after recovering from the damage. This allowed the smooth production of the cores at the rate of one per day. Thanks to this production, a sufficient number of magnetic cores has been manufactured for testing high-gradient accelerating cavities comprised of these cores. These ultrahigh-performance large cores can significantly improve the accelerating gradient*4 even further. The resultant high-gradient accelerating cavities can contribute not only to improving the performance of J-PARC but also to downsizing future proton/ion accelerators, thereby leading to cost reductions for constructing and operating these accelerators.

Upgrading the accelerator for higher beam intensity and higher gradient of accelerating cavities

J-PARC’s 50-GeV synchrotron has been producing beams with a power of ~ 150 kW by accelerating a proton beam every three seconds. Further improvement in beam intensity is expected. The key to this development is the following two technologies: the technology of increasing the number of particles to be accelerated at one time and that of reducing the acceleration interval. J-PARC aims to achieve a beam power of 750 kW based on these two technologies. In order to reduce the acceleration interval, higher accelerating voltage is needed, which requires a larger number of accelerating cavities; however, space is limited and thus cavities with higher accelerating gradient must be developed. Ultrahigh-performance magnetic cores developed herein are the basis for significantly improving the performance of accelerating cavities.

Toward a higher gradient of accelerating cavities

J-PARC has already achieved the world’s best accelerating gradient using accelerating cavities with high-performance metal magnetic cores (Figure 1). Successful high-volume production of large ultrahigh-performance magnetic cores allows us to significantly improve such high-performance accelerating cavities with high accelerating gradient even further (Figure 1). The key to improving the accelerating gradient is the impedance*5 of a magnetic core. The magnetic core that was mass-produced at J-PARC has the shape of a torus with a diameter of 80 cm, a thickness of 2.5 cm, and a weight of 60 kg. This core has approximately two times higher impedance compared to conventional high-performance cores, allowing the reduction in the number of the required magnetic cores and consequently improving the accelerating gradient. Moreover, the new core can produce higher RF voltage with the limited input power. This success enabled us to generate higher accelerating voltage without re-constructing power supplies or RF amplifiers. Accelerating cavities with a higher gradient can not only improve the performance of J-PARC but also lead to the downsizing and cost reduction of future proton/ion accelerators.

Metal magnetic cores

Figure 3 depicts a large ultrahigh-performance magnetic core manufactured at J-PARC. The thickness of this core is only 70% of that currently used at J-PARC, but its impedance is far greater. This metal magnetic core is manufactured by heat-processing an amorphous material to form nanometer-sized crystals. Applying a strong magnetic field during this heat treatment can produce a magnetic material, in which the axes of easy magnetization*6 of the resultant crystals are aligned parallel to the direction of the magnetic field. Manipulating the direction of this applied magnetic field allows us to align the axis of easy magnetization orthogonal to the direction of the RF magnetic field produced inside the magnetic core during accelerating beams. This causes the nanocrystals to be in the state of low RF power loss. A large electromagnet and a large heat-treating furnace used for manufacturing these cores are shown in Figure 4. These electromagnet and furnace are located in J-PARC’s Hadron Experimental Facility, which were damaged especially extensively by the Great East Japan Earthquake of 2011; however, full cooperation from members of the hadron section and cryogenics section allowed the high-volume production of the cores. The key step in the production of the magnetic core is the process of forming nanometer-sized crystals from amorphous materials. In order to examine this process in detail, the latest technology called “muon spin rotation, relaxation, and resonance (μSR)*7Ehas been utilized at high temperatures, via the world’s most powerful muon beams available at the Materials and Life Science Experimental Facility of J-PARC.

High-volume production of magnetic cores

In order to raise the intensity and performance of J-PARC to a higher level, accelerating cavities using ultrahigh performance metal magnetic cores are needed. To this end, more than 100 magnetic cores are required. Therefore, it was necessary to demonstrate whether the high-volume production can be achieved by using a combination of a large electromagnet and a large furnace. This was confirmed by the fact that one magnet core was manufactured per day without any problems in a high-volume production test for two weeks. In the future, as shown in Figure 1, we aim to achieve higher performance in high-gradient RF accelerating cavities using these ultrahigh-performance magnetic cores. Moreover, we plan to establish a full-fledged, high-volume production system by combining another large electromagnet with the furnace used here.





Glossary

*1 Japan Proton Accelerator Research Complex (J-PARC)
A collective term for the complex of proton accelerators and shared-use facilities that were constructed in 2001 in Tokai, Ibaraki prefecture by the High Energy Accelerator Research Organization and the Japan Atomic Energy Agency as a joint project. It has been operated since 2001. Using secondary particles such as neutrons, muons, mesons, and neutrinos produced by colliding accelerated protons with nuclear targets, leading-edge science research and industrial applications in materials and life science as well as nuclear and particle physics are carried out there.
*2 Metal magnetic cores
These are the metal magnetic rings that are placed inside accelerating cavities. Herein metal magnetic cores mean magnetic materials, such as amorphous or nanocrystalline magnetic materials. Metal magnetic cavities (magnetic alloy-loaded cavities) were first manufactured by loading amorphous materials at the Saclay Nuclear Research Centre in the 1980s; however, a voltage gradient was low and a mechanism to synchronize the frequency of applied RF waves to the acceleration frequency was required. In Japan the development of metal magnetic cavities began in 1995 at the former Institute for Nuclear Study of the University of Tokyo, and a high voltage gradient was achieved by using nanocrystalline soft magnetic materials. Furthermore, these cores have been used in magnetic cavities that can stably work for accelerating beams by upgrading the manufacturing processes of cores at J-PARC. Currently, RF acceleration systems that have these metal magnetic cores, which are being used at J-PARC, are installed in many clinical accelerators in Japan and around the world, as well as used in accelerating lead ions at the European Organization for Nuclear Research (CERN), contributing to heavy ion experiments at the TeV-enegy range (1 TeV = 1015 eV).
*3 Radio-frequency (RF) accelerating cavities
Devices to accelerate particles, such as protons or electrons, using RF waves. In proton accelerators, the speed of protons changes as they are being accelerated, and therefore RF has to be adjusted. At the rapid cycling synchrotron (RCS) and the main ring (MR) of J-PARC, RF has to be adjusted between 0.94 MHz and 1.67 MHz and between 1.67 MHz and 1.72 MHz, respectively. In particular, at RCS protons are accelerated from 181 MeV to 3 GeV within just 2/100 seconds (the world quickest synchrotron accelerator), which requires a high-speed, precise control of the accelerators. In the past, ferrites have been installed in proton accelerators and the resonance frequency of accelerating cavities was being synchronized with the energy of protons during acceleration in order to control the frequency of RF waves. In contrast, at J-PARC researchers have developed a system suitable for accelerating high-intensity beams without using such a synchronization mechanism by installing metal magnetic cores. At the same time, using such high-performance magnetic cores can improve the accelerating gradient by more than 2-fold compared to that of existing accelerators, thereby solving the space/cost challenge, that is, constructing 3-GeV RCS with a circumference of only 300 m.
*4 Accelerating gradient
The accelerating gradient is determined by dividing the RF voltage of an accelerating cavity by its length. With a higher accelerating gradient, higher beam energy can be achieved per unit length and thus the total length of an accelerator can be reduced for a fixed beam energy. Accelerating gradient of existing proton/ion accelerators is around 15 kV/m as shown in Figure 1. A problem in developing high accelerating gradient is in the fact that a magnetic core (ferrite) is saturated at high accelerating voltages, significantly reducing the performance (impedance*5) of cavities. In particular, ferrites are easy to saturate in a high-intensity synchrotron with a high repetition rate that requires the large diameter of a beam pipe, limiting the maximum accelerating gradient to approximately 8 kV/m. To overcome this difficulty, J-PARC employed metal magnetic cavities that are hardly saturated and achieved accelerating gradient of at least 20 kV/m. In addition, we believe that the ultrahigh-performance magnetic cores will be able to drastically improve the world-best accelerating gradient at J-PARC even further. Moreover, this technology is likely to be utilized widely in various fields, including medicine, for downsizing accelerators, for example for cancer therapy.
*5 Impedance
Impedance is a measure used to determine the performance of accelerating cavities. It is defined by V2/2P, where V is a radio-frequency (RF) voltage and P a power loss. In proton accelerators, in order to obtain the RF voltage necessary for high-gradient acceleration, a larger number of magnetic cores need to be installed in individual cavities to obtain higher impedance. The utilization of high-impedance cores developed in the present research makes it possible to achieve high accelerating voltage with a smaller number of cores, which implies a shorter acceleration length. Moreover, existing accelerators, including J-PARC, can benefit from such an impedance improvement in that the higher accelerating voltage can be obtained with the same RF sources, consequently improving overall performance of accelerating cavities in general.
*6 Axis of easy magnetization
The axis of easy magnetization is an axis that is easy to be magnetized in a crystal when an external magnetic field is applied, while the axis of hard magnetization is that which is more difficult to be magnetized.
*7 Muon spin rotation, relaxation, and resonance (μSR)
μSR is a technique for studying physical properties of materials using muons. Muons have a magnetic moment, which is like a property of ordinary magnets. In other words, muon is like an atomic scale compass. Moreover, the magnetic moment of a muon is related to the muon spin. In the process of producing muons, their magnetic moment (spin) is completely aligned in a particular direction, which is very convenient for many experiments. This property can be used to study magnetic properties in a material at the atomic scale, by measuring how muon spins, which were completely aligned at the time of injection into a material, change while traveling through it. This technique is called “muon spin rotation, relaxation, and resonance (µSR),Ewhich works just as a “magnetic microscope.EμSR can be utilized to examine the formation of nanocrystals at high temperature.

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