Highlights

Associate Professor Yoshihisa Iwashita Awarded the Nishikawa Prize by the Foundation for High Energy Accelerator Science
2009. 6. 25

 

On March 23rd, 2009, Associate Professor Yoshihisa Iwashita (Advanced Research Center for Beam Science, Institute for Chemical Research, Kyoto University) was awarded the Nishikawa Prize by the Foundation for High Energy Accelerator Science, Japan.

This prize is awarded to researchers or technologists for published research on high energy accelerators or on experimental equipment used in high energy accelerators that has been judged of outstanding originality and recognized internationally to be of excellent quality.

Professor Iwashita was awarded this prize for "developing and applying an optical system for examining the surfaces of superconducting high frequency accelerator cavities." He won the prize jointly with Dr. Hitoshi Hayano (High Energy Accelerator Research Organization) and Mr. Yujiro Tajima(formerly of the Institute for Chemical Research, Kyoto University; presently of Toshiba Corporation).

 

< Background of Professor Iwashita's Research>
The International Linear Collider (ILC) is currently being constructed through a great international collaboration. The purpose of this accelerator is to help solve some of the most fundamental problems in physics, such as elucidating the conditions of the very early universe and the origin of mass. It will be the largest and highest energy electron/proton accelerator in history, consisting of an extremely precise experimental system constructed along the entire length of an approximately 40 km tunnel. Presently, the largest accelerator in the world is the Large Hadron Collider (LHC) at the CERN Research Center, located in a suburb of Geneva, Switzerland [1]. The LHC is a circular accelerator with a circumference of approximately 27 km (which is nearly equal to the length of the Yamanote train line running through Tokyo). The problem encountered when using this type of accelerator is that in order to keep particles on the circular path, a magnetic field must be used to bend their trajectories. As a result of this bending, the particles loose energy in the form of radiated light. This introduces a limit on the amount by which the particles can be accelerated. In a linear collider, there is no such problem, because the particles follow a linear trajectory, being accelerated down the length of the accelerator in a single run (see Figure 1) [2]. Most of the entire length of the facility is made up of a superconducting accelerator (see Figure 2). In order to accelerate the particles within the limited length of the accelerator, it is necessary to make the accelerating electric field along the accelerator as large as possible (high voltage). Until recently, the tubes for such accelerators were made of metal substances, usually copper. However, their use requires a very large amount of high frequency electrical power. In the ILC, instead of metal tubes, superconducting tubes with niobium surfaces will be used. As is well known, the electrical resistance of a superconducting material with respect to DC current is zero, and even for high frequency AC current, the resistance is very low. Thus, using such a material allows for a great reduction in the use of electrical power.

 

Figure 1: Plan for the International Linear Collider.

Figure 1: Plan for the International Linear Collider.


Figure 2: Superconducting accelerator tunnel.
Figure 2: Superconducting accelerator tunnel.

< Success of the Research>
Because this accelerating apparatus relies on the superconducting nature of the accelerator tube, if a defect appears on the internal surface of this tube, that region will become non-superconducting, and as a result, heat will be generated there. This will cause the temperature of the surrounding area to increase, and as a result, it too will become non-superconducting. If this happens, the large electrical potential along the length of the accelerator cannot be sustained. For this reason, it is very important to maintain the interior surface of the accelerator tube. Because of the complicated structure of the superconducting cavities, until recently, this surface was inspected using such low-resolution devices as gastrocameras. There are several types of problems that can prevent the realization of a strong electric field. In particular, it has been realized that it is necessary to have a camera system operating in the region of visible light that has sufficient resolution to allow detection of defects in the niobium surface of size on the order of 100 microns [3, 4].


Because the inside surface of the accelerator tube undergoes a processing that includes electropolishing (EP), other than the grain boundaries, unevenness in the surface on a scale of several microns is almost entirely eliminated, and it is like a mirror, even on this microscopic scale. The key to observing this surface was to implement the proper lighting. This was accomplished by wrapping the cylinders with flexible electroluminescent lighting (EL) sheets (see Figure 3).

 

Figure 3: EL sheet attached to the outside of a cylinder.
Figure 3: EL sheet attached to the outside of a cylinder.

During an RF power test, the system is equipped with many temperature sensors, which allow for measurement of the temperature distribution in the tube (T-map). With this, several heat source regions were detected. However, despite this fact, until recently, it was not possible to find the defects causing the problem. As a result, the accelerator exhibited very poor performance. This problem was solved using the camera system developed by Professor Iwashita. As shown in Figure 4, three 400-600μm defect regions were discovered. The biggest of these turned out to be a pair of defects separated by approximately 1 mm that with the T-map appeared to be a single defect. Until they were actually observed, nobody believed that such defects existed on the internal surface of the tube, and their discovery astonished many researchers around the world. With this discovery, there has been a new standard of performance realized. Also, although with this kind of planer observation, information about unevenness of the surface cannot be obtained, because the surface of the cavity is extremely smooth, measuring the slope at a point on the surface, the height can also be determined.
Figure 4: Images of the areas on the interior surface indicated by the T-map indicated in which defects were found. The stripe patterns are the grain boundaries.
Figure 4: Images of the areas on the interior surface indicated by the T-map indicated in which defects were found. The stripe patterns are the grain boundaries.

Thus, this camera system can be used to improve the performance of the collider apparatus. It is also expected that this camera will yield increased efficiency for cavity production, and thus improve cost performance. It has already been utilized at the DESY and FNAL facilities, and there are several requests for more cameras. For this reason, we too are pursuing improved performance. The applications to superconducting cavities include, in addition to ILC, the European XFEL and other next-generation radiation source facilities. The range of application of this camera system thus continues to broaden.

 

References
[1] http://lhc.web.cern.ch/lhc/
[2] http://www.linear-collider.org/
[3] Y.Tajima, Y.Iwashita, H.Hayano: Journal of the Particle Accelerator Society of Japan: Development of optical inspection system of L-band SRF cavity Vol.5, No.1, p.41-49, 2008 (in Japanese)
[4] Y.Iwashita, H.Hayano, Y.Tajima: Development of a High Resolution Camera and Observations of Superconducting Cavities, Phys. Rev. ST Accel. Beams, 11, [093501-1-6], 2008