KEK Accelerator Laboratory

We developed a two-beam interdigital-H-type radio frequency quadrupole (IH-RFQ) linac as a prototype of a multibeam IH-RFQ for high intensity heavy ion acceleration in the low energy region. This linac has two sets of RFQ electrodes within an IH-type resonant frequency cavity that is a power-efficient structure for low energy beam acceleration. The linac can accelerate two beams in parallel in one cavity with a reduction in the coulomb repulsive force (the space charge effect) between the accelerated heavy ion particles. The resonance frequency and the $Q$ factor of the linac were found to be 47 MHz and 5900, respectively. We also developed a two-beam laser ion source with a direct plasma injection scheme as an injection system for the two-beam IH-RFQ linac and built a system to demonstrate the use of the two-beam IH-RFQ linac. Using this linac system, we were able to accelerate carbon ions from 5 to $60\text{ }\text{ }\mathrm{keV}/\mathrm{u}$ and generate an output beam current of about 108 mA ($2\ifmmode\times\else\texttimes\fi{}54\text{ }\text{ }\mathrm{mA}/\mathrm{channel}$). A coherency between the two beams, derived from the imbalance of the beam loading, was observed in the acceleration test with carbon ions.

Abstract The SuperKEKB is an electron-positron collider consisting of two storage rings: the 4 GeV-positron low-energy ring and the 7 GeV-electron high-energy ring. The impedance of the rings has been modeled using electromagnetic simulation codes and used to study the single-bunch collective instabilities. The wake potentials are exported from the impedance model and included in a particle tracking simulation code, PyHEADTAIL. In this paper, we review the impedance modeling and the results of the collective instability simulations.

Electropolishing process had been considered to cause hydrogen absorption that could result in hydrogen Q‐disease in niobium superconducting RF cavities. Recently, however, we found that hydrogen Q‐disease did not always occur in a L‐band niobium RF cavity treated by continuous electropolishing (EP) only. We consider mechanical grinding as a very powerful method to eliminate surface defects. Therefore, our surface treatment process is a combination of mechanical grinding and EP. We developed a simple and fast mechanical grinding method, centrifugal barrel polishing method (CBP), as the pre‐treatment for the EP. We applied a combination of CBP and EP to a L‐band niobium cavity and found the result showing a heavy hydrogen Q‐disease. We found out that hydrogen absorption occurred during the CBP, and the absorbed hydrogen came from the water used during the CBP. Hence, we replaced the water by a liquid without hydrogen as its component. As the result, we successfully prevented hydrogen absorption in the CBP. However, the combination of hydrogen‐free CBP and EP resulted in a heavy hydrogen Q‐disease again. The hydrogen might have been absorbed through surface defects caused by the CBP. We found hydrogen Q‐disease did not occur by the combination of hydrogen‐free CBP and chemical polishing (CP). CP acid contains nitric acid functioning as an oxidiser. EP acid doesn’t contain chemicals that function as oxidiser. This suggested that oxidation helps preventing hydrogen absorption. In the EP, oxidization occurs only when electric voltage is applied. There are moments during the EP, when electric voltage can not be applied (at the beginning and at the end), which in turn stops oxidation so that hydrogen absorption would occur. To assure the effect of the continuous oxidization, we put a small amount of nitric acid in the EP acid, and the additional oxidizer prevented hydrogen absorption successfully.

SuperKEKB, which is an upgrade of the KEKB B-factory (KEKB), is a next-generation high-luminosity electron-positron collider. The construction started on July 2010, and is still underway. In order to achieve its extra high performance (about 40 times the luminosity of the KEKB), many upgrades of the vacuum system must be performed. More than 1200 beam pipes are being replaced with new one with antechambers. To deal with a serious electron cloud issue expected in a positron ring, several types of countermeasures are adopted such as a titanium nitride (TiN) coating, solenoid field, clearing electrode and groove structure. More than 920 beam pipes had been delivered, and the pre-baking and TiN coating of them are finished in KEK so far. About 500 beam pipes are already installed in the SuperKEKB and the construction work of the vacuum system will be completed by the end of 2014.

In order to obtain 3D mapping of the hydrogen distribution in solids in-situ, the nuclear reaction analysis (NRA) measurement has been carried out by using a tapered glass capillary to focus and introduce the incidence beam to the sample under atmosphere. A Y thin film covered with Pd was used for this measurement. In the N2 gas, hydrogen absorbed by the Y film was observed at a pressure up to 8×103 Pa. In the H2 gas and air, on the other hand, substantial γ-ray signals were observed at 15N energies of 6400~7200 keV, which obscured the signal originating from the Y sample. The origin of this signal is discussed.

The monitoring of the beam loss distribution along the accelerator is important in order to prevent damage of the vacuum components, and, in addition, to suppress the unnecessary irradiation of the accelerator elements. As it is not easy to construct the readout system to be synchronised to a fast timing signal, such as beam injection, a new 64-ch ADC system has been developed that samples the output of the loss monitor signal integrator with a fairly fast rate and automatically keeps the peak, mean and minimum of the data. The performance of the ADC system will be shown. The control system configuration that reads and resets the hardware interlock signal from the loss monitor signal integrator for the machine protection system (MPS) will also be presented.

In the slow extraction of the JHF 50GeV ring, about 1 is expected to hit the ESS(Electro Static Septum). The beam loss in the scattering is about 1% corresponding to the beam power of 7.5kW. Considering the tolerable beam loss(0.5∼1W/m), careful study of the beam loss of the scattering beam is indispensable. To evaluate the radiation level around ESS, simulation study with MARS14(00), was carried out. In addition, the beam loss distribution in the ring was studied combining MARS14(00) and single particle tracking. The result shows that the radiation level around ESS is suppressed to the tolerable level. In addition, it was found that the actual beam loss is much smaller than the loss of 1% which hit the ESS wire.

可視光から真空紫外 光までがやっとであり、X 線はほとんど発生し ない。最初の放射光利用実験は、1963 年にア メリカ標準規格局の 180 MeV シンクロトロン において、主に真空紫外領域の光で行われた。 その後の 1970 年代、素粒子原子核実験用の高 エネルギー加速器に放射光実験ビームライン が作られ、X 線領域の放射光利用実験も広まっ ていった。その様な、素粒子原子核実験の合間

Recent activities of the eight divisions (Applied Surface Science, Electronic Materials & Processing, Nanometer Structures, Plasma Science & Technique, Surface Engineering, Surface Science, Thin Film, and Vacuum Science and Technology) of the International Union for Vacuum Science, Technique and Applications (IUVSTA) are reported.

In the KEKB injector linac, a two-bunch acceleration scheme has been used for doubling the positron injection rate to the KEKB low-energy-ring (LER). In this operation mode, the multi-bunch transverse wake field caused by the first bunch affects the beam orbit of the second bunch. In the KEKB linac, an orbit correction method based on the average minimum of two-bunch orbits has been adopted, and has worked stably. However, a new two-bunch orbit correction method is strongly required to make the loss of charge less. We propose a new two-bunch orbit correction method based on a local bump method. In this scheme, some local bumps are intentionally constructed in a low-energy area. Adjusting the local bump height can control the wake field strength affecting the second bunch. In this paper, we report on the results of a preliminary beam test to confirm that this new method is useful

The performance of existing and proposed collider is often limited by the beam-beam interaction. The tune shift and spread from head-on and long-range beam-beam interactions for both round and flat Gaussian beam have been studied with analysis method in this paper. These formulas are useful for multi-bunch colliders where both short-range and long-range beam-beam interactions should be considered.