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.

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.

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.

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.

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 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