Yoshihiro Shobuda

Realizing a high-intensity low-emittance beam in the J-PARC 3-GeV RCS

The J-PARC 3-GeV rapid cycling synchrotron is now developing beam studies to realize a high-intensity lowemittance beam with less beam halo. This paper presents the recent experimental results while discussing emittance growth and its mitigation mechanisms.

Rigorous formulation of space-charge wake function and impedance by solving the three-dimensional Poisson equation

In typical numerical simulations, the space-charge force is calculated by slicing a beam into many longitudinal segments and by solving the two-dimensional Poisson equation in each segment. This method neglects longitudinal leakage of the space-charge force to nearby segments owing to its longitudinal spread over 1/γ. By contrast, the space-charge impedance, which is the Fourier transform of the wake function, is typically calculated directly in the frequency-domain. So long as we follow these approaches, the longitudinal leakage effect of the wake function will remain to be unclear. In the present report, the space-charge wake function is calculated directly in the time domain by solving the three-dimensional Poisson equation for a longitudinally Gaussian beam. We find that the leakage effect is insignificant for a bunch that is considerably longer than the chamber radius so long as the segment length satisfies a certain condition. We present a criterion for how finely a bunch should be sliced so that the two-dimensional slicing approach can provide a good approximation of the three-dimensional exact solution.

Recent Progress of 1-MW Beam Tuning in the J-PARC 3-GeV RCS

This paper presents the recent progress of 1-MW beam tuning in the J-PARC 3-GeV RCS, especially focusing on our approaches to beam loss issues.

The Path to 1 MW: Beam Loss Control in the J-PARC 3-GeV RCS

The J-PARC 3-GeV RCS started a 1-MW beam test in October 2014, and successfully achieved a 1-MW beam acceleration in January 2015. Since then, a large fraction of our effort has been concentrated on reducing and managing beam losses. In this paper, recent progresses of 1-MW beam tuning are presented with particular emphasis on our approaches to beam loss issues.

Analytical Estimation of the Field Modulation during the Injection Period of the 3 GeV RCS in J-PARC

A ceramic chamber is utilized at the 3 GeV Rapid Cycling Synchrotron (RCS) in Japan Proton Accelerator Research Complex (J-PARC). To conduct the image currents on chambers, the outside of the chamber is surrounded by copper stripes equipped with capacitors. When magnetic fields are excited outside the chamber, the currents on the stripes generate rippling fields in the chamber. A three-dimensional theory is developed to cope with the field oscillations. In order to avoid adverse effects on the beam from the field oscillations, we found that the positions of the stripes with the capacitors should be as identical as possible among the different chambers.

Comparison between Measurements and Orbit Code Simulations for Beam Instabilities due to Kicker Impedance in the 3-GeV RCS of J-PARC

The transverse impedance of the extraction kicker magnets is the most dominant beam instability source in the 3GeV RCS of J-PARC. The beam instability occurs when the chromaticity is fully corrected during acceleration but on the other hand full chromatic correction only at the injection energy makes no instabilities at least for a beam power up to 500 kW. Recently, we have succeeded introducing realistic time dependent impedances in the ORBIT simulation code and using that the corresponding simulation results are found to be quite consistent to those with measurements. As a result, the simulation was also performed for the designed 1 MW beam in order to check beam instability scenario for such a high intensity beam. The beam instability occurs even a full chromatic correction is done only at the injection energy but betatron tunes are fixed during acceleration process. However, the situation can be avoided by utilizing a proper tune manipulation so as to stabilize the beam. The measurements results and comparison between corresponding simulation results for equivalent beam power up to 500 kW including 1 MW simulation results are presented.

Extension of napoly integral for transverse wake potentials to general axisymmetric structure

The Napoly integral for the wake potential calculations in the axisymmetric structure is a very useful method because the integration of Ez field can be confined in a finite length instead of the infinite length by deforming the integration path, which reduces CPU time for the accurate calculations. However, his original method could not be applied to the transverse wake potentials in a structure where the two beam tubes on both sides have unequal radii. In this case, the integration path needs to be a straight line and the integration stretches out to an infinite in principle. We generalize the Napoly integrals so that integrals are always confined in a finite length even when the two beam tubes have unequal radii, for both longitudinal and transverse wake potential calculations. The extended method has been successfully implemented to ABCI code.

ABCI progresses and plans: Parallel computing and transverse Shobuda-Napoly integral

In this paper, we report the recent progresses of ABCI. First, ABCI now supports parallel processing in OpenMP for a shared memory system, such as a PC with multiple CPUs or a CPU with multiple cores. Tests with a Core2Duo (two cores) show that the new ABCI is about 1.7 times faster than the non-parallelized ABCI. The new ABCI also supports the dynamic memory allocation for nearly all arrays for field calculations so that the amount of memory needed for a run is determined dynamically during runtime. A user can use any number of mesh points as far as the total allocated memory is within a physical memory of his PC. As a new and important progress of the features, the transverse extension of Napoly integral (derived by Shobuda) has been implemented: it permits calculations of wake potentials in structures extending to the inside of the beam tube radius or having unequal tube radii at the two sides not only for longitudinal but also for transverse cases, while the integration path can be confined to a finite length by having the integration contour beginning and ending on the beam tubes. The future upgrade plans will be also discussed. The new ABCI is available as a Windows stand-alone executable module so that no installation of the program is necessary.