Abstract A generalized theory of beam impedance for ceramic breaks has been developed by introducing two types of effective impedance that account for accelerator environments and the capacitors attached to the ceramic break. For a ceramic break with a titanium nitride (TiN) coating installed in an accelerator, the setup conditions are characterized by the effective impedance determined from scattering matrices, for which a theoretical basis is also provided to calibrate the beam-induced voltage on the ceramic break. Using the calibration scheme, the beam distribution with high-frequency components can be recovered from the beam-induced voltage modulated by the surrounding conditions. For a TiN-coated ceramic break covered by capacitors-whose direct numerical simulation requires an excessively fine mesh-the capacitor effect is incorporated by the other effective impedance. For the radiation fields, analytical results for a floated ceramic break in an open space can be obtained independently of capacitors or TiN coating. A benchmark test between analytical results and simulations is conducted using a ceramic break with neither TiN coating nor capacitors to cement the foundation of the theory.

The main source of beam instability in the J-PARC 3-GeV RCS is the impedance of the eight installed kickers. This arises because one end of each kicker magnet is shorted while the other end is left open during beam acceleration. The shorted-end configuration provides the benefit of power savings during beam extraction from the RCS. However, it also excites beam instability. To retain the energy-saving benefit while suppressing beam instability, we developed a diode stack with resistors and inserted it at the open ends of four kickers. This configuration effectively suppresses beam instability for smaller-emittance beams, which are delivered to the MR at J-PARC.

In general, optimizing chromaticity and transverse feedback parameters is important in suppressing beam instabilities for stable operation in high-intensity proton machines. Meanwhile, space charge effects impact the intrabunch motion of the proton bunch within a large chromaticity region in the machines, such as the main ring (MR) in the Japan Proton Accelerator Research Complex (J-PARC). To address this issue, the decoherence and recoherence of the transverse motion of the particles comprising the proton bunch are investigated. The analysis reveals that the space charge effects have a significant influence on the recoherence period through the chromaticity. Nevertheless, the relationship between the maximum frequency in the bunch and the chromaticity is not affected by the space charge effects. These findings are demonstrated with particle tracking simulations including the direct and indirect space charge effects, and the impedance source of the J-PARC MR. Furthermore, we illustrate the influence of the indirect space charge effect on particle motion by examining the excitation patterns of radial and head-tail modes.

Beyond 1 MW operation of the J-PARC RCS The 3-GeV Rapid Cycling Synchrotron (RCS) of the Japan Proton Accelerator Research Complex (J-PARC) has already been achieved the designed 1 MW operation to the Material and Life Science Experimental Facility (MLF). However, to cope with the gradually getting faster operation cycle of the main ring synchrotron sharing more beam requires RCS to accelerate more than 1 MW beam per pulse for the MLF to ensure net 1 MW beam power at the MLF. Moreover, the beam sharing to the under designed 2nd MLF target facility has also to be considered. As a result, the next goal is to realize 1.5 MW beam power first and continue for 2 MW or even more. This will be done by injecting more particles in the RCS by increasing both peak current and pulse duration of the injection beam. Beam dynamics issues and possible scenarios to realize far beyond 1 MW in the RCS are presented.

When computing the space-charge tune shift for a relativistic bunched beam within a cylindrical chamber, mirror currents for a coasting beam, initially introduced to replace the chamber wall, are employed. Subsequently, the obtained result is extended to encompass the bunched beam, taking into account the bunching factor which quantifies the distribution of bunches around the accelerator ring. In the process of derivation, the terms that characterize the bunch length are intuitively integrated into the formula. As a result, the validity of this approach has never been established. This study provides the derivation of the space-charge tune shift formula for both relativistic and nonrelativistic bunched beams right from the outset, employing the Green function formalism. Subsequently, it is compared with the earlier formula derived using mirror currents.

The 3-GeV RCS (Rapid Cycling Synchrotron) of J-PARC (Japan Proton Accelerator Research Complex) at present operates at a high intensity beam near to 1 MW beam power. The beam loss and the corresponding residual radiation are key issues for beam intensity ramp up. Based on detail numerical simulations and systematic beam studies the beam loss has been well mitigated to a minimum level. The residual beam loss at 1 MW beam power is mostly due to unavoidable foil scattering of the circulating beam during injection. We have identified almost all major beam loss sources and optimized to minimize the beam loss for achieving a stable operation at 800 kW beam power since April 2022.

The linac and the 3 GeV rapid-cycling synchrotron (RCS) at the Japan Proton Accelerator Research Complex (J-PARC) were designed to provide 1-MW proton beams to the following facilities. Due to the improvement of the accelerator system, we accelerated a 1-MW beam with a small beam loss. The lack of anode current in the radiofrequency (RF) cavity, rather than beam loss, limits the RCS beam power. Recently, we developed a new acceleration cavity that can accelerate a beam with a low anode current. This new cavity enables us to reduce the requirement for the anode power supply and accelerate a beam of more than 1 MW. We considered how to achieve beam acceleration beyond 1 MW. So far, a beam of up to 1.5 MW is expected to be accelerated after replacing the RF cavity. We also studied to achieve an up to 2 MW beam in J-PARC RCS.

Ceramic breaks are used in synchrotrons for many purposes. For example, they are inserted between the Multi-Wire Profile Monitor (MWPM) on the injection line at the Rapid Cycling Synchrotron (RCS) in J-PARC to completely prevent the wall currents accompanying beams from affecting the MWPM. On the other hand, from the viewpoint of suppressing beam impedances and the radiation fields from the ceramic breaks, it would be preferable that the inner surface of the ceramic break is coated with Titanium Nitride (TiN), or covered over capacitors. In this report, we measure the radiation fields from the ceramic break with and without capacitors as well as the beam profile and investigate the effect of the ceramic breaks on the measurements.

At the Rapid Cycling Synchrotron (RCS) in Japan Proton Accelerator Research Complex (J-PARC), theoretical predictions have indicated that the kicker impedance would excite the beam instability. A 1-MW beam with large emittance can be delivered to the Material and Life Science Experimental Facility through suppression of the beam instabilities by choosing the appropriate machine parameters. However, we require other high-intensity and high-quality smaller emittance beams (than the 1-MW beam) for the main ring. Hence, we proposed a scheme for suppressing the kicker impedance by using prototype diodes and resistors, thereby demonstrating the effect on the kicker impedance reduction. However, the J-PARC RCS must be operated with a repetition rate of 25 Hz, which urged us to consider special diodes that are tolerant to heating. After developments, we have demonstrated that the special diodes with resistors can suppress the beam instability by reducing the kicker impedance. Enhanced durability of the prototype diodes and resistors for the 25-Hz operation was also realized. Moreover, the new diodes and the resistors have a negligible effect on the extracted beam from the RCS. From a simulation point of view, the scheme can be employed for at least 5-MW beam operation within the stipulated specifications.

In the 3-GeV RCS (Rapid Cycling Synchrotron) at J-PARC (Japan Proton Accelerator Research Complex), multi-turn H− charge-exchange injection is performed by using a thin stripped foil. The residual radiation at the injection area caused by the uncontrolled beam loss occurred by foil scattering of the circulating beam is a serious issue for regular maintenance works. In addition, the beam loss at the collimator section and its downstream caused for a large emittance beam also should be reduced, especially at high intensity operation. For that purpose we have minimized injection beam size and implemented a smaller size stripper foil. The circulating beam hitting rate is reduced by using a smaller foil, while an optimized vertical angle of the smaller injection beam for vertical transverse painting also gave a reduction of the circulating beam emittances and resulted a further significant beam loss mitigation at the collimator section and its downstream. As a result, the residual radiation after user operation at 700 kW beam power was also measured to be significantly reduced.

In the Japan Proton Accelerator Research Complex, the purpose of the 3 GeV rapid cycling synchrotron (RCS) is to accelerate a 1 MW, high-intensity proton beam. To achieve beam operation at a repetition rate of 25 Hz at high intensities, the RCS was elaborately designed. After starting the RCS operation, we carefully verified the validity of its design and made certain improvements to establish a reliable operation at higher power as possible. Consequently, we demonstrated beam operation at a high power, namely, 1 MW. We then summarized the design, actual performance, and improvements of the RCS to achieve a 1 MW beam.

In many cases beam coupling impedances or wake fields are calculated with computer simulator such as CST Studio Suite, GdfidL Electromagnetic Field Simulator and so on. But evaluation with the stretched-wire method is still very useful by its flexibility to the change of the device-under-test configuration, speed to get results and, more than anything, its accessibility on the real devices. One of the drawbacks in the practical procedure, difficulty of Thru-Reflect-Line calibration, is overcome using the calibration method recently introduced in high frequency vector network analyzers, "2-X THRU de-embedding". Here the procedure is explained in detail. The method has been successfully applied to RF cavity and FX kicker measurement in the J-PARC MR.

The beam power of the main ring of the Japan Proton Accelerator Research Complex (J-PARC) is currently being increased. For high-power beam realization, it is essential to suppress the beam instability that limits the beam power and to estimate and enact countermeasures against the beam coupling impedance of individual devices, which can affect the collective motion of the beam. Therefore, we promote the identification and the reduction of impedance. The fast-extraction (FX) septum magnet will be replaced with another magnet that copes with higher numbers of repetition cycles. Despite their different structures, both septa demonstrated a large impedance in estimates performed by the CST studio suite wake-field solver. As the new septum magnet has been completely manufactured, we considered reducing its impedance by installing additional components. These components must meet strict installation requirements: they must be easy to install on the septum magnet, must not protrude into the beam pipe connected to the septum magnet (to avoid interfering with maintenance), must not disturb the wide horizontal aperture of the septum magnet, and must not affect the magnetic field. The widely used taper impedance-reduction method would be effective but violates the second requirement above (i.e., protrusion into the beam pipe). We found that by attaching a copper plate and SiC to the flange of the septum magnet, we could effectively reduce the impedance while satisfying all installation requirements. The copper plate on the flange reduces the impedance below the cut-off frequency. Moreover, when SiC was loaded, the remaining impedance was three times lower than when using the copper plate alone. After applying this method to the new septum magnet, the maximum longitudinal impedance was reduced to 1% of the value without countermeasures, largely improving the beam stability condition. We also estimated the required thickness of SiC and the calorific value. This method saves space and is installed by simple attachment to the flange, regardless of the shape of the beam pipe. Therefore, it is also applicable to other devices.

When the skin depth is greater than the chamber thickness for relativistic beams, the two-dimensional longitudinal resistive-wall impedance of a cylindrical chamber with a finite thickness decreases proportionally to the frequency. The phenomenon is commonly interpreted as electromagnetic fields leaking out of the chamber over a frequency range. However, the relationship between the wall current on the chamber and the leakage fields from the chamber is unclear because the naive resistive-wall impedance formula does not dynamically express how the wall current converts to the leakage fields when the skin depth exceeds the chamber thickness. A prestigious textbook re-expressed the resistive-wall impedance via a parallel circuit model with the resistive-wall and inductive terms to provide a dynamic picture of the phenomenon. However, there are some flaws in the formula. This study highlights them from a fundamental standpoint, and provides a more appropriate and rigorous picture of the longitudinal resistive-wall impedance with finite thickness. To demonstrate their physical meaning, we re-express the longitudinal impedance for non-relativistic beams, as well as the transverse resistive-wall impedance including space charge impedance based on a parallel circuit model.

Mössbauer spectra of human liver ferritin and its pharmaceutical analogues Ferrum Lek and Maltofer® measured at various temperatures within the range of 295–83 K were fitted using five quadrupole doublets related to different 57Fe microenvironments in various layers/regions of the ferrihydrite and akaganéite iron cores. The observed anomalous temperature dependences of some Mössbauer parameters were considered as a result of low temperature structural rearrangements in different layers/regions in the iron core.

At the injection area of the RCS in J-PARC, the interaction between the copper stripes (RF-shields) on the ceramic chambers and the external magnetic fields modulates the magnetic fields in the chamber, causing beam losses for a special tune. A ceramic chamber spirally covered by the stripes is a candidate to mitigate the modulations. In this report, we numerically and experimentally investigate how the interaction is suppressed, while sustaining the beam impedance enhancement within tolerable at the RCS.

The J-PARC 3-GeV rapid cycling synchrotron (RCS) has recently achieved a 1-MW beam operation with considerably low fractional beam loss of a couple of 10(-3) as a result of continuous efforts iterating experiments and numerical simulations. This success of the 1-MW beam operation opened a door to further beam power ramp-up beyond 1 MW; we are now promoting 1.2 similar to 1.5-MW-equivalent high-intensity beam tests looking ahead to future upgrades at J-PARC. In this article, we first review the current status of beam loss in the 1-MW beam operation, then presenting the recent results of the 1.2-MW beam tests with particular emphasis on our approaches to beam loss issues. The beam intensity limit of the RCS is also discussed with well-established numerical simulations.

The beam impedance of a ceramic break with titanium nitride (TiN) coating consists of three electric components in parallel: resistive wall term caused by TiN, radiation term, and capacitive term made by the ceramic itself. The entire wall current continues to run in the thin TiN even when the skin depth is much larger than the chamber thickness, except for the extremely thin TiN satisfying the condition that the radiation loss from the ceramic break becomes lower than the energy loss due to the dc current on the thin TiN. This characteristic is useful in developing a wall current monitor with an improved frequency response. This study demonstrates the feature of the ``ceramic break'' monitor up to a few GHz from the theoretical and measurement points of view.