In the main ring (MR) at the Japan proton accelerator research complex (J-PARC), beam-based alignment (BBA) is essential for precise calibration of beam position monitors relative to quadrupole magnet centers. With the upgrade of the beam position monitor (BPM) system for better precision scheduled in this summer, frequent and accurate BBA will become increasingly important to optics and orbit correction for realizing less beam loss and stable operation. In this study, a fast BBA method is experimentally investigated and compared with conventional approaches. The fast method uses orbit modulation within a single measurement to efficiently extract offset information. Experimental results show that the fast approach achieves alignment accuracy comparable to the conventional method while significantly reducing the required measurement time. These results demonstrate the feasibility of regular beam-based alignment measurements and provide a path toward more precise optics correction in the MR.

Magnetic-Alloy-loaded cavities have been used for many applications; beam accelerations of high-intensity proton and heavy ion beams, beam manipulations, medical accelerators and anti-proton decelerations. The material has a large permeability and the cavities have bandwidth below approximately 10 MHz. Using an external inductor for reducing the effective inductance of a cavity system, the cavity bandwidth can be moved beyond 10 MHz. The higher harmonic cavity is required in J-PARC Main Ring to enlarge the longitudinal beam emittance before reaching the flat-top energy. For the slow extraction, the emittance growth will be inevitable to suppress the beam instability. For Hyper-Kamiokande neutrino experiment, high-intensity beam with lower peak current will be required to avoid the event-pile-up at a new intermediate detector (IWCD). In this paper, we present the emittance control scenario with the cavity, beam effects on it, and design of a new VHF RF system.

The J-PARC Main Ring plans to increase its beam power from 830 kW to 1.3 MW for the Hyper-Kamiokande neutrino experiment by 2028. To enable this higher-power operation, the RF system has been upgraded by increasing both the number of cavities and the anode current of the tetrode tubes. However, the required anode current is now approaching the maximum capability of the tubes, and several associated power-supply systems are also nearing their operational limits. Traditionally, the anode current has been estimated using a phasor diagram approach. As an alternative, we evaluate the current using LTspice simulations, which can incorporate the vacuum-tube characteristics, cavity impedance, and beam loading. We have constructed an LTspice model of the Main Ring RF system that includes the impedance of the magnetic alloy loaded RF cavity, two 600 kW tetrode tubes, and the beam current corresponding to eight bunches with a total intensity of 3.3 \times 10^{14} protons. This allows us to estimate not only the anode current but also the screen grid currents, and the simulation results have been compared with measurements. In this paper, we present LTspice based RF system calculations for the Main Ring and evaluate the RF power requirements necessary to achieve 1.3 MW beam operation.

In a rapid cycling synchrotron, sinusoidal excitation using an LC resonant circuit is commonly used as the current pattern for the main magnets. In this case, dB/dt reaches its maximum value at the 1/4 cycle period, and the time variation of dB/dt is important in determining the required RF acceleration voltage. Multi-harmonic excitation can be considered as a method to reduce the time variation of the bending magnetic field. Mathematically, a 12.5% of 2ndharmonic can suppress dB/dt to 75%, i.e., B(t) = Bo + B1(cos(ωt)+0.125sin(2ωt)), which also leads to suppression of RF peak voltage. In this study, we compare the advantages of sinusoidal excitation and harmonic excitation using the J-PARC RCS as an example.

The J-PARC Main Ring (MR) has achieved a high extraction efficiency above 99.5% during 30 GeV slow extraction at the current beam power of 92 kW (8.1x10^13 ppp). However, at beam powers above 30 kW, we observed ring-wide beam loss due to transverse beam instability associated with vacuum pressure rise and electron cloud, believed to be triggered by longitudinal microwave structure in the beam. To achieve stable operation, we implemented phase offset injection into RF buckets and a two-step RF voltage reduction technique for debunching, in addition to chromaticity control. The generation of the longitudinal microwave structure and coherent transverse oscillation by the electron cloud during debunching has been investigated using a newly developed simulation code. The simulation predicts the generation of the longitudinal microwave structure under known ring coupling impedances. The code models the electron cloud–beam interaction as a transverse impedance derived from an assumed neutralization factor. The simulation results will be compared with measurements from a wall current monitor and a beam position monitor. Through this study, we will explore further instability mitigation strategies toward higher beam intensity operations planned for the future.

In the main ring (MR) at the Japan proton accelerator research complex (J-PARC), beam-based alignment (BBA) is essential for precise calibration of beam position monitors relative to quadrupole magnet centers. With the upgrade of the beam position monitor (BPM) system for better precision scheduled in this summer, frequent and accurate BBA will become increasingly important to optics and orbit correction for realizing less beam loss and stable operation. In this study, a fast BBA method is experimentally investigated and compared with conventional approaches. The fast method uses orbit modulation within a single measurement to efficiently extract offset information. Experimental results show that the fast approach achieves alignment accuracy comparable to the conventional method while significantly reducing the required measurement time. These results demonstrate the feasibility of regular beam-based alignment measurements and provide a path toward more precise optics correction in the MR.

The Fermilab Booster will accept a 600 µs beam pulse from the new superconducting Linac for PIP-II operations. The Booster is a rapid-cycling synchrotron that uses a resonant magnet circuit ramping at 15 Hz. For PIP-II, the cycle rate will increase to 20 Hz, and the injection pulse length will expand from 40 µs to 600 µs due to the lower output current from the new Linac. Because the Booste main bending field follows a sinusoidal waveform, the magnetic field is not constant during the extended injection window. The longer pulse length and higher repetition rate modify the beam orbit and can lead to increased beam losses. The Booster contains 48 dipole-corrector packages distributed across its 24 periods. Each package includes horizontal and vertical dipoles, quadrupole, sextupole, skew-quadrupole, and skew-sextupole elements. By driving the dipole correctors with an appropriately shaped sinusoidal waveform during injection, we can compensate the changing main field and create an effectively flat bending field—referred to as *flat injection*. Over the past several years, machine studies have been performed to characterize the required correction fields and to determine the corresponding power-supply specifications needed for PIP-II operation. In this presentation, we will summarize the study results and discuss the estimated magnetic field requirements and power supply parameters for achieving flat injection in the Booster.

A fundamental challenge in large-scale accelerator operation is to infer internal machine states from limited monitoring. In the J-PARC linac, errors in RF cavities lead to momentum drifts observable through beam phase monitors. Although the number and placement of monitors satisfy the solvability condition, the cumulative and sequentially coupled nonlinear nature of beam dynamics makes the inverse problem numerically ill-conditioned. Our initial machine-learning study demonstrated that a multilayer perceptron can accurately model the forward mapping from cavity errors to observed phases. However, the inverse model, which predicts cavity errors from monitor signals, failed under the non-redundant condition where each cavity was followed by only one phase probe. In contrast, when at least two probes were available (redundant case), the inverse prediction became accurate. This brought to light a fundamental limitation: what appeared to be a dependence on monitor redundancy was, in fact, a consequence of the model’s limited capacity to represent cumulative dependencies and of the insufficient information content in the training data. We reformulated the problem within an encoder/decoder framework, which treats the accelerator as a causal sequence of coupled elements. The inverse prediction succeeded in the non-redundant case. This reveals that physical irreversibility can be effectively mitigated when the model captures the hidden correlations underlying the sequential process.

The 3GeV Rapid-Cycling Synchrotron (RCS) at J-PARC supplies the proton beam to the Materials and Life Science Experimental Facility (MLF), where it is utilized for the production of secondary particle beams such as muons and neutrons. Secondary particle beams are used in experiments in condensed matter physics, polymer chemistry, biophysics, and other fields. Experiments requiring high time resolution demand an RCS extraction beam with a short-bunch width. The RCS extraction beam is shortened through bunch rotation driven by a manipulation of the fundamental RF voltage. The present study investigated the optimization of this method. In addition, the effectiveness of employing a second-harmonic RF voltage in combination was evaluated. Detailed results on short-bunch extraction in the RCS are reported in this presentation.

The J-PARC Main Ring (MR) is a high-intensity proton synchrotron, which accelerates protons from 3 GeV to 30 GeV. In addition to protons, we are considering accelerating heavy ions to GeV/u energies in the MR as part of the J-PARC heavy-ion program (J-PARC HI). The heavy ions will be injected from the new heavy-ion injector into the J-PARC Rapid Cycling Synchrotron (RCS) and delivered to the MR. As the first stage of the program, Au ions are considered the ion species to accelerate. A full stripping beam of Au ions ($^{197}$Au$^{79}$ ) with an energy of 500 MeV/u is injected from the RCS to the MR. Au ions are accelerated up to 11.5 GeV/u and delivered to the hadron experimental facility. Since the change in revolution frequency during acceleration of Au ions is larger than that for protons, additional cavities dedicated to ion acceleration or modifications to the existing RF cavity will be needed to cover a wider frequency range. To estimate the RF system requirements for accelerating Au ions in the MR, we conducted a longitudinal beam dynamics simulation. In this presentation, we present the simulation results with various harmonic numbers and acceleration times.