Development and Characterization of Pre-bunching and Re-bunching systems for the RAON heavy ion accelerator

Abstract

This thesis presents the development and commissioning of a pre-bunching system and re-bunching system for the Rare Isotope Accelerator complex for ON-line experiments (RAON) heavy ion accelerator. The pre-bunching and re-bunching systems were designed to optimize beam properties for Time Of Flight (TOF) experiments at the RAON low-energy experiment facilities, i.e., the Korea Broad Acceptance Recoil spectrometer and Apparatus (KoBRA) and the Nuclear Data Production System (NDPS). The pre-bunching system was developed to reduce the repetition rate of the ion beams to several hundred kHz (for NDPS) or several MHz (for KoBRA) to enable accurate TOF experiments. If the ion beam repetition rate is too high, the secondary particles, which is generated from the collision between primary beams and the production target, are overlaped, making it difficult to distinguish between beam bunches and perform a accurate TOF experiment. The pre-bunching system incorporates a fast chopper and a Double Gap Buncher (DGB). A high-voltage square wave or sinusoidal RF wave is applied to the fast chopper to deflect the beam, allowing the reduction of the beam repetition rate. The DGB further shortens the bunch length of the beam for proper operation of the RFQ. The operation of the pre- bunching system was simulated using SIMION code, and based on these simulation results, beam-based commissioning was attempted. Commissioning experiments using an Ar8+ ion beam demonstrated the system's capability to reduce the repetition rate to the desired level of 2.03125 MHz. The re-bunching system is designed to shorten the bunch length of the primary beams before they reach the experimental target. To determine the optimal type of rebunchers, a simplified beam simulation was conducted using Python code. For beams with energies ranging from 1 to 5 MeV/u, a normal conducting Quarter Wave Resonator (QWR) type cavity has been selected as the rebuncher. For bunching beams with energies above 5 MeV/u, a normal conducting Interdigital H-mode Drift Tube Linac (IH-DTL) has been chosen as the rebuncher. Since beam experiments in the near future are expected to frequently utilize beams with energies above 5 MeV/u, the IH-DTL type rebuncher was prioritized for manufacture and installation at beamline. The successful application of 22 kW RF power to the IH-DTL rebuncher confirmed its efficacy in achieving the desired bunch lengths. Finally, the Capacitive Pick-Up type Bunch Shape Monitor (CPU-BSM) was developed and optimized to provide non-destructive diagnostics of the beam's longitudinal profile. Unlike traditional bunch shape monitors, the CPU-BSM offers minimal interaction with the beam, allowing for continuous monitoring without disrupting the experimental conditions. The design optimization included ensuring a wide operational frequency range and mitigating impedance mismatches. Bench tests and simulations demonstrated the CPU-BSM's ability to accurately reconstruct the bunch shape from the detected signals. Additionally, the performance of the CPU-BSM was validated through bunch shape measurement experiments using proton beams, demonstrating its effectiveness.