Development of a Surface Muon Production Target System with Thermal Analysis Using RF Heating

Abstract

Muon spin spectroscopy, known as μSR, is an experimental technique in condensed matter physics for investigating a material's magnetic and electronic properties at the microscopic level, shedding light on phenomena like superconductivity and magnetism. The more intense surface muons beams will allow beamline extensions and new types of experiment to be done. As the first μSR facility in Korea, our primary objective was to design and fabricate a target system configuration with a new disk-shaped target that maximizes surface muon production per proton current by extending the proton-target interaction length beyond that of existing facilities. Generally, commissioning of target systems is conducted by operating with a low beam power at first. Facing the challenge that the proton beam from SCL2 is not provided yet, the desired performance of the target system was successfully achieved through an optimization process that ensured a margin of safety and precision. Moreover, a new method for heat analysis of target systems without the use of radiation was introduced. For the prediction of surface muon yield and the amount of heat from proton- nucleus interactions, GEANT4 and Monte Carlo N-Particle (MCNP) simulation toolkit was utilized. The result of particle transport simulation was verified and calibrated by comparing to the experiment results published by PSI. Calculation of temperature distribution was performed using ANSYS mechanical software. Dimensions of each component, coolant flow rates, and target rotation speed were optimized by ANSYS software to satisfy the design criteria. The designed target system composed of the disk-shaped target with 20 cm radius, copper plates surrounding the target, alignment rails, a rotation system with a magnetic clutch, a coolant system, and shielding blocks. 4.736 × 10^7 surface muons per second could be expected at the entrance of the beamline with a 40-kW proton beam, which is sufficient to satisfy the surface muon yield criterion. The maximum temperature was 1307.5 °C with a 100-kW proton beam, which is satisfying the temperature criterion of 1626 °C. Thermal stress distribution of the target system is significantly lower than the stress criterion. Effective radiation dose for workers during maintenance was assessed and found to be within acceptable limits. The designed target system was fabricated and installed at RAON. In order to control the overall operation of the facility in remote, a control system was developed based on the EPICS. As a commissioning process, the target was heated by RF heating method which can heat only the desired volume of the graphite target above 1000 °C without any contact in vacuum and without radiation. The experimental data were compared to ANSYS results calculated by the same methodology to the design process. Since the ANSYS temperature calculation results are consistently higher within 7% than the experimental values, the ANSYS calculation methodology was thought to be carried out conservatively. This study introduces a process of designing and fabricating a target system with thermal stability while considering sufficient safety margins, aiming to achieve the longest proton-nucleus interaction length with a large acceptance solenoid. Methods have also been proposed to minimize errors inherently generated by particle transport simulations and numerical analysis software. Additionally, the introduction of a remote-control system and a detailed analysis of worker radiation exposure during maintenance periods ensured safety. Furthermore, experiments using radio-frequency heating were performed for heat analysis when a proton beam is not available and found out that ANSYS calculation methodology during the design process is sufficiently realistic and conservative by comparing the experimental results. The design approach and novel commissioning method introduced in this study help minimizing trial and error in the design and fabrication process, thus reducing costs, and are expected to have applicability not only to target systems in μSR facilities but also to the design of target systems for other particle production purposes.