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As a specific device for light production, undulators have been researched and developed since the third generation of synchrotron photon sources. Nowadays, superconducting undulators become a research hotspot. However, the cryostat, which is used to create a liquid helium temperature environment, often causes the failure of the superconducting undulator. In this paper, a cryostat for a superconducting undulator is designed and investigated. First, a new refrigeration distribution is proposed that can provide more excess cooling capacity at 4.2 K temperature. For the cooling of the superconducting magnet, a liquid helium circulation loop based on the thermosiphon effect is designed, which has no pump or any other moving parts. Next, based on high-temperature superconducting technology, six binary current leads are used to decrease the heat load. The beam chamber is cooled below 20 K to reduce possible effect on the magnet. In addition, for the tubes that connect the 4.2 K helium tank to the room temperature component, the thermoacoustic oscillation is studied. In the experiment, the superconducting magnet could be cooled and maintained at 4.2 K depending on the thermosiphon loop. No helium was discharged when the magnet went through a quench which generated large heat in a short time. There was no liquid helium consumption and the excess cooling capacity reached 2.2 W. The maximum magnet current is as high as 470 A. This study can provide a valuable reference for the development of superconducting undulator cryostats. [ABSTRACT FROM AUTHOR]

Superconducting undulators generally depend on a liquid helium cryostat. If the cryostat can liquefy the boil-off helium completely, a large helium refrigerator can be avoided, and a large amount of electrical energy can be saved. However, many studies are limited by the excessively high heat load. In this work, systematic thermal analysis and optimization are carried out to minimize the total heat load so that it can be covered by the cooling capacity. Based on the analysis, one single thermal shield is set in the cryostat, and thermal interruptions are widely adopted together. Moreover, the cooling capacity matches the heat load well, and the excess cooling capacity is increased significantly. In the experiment, the cryostat was tested with no magnet, a small magnet and a full-scale magnet respectively so that the heat load could be analyzed step by step. Finally, zero liquid helium consumption was achieved with the full-scale superconducting magnet, which reached a current of 450 A. The excess cooling capacity accounts for approximately 40 % of the total cooling capacity, which is as high as approximately 2.0 W. This study can serve as a reference for the research of superconducting undulator and other magnet cryostats. • Zero liquid helium consumption is realized with a 1.5 m superconducting undulator. • Various heat load parts are analyzed, measured and investigated clearly. • Thermal interruptions play an important role in heat insulation of the cryostat. • The mechanical structure is simplified by one single thermal shield. • The cryostat can provide a large temperature margin for the superconducting magnet. [ABSTRACT FROM AUTHOR]

• Thermal imbalance is the main factor which breaks the pressure stability of the subcooled liquid nitrogen circulation. • The more precise the thermal balance is, the more stable the pressure will be. • Bypass flow cooling method simplifies the circulation, which has much potential. • Peak-to-peak fluctuation can be less than ± 4.0 mbar and the RMS of the fluctuation less than 1.0 mbar. The High Energy Photon Source (HEPS) is a fourth generation radiation source, where lots of optical equipment are cooled down to the liquid nitrogen temperature for the better performance. The subcooled liquid nitrogen circulation is a very advantageous cooling method of monochromators and undulators, which are sensitive to fluid pressure fluctuations. Although the pressure stabilization matters a lot but it has rarely been studied. In this paper, the causes of pressure fluctuations are analyzed, and accordingly the principle of thermal balance is proposed. Based on the analysis, three pressure stabilization methods were suggested and tested. A partially liquid nitrogen filled tank was connected to the subcooled liquid nitrogen circulation serving as a pressure fluctuation buffer. The side effect is that the heat load of the tank raises the pressure gradually. Therefore, the cold source buffer and its induced heat load need good compromise. The tank heat load was cooled by saturated nitrogen, bypass liquid nitrogen and direct gas absorption: The saturated nitrogen reuses the sensible heat of cold gas; the bypass liquid nitrogen avoids the influence of circulation heat load variation and the gas absorption prevents the heat exchanger in the tank. The test results show that the peak-to-peak value of the pressure fluctuation can be controlled in the level of ± 4.0 mbar and the RMS value less than 1.0 mbar. The pressure affect factors were empirically studied. [ABSTRACT FROM AUTHOR]

The 166.6 MHz radio frequency (RF) system with five superconducting HOM damped cavities is adopted to accelerate the 200 mA electron beam in the storage ring of the High Energy Photon Source (HEPS). The 166.6 MHz cavity cryomodules have been designed, constructed and horizontal tested at IHEP since the beginning of 2019. The thermal performances of the cryomodule are analyzed by numerical simulations which take into account the main contributors to the static heat loads at various temperature levels. The experiment investigations including the cool down, operating with RF power and static heat load measurements of the cryomodule are performed in the liquid helium cryogenic system. The calculated static heat loads have been compared with the experimental measurements, and the results show that deviations between the calculated heat loads and available experimental measurements at 4.2 K temperature level are within a small range, which indicates that thermal simulations are accurate. The detailed operating parameters in the cryogenic environment are reported. [ABSTRACT FROM AUTHOR]

The Chinese Accelerator Driven Sub-critical system (C-ADS) Injector-I is designed and constructed by the Institute of High Energy Physics (IHEP) of the Chinese Academy of Sciences (CAS) in Beijing, China. The test cryomodule (TCM), housing two 325 MHz spoke cavities and two solenoids and operating at 2 K, has been designed, assembled and commissioned since the beginning of 2012. The C-ADS Injector-I linac with the TCM successfully accelerated the proton beam to 3.68 MeV with the peak current of 10.1 mA in pulse mode. It is the world's first prototype of spoke cavity cryomodule with beam, and is an important step in the qualification of the cryomodule before series productions. This paper describes the detail of the design, operation and thermal performances of the prototype cryomodule. The analysis takes into account the heat loads of the main components (support posts, multilayer insulation and cavities) at various cryogenic temperature levels. The estimated heat loads of the cryomodule have been compared with the available experimental measurements, and the cryogenic deformations by simulation is verified by the experimental measurements by the wire position monitors (WPMs). The operation experiences including the cool down, alignment and heat load measurement are reported. • The 325 MHz spoke cavity prototype cryomodule has been designed, assembled and commissioned. • The thermal performance of the bottom-supported cryomodule has been proved firstly worldwide. • The cryomodule can be in a stable operation at 2 K and 31 mbar with beam. • The operation experiences including the cool down, alignment and measurement of heat loads are reported. • The linac with cryomodule had been accelerated successfully to 3.68 MeV@10.1 mA in pulse mode. [ABSTRACT FROM AUTHOR]