Operation and Control of Helium Subcooling System of LHD Helical Coils During Change of Rotational Speed of Cold Compressors

Abstract

Helical coils of the Large Helical Device (LHD) are large scale superconducting magnets for fusion plasma experiments. The cooling system was upgraded in 2006 to improve the cryogenic stability of the coils by subcooling supplied helium. After the upgrade, the subcooled helium of designed mass flow rate of 50 g/s at designed temperature of 3.2 K has been supplied to the coils stably during steady state subcooling operations. The supplied helium is subcooled at a heat exchanger in a saturated helium bath. A series of two centrifugal cold compressors are utilized in the subcooling system to reduce the bath pressure and temperature. It is necessary to operate the cold compressors within a range of appropriate mass flow rate in order to escape from surge and choke.When the rotational speed of the cold compressors increased/decreased, the mass flow rate of the cold compressors varied. Thus, it is important to comprehend the transient performance of the cold compressors in those cases and to control the mass flow rate for the purpose of the safe operations. In the present study, the characteristics of the system were investigated and automatic operating methods with a heater in the bath and a bypass valve were developed during the change of the rotational speed of the cold compressors. Consequently, the reliable subcooling operations of the system have been achieved over 6,000 hours for three years.

Operations of the Helium Subcooling System for the LHD Helical Coils during Ten Plasma Experimental Campaigns

“…and the choke for mechanical soundness and stable operation. Therefore, it is important to understand the performance of the cold compressors for the reliable operation of the cooling system, especially the transient characteristics against large disturbance due to heat load generated in superconducting coils [10][11][12][13]. In the present study, the thermal hydraulic behavior of the helium subcooling system and the operational performance during ten plasma experimental campaigns are reported.…”

“…The set value of the rotational speed of the cold compressors is 95 %, corresponding to the rated rotational speed of approximately 90,000 rpm. The helium mass flow rate of cold compressors is automatically controlled to be 16 g/s by the heater in the saturated helium bath in order to operate the cold compressors at the rated operating point [13,14]. Figure 3 shows the bath temperature and the inlet temperature of the coils with the mass flow rate of the cold compressors.…”

Present status and one upgrading method with a cold compressor of the EAST sub-cooling helium cryogenic system

“…Detailed specifications of LHD cold compressors are shown in table 3. With an excellent cold compressor control system, the LHD upgraded sub-cooling helium system has achieved reliable operations over 6000 h for three years [9].…”

“…With the development of cryogenic technology and the requirement of higher magnetic fields, a large number of cryogenic systems below 4.2 K are utilized in tokomaks, particle accelerators, colliders, etc [4][5][6][7][8][9]. As shown in table 1, it is a trend to adopt cold compressors, based on cold compression or mixed compression, to obtain sub-cooling or superfluid helium in the cryogenic systems below 4.2 K. In order to improve the reliability and stability of the EAST sub-cooling helium cryogenic system in the future, this research explores an upgrading method.…”