Cryogenic systems for two superconducting cyclotrons under development at CIAE.

Two proton therapy superconducting cyclotrons using NbTi superconducting coils and warm iron yokes as main magnet, namely the CYCIAE-230 and the CYCIAE-250, are currently under development at China Institute of Atomic Energy (CIAE). In this paper, the cryogenic systems based on the liquid helium zeroboiling- off technology for the CYCIAE-230 and the CYCIAE-250 superconducting cyclotrons is described. At first, the structure of the cryostat with helium gas re-condenser has been illustrated, and then the heat loads from conduction, thermal radiation and convection have been calculated. Based on performed calculations, 4 two-stage GM coolers with total cooling power of 5.5 W @ 4.2 K and 2 single-stage GM refrigerators with the total cooling power of 170 W @ 77 K are installed on the helium re-condenser and the thermal shield of the cryostat respectively to maintain the cryogenics needed by the NbTi superconducting coils. The cooling down test results of the cryogenic system of CYCIAE-230 show that the superconducting magnet can achieve 4.2 K in 65 hours. The highlights of over a year’s continues operation of the cryogenic system are also presented.