Experimental investigation on sub-miliKelvin temperature control at liquid hydrogen temperatures

Temperature control around 20 K with a tolerance of better than 1 mili-Kelvin is challenging but essential for applications like cooling deuterium/tritium target in fusion ignition experiments. To explore the practicability of cryogenic temperature control on this level with a cryocooler as the cold source instead of a cryogen bath, experimental investigations were conducted to approach the best performance by optimizing configurations of effective measures. A special heat sink was fabricated and attached to the second-stage cold head of a G-M cryocooler which could reach temperatures down to 2.5 K and provide 13 W cooling power at 18 K. A thermal reservoir and/or temperature transmission damping were incorporated, as well as a manganin resistance wire electrical heater attached to the heat sink. Experimental results show that an optimal stability of the temperature of the shielded heat sink within ±0.4 mK at 20 K was achieved by choosing proper excitation for the sensors, control mode for the heater and capacity for the thermal reservoir.