Solid hydrogen is more dense than the liquid phase, so freezing does not endanger the mechanical integrity of a closed system. The chief hazard is that relief routes out of the system will become clogged with hydrogen ice, making the behavior of the system during a warmup unpredictable. However, since we are using 15 K coolant, while the hydrogen freezing point is about 13.8 K, the hydrogen target should not get frozen. The freezing point of deuterium is higher than that of hydrogen and higher than the temperature of the gas used for cooling (15 K).There is a chance that the deuterium target can freeze.
The coolant flow through the three target heat exchangers is connected in parallel for the three target loops. The entire target system will be run so that it represents a constant heat load on the ESR. For instance, the ESR will deliver a constant mass flow of helium cryogen at a constant temperature, about 15 K, and the coolant will be returned at an approximately constant but higher temperature, usually about 20 K.
The targets are always temperature regulated by temperature controllers. Also a high power heater will be in the PID loop to compensate any large temperature fluctuations to keep the temperature constant. In the unlikely event that the target temperature drops too low, an alarm will sound and the target operator will turn down the corresponding J-T valve(s).