Stable operation of cryogenic system at 2K during the RF or magnet power variations

In the last 40 years several large laboratories, e.g. CERN, DESY, TJNAF, constructed the accelerators with large number of superconducting (sc) cavities or sc magnets operating at 2 or 4 K temperature levels. The sc cavity operation requires more stringent ranges of pressure variation than ones of sc magnets. In comparison to small testing benches or single cavities modules, for large accelerators a complicated controlling of refrigerator and cryogenic system is required. In the present paper, a review of controlling principles and loops, typically applied for the controlling of liquid helium (LHe) level and pressure for large accelerators is presented. Other possible control loops, e.g. cascade control or multivariable feed-back (or with feed-forward option), are discussed. Activities related to the commissioning of control loops and some supporting measurements are mentioned. Experimental data on investigation of gas/liquid thermal equilibrium are also presented.