The application of mid-ranging control to improve thermal disturbance rejection for cryogenic THD/DT layering at the National Ignition Facility

This paper presents a cryogenic temperature control technique that demonstrated good disturbance rejection at the National Ignition Facility. Temperature excursions must be minimized to maintain DT fuel layer symmetry needed for ignition. The control scheme known as mid-ranging control effectively used two manipulated inputs and one output, which differs from traditional single-input-multiple-output control schemes like cascade control. Each input had different power constraints and substantially different dynamic effects on the output. One input acted as a bulk heat source whereas the other acted as a trimming heat source. During an upset, the controller manipulated both input variables simultaneously to maintain the desired temperature. The mid-ranging controller was tuned using an extension of the well-known Ziegler-Nichols (ZN) method. The derived SIMC tuning rules were not as attractive since, to be applied, they required prior knowledge of the target's thermal dynamics. The resulting control scheme rejected large and sudden increases in thermal loading quicker than a more conventional scheme. The technique and tuning equations may be applied to similar cryogenic control problems.