A plasma reactor having a reactor chamber and an electrostatic chuck with a surface for holding a workpiece inside the chamber includes a backside gas pressure source coupled to the electrostatic chuck for applying a thermally conductive gas under a selected pressure into a workpiece-surface interface formed whenever a workpiece is held on the surface and an evaporator inside the electrostatic chuck and a refrigeration loop having an expansion valve for controlling flow of coolant through the evaporator. The reactor further includes a temperature sensor in the electrostatic chuck and a memory storing a schedule of changes in RF power or wafer temperature. The reactor further includes a thermal model capable of simulating heat transfer between the evaporator and the surface based upon measurements from the temperature sensor, and a control processor coupled to the thermal model and to the memory and governing the backside gas pressure source in response to a prediction from the model of a change in the selected pressure that would compensate for the next scheduled change in RF power or implement the next scheduled change in wafer temperature.