Helium II thermal counterflow at large heat currents

Data for the temperature and pressure differences produced by the thermal counterflow of helium II in long capillaries are given and shown to define a region of nonlinear flow distinct from that described by Vinen. We suggest a simple modification of the Vinen theory associated with a secondary flow in the normal fluid and in good agreement with our data. The critical heat current for the onset of the secondary flow is shown to define a stability region in analogy to second-order phase transitions.