Influence of the Steady Background Turbulence Level on Second Sound Dynamics in He II

We report measurements on the dependence of the second sound breakpoint energy on the steady background turbulence in He II at 1.7 K. We have determined the breakpoint energy for two pulsed heat flux values (100 and 200 kW/m2) at different steady background levels (up to 2.6 kW/m2). The experiment consists of a 175 mm long channel having a cross section of 242 mm2, which is equipped with a thin film Ni-Cr heater at its bottom end and two superconducting filament thermometers in the path of the heat pulses, respectively 25.4 and 127 mm above the heater. The heater both generates a steady heat flux producing background turbulence in the liquid column and transmits finite amplitude heat pulses by superposing them on the background heat flux. The second sound pulse travels through the He II in the channel and is detected by the thermometers. The breakpoint energy is determined by analyzing the raw pulse signals recorded. We can subsequently obtain the dependence of the breakpoint energy on the background heat flux. This determines the energy limit that second sound pulses can carry under different background turbulence conditions