This study was carried out to clarify the difference in the boiling heat transfer modes in He II. Experiments were performed using a cryostat at a wide range of hydrostatic pressures in He II, from atmospheric to saturated vapor pressure. A thin stainless steel foil heater was used to initiate film boiling and simultaneously served as a temperature sensor as well. According to experimental results it was confirmed that the two film boiling modes appear above the lambda pressure: the strongly subcooled film boiling mode and the weakly subcooled film boiling mode. Measurements of the heater temperature variations indicated the difference between the thermo-fluid dynamic boiling features of the weakly subcooled film boiling and that of the noisy film boiling. In case of the weakly subcooled film boiling, the heat transfer coefficients become smaller as pressure increases. However, during the strongly subcooled film boiling, pressure has nearly no affect on heat transfer coefficient. Note that the peak in the heat transfer coefficient appears at a pressure of about 8 kPa, which is in the transition region between the weakly subcooled and the noisy film boiling modes.

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A mechanism producing microwave-induced steps crossing the zero-current axis of I-V characteristics of Bi₂Sr₂CaCu₂O_8+x (BSCCO) intrinsic Josephson Junctions under a microwave irradiation was investigated by a numerical simulation. On the basis of both inductive and charge coupling effects in high-T_c superconductors, the numerical simulation using the coupled sine-Gordon equations for 13 intrinsic Josephson junctions was performed under 5- and 20-GHz microwave irradiation without a dc bias current. The simulation demonstrated an ac Josephson current with a harmonic frequency of the microwave and a dc voltage induced by the current. This implies that the microwave-induced steps are not produced by a flux flow but originate from the inverse ac Josephson effect produced by the current with the harmonic frequency.

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