Seiichi Takahashi

Parallel electric fields in nonlinear magnetosonic waves

The electric field parallel to the magnetic field, E‖, in nonlinear magnetosonic waves is studied theoretically and numerically. In the calculation of E‖ based on the conventional reductive perturbation method, the terms related to the magnetic pressure cancel, and E‖ is proportional to the electron temperature Te. With a modified perturbation scheme assuming that the wave amplitude is in the range (me∕mi)1∕2<ϵ<1, an expression for E‖ is obtained that is proportional to the magnetic pressure in a cold plasma. Its integral along the magnetic field, F=−∫E‖ds, is proportional to ϵ2mivA2. One-dimensional, fully kinetic, electromagnetic particle simulations verify the theoretical predictions for small-amplitude waves. Further, they demonstrate that eF becomes of the order of ϵ(mivA2+ΓeTe) in large-amplitude [ϵ∼O(1)] oblique shock waves. These theory and simulations indicate that E‖ in magnetosonic waves can be strong in a strong magnetic field.

Parallel electric fields in nonlinear magnetosonic waves in an electron-positron-ion plasma

The electric field E∥ along the magnetic field B in nonlinear magnetosonic waves in a three-component plasma is studied with theory based on a three-fluid model and with fully kinetic, electromagnetic, particle simulations. The theory for small-amplitude (ϵ⪡1) pulses shows that the integral of E∥ along B, F=−∫E∥ds, is proportional to ϵ(pe0−pp0) in warm plasmas, where pe0 and pp0 are, respectively, the electron and positron pressures, and proportional to ϵ2mivA2∕(1+vA2∕c2)3 in cold plasmas, where vA is the Alfven speed. These predictions are verified with simulations. Furthermore, for shock waves with ϵ∼O(1), simulation values are consistent with the phenomenological relation ne0eF∼ϵ(ρvA2+Γepe0)(ni0∕ne0), where ρ is the mass density and Γe is the specific heat ratio. These results indicate that E∥ can be strong in strong magnetic fields.

The effect of parallel electric field in shock waves on the acceleration of relativistic ions, electrons, and positrons

The effect of an electric field E∥ parallel to the magnetic field B on particle acceleration in shock waves is studied. With test particle calculations, for which the electromagnetic fields of shock waves are obtained from one-dimensional, fully kinetic, electromagnetic, particle simulations, the motions of relativistic ions, electrons, and positrons are analyzed. In these simulations, the shock speed vsh is taken to be close to c cos θ, where θ is the angle between the external magnetic field and wave normal, and thus strong particle acceleration takes place. Test particle motions calculated in two different methods are compared: In the first method the total electric field E is used in the equation of motion, while in the second method E∥ is omitted. The comparison confirms that in the acceleration of relativistic ions E∥ is unimportant for high-energy particles. For the acceleration of electrons and positrons, however, E∥ is essential.

A high temperature Tian-Calvet type calorimeter and an analysis of the baseline fluctuation

Abstract A high-temperature Tian-Calvet type calorimeter has been constructed and the baseline fluctuation due to the change in the temperature at the main heater has been analysed. After applying a temperature oscillation at the main heater, temperatures at various positions in the calorimeter were measured as a function of time and were compared with those calculated according to a model. Temperature oscillations at the main heater were found to cause fluctuations of the baseline through deviations from the symmetry between the two calorimetric units, which was also simulated by the model calculation. It is suggested from these results that the presence of an air-containing region in the calorimeter is not desirable, the size of thermal insulators within the main heater should be as large as practical, and the thermal diffusivity of the thermal insulators should be as small as possible so as to decrease the fluctuations of the baseline due to the outer temperature fluctuations; it is also suggested that the baseline could be stabilized by correcting for deviations from symmetry.

Phase transitions in U3O8−z: I, heat capacity measurements

Abstract The heat capacity of U 3 O 8− z with various O/U ratios was measured in the range from 250 to 750 K, and λ-type heat capacity anomalies were found in each sample. The transition temperatures were 487 and 573 K for UO 2.663 , 490 and 576 K for UO 2.656 and 508, 562 and 618 K for UO 2.640 . The entropy changes of the transitions were 0.44 and 0.39 J K −1 mol −1 for UO 2.663 , 0.58 and 0.47 J K −1 mol −1 for UO 2.656 and 0.62, 0.51 and 0.25 J K −1 mol −1 for UO 2.640 , increasing as O/U decreases. The enthalpy change due to the transition varied linearly with the transition temperature except for UO 2.640 , showing the presence of the same mechanism of phase transition among the samples with various O/U ratios. The mechanism of the phase transition was discussed on the assumption that the transition is originated from the order-disorder rearrangement of U 5+ and U 6+ with a consequent displacement of atoms, similarly to the case of U 4 O 9−y .

Simulation studies of positron acceleration in a shock wave in a nonuniform external magnetic field

Positron acceleration in a shock wave in an electron-positron-ion plasma is studied with one-dimensional, fully kinetic, electromagnetic particle simulations, with particular attention paid to the effect of inhomogeneity of external magnetic field B0. First, acceleration to γ ∼ 104, where γ is the Lorentz factor, is demonstrated for a shock wave in a uniform B0 with the shock speed νsh close to c cos θ, where c is the speed of light and θ is the angle between B0 and the wave normal. The acceleration is not saturated till the end of the simulation run. Then, the effect of nonuniformity of B0 is investigated: Comparisons are made between the case in which the difference (νsh − c cos θ) at the shock front changes from negative to positive values as the shock wave propagates and the case with this relation reversed. The latter is found to create a greater number of high-energy particles than the former.