Journal of Applied Physics

When a plasma is of finite transverse cross section, space-charge waves may propagate even in the absence of a drift motion or thermal velocities of the plasma. Some of the properties of these space charge waves have been investigated by regarding the plasma as a dielectric and solving the resulting field equations. The effect of a steady axial magnetic field is considered, but motion of heavy ions and electron temperature effects are neglected. Waves are found to exist at frequencies low compared with the plasma frequency as well as waves with oppositely directed phase and group velocities (backward waves).

Many of the features of these waves have been verified experimentally by measuring phase velocity and attenuation of waves along the positive column of a low pressure mercury arc in an axial magnetic field. Measurements of electron density have been made using these waves and the results are compared with those obtained by other methods. An interesting feature of these measurements, of value in plasma diagnostics, is that they can be made with frequencies which are small compared with the plasma frequency.

Thermohydraulic explosion, caused by direct contact of hot liquids with cold water, represent a major danger of volcanism and in technical processes. Based on experimental observations and nonequilibrium thermodynamics we propose a model of heat transfer from the hot liquid to the water during the thermohydraulic fragmentation process. The model was validated using the experimentally observed thermal energy release. From a database of more than 1000 experimental runs, conducted during the last 20 years, a standardized entrapment experiment was defined, where a conversion of 1 MJ∕kg of thermal energy to kinetic energy within 700μs is observed. The results of the model calculations are in good agreement with this value. Furthermore, the model was found to be robust with respect to the material properties of the hot melt, which also is observed in experiments using different melt compositions. As the model parameters can be easily obtained from size and shape properties of the products of thermohydraulic explosions and from material properties of the hot melt, we believe that this method will not only allow a better analysis of volcanic eruptions or technical accidents, but also significantly improve the quality of hazard assessment and mitigation.

Isotropic nanocrystalline exchange coupled FeNdB magnets with enhanced remanence were produced using the melt spinning procedure. Starting from nearly single phase Fe14Nd2B magnets, the content of additional α-Fe in composite magnets was stepwise increased up to 40 vol % by reducing the Nd and B content. The maximum remanence of JR=1.25 T was found in composite magnets containing 30 vol % α-Fe, the maximum energy product reaches the value (BH)max=185 kJ/m3, whereas the coercive field is μ0HC=0.53 T for this composition. The microstructural investigations of the composite magnets reveal two characteristic maxima in the grain size distribution at about 15 and 25 nm corresponding to the α-Fe phase and to the Fe14Nd2B phase, respectively. The influence of the exchange coupling on the coercive field can be described by a microstructural parameter αex. From the temperature dependence of the coercive field the microstructural parameters αKαex and Neff are determined.

It was recently noted that the dynamic domain structure in a ferromagnetic material may differ markedly from that observed under static conditions. In this investigation the dynamic size of magnetic domains in a rectangular single-crystal specimen of an iron-3% silicon alloy (2.5 cm×3.3 cm×0.22 mm with a (001) [100] orientation) was measured using the Kerr magneto-optic effect together with high-speed cinematography. Under dynamic conditions of a 100 Hz sinusoidal induction of saturation amplitude, the average domain width was reduced from the average static width of 2½ times the sheet thickness to about ½ this value. The finer dynamic domain structure could be ``frozen in'' by turning off the applied field during the middle of the magnetization cycle, but the original coarser structure always returned following an ac demagnetization. Measurements in the frequency range 20–100 Hz showed that above a threshold frequency domain width decreased with frequency as (f)−1/2. With a varying amplitude of induction the domain width decreased as (Bmax)−1, above a threshold induction.