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In the previous experiment by Li et al., [Phys. Plasmas 19, 113511 (2012)], a deep virtual cathode was measured within an ion sheath near a negatively biased stainless steel plate immersed in an ion-beam-plasma system. The appearance of a virtual cathode was attributed to secondary electrons produced by the high speed ion beam instead of the plasma electrons since these electrons are depleted in the sheath. This paper presents a theoretical model of the sheath structure in the ion-beam-plasma system near an electron-emitting wall. The results show that the presence of the ion beam will compress the whole sheath and make it more difficult to form the virtual cathode, i.e., it causes the increase in the threshold density of the emitted electrons at the wall needed to form the virtual cathode. When comparing with the previous experimental results, it is found that the needed secondary electron yield is unrealistically high in order to obtain the experimentally measured depth of the virtual cathode. Possible experimental uncertainties are discussed concerning the use of an emissive probe when it is too close to the wall.

In inhomogeneous dusty plasmas, the ion–dust collisions are nonuniform because of the dependence of the collision rates on plasma densities and the grain charge. The propagation of dust–ion–acoustic solitons in such dusty plasmas is studied using the reductive perturbation method. It is shown that either compressive or rarefactive solitons can exist depending on whether the ion-to-electron density ratio is greater or less than a critical value. The interplay between the effects of the plasma inhomogeneity and the spatially varying collision rates can result in the amplitude distribution from decrease to increase when the soliton propagates from high to low density regions. Different from the homogeneous case, the velocity and width of the soliton increase with distance instead of following the changes of the amplitude.

In a magnetized plasma sheath, strong velocity shear exists owing to the three-dimensional nature of ion velocity. Thus, the ion viscosity should have an important effect on the sheath structure, which has not been studied. This article presents the study of the effect of ion shear viscosity on the sheath in an oblique magnetic field within the framework of classical cross-field transport. It is shown that the inclusion of the shear viscosity in the ion momentum equation results in a significant reduction in the sheath thickness. It is also shown that the “generalized Bohm criterion” is not affected by the shear viscosity within the present model. However, additional boundary conditions such as the velocity shear arise in the viscous case. The appropriate boundary conditions are formulated, accounting for E × B and diamagnetic drifts at the sheath edge, which affects the criterion and sheath profiles.

We investigated the effect of thermal relaxation on reversible transverse susceptibility (RTS), χ, of single domain Stoner–Wohlfarth particles. We studied such an effect as a function of the orientation β of the easy axis with respect to the drive field (the z axis) within a non‐interacting bi‐state model. At β=00 one prominent cusp occurs for both χt1 and χt2 along the direction perpendicular to the drive field at H≂HC

Field angles and sweep rate dependence of coercivity of the Stoner–Wohlfarth (S‐W) particles are studied based on the decay‐rate model. An analytic expression of the hysteresis loops based on the model is obtained as a function of applied field for various frequencies at various field angles with respect to the easy axis. Variation of the coercivities versus the sweep rate (measurement frequency), field angles, anisotropy constant, activation volume, and the combination thereof, and the distribution of easy axis, etc., have been obtained. Comparison with the thermal‐equilibrium and S‐W hysteresis loops are made. Coercivities of such a system generally increase with increasing sweep rate, anisotropy constant and activation volume, etc., hence decreases with increasing sweeping period. As a function of the field angles, the coercivity curve turns flat with increasing field angles and sweeping period.

Blocking temperature, energy barrier, and reversal field variation of interacting fine magnetic particles display similar characteristics as a function of the bonding angle and interparticle distance in the context of the dipole interaction. The energy barrier and reversal field exhibit local maxima at the bonding angles β=0° and π/2 and a global minimum at β≃60°. Thus, the sample average of these quantities is found to reduce with decreasing interparticle distance, in agreement with the latest experimental data. For a system with easy-axis misorientation, magnetization reversal will traverse along the direction in which the misorientation angle and bonding angle have the same sign.

Energy barrier height and coercivity variation of a pair of interacting particles has been studied as a function of the bonding angle β for 0°⩽β⩽π/2 and applied field using the fixed step Monte Carlo simulation approach supplemented by the perturbation method. Coercivity Hc(β) of the system has local maxima at β=0 and π/2, and a global minimum at β≃65°, and remains depressed compared to the noninteracting case for β⩽30° at any temperature. The barrier height behaves similarly. Due to the random nature of distribution dipole interaction, on average, leads naturally to a reduction in coercivity and energy barrier height in comparison with the noninteracting case.

The response of a system to an infinitesimal perturbation becomes singular if the system changes its degree of metastability, i.e., if it has an inflection point. This theory is used to calculate the reversible transverse susceptibility χt of a single domain particle executing a major hysteresis loop under the influence of a field H(t) applied at an angle β to the easy axis. Thermal relaxation is taken into account and the response of the nonequilibrium system to the infinitesimal field h(t)=h sin ωt, h⊥H, is found. The resultant χt is studied as a function of β, temperature, and of the frequency Ω of the biasing field H.

The characteristics of a weakly collisional sheath in a warm electronegative plasma in the presence of an oblique magnetic field are investigated using a fluid model including the effects of ionization and ion-neutral collisions. The general sheath criterion imposed on the entrance velocity component of the positive ions perpendicular to the wall at the sheath-presheath edge is derived and discussed. It is shown that the boundary conditions are crucial to the sheath structure. Without including the entrance velocity components parallel to the wall, a pulse-like structure in the positive-ion density distribution near the sheath-presheath edge appears if the magnetic field is strong. With inclusion of all velocity components at the edge, the pulse-like structure disappears, resulting in a smooth sheath profile. It is also found that increasing the temperature and decreasing the concentration of the negative ions will increase the sheath thickness, and increasing the magnetic field will decrease the sheath thickness.

The potential profiles near a negatively biased stainless steel plate or mesh immersed in an unmagnetized weakly collisional plasma were investigated experimentally. The results show that deep virtual cathode structures arise inside the sheath region in front of the plate caused by secondary electrons produced by streaming ions hitting the surface of the plate. The characteristics of the virtual cathode depend on the incoming ion energy, neutral gas pressure, and bulk plasma density. In addition, the replacement of the plate with the mesh results in the suppression of the virtual cathode because of the equivalent reduction in the secondary electron yield.

The measured potential profiles of unmagnetized plasma sheath near a stainless steel plate exhibit deep virtual cathode structures caused by secondary electrons produced by high-speed ions hitting the surface of the plate. The depth and thickness of the virtual cathode depend on the ion streaming energy and gas pressure. The experimental results are in agreement with numerical calculations.

The diffusion velocity of an inhomogeneous unmagnetized plasma is measured by means of the phase velocities of ion-acoustic waves propagating along and against the direction of the plasma flow. Combined with the measurement of the plasma density distributions by usual Langmuir probes, the method is applied to measure the ambipolar diffusion coefficient and effective ion collision frequency in inhomogeneous plasmas formed in an asymmetrically discharged double-plasma device. Experimental results show that the measured flow velocities, diffusion coefficients, and effective collision frequencies are in agreement with ion-neutral collision dominated diffusion theory.