Experimental Studies on Low-Pressure Plane-Parallel Hollow Cathode Discharges

The behavior of a fluidized bed of fine magnetite particles as affected by a cross-flow magnetic field is investigated. A distinct feature of this naturally cohesive powder, as compared to noncohesive magnetic grains usually employed in magnetofluidized beds, is that the fluidized bed displays a range of stable fluidization even in the absence of an external magnetic field. Upon application of the magnetic field, the interval of stable fluidization is extended to higher gas velocities and bed expansion is enhanced. We have measured the tensile strength as affected by application of the external magnetic field according to two different operation modes. In the H off-on operation mode, the bed is driven to bubbling in the absence of external magnetic field. Once the gas velocity is decreased below the bubbling onset and the bed has returned to stable fluidization due to natural cohesive forces, the field is applied. In the H on-on mode, the field is maintained during the whole process of bubbling and return to stable fluidization. It is found that the tensile strength of the naturally stabilized bed is not essentially changed by application of the field ( H off-on) since the magnetic field cannot alter the bed structure once the particles are jammed in the stable fluidization state. Magnetic forces within the bulk of the jammed bed are partially canceled as a result of the anisotropic nature of the dipole-dipole interaction between the particles, which gives rise to just a small increment of the tensile strength. On the other hand, when the field is held on during bubbling and transition to stable fluidization ( H on-on mode), the tensile strength is appreciably increased. This suggests the formation of particle chains when the particles are not constrained due to the dipole-dipole attractive interaction which affects the mechanical strength of the stably fluidized bed. Experimental data are analyzed in the light of theoretical models on magnetic surface stresses.

In this work, a theoretical model for the cathode sheath using experimental data of a low pressure hollow cathode discharge (HCD) is derived considering it to be non-collisional. Secondary electrons emitted from the cathode surface are taken into account on the discharge model and their influence on the theoretical sheath potential profile is investigated. The plasma parameters and the floating potential profile along the discharge axis were inferred from the current-voltage characteristics of a single Langmuir probe positioned at the inter-cathode space of the HCD. For a low pressure HCD, typical values of the electron density and electron temperature are ne ≈ 1016 m−3 and Te = 4 eV, respectively. By using the probe data the floating potential profile was determined to verify the position of the plasma-cathode sheath interface and to promote a qualitative discussion between theoretical and experimental results.

Magneto-active elastomers (MAE) are a new branch of smart materials that consist of hard-magnetic particles such as barium ferrite in an elastomer matrix. Under the application of a uniform magnetic field, the MAE material undergoes large deformation as the material bends due to magnetic torques acting on the distribution of hard-magnetic particles. This behavior demonstrates the potential of MAEs to act as remote actuators. MAEs vary from magnetorheological elastomers (MRE) which use soft-magnetic iron particles in place of the hard-magnetic particles and they are driven by magnetic interactions between particles. In this work, MAEs were fabricated using 30% v/v 325 mesh M-type barium ferrite (BaM) particles in Dow Corning HS II silicone elastomers. Prior to curing, the samples were placed in a uniform (∼2 Tesla) magnetic field to align the magnetic particles and produce a magnetization oriented in the direction of the applied magnetic field. The specimens were bonded to a passive poldymethylsiloxane (PDMS) substrate to form a two-segment accordion structure where the MAEs with magnetization, M, were placed in opposing orientations a prescribed distance apart. The application of a uniform magnetic field perpendicular to the magnetization of the undeformed MAEs would result in a bend (on the PDMS) that is dependent upon the orientation of the magnetic particles and the direction of the applied field. This behavior of the composite structure highlights the ability of the MAE material to perform work. Experimental testing of the MAEs used a two-segment accordion structure with fixed boundary-conditions on both ends of the PDMS substrate and a uniform magnetic field was applied to the structure. The resulting deformation roughly represented either a mountain or valley fold (dependent upon the orientation of the applied field). The resulting axial force was observed and compared to computational simulations which utilized numerical techniques to develop approximate solutions. This procedure was repeated with a prescribed displacement on one of the two fixed boundary conditions to induce bending prior to the application of a uniform magnetic field. Results show a decrease in magnetic work potential with increases in the aforementioned prescribed displacement; results also show an increase in magnetic work potential with increases in the applied magnetic field.

The application of magnetic field at low pressure for optimal laser-induced plasma spectroscopy

The structural changes in liquid crystal (LC) polymer vesicles triggered by changes in temperature and the application of a magnetic field were investigated by SANS, cryo-TEM, SEM and high sensitivity DSC. PEG-b-PA444 and PEG-b-PMAazo444, two block copolymers with side-on nematic hydrophobic blocks, were used to form vesicles with a radius of 450–500 nm and bilayer membrane thickness of 10–15 nm at room temperature. Upon heating the membrane thickness, d, started to increase dramatically from a temperature (∼55 °C) above Tg but below TNI of the LC polymer block, and reached up to 120 nm at T > TNI. The vesicles transformed into thick-walled capsules. The thickness of the membrane was inconsistent with a bilayer structure and surprisingly the capsules were stable even for temperatures above TNI. As the PEG chains should partially dehydrate with temperature, we propose that the membrane reorganized into a structure consisting of microphase separated LC and PEG domains. Analysis of changes in structural parameters such as the internal aqueous volume and the polymer membrane volume suggest that capsule scission and fusion also occurred during this transition. The polymersomes were also sensitive to applied magnetic fields and for some temperatures the application of a strong magnetic field (1.4 T) substantially increased the membrane thickness due to the induced mesogen alignment.

The Origin of Bulk Magnetoresistivity in Manganites

We report synchrotron x-ray powder diffraction measurements of Ce${}_{0.8}$La${}_{0.1}$Th${}_{0.1}$ subject to pulsed magnetic fields as high as 28 T. This alloy is known to exhibit a volume collapse upon cooling at ambient pressure, which is a modification of the $\ensuremath{\gamma}$-$\ensuremath{\alpha}$ transition in elemental cerium. Recently, it has been suggested on the basis of field-cooled resistivity and pulsed-field magnetization measurements that the volume collapse in this alloy can be suppressed by the application of magnetic fields. Conversely, our direct diffraction measurements show a robust collapsed phase, which persists in magnetic fields as high as 28 T. We also observe nanoscale disorder in the collapsed phase, which increasingly contaminates the high-temperature phase upon thermal cycling.

YbCu5−xAlx provides the possibility to tune ground state properties by a change of the valence due to the Cu/Al substitution, by pressure as well as by the application of a magnetic field. Near to the critical concentration xcr≈1.5 non-Fermi-liquid properties (NFL) are obvious, obeying hyperscaling. If magnetic order sets in for x>1.5, the application of moderate magnetic fields quenches order and again NFL features become evident. Hyperscaling in this case indicates strongly interacting spin fluctuations.

Moll and Ornstein and later Miesowicz and Jezewski observed that temperature changes were induced in nematic mesophases in the vicinity of solid interfaces by application or removal of magnetic fields. Such phenomena have been considered to be entropy effects associated with the ordering produced by the interface and the magnetic field. In the present work, these experiments have been conducted with two materials, p-azoxyanisole and p-n-decyloxy benzoic acid, but measurements of sample temperature were extended to much larger times and in larger volume that is relatively free of interfaces than those used by the earlier workers. It was found that the temperature rise upon application of a magnetic field was permanent rather than transient; the sample returned to the surroundings temperature only after the field was removed. Thus, a magnetic field causes a continuous generation of heat in a nematic mesophase, and this cannot be attributed simply to ordering effects.

Magnetic nanoparticles have been widely used in nanobiomedicine for diagnostics and the treatment of diseases, and as carriers for various drugs. The unique magnetic properties of “magnetic” drugs allow their delivery in a targeted tumor or tissue upon application of a magnetic field. The approach of combining magnetic drug targeting and gene delivery is called magnetofection, and it is very promising. This method is simple and efficient for the delivery of genetic material to cells using magnetic nanoparticles controlled by an external magnetic field. However, magnetofection in vivo has been studied insufficiently both for local and systemic routes of magnetic vector injection, and the relevant data available in the literature are often merely descriptive and contradictory. In this review, we collected and systematized the data on the efficiency of the local injections of magnetic nanoparticles that carry genetic information upon application of external magnetic fields. We also investigated the efficiency of magnetofection in vivo, depending on the structure and coverage of magnetic vectors. The perspectives of the development of the method were also considered.

Ferromagnetic Fe or Co was electrodeposited into the pores in aluminum anodic oxide film to produce a magnetic anodic film, and the effects of magnetic field during electrodeposition on the deposition characteristics of the metals and coercivity of the produced anodic film were investigated. Fe was electrodeposited into the pores in anodic film using a mixed solution (pH=3.8) of FeSO4(NH4)SO4.6H2O (0.20 mol/l) and H3BO4 (0.48 mol/l) at 20 °C with constant applied voltage of 10 V, and Co using a mixed solution (pH=4.5) of CoSO4,7H2O (0.10 mol/l) and H3BO4 (0.48 mol/l). It has been found that for the Fe-deposited anodic film the coercivity increased with increasing length of deposited Fe particles and decreased with increasing diameter (or porosity). The application of magnetic field during electrodeposition of Fe had little influence on the coercivity of the anodic film. For the Co-deposited anodic film the coercivity increased with increasing length of deposited Fe particles and increased unexpectedly with increasing diameter. The perpendicular coercivity of the Co-deposited anodic film was significantly increased by the application of magnetic field during the electrodeposition. The unexpected coercivity increase with diameter and the increase of the perpendicular coercivity of Co-deposited film by the application of magnetic field were closely related to the preferred orientation of Co crystals in the anodic film.

Model of microsecond pulsed glow discharge in hollow cathode for mass spectrometry

Insights into the effect of hollow cathode with external injection of fast ions in a sputter-type negative ion source

XPERIMENTAL studies were made on the properties of the plasma stream emanating from a pulsed MPD arcjet, expecially with respect to the quasi-steady operation.14 From diagnostic measurements, a large fraction of the discharge current was found to extend in the plasma downstream of the electrodes. It has been questioned whether this extending current is desirable in the operation of an MPD arcjet. The purpose of the present work is to answer this question with respect to the establishment of the quasi-steady operation. The quasi-steadiness of the operation should be inferred from the properties of the plasma stream as well as the current and voltage of the arc discharge. Contents The pulsed MPD arcjet used in the present experiment is similar to that described in Ref. 1. The maximum discharge current is 12.5 ka and the maximum power is about 2 Mw. The discharge current and voltage are steady and last for more than 800 jusec. An annular coil is mounted concentric to the arcjet system with its center plane at the front surface of the anode. The maximum strength of the magnetic field is 20 kgauss at the center of the coil. The working gases are hydrogen, helium and argon. The mass-flow rate m is variable—for instance, from 50 mg/sec to 300 mg/sec for hydrogen. The MPD arcjet is installed on an end flange of a vacuum tank which is 1.5 m in diameter and 2.8 m in length. The metal surface inside the tank is completely covered with Mylar sheet in order to prevent the discharge current from flowing in the tank wall. The tank pressure before each run is about 10 ~5 torr. The working gas injection phenomena was studied to determine the trigger time of the arc discharge so that the amount of the neutral gas injected before triggering the arc discharge could be minimized, and also to find how a quasi-steady injection of the working gas is established in the discharge region. To this end, the pressure was measured in the jet of neutral gas by a newly developed fast ionization gauge.5 From the gauge signal, the gas pressure was found to rise in about 600 jusec after opening the gas ports and reach a constant value. A steady state lasts for 3 ~ 5 msec. Figure 1 demonstrates the time variation of the pressure distribution along the center line in the jet of neutral argon. The origin of time in this figure is taken arbitrarily and the normal trigger time is 1 msec in this case. As the time increases, the experimental curve tends to follow a Z~2 law. The coordinate Z is taken along the center line and measured from the anode surface. The pressure dependence on Z~2 law is predicted from the spherically expanding steady flow at frozen

This article investigates the impact of a sudden application or sudden withdrawal of a magnetic field on an unsteady MHD Couette flow formation in a parallel plate channel. The governing momentum equation is derived and solved exactly in Laplace domain using the Laplace transform technique with the necessary initial and boundary conditions to capture the present physical situation for the cases; sudden application or sudden withdrawal of a magnetic field. Due to the complexity of the solution obtained, the Riemann-sum approximation technique is used to transform the Laplace domain to time domain. During the course of graphical and tabular representations, results show that the Hartmann number, time and nature of application of a magnetic field play an important role in the transition from hydrodynamic to magnetohydrodynamic flow and vice-versa. Also, fluid velocity steady-state solution is independent on whether the magnetic field is fixed relative to the moving plate or to the fluid for sudden withdrawal of magnetic field. In addition, the application of a sudden magnetic field leads to a delay in the attainment of steady-state solution.