Large Skin Depth Research Articles

Microwave Inversion of Leaf Area and Inclination Angle Distributions from Backscattered Data

The backscattering coefficient from a slab of thin randomly oriented dielectric discs over a flat lossy ground is used to reconstruct the inclination angle and area distributions of the discs. The discs are employed to model a leafy agricultural crop, such as soybeans, in the L-band microwave region of the spectrum. The distorted Born approximation, along with a thin disc approximation, is used to obtain a relationship between the horizontal-like polarized backscattering coefficient and the joint probability density of disc inclination angle and disc radius. Assuming large skin depth reduces the relationship to a linear Fredholm integral equation of the first kind. Due to the ill-posed nature of this equation, a Phillips-Twomey regularization method with a second difference smoothing condition is used to find the inversion. Results are obtained in the presence of 1 and 10 percent noise for both leaf inclination angle and leaf radius densities.

Kinetics of plasmas and melting induced in silicon and germanium by nanosecond laser pulses

We have performed time-resolved infrared reflectivity measurements at 5.3 and 10.6 \ensuremath{\mu}m to determine the plasma and melting kinetics of intrinsic, crystalline silicon and germanium following excitation by 25-ns 0.53- and 1.06-\ensuremath{\mu}m laser pulses. Below the threshold of melting, the maximum non-equilibrium plasma density is ${10}^{20}$ ${\mathrm{cm}}^{\ensuremath{-}3}$ in both materials for etched samples. For germanium, the results were compared with a numerical modeling of the plasma which employed only known optical and thermal properties. The solid-state plasma kinetics were found to be consistent with conventional models using the usual generation, diffusion, and recombination rates of lower-density plasmas. The only unusual feature is the apparent observation, in some circumstances, of reduced plasma diffusion or possibly confinement in the presence of the strong temperature-induced band-gap gradients. Above the threshold for melting we have taken advantage of the large skin depth (800 \AA{}) at infrared wavelengths to determine melt-front kinetics for depths up to 1000 \AA{}. Results of this contactless technique are in good agreement with earlier measurements of Galvin et al. [Phys. Rev. B. 27, 1079 (1983)] who pioneered the use of a dc conductivity technique in melt-front measurements.

Magnetic guiding, focusing and compression of an intense charge-neutral ion beam

The motion of an intense, charge- and current-neutralized ion beam propagating axially into a solenoidal magnetic field in vacuum is shown to be described by one of two models depending on whether or not the beam magnetic skin depth c/ωpe exceeds half the beam radius. For a less-intense beam having a large skin depth, a new theoretical model is presented which shows that a solenoidal field penetrates to the beam axis and acts as a lens. If a minimum beam density is exceeded, the electrons and ions are brought to a common focus as a result of quasineutrality. The collective focal length is the geometric mean of the focal lengths for the two species when traveling alone. For an intense beam having a small skin depth, the applied field is excluded from the center of the beam channel by a skin current and the resulting magnetic pressure gradient compresses the beam. This case is similar to the theta pinch and is described by a snowplow model. The two models are verified by experiments with a 150 kV, 0.5–5 A/cm2 proton beam from a magnetically insulated diode. Possible applications in magnetic fusion, inertial fusion, and magnetospheric physics are discussed.

Infrared Measurement of Carrier Density, Lattice Temperature and Melt Depth During Nanosecond Pulsed Laser Annealing of Silicon and Germanium

ABSTRACTWe have performed infrared reflectivity measurements using 5.3 and 10.6μm probes to determine the plasma density, lattice temperature and melting kinetics of silicon and germanium following excitation by 25 nanosecond, 0.53 and 1. 06μm pulses. Below the threshold of melting, the maximum plasma densities are approximately 1020 cm−3; for both materials; these values and the spatial and temporal evolution of the plasma are consistent with well known generaticn, diffusion and recombination processes. Above the threshold for melting we have taken advantage of the large skin depth at infrared wavelengths to determine the melt front kinetics for depths up to 1000 Å using a contactless technique.

A boundary integral method for eddy current flow around cracks in thin plates

A boundary element method which employs a Green's function for a crack has been developed to calculate the induced eddy current flow around cracks in thin conducting plates. The theoretical equations employ a stream function for the current density vector and is equivalent to the electric field vector potential method. A low frequency or large skin depth approximation leads to a Poisson equation for steady harmonic inductor fields. Induced currents around a crack in a square plate due to a uniform inductor field for various crack positions and sites have been calculated in this paper. The effect of the relative position and length of the crack, with respect to the plate width, on the eddy current density near the tips of the crack is given special attention. These results may be useful to simulate eddy current flow detection phenomena.

Infrared measurements of induced eddy currents in sheet conductors

An infrared scanning system is used to visualize eddy current distributions in thin sheet-like conductors. Results show that the infrared technique can detect eddy current related temperature changes as small as 0.2°C induced by alternating, pulsed and convective magnetic fields. The effects of magnetic field frequency are examined as well as a comparison of alternating and pulsed field induction of currents. Comparison of infrared measurements with results from the more conventional search coil technique show good agreement. A theoretical model is developed which can predict the induced currents and Joule heating thermal patterns in the large skin depth limit. This theoretical model includes heat conduction effects and shows that isotherm patterns are close to current density patterns when heat conduction can be neglected. Thus the method works well for pulsed current applications. The model is applied to the induction of currents in an electrodynamic levitation problem of a long rectangular coil over a sheet conductor track. The numerical predictions of eddy current thermal patterns show good agreement with the results from the infrared technique. These results show an increase in eddy current density when the lift coil moves close to the edge of the conductor track. Eddy current densities in linear motor secondaries have also been detected using this method. This experimental method should have applications to magnetic levitation and magnetic fusion technologies.

A Quest for a Controllable ULF Wave Source

Natural geomagnetic phenomena indicate that wave generation and amplification processes occur spontaneously in the magnetosphere and may be available under certain conditions to permit the frequency band near 1 Hz to be used for communications. An ionospheric duct provides low-loss propagation for waves in this band to distances of thousands of kilometers, and their large skin depth in seawater permits penetration to several hundred meters below the ocean surface. Several methods by which the generation of waves in this band might be possible are described. Methods involving the modulation of ionospheric conductivity include the injection of plasma clouds into the ionosphere and heating of the lower E region with powerful radio transmitters. Methods involving nonlinear processes in the magnetosphere include cold plasma injection and very low frequency (VLF)/ultralow frequency (ULF) three-wave interactions.

Measurement of Volume and Resistivity Changes in High-Pressure Experiments by an A-C Inductive Technique

A coil wound on a sample core subjected to high pressures may be used to measure compressibility and electrical resistivity of the core. In a first approximation, the coil reactance reflects core volume, and the resistance reflects core resistivity. The exact impedance of the coil in the general case depends on the ratio of skin depth to sample size. With large skin depths, the sensitivity to volume changes is high, and with small skin depths, sensitivity to resistivity is high. Operation in these two cases was demonstrated for four materials of widely different hardness: barium, bismuth, titanium, and tungsten. In addition, operation was demonstrated at high temperatures by detection of the alteration of pyrophyllite to coesite plus kyanite. Permeability as a function of pressure was shown for an iron-nickel alloy. The chief advantage of the coil method is the remote, nonmechanical measurement of volume changes. In addition, it offers greater versatility to favor many different types of sample material.

High Pressure Polymorphic Transitions Studied by Audio- and Radio-Frequency Techniques

Volume and resistivity changes associated with polymorphic transitions in solids under pressure have been studied by an ac inductive method. A small coil is wound over a core of the sample, and the electrical impedance of the coil determined as a function of pressure to 65 kbar. The coil method was demonstrated in three electrically different operating regions: large skin depth, small skin depth, and an intermediate case. With large skin depth, the inductance of the coil reflects the volume of the sample core. With small skin depth, the reactance and resistance of the coil represent a mixture of volume and resistivity information, but the data may be easily analyzed to yield resistivity alone. In the intermediate case, volume and resistivity are separately displayed by the inductance and resistance, respectively. The coil method offers the advantage that volumetric transitions may be studied remotely, with no moving mechanical indicators. The sensitivity to volume change was demonstrated to be at least 0.1%.