Index of content:
Volume 67, Issue 11, 01 June 1990

Methods of calculating the Barkas‐effect correction to Bethe–Bloch stopping power
View Description Hide DescriptionThe low‐velocity Barkas‐effect correction term for the Bethe–Bloch stopping‐power formula has been calculated with each of the three extant formalisms for targets with atomic numbers from 1 to 18 and for projectile velocities from 0.025c to 0.3c. Comparison of each with measurements on aluminum revealed which formalism provided the best fit over three different projectile velocity intervals. The method yielding the generally superior agreement was used to calculate the stopping power of each element (except Li, F, Na,Mg, P, and Cl) for protons and alpha particles with energies between 0.5 and 5.0 MeV. Results corresponded very closely to those of an existing fit to most known experimental data.

Recombination and dissociation of H_{2} ^{+} and H_{3} ^{+} ions on surfaces to form H_{2}(v‘): Negative‐ion formation on low‐work‐function surfaces
View Description Hide DescriptionThe recombination and dissociation of H^{+} _{2} and H^{+} _{3} ions incident upon metal surfaces leads to H, H_{2}(v‘), and H^{−} products rebounding from the surface. A four‐step model for H^{+} _{2} ‐ion recombination generates H_{2}(v‘) via resonantelectron capture through the b ^{3}Σ^{+} _{ u } and X ^{1}Σ^{+} _{ g } states. A molecular trajectory analysis provides final‐state H_{2}(v‘) distributions for incident energies of 1, 4, 10, and 20 eV. The calculated H_{2}:H^{+} _{2} yields compare favorably with the observed yields. A similar four‐step model for incident H^{+} _{3} proceeds via resonant capture to form the H_{3}(2p ^{2} E’→2p ^{2} A _{1}) ground state, in turn dissociating into H+H_{2}(v_‘), with the fragment molecule rebounding to give the final H_{2}(v‘) distribution. Comparing the final populations v‘≥5 for incident H^{+} _{2} or H^{+} _{3} shows that the H^{+} _{3} ion will be more useful than H^{+} _{2} for H^{−} generation via dissociative attachment. Molecular ions incident upon low‐work‐function surfaces generate additional H_{2}(v‘) via resonantelectron capture through excited electronic states and provide two additional sources of H^{−} production: Direct H^{−} production by H dissociation products rebounding from the surface and H^{−} production through the formation of H^{−} _{2} in the surface selvage that in turn dissociates into H+H^{−}. The H^{−} _{2} in the selvage is formed by resonant capture to the low‐lying vibrational levels of H_{2}(v‘), and complements dissociative attachment to high‐lying levels in the discharge. The H, H_{2}(v‘), and H^{−} yields are inventoried for H^{+} _{3} incident upon barium surfaces.

Electromagnetic radiation from doubly rotated piezoelectric crystal plates vibrating at thickness frequencies
View Description Hide DescriptionExact solution of the three‐dimensional linear equations of piezoelectromagnetism is obtained for doubly rotated piezoelectric crystal plates surrounded by vacuum and excited by face traction. A generalized Poynting’s theorem is derived for general media in which electromagnetic and mechanical fields interact with each other. For linear piezoelectric crystals it is shown that the generalized theorem may still be interpreted as an energy theorem, and hence densities of energy stored in the electric, magnetic, and elastic strain fields can be identified. Radiated power, per unit surface area and averaged over the period, and induced strain and electric fields in the middle plane of the plate are calculated for doubly rotated quartz plates whose cut orientations follow the upper and lower loci of zeros of the first‐order temperature coefficient of frequency of the x _{1} ‐thickness‐shear mode. Quality factors and partition of stored energies are also examined.

On the magnetic (electric) field associated with semi‐infinite slabs in front of conducting or insulating planes
View Description Hide DescriptionSystems consisting of two‐dimensional semi‐infinite slabs in front of conducting or insulating planes are examined. The slabs which are symmetric may either be thick or thin and of rather general shape. Some of these systems may be identified with magnetic recording head systems, while others are complementary to systems examined by Sproston [IEE Proc. 1 2 9, 619 (1982)] and Gibbings [J. Electrostat. 6, 121 (1979)]. Four examples are treated giving the behavior of a dimensionless intensity over their geometrical surfaces. On the conducting plane of certain types of systems, it is seen that the intensity may be approximated quite well by that due to an ideal rectangular slab and also that it decreases along the plane faster than that due to the ideal slab.

Nematic liquid crystal clad tapered optical fiber with temperature sensing properties
View Description Hide DescriptionWe discuss the thermal and optical properties of the lyotropic liquid crystal decylammonium chloride/water/NH_{4}Cl (DACl‐LLC) from −40 to 80 °C. Combined use of DACl‐LLC and highly tapered monomode optical fibers allows sensing of temperature (from room temperature up to 65 °C).

Thermal modeling of laser‐addressed liquid‐crystal displays
View Description Hide DescriptionOptical‐absorption calculations and finite‐element methods are used to calculate time‐dependent temperature profiles in two contrasting laser‐addressed liquid‐crystal displays. It is shown that the presence of conductingelectrode layers has a significant effect on the temperature profiles both by affecting the optical‐absorption characteristics of the cell and the resulting thermal conductivity. It is shown that efficient optical absorption does not necessarily result in the best cell‐addressing performance.

Temperature dependence of the harmonic distortion in InGaAsP distributed feedback lasers
View Description Hide DescriptionMeasurements are reported of the second‐harmonic distortion in 1.3‐μm InGaAsP distributed feedback lasers operating at different temperatures. A minimum of the second‐harmonic distortion exists at a particular bias current when the laser is operating at low temperature (10 °C). The minimum shifts to a higher bias current when the operating temperature is raised. At an even higher temperature, the minimum dissappears. A temperature‐dependent model of the second‐harmonic distortion, based on an equivalent circuit analysis, is presented to explain the experimental results.

Intensity dependent photorefractive properties of BaTiO_{3}
View Description Hide DescriptionWe have used the light‐induced grating erasure technique to measure the photorefractive properties of pure and irondoped BaTiO_{3}. Our experiments were performed using an anisotropic configuration which forbids beam coupling and self‐diffraction between the writing beams, resulting in plane parallel intensity fringes that do not change with time. This allowed direct measurement of the charge transport processes without any feedback on the grating due to wave‐mixing processes. The results from these experiments show that the photorefractive parameters known as the trap density N _{pr} and the mobility‐recombination time product μτ_{ r }, vary with intensity and are not material constants.

Transverse effects and enhanced dispersive optical bistability in a nonlinear Fabry–Pérot filled with a self‐focusing medium
View Description Hide DescriptionThis paper is concerned with transverse effects arising from diffraction in a nonlinear Fabry–Pérot filled with a self‐focusing medium. It is shown that the resulting bistable loop is ‘‘enhanced’’ with respect to the plane‐wave one. This result contrasts with the self‐defocusing case where optical bistability disappears when decreasing the width of the input beam.

Novel types of surface acoustic wave microreflectors: Performance analysis and simulations
View Description Hide DescriptionSurface acoustic waves for micrograting reflectors have been characterized. Based on the perturbation theory, eight different types of structures on an acoustic waveguide were analyzed. Results of simulations of all eight types of corrugation structures were evaluated in order to find the least leaky waveguide, the most efficient reflector (with minimum necessary perturbations), and the optimal mode shape for improved performances. General design curves are presented in order to illustrate the behavior of the incident and reflected waves under a variety of structural conditions. Analytic expressions for the calculations of the mode amplitude and mode shape, and for general acoustic corrugations are derived and then the simulations results are presented.

A perturbation approach to mixed boundary‐value spherical problems
View Description Hide DescriptionThe solution of a special form of dual series equations involving Legendre functions is presented in the framework of the perturbation method. This leads to a series whose structure is much simpler than that of the alternative integral form solution, and numerically much more practical. Several examples of physical problems in the areas of electromagnetics and hydrodynamics, which are represented by these types of dual equations, are discussed.

Potential flow past an open spherical cavity
View Description Hide DescriptionThe potential flow of an ideal fluid in the presence of an open spherical cavity, with a circular opening and enclosing a concentric sphere, is analyzed in the framework of the theory of dual series equations. It is shown that the flowequations can be reduced to a pair of dual series equations whose exact solution is presented in the form of a perturbation series. The solution inherently contains the correct behavior near the edge of the aperture, i.e., that required by Meixner’s conditions. Various properties of the flow, in particular its separation at the solid boundaries, are discussed and illustrated by numerical results.

Melt‐front velocity in laser‐induced melting
View Description Hide DescriptionLaser melting processes have been studied extensively, particularly for semiconductor substrates. Values for the velocity of the melt front have been determined by several experimental methods, and also calculated in numerical simulations of the melting processes. The velocity of the melt both during melting and recrystallization is of direct consequences for the material properties of the laser treated zone. Hence, a clear understanding of the physical parameters involved is essential. For laser pulses of a Gaussian shape, and whose duration is longer than a few tens of nanoseconds, expressions are derived for the melt‐front velocity for the general case and for the limiting case of a point source. In either case the velocity turns out to be nonconstant. Hence, experimentally reported values may only be regarded as indicative of the maximum velocity achievable. The simple closed‐form analytical expressions obtained in the present study are amenable for a direct analysis of relevant experimental results. Comparison made with some available data reveals a general agreement between theory and experiment. Ultra high‐speed photography is one possible technique that may enable observation of the varying velocity of the lateral melt front.

On the effect of asymmetries on the jet from a linear shaped charge
View Description Hide DescriptionThe jet from a linear‐shaped charge is formed by the collapse of the wedge‐shaped liner brought about by the passage of the detonation wave through the surrounding explosive. There is a variety of possible sources of asymmetry which can affect this process. The thickness distributions along each face of the liner may vary. The detonation wave may traverse the liner asymmetrically. The quantity or other properties of the explosive adjoining the liner arms may differ. In such situations the velocities imparted to opposite liner elements at equal distances from the intersection of the arms will in general be different. Thus a pair of liner elements which do meet will in general have started from different distances down their respective liner arms, and will not meet on the axis of symmetry of the charge. This effect causes the process of formation of the jet at the meeting point to occur asymmetrically. This paper presents an investigation of this phenomenon. We generalize the classical analysis of the symmetric steady‐state collapse of a liner to include the asymmetries we have described. We present an extension of the classical jet formation theory to the asymmetric case by making several simple physical assumptions. We consider an example of a typical asymmetry in liner thickness and recover values for the off‐axis velocity of the jet of the order observed in experiments.

Theory of multistage intense ion‐beam acceleration
View Description Hide DescriptionWe present an analytic theory for magnetically insulated, multistage acceleration of high‐intensity ion beams, where the diamagneticeffect due to electron flow is important. Our theory is an extension of the single‐stage diode theory developed by Desjarlais [Phys. Rev. Lett. 5 9, 2295 (1987)], based on a self‐consistent calculation of the virtual cathode position, which has been successful in modeling Applied‐B ion diode experiments on several accelerators. The new theory incorporates a finite injection energy q W for the beam ions. We have found a critical voltage V _{1}(W) that corresponds to V _{*} of the single‐stage theory. As the voltage approaches V _{1}, unlimited beam‐current density can penetrate the gap without the formation of a virtual anode because the dynamic gap goes to zero. At voltages lower than V _{1}, a sufficiently large injection current will cause the formation of a virtual anode in response to the large beam space charge. Furthermore, we have found that unlimited beam current can penetrate an accelerating gap operated above a second critical voltage V _{2}(W). At voltages below V _{2}, there is a maximum steady‐state current that can be transmitted through the gap. The critical voltage V _{2} is smaller than V _{1} and is unique to the multistage theory. If fluctuations allow electron transport across magnetic field lines so that any virtual anode is neutralized, V _{2} goes to zero for all beam injection energies. This effect can be used to test the importance of field fluctuations on the electron dynamics in magnetically insulated ion acceleration gaps.

A tuned Langmuir probe for measurements in rf glow discharges
View Description Hide DescriptionMeasurements of charged‐particle concentrations and the electron energy distribution function(EEDF) have been made in Ar and SF_{6}glow discharges using a tuned Langmuir probe technique. A simple passive circuit connected to the probe when properly tuned increases the impedance between the probe and ground, thereby forcing the probe to follow the instantaneous plasma potential. In this manner, rf‐induced distortion of the probe characteristic is mitigated. At 13.56 MHz the electron collection characteristic of a detuned probe is distorted by rf interference; the ion collection characteristic is unaffected. The EEDF is highly non‐Maxwellian in argon discharges, but quite Maxwellian in SF_{6} discharges. The mean electron energy increases with decreasing pressure and increasing power in argon discharges, but is independent of pressure and power in SF_{6} discharges. The measureddistribution functions and charged particle concentrations are in good agreement with calculations.

Copper emissions from pulsed discharges in CF_{4}/O_{2} and Ar
View Description Hide DescriptionEmission from atomic copper and CuF is characterized in a pulsed dc discharge in CF_{4}/O_{2}/Ar with a copperelectrode. Similar emission is seen from neat Ar discharges, provided that a F‐ (or H_{2}O‐) containing plasma has been operated first. Near the electrodesurface emissions from CuF and lower‐energy states of Cu are observed. In the bulk of the plasma, additional emission is seen from more energetic states of Cu at the end of the pulse. Lifetime and energy considerations suggest that much of the Cu emission seen in Ar plasmas may arise from collisions between argon metastables and some molecular species, such as CuF.

Ion extraction characteristics by an electric field on laser‐produced barium plasma
View Description Hide DescriptionTime evolution of a two‐dimensional ion‐density distribution in a bariumplasma produced by simultaneously illuminating a tuned dye laser light and an excimer laser light has been measured by means of laser‐induced fluorescence technique to investigate the ion extraction characteristics by an electric field. When the plasma is produced between two parallel plates with a potential difference, the ion behavior is governed by three factors: bulk plasma flow,plasmadiffusion, and ion extraction by the electric field. The flowvelocity of the bulk plasma is equal to the mean velocity of the atoms emanating from a hot source. The expansionvelocity of the plasma boundary into a vacuum agrees well with the ambipolar expansionvelocity corresponding to that of an ion acoustic wave, while the diffusion coefficient obtained from the density decay rate is smaller by a factor of about 8 than the ambipolar expansion coefficient for the steady state because of low collision frequency between ions. Enhancement of the ion extraction rate by the electric field is supplied by retrograde motion of the plasma edge which contributes to reduction of the ion extraction time.

A laser‐driven electron injector for laser acceleration experiments
View Description Hide DescriptionWe report a novel technique for producing a source of high‐energy (0.1–1.0 MeV) electrons for studies of laser acceleration concepts. We have designed, built, and implemented a laser‐driven electron injector that is simple and compact and can be optically synchronized with a short‐pulse laser beam.

Anomalous elastic behavior in superlattices of twist grain boundaries in silicon
View Description Hide DescriptionThe elastic constants and moduli of superlattices of high‐angle twist grain boundaries on the two densest crystallographic planes of silicon are calculated using Stillinger and Weber’s three‐body potential. While in both cases the Young’s and shear moduli are found to be softened, the Poisson ratios and some elastic constants, in particular C _{3} _{3} (in the direction of the interface‐plane normal), are found to be hardened. It is shown that the elastic behavior is determined by the structural disorder at the interfaces, and that it cannot be understood in terms of the dimensional changes of the system alone. A comparison with similar calculations for metallic superlattices elucidates the role of the covalent nature of bonding of silicon on its elastic behavior.