摘要:

The 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.

ISSN:0021-8979    

DOI:10.1063/1.345104

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

The 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 flow equations 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.

ISSN:0021-8979    

DOI:10.1063/1.345105

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Laser 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.

ISSN:0021-8979    

DOI:10.1063/1.345106

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

The 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.

ISSN:0021-8979    

DOI:10.1063/1.345107

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

We present an analytic theory for magnetically insulated, multistage acceleration of high‐intensity ion beams, where the diamagnetic effect due to electron flow is important. Our theory is an extension of the single‐stage diode theory developed by Desjarlais [Phys. Rev. Lett.59, 2295 (1987)], based on a self‐consistent calculation of the virtual cathode position, which has been successful in modeling Applied‐Bion diode experiments on several accelerators. The new theory incorporates a finite injection energyqWfor the beam ions. We have found a critical voltageV1(W) that corresponds toV*of the single‐stage theory. As the voltage approachesV1, 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 thanV1, 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 voltageV2(W). At voltages belowV2, there is a maximum steady‐state current that can be transmitted through the gap. The critical voltageV2is smaller thanV1and is unique to the multistage theory. If fluctuations allow electron transport across magnetic field lines so that any virtual anode is neutralized,V2goes 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.

ISSN:0021-8979    

DOI:10.1063/1.345108

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Measurements of charged‐particle concentrations and the electron energy distribution function (EEDF) have been made in Ar and SF6glow 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 SF6discharges. The mean electron energy increases with decreasing pressure and increasing power in argon discharges, but is independent of pressure and power in SF6discharges. The measured distribution functions and charged particle concentrations are in good agreement with calculations.

ISSN:0021-8979    

DOI:10.1063/1.345109

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Emission from atomic copper and CuF is characterized in a pulsed dc discharge in CF4/O2/Ar with a copper electrode. Similar emission is seen from neat Ar discharges, provided that a F‐ (or H2O‐) containing plasma has been operated first. Near the electrode surface 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.

ISSN:0021-8979    

DOI:10.1063/1.345110

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Time evolution of a two‐dimensional ion‐density distribution in a barium plasma 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, plasma diffusion, and ion extraction by the electric field. The flow velocity of the bulk plasma is equal to the mean velocity of the atoms emanating from a hot source. The expansion velocity of the plasma boundary into a vacuum agrees well with the ambipolar expansion velocity 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.

ISSN:0021-8979    

DOI:10.1063/1.345111

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

We 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.

ISSN:0021-8979    

DOI:10.1063/1.345112

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

The 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 particularC33(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.

ISSN:0021-8979    

DOI:10.1063/1.345113

出版商:AIP    

年代:1990

数据来源: AIP