摘要:

We propose a new type of inverse problem, in which the spatial distribution of the relative value of a physical parameter can be determined only from the distribution of another physical variable in the region of interest if there is no source in that region. The inverse problem proposed here has two features different from conventional remote probing type problems. One is that the physical variable data are given throughout the region of interest although the number of data variables is insufficient to determine directly the physical parameter of interest, while only remote data are given in a conventional problem. The other is the mathematical structure of the inverse problem: this new inverse problem yields a spatial differential equation on the target parameter, while the conventional problem becomes an integral equation on the target parameter. To show the nature of the problem, we formulated an illustrative inverse problem on the steady‐state electric current field, in which the spatial distribution of the relative conductivity can be determined from the distribution of either the potential or the current density. We give examples of computational results obtained when using the simulated potential data and the current density data. This proposed formalism for a new type of inverse problem could provide a new approach and open up new measurement schemes in many fields of science and engineering. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.362764

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

We have performed an experiment to optically cool and compress a thermal beam of sodium atoms by employing a two‐dimensional version of a magneto‐optical trap. The beam compressor employs a tapered transverse magnetic field gradient formed by four pairs of permanent magnets. We measure the efficiency of collimation and compression as function of longitudinal velocity group. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.362774

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The backscattering of an electromagnetic missile from a perfectly conducting curved obstacle is investigated. The obstacle is assumed to have zero curvature just at the point of reflection of the incident pulse. The asymptotic dependence of the backscattered energy is sought, as the distance separating the obstacle from the source of the incident pulse tends to infinity. The backscattered energy is found to depend on the rate at which the energy spectral density of the current pulse at the source decays with increasing frequency, as well as on the degree of flatness of the obstacle at the point of reflection. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.362769

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

We present an analysis of the steady state optical response of two‐dimensional optical superlattices with Kerr nonlinearity. We consider the effects of the modification of the refractive index modulation strengths in the superlattices and the angular deviation of incoming radiation from Bragg conditions. We find that under Bragg incidence the system displays optical bistability only when the modulation ratio exceeds a certain threshold. We also predict that optical bistability can disappear under non‐Bragg incidence. Physical considerations are presented and discussed on how to optimize the chances for observing optical bistability in such a superlattice. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.362782

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Methods are developed to estimate the adhesion and surface free energies of compliant materials from the contact deformations of cylindrical lenses with flat sheets. Some important differences are found between the cylindrical contact studied here and the widely studied geometry of spherical contact. For example, while the pull‐off force is completely independent of the elastic constants (K) of the materials for spherical contacts, the pull‐off force for cylindrical contact is proportional toK1/3. Furthermore, for cylindrical contacts the contact width at separation reaches to a value of 39% of the width (a0) at zero load, whereas the corresponding value is 0.63a0for spherical contact. The feasibility of using cylindrical contacts to estimate the surface and adhesive energies of polymers was investigated using elastomeric polydimethylsiloxane (PDMS) as a model system. PDMS was used in two ways: (1) unmodified and (2) with its surface hydrolyzed with dilute hydrochloric acid. Significant hysteresis of adhesion was observed with the hydrolyzed PDMS surfaces due to H‐bonding interactions, which appeared to depend on normal stress. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.362819

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

This work studies dynamics of melting in current‐carrying conductors. It is shown that during equilibrium melting, when the dynamics of a phase‐transition front are determined by a heat balance, there exists a single‐valued correlation between the rate of inductance change and a temperature at the phase‐transition front. It is demonstrated that, although surface melting begins at temperatures less than the melting temperature of the current‐free conductor, corrugation of a phase‐transition front occurs at this temperature. It is shown that the nucleation rate of nuclei flattened in the direction normal to a conductor’s axis is negligibly small. Current dynamics at the stage of melting are analyzed for the case of a fixed external voltage. It is demonstrated that there exists an instability stage at which an excitation of the electric current occurs. Conditions for the experimental observation of the predicted phenomena are discussed. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.362793

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Surface‐wave‐sustained discharges (SWDs) form a particular class of high frequency (HF) discharges: their HF sustaining field is provided by a traveling wave that transfers energy as it propagates along the discharge column, yielding a plasma column with an axially decreasing electron density. SWDs have proved to be ideal for investigating experimentally and theoretically both the HF field and discharge aspects of HF plasma sources at reduced gas pressure. In this article, SWDs are utilized at atmospheric pressure to gain insight into the departure from thermodynamic equilibrium (TE) of HF sustained discharges. This departure is found to increase significantly as the electron density decreases along the plasma column whereas the gas temperature and the power absorbed per electron do not vary axially. The two‐temperature plasma model provides an adequate description of this departure from TE. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.362748

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

An energy‐resolving quadrupole mass spectrometer (E‐QMS) was assembled underneath the powered electrode of a diode reactive ion etcher. The plasma ions reach the E‐QMS through an orifice in the powered electrode with a diameter of 100 &mgr;m. The ion energy distributions (IEDs) of ionic species from SF6plasmas in the pressure range of 0.1–1.5 Pa for dc bias potentials between 50 and 300 V and a rf of 13.56 MHz were investigated. The IEDs always show a saddle shaped peak at an energy corresponding to a total potential drop across the sheath given byUSh=Udc+UP, whereUdcis the dc bias potential andUPis the time averaged plasma potential. In the energy range from 0 eV toeUdcthere are multiple peaks in the IEDs of SF+x(x=1..5), whereas the F+, F+2, and S+IEDs show only a single peak in this range. These peaks are the result of ion generating collisions in the sheath. On pressure variation the IEDs do not change significantly. We also measure IEDs of negative ions. The IEDs of these ions show a broad distribution with an intensity maximum appearing at the half ofUPand a width corresponding to max[UP(t)], whereUP(t) is the time varying plasma potential. These correlations suggest that these ions originate from the plasma bulk. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.362761

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Sheath expansion into a cylindrical bore is studied for radii larger than the ion‐matrix overlap radius in the context of plasma‐based ion implantation. The temporal dependence of the sheath width, sheath speed, and ion impact energy and the dependence of the maximum and average ion impact energy on the radius of the bore are computed. It is found that the sheath speed increases as the radii decreases, and that it is, in all cases, greater than the planar sheath speed. After the sheath has reached the axis, the potential drop across the bore decays rapidly. Consequently, the ion impact energy first increases and then decreases in time. Both the maximum and average impact energy increase with radius. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.362771

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

We report morphological investigations of polypyrrole thin films deposited by means of a self‐aligning vapor phase technique onto glass, silicon and silicon dioxide substrates, coated with an oxidizing precursor. The variation of the deposition parameters allows the control of the film microstructure which can be fibrillar and strongly anisotropic or globular and tendentially isotropic. Patterning of the precursor by electron‐beam lithography allows the production of submicron wide lines as shown by both the scanning electron microscope and the atomic force microscope. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.362772

出版商:AIP    

年代:1996

数据来源: AIP