摘要:

Shock‐heated plasmas were produced by focusing 1‐ns 1.06‐&mgr;m laser pulses to intensities of 2×1012W/cm2on 100‐&mgr;m‐diam lithium wires. Interferograms and shadowgrams produced with 1‐ns 0.53‐&mgr;m pulses indicate two distinct phases in the plasma expansion. The first phase is a spherical core of dense plasma produced by a shock propagating through the wire, normal to the axis of the wire, and along the laser beam. Interferometric measurements of the expansion and drift velocity of this phase indicate that the initial pressure behind the shock was 1.4–1.9 Mbar, the compression was 2.6–4.2, the shock velocity was (2.1–2.2) ×106cm/s, and the fluid velocity was (1.3–1.6) ×106cm/s. The second phase is an approximately cylindrical column of partially ionized material produced by shocks propagating along the wire. Shadowgrams show that the velocities of these shocks were greater than 1.7×106cm/s.

ISSN:0021-8979    

DOI:10.1063/1.324331

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

Values of the ionization (&agr;/N) and attachment (&eegr;/N) coefficients have been determined from spatial current growth measurements in uniform fieldsEfor a CO2 : N2 : He (1 : 2 : 3) mixture and for pure CO2over the ranges 4.66×10−16⩽E/N⩽8.48×10−16V cm2and 7.63×10−16⩽E/N⩽9.89×10−16V cm2, respectively, whereNis the gas density. The limiting values ofE/N, at which &agr;=&eegr;, are found to be (4.66±0.14) ×10−16V cm2and (8.20±0.16) ×10−16V cm2for the CO2 : N2 : He mixture and for CO2, respectively. The present values of the coefficients are in reasonably good agreement with those calculated from a solution of the Boltzmann equation using electron‐collision cross sections. Nevertheless, the present results indicate that even closer agreement would be obtained if the attachment cross section used in the calculations is reduced by about 15&percent;.

ISSN:0021-8979    

DOI:10.1063/1.324332

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

The energy carried by particles and low‐energy x rays resulting from irradiation of laser fusion targets with 0.5–1‐TW 1.06‐&mgr;m lasers has been measured. The energy distributions were obtained from measurements at discrete locations using plasma calorimeters. The data have been integrated to obtain the absorbed energy, and these values compared to scattered‐light calorimetry and optical‐energy balance.

ISSN:0021-8979    

DOI:10.1063/1.324334

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

The radiation and reception characteristics of the oblate spheroidal antenna for electron plasma waves are theoretically investigated. The analysis is carried out as a boundary‐value problem. The formulas for the radiation and reception characteristics such as radiation impedance, electron charge distributions, radiated wave potential, directional properties, and receiving voltage of the oblate spheroidal antenna are analytically obtained. As a result, it is concluded that the radiation and reception characteristics of the antennas are not uniquely determined bykpa(kpis the wave number of an electron plasma wave, andais the radius of the circular‐plate antenna), but are determined by two out of three factors,kpa, &zgr; (radius divided by Debye length), and &ohgr;/&ohgr;p(angular signal frequency to angular plasma frequency). This conclusion is in marked contrast to the conventional theory in which the charge distribution on the antenna is assumedapriorias uniform and, thus, the antenna characteristics are uniquely determined bykpa. It is claimed that the experimental results obtained hitherto support the present new theory.

ISSN:0021-8979    

DOI:10.1063/1.324311

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

An electrical discharge between two ring electrodes embedded in the mantle of a cylindrical chamber is considered, in which the plasma in the anode and cathode regions rotates in opposite directions under the influence of an external axial magnetic field. The associated boundary‐value problem for the coupled partial differential equations describing the azimuthal velocity and radial current‐density fields is solved in closed form. The velocity, current density, induced magnetic induction, and electric fields are presented for typical Hartmann numbers, magnetic Reynolds numbers, and geometry parameters. The discharge is shown to produce anodic and cathodic plasma sections rotating at speeds of the order 106cm/sec for conventional magnetic field intensities. Possible application of the magnetoactive discharge as a plasma centrifuge for isotope separation is discussed.

ISSN:0021-8979    

DOI:10.1063/1.324312

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

Conservation of energy for the temperature field is solved jointly with conservation of charge for the electrostatic potential for an arc in which the effects of convection and radiation are neglected. The electrical conductivity is modeled by a hyperbolic tangent function. Prolate spheroidal coordinates are used to emphasize the effects of electrode shape. Electrodes are chosen as hyperboloids of revolution (varying from needlelike to planar) and the bounding wall is a prolate spheroid. Approximate solutions are obtained by Galerkin’s method. Current‐voltage characteristics, arc shapes, and heat transfer to the electrodes and bounding wall are obtained. We find that current‐voltage characteristics are sensitive to electrode geometry and temperature. Blunt and hot electrodes yield monotonically increasing characteristics. Cooling the electrodes and increasing their tip curvature both act to increase the voltage to drive a given current, especially for small currents, giving characteristics with distinct maxima and minima. The blunter the cathode, the larger the potential difference to drive a given current. The results also indicate that equipotentials are very close to coordinate lines, justifying an assumption of a one‐dimensional electric field made elsewhere.

ISSN:0021-8979    

DOI:10.1063/1.324313

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

The two‐dimensional variable‐property arc has been studied through the use of the tanh‐conductivity model. Results that describe the thermal and electric arc characteristics for various values of the electrode temperatures and aspect ratios are given. The numerical evaluation is carried out by the use of a Galerkin technique. The results exhibit several novel and interesting features depending on the arc parameters. For large aspect ratios (ratio of the interelectrode distance to that between the bounding walls) and small electrode temperatures, the current–electric‐field characteristics tend toward those of a slender arc. However, at a given aspect ratio with large enough electrode temperatures, the distinct minimum noted in the slender‐arc characteristics does not occur. Also, for a given aspect ratio and large enough differences in electrode potential, the electric‐field–current characteristic is nearly linear and is independent of the electrode temperature. The transverse electrostatic potential is found to have no significant variation in cross‐sectional planes. The qualitative nature of the thermal characteristics are similar to those of a constant‐property arc although significant differences in quantitative results exist. Wall and electrode heat transfer rates are provided.

ISSN:0021-8979    

DOI:10.1063/1.324314

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

Coupling of thermal vibrations was introduced into the calculation of the reduced intensity function for two model amorphous structures by using coupling coefficients which are a function of interatomic distance. A microcrystallite model of amorphous silicon and an ion cluster model of amorphous calcium phosphate were examined. Since coupling factors appropriate to these materials were not available, data from several different materials was used to estimate the importance of such coupling. Splitting of certain peaks in the diffraction pattern at high angles was found to be reduced, giving better agreement between calculated patterns and measurements on actual materials. These results suggest that coupled thermal vibrations and other random perturbations which affect large interatomic distances more than smaller ones may have a significant affect on the diffraction pattern.

ISSN:0021-8979    

DOI:10.1063/1.324315

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

The (110) and (111) planes of monocrystalline nickel targets have been bombarded with 500‐keV4He+ions under channeling conditions for a total dose of 6×1018ions/cm2and for target temperatures ranging from room temperature to 950 °C. The results indicate a decrease in average blister diameter with increase in target temperature. Large‐scale exfoliation of blister skin occurs at 300 °C, and, above 850 °C, holes are observed on the surface.

ISSN:0021-8979    

DOI:10.1063/1.324317

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

The effect of Te doping on the growth of Al0.36Ga0.64As films has been studied. Variables investigated were Te concentration, growth rate, growth temperature, and heat treatment of the resultant films. Structural defects occur in Te‐doped films grown from 772 °C with a free‐carrier concentration as low asn=4×1017/cm3. A free‐carrier plateau is reached atn?2×1018/cm3. Surprisingly, electrically active Te is incorporated in Al0.36Ga0.64As at three distinct distribution coefficients. Focusing on the same variables in the temperature regime from 772 to 836 °C, transmission electron microscopy shows that high Te doping causes very high concentrations (109/cm2) of small dislocation loops. These loops occur both over a wide range of growth solution doping at growth rates of 12 and 120 A˚/sec. After heat treatment of these films at 820, 860, and 920 °C, substantially larger loops are observed. These loops are found to be extrinsic. In addition, the larger of these loops have small particles attached to them. The nature of the attached particles could not be determined. During heat treatment at 930 °C, dislocation loops could not be nucleated. Since Al2Te3melts at 895 °C, it is assumed that below 895 °C Al2Te3is the principal nucleating agent for the observed loops.

ISSN:0021-8979    

DOI:10.1063/1.324318

出版商:AIP    

年代:1978

数据来源: AIP