摘要:

The expansion of a laser‐produced Be plasma is studied by comparing hydrodynamical model calculations with experimentally measured distributions of each charge state as a function of energy. The plasma is generated with 1–10‐J 200‐ps pulses from the LPARL Nd‐glass laser facility, and the distributions are obtained using a Thomson parabolic spectrograph. The hydrodynamic equations include flux‐limited thermal conductivity and collisional‐radiative recombination and ionization. For the proper choice of the initial parameters, we obtain qualitative agreement between the model prediction and the experimentally measured distributions.

ISSN:0021-8979    

DOI:10.1063/1.325539

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

The generation of pulsed high‐energy‐density plasmas by electromagnetic implosion of cylindrical foils (i.e., imploding liners or hollowZpinches) has been investigated experimentally and theoretically at the Air Force Weapons Laboratory. The experimental studies involve discharging a 1.3‐&mgr;sec 1.1‐MJ capacitor bank through 7‐cm‐radius 2‐cm‐tall 3–30‐mg cylindrical foil liners. Typical discharge parameters are 7–12‐MA peak current and 1–1.5‐&mgr;sec current rise time. Current and voltage waveforms indicate strong coupling of the load to the capacitor bank, and analysis of the waveforms indicates good implosion of the current sheath. Optical‐ and magnetic‐probe measurements are consistent with 1–2‐cm thickness of the imploding plasma shell and with final implosion velocities ∼15–20 cm/sec. Radiation‐diagnostic measurements indicate ultrasoft x‐ray yields ∼50–100 kJ with the FWHM of the photon pulse ∼80–100 nsec. The radiation data is consistent with a quasiblackbody spectrum (T∼30–50 eV) comprising most of the energy, with additional higher temperature and optically thin spectral components. Al11+and Al12+line and recombination radiation is frequently observed. Comparison of electrical, magnetic, and radiation data with one‐dimensional MHD and two‐dimensional MHD calculations is presented. The prospects for improving the performance with the present energy source and scaling to larger energy sources are briefly discussed.

ISSN:0021-8979    

DOI:10.1063/1.325540

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

The primary‐electron bias voltage and the neutral‐pressure dependence of plasma density in a multidipole device are investigated. Experimental data are described by a simple model which takes into account the production of multiple electron‐ion pairs by each ionizing primary electron.

ISSN:0021-8979    

DOI:10.1063/1.325541

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

In the design of the neutral‐beam injectors for large tokamaks such as JT‐60 planned in Japan Atomic Energy Research Institute, the optimum configuration of extraction grids and the operating conditions were studied numerically for a two‐stage acceleration in a high‐power ion source. Results of the calculations show the feasibility of extracting a proton beam with energy 75 keV and current 15 A of about 0.6° divergence from the 12‐cm‐diam extraction grids of transparency 40&percent;. Beam divergence in the two‐stage configuration depends largely on the field intensity ratio; the optimum ratio that gives a minimum beam divergence is about 0.3–0.5.

ISSN:0021-8979    

DOI:10.1063/1.325542

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

The present analysis gives the influence of the Hall currents on the growth of a boundary layer in a parallel‐plate channel in the presence of a transverse traveling magnetic field. The analysis is made by using a perturbation technique in terms of the iteraction parameter. The boundary layer thickness as well as the skin‐friction coefficient decreases due to the Hall currents.

ISSN:0021-8979    

DOI:10.1063/1.325543

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

The propagation of an axisymmetric laser beam in a plasma column having a radially parabolic electron density distribution is examined. First, an extended paraxial procedure is developed for the case of an axially uniform waveguide. It is shown that the essential feature of an alternate focusing and defocusing beam is retained, but that the intensity distribution is cumulatively modified at the foci and at the outer portions of the beam as compared to that of the paraxial case. Second, some general features of paraxial beam propagation are examined for the case of axially varying waveguides. Finally, laser plasma coupling is examined for the case when laser heating generates a density distribution that is radially parabolic near the axis and when the energy absorbed over a focal length of a plasma lens is small. It is shown that stable or unstable beam propagation depends upon the relative magnitude of the density fluctuations which exist in the axial variation of the waveguides as a result of laser heating. When the fluctuations are small, the propagation is stable, and a simple algebraic expression is obtained which relates the beam diameter to the axially slow averaged variation in the waveguide. When the fluctuations are large, the propagation stability can be determined only by consistently combining plasma dynamics and beam propagation to interrelate the axial variation of the beam to that of the waveguide. In this case of beam propagation in a time‐varying waveguide, it is shown that the global stability of the propagation depends upon the initial fluctuation growth rate compared to the initial time rate of change in the radial curvature of the waveguide.

ISSN:0021-8979    

DOI:10.1063/1.325544

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

Cylindrical crystals are generally found in biological structures where the structural units are packed in two‐dimensional lattices. Dislocations are believed to be responsible for the movement and shape changes of cylindrical crystals in many biological systems. We investigated the dislocation interactions in thin cylindrical crystals. The elastic field of a dislocation dipole with Burgers vectors normal to the cylinder axis is obtained by employing a first‐order shallow‐shell theory. The variation of climb force as a function of dislocation separation distance is expressed in a closed form. The significance of the present finding as related to the study of contractile mechanisms in biological systems is also discussed.

ISSN:0021-8979    

DOI:10.1063/1.325545

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

Polycrystalline x‐ray diffraction data are reported for three high‐pressure phases of InSb. InSb‐II patterns are indexed on the basis of a body‐centered tetragonal lattice with unit cell parametersa=5.810 A˚ andc=3.136 A˚ atP∼4.3 GPa andT≳100 °C; a unique space group has not been assigned. Possible space groups and structures have been determined for the other high‐pressure phases. Contrary to published results, InSb‐III is orthorhombic witha=5.712 A˚,b=5.357 A˚,c=3.063 A˚ at 11.5 GPa and room temperature, the space group is eitherPmmmorPmmn, andZ=2. InSb‐IV is also orthorhombic witha=2.919 A˚,b=5.618 A˚, andc=3.066 A˚ atP=4.4 GPa and room temperature; the space group isPmm2, andZ=1. Atomic coordinates and appropriate bond angles are also reported.

ISSN:0021-8979    

DOI:10.1063/1.325546

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

Electron mobility, minority‐carrier lifetime, and misfit‐dislocation density are measured along the layer thickness direction near the interfacial regions of GaAs crystals grown on GaAs and Ge substrates by vapor phase epitaxy. It is observed that the mobility and lifetime decrease, while the dislocation density increases exponentially, as the layer thickness is decreased. These phenomena are found to be consistently caused by out‐diffusion of interstitial Ga vacancies introduced at the epitaxial layer/substrate interface in the stage prior to growth or in the first stage of growth. The diffusion coefficient of the interstitial Ga vacancy (off‐centered Ga atom) is estimated to be 3.6×10−5 exp(−11800/T) cm2/sec based on the experimental results.

ISSN:0021-8979    

DOI:10.1063/1.325547

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

Radiation damage in neutron‐transmutation‐doped (NTD) silicon, irradiated to introduce 5×1013to 6×1016phosphorus cm−3, has been studied by electrical property measurements. The experimental results indicate that thermal‐neutron‐induced (n,&ggr;) recoil‐type damage can be annealed at 400 °C. The nature of any remaining lattice defects and their annealing behavior above 400 °C is a function of the fast‐neutron fluence. Small defect clusters are present in Si irradiated with a light‐to‐moderate fast‐neutron fluence (?5×1018n cm−2), and temperature‐dependent Hall coefficient measurements indicate that at least two deep acceptor levels and one deep donor level are formed during annealing. One of these acceptor levels anneals at ∼450 °C, and the other two levels anneal at ∼550 °C. A shallow acceptor level near the valence band that anneals at 750 °C is also observed. Larger defect clusters which reduce the electron mobility tremendously and distort the band structure are formed in heavily irradiated Si (5×1018to 1020n cm−2). Virtually all of the electrically detectable radiation damage in NTD Si irradiated with a fast‐neutron fluence up to 1020n cm−2can be removed by annealing at 750 °C for 1/2 h. There is some indication that a minority‐carrier‐recombination effect remains even after such annealing.

ISSN:0021-8979    

DOI:10.1063/1.325548

出版商:AIP    

年代:1978

数据来源: AIP