摘要:

Tomographic reconstructions of x‐ray emission from hot electrons have been carried out in the thermal barrier region of the GAMMA 10 plasmas. Here, the first application of two sets of the 50‐channel microchannel plates have been performed using their detailed calibration data as a function of x‐ray energy and incident angle [Rev. Sci. Instrum.59, 256 (1988);59, 2453 (1988);60, 368 (1989)]. The x‐ray reconstructed signals under standard thermal barrier operating conditions indicate a good axisymmetric radial profile peaked on the magnetic axis. This symmetric profile is important for preventing the formation of a local anisotropic electric field, which may cause particle confinement degradation, and its peaking profile is desirable for thermal barrier potential formation in the core plasma region. When we have relocated the second‐harmonic electron cyclotron layers (&ohgr;=2&OHgr;e) in two different ways (moving them out radially, or axially away from the midplane), tomographic reconstructions show hollow x‐ray profiles in both cases. These may be formed due to theE×Brotations of the hot electrons produced near &ohgr;=2&OHgr;e. These applications of x‐ray tomography to two‐dimensional radial profile observations along with the axial x‐ray profile measurement give information that the production mechanism of these hot electrons is ascribed to second‐harmonic electron cyclotron heating.

ISSN:0021-8979    

DOI:10.1063/1.345639

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

The electron flow in a planar‐anode diode having an extended anode‐cathode gap operating on the HERMES III accelerator is characterized and compared with predictions of a computational model. The model combines a particle‐in‐cell code with Monte Carlo radiation transport. The comparisons confirm the model and show that the diode provides both a matched load and a versatile large‐area source of &ggr; rays for the study of nuclear radiation effects. Electrical and spatial parameters of the beam at the diode and the downstream radiation fields from a graphite target are presented as a function of the anode‐cathode gap.

ISSN:0021-8979    

DOI:10.1063/1.345640

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

A three‐dimensional theoretical model based on first principles has been developed to predict the characteristics of mass, momentum, energy, and electrostatic potential transport in high‐pressure mercury arcs confined in a quartz arctube. The model is utilized to systematically investigate the impact of convection on the transport process by including and excluding the gravity effect. Strong three‐dimensional convection flows with multiple contrarotating vortices have been identified. These vortices substantially change the energy balance within the arc, causing highly nonuniform gas temperature distribution and lowering the maximum gas temperature. Geometrical modifications of the arctube such as wall contour curvature and electrode offsets do not change the strength of convection but can produce better overall temperature uniformity within the arctube by accommodating the upward‐moving tendency of temperature contours caused by convection. In agreement with the experimental measurement, the model predicts that the arctube curvature can cause large differences of wall temperature profiles, including shape, level, and locations of peak values.

ISSN:0021-8979    

DOI:10.1063/1.346097

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

The velocity of cathode spots with nonretrograde motion in a closed magnetic field has been found to have a component normal to the direction ofj×B, and the trajectory of the cathode spots in this field configuration is not stable, in contrast to the trajectory for retrograde motion. This component of the spot motion can be understood, as has been predicted, by consideration of the Lorentz force acting on a positive space charge, which has been proposed to be associated with cathode spots. The behavior of the motion normal toj×Bof cathode spots with retrograde velocities can be explained by the same model. Further results presented are consistent with a positive space charge also being associated with anode spots.

ISSN:0021-8979    

DOI:10.1063/1.345641

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Two energies are identified that define the x‐ray emission characteristics ofZ‐pinch array implosions. One, the kinetic energy per ion, is intensive, and the other, the kinetic energy per centimeter, is extensive. From a series of one‐dimensional axisymmetric hydrodynamic calculations, we have calculated the dependence of the x‐ray emission from aluminum implosions above 1 keV on these energies. These calculations are carried out for a specially chosen theoretical case where the kinetic energy that is generated during implosion is converted to thermal energy and x rays during the plasma collision on axis in the absence of current. In this case, we determine theI4toI2transition of the scaling of emission with peak current,I, as a parametric function of the kinetic energy per ion. We also determine a functional dependence of the emission on this energy when the mass of the imploded aluminum array is held fixed. It is seen that the ability of the plasma to radiate large amounts of energy in eitherI4orI2regimes is strongly dependent on the mass loading. Finally, some arguments are presented on how the breakpoint betweenI4andI2scaling is expected to scale when the atomic number of the array load is varied.

ISSN:0021-8979    

DOI:10.1063/1.345642

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Amorphous poly‐ether‐ether‐ketone (PEEK) films have been implanted with a variety of ions (He, N, F, As, Xe and I) in the energy range 50 keV to 32 MeV. At the lower end of this range, the dependence of the electrical conductivity of the PEEK on the dose and ion species has been explained in terms of a simple model of electronic and nuclear excitation effects. Implantations in the MeV energy range yielded a surface layer on the PEEK with a high conductivity [up to 2.5 (&OHgr; cm)−1] and a moderate hardness (320 knoop, 1‐g load). Evidence for diffusion of iodine implanted at the highest energy has been found. The role of the uniform iodine concentration throughout the implanted layer in the prevalent conduction mechanism is not known at present.

ISSN:0021-8979    

DOI:10.1063/1.345622

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Molecular‐beam epitaxy was used to grow vertical silicon structures with extremely sharp doping transitions. Hereby a series of modulation‐doped multilayer structures was prepared which had successively reduced period lengths from 100 down to 4 nm. These test samples are appropriate to study the depth resolution of standard electrical and chemical profiling techniques. Secondary ion mass spectrometry is able to resolve doping modulations down to 4 nm. Electrical measurement techniques are fundamentally limited by the out‐diffusion of carriers from the highly to the lowly doped layers. Such effects become of increased importance in small period superlattices. Spreading resistance is only able to resolve the doping modulations of the test sample with the largest period length (100 nm).

ISSN:0021-8979    

DOI:10.1063/1.345623

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

We have studied the sequence of phase transformations induced in the Zr‐Al binary system by mechanical alloying of mixed Zr and Al powders. The structure of these materials has been studied by transmission electron microscopy and by x‐ray diffraction measurements. Three different metastable phases have been found experimentally with variation of the initial compositionxAl: (1) a nanocrystalline supersaturated solid solution of &agr;‐Zr forxAl≤0.15, (2) an amorphous phase for 0.15<xAl≤0.4, and (3) a metastable face‐centered‐cubic phase forxAl=0.5 with a grain size of 4 nm. The crystallization reaction of the amorphous phase was monitored by differential scanning calorimetry, and the kinetics of the reaction have been examined as well. A possible explanation based on thermodynamic arguments is given for the defect‐driven vitrification of the crystalline Zr phase.

ISSN:0021-8979    

DOI:10.1063/1.345624

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Scanning tunneling microscopy has been used to investigate changes in a vacuum‐annealed gold surface induced by the application of voltage pulses to the tip. After a 2‐V 1.5‐s pulse a hole several hundreds of angstroms wide and of similar depth was generated, the responsibility being ascribed to ion arcing. We present a time‐lapse sequence showing its decay process, from which surface‐diffusion velocities between 4 and 0.1 A˚ s−1have been determined. From the new surface, a scan showing atomic resolution has been obtained, giving different interatomic spacings (S) and atomic diameters (D) in two lattice directions at an angle of 60°. These areS=2.1 A˚ andD=2.0 A˚ for the short axis andS=3.1 and 3.8 A˚ andD=2.7 A˚ for the long axis.

ISSN:0021-8979    

DOI:10.1063/1.345598

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Amorphous C‐Ge multilayers have been produced with periods varying from 2.6 to 12.5 nm. We have studied the multilayers using Raman spectroscopy and dc conductivity measurements and found that their properties change significantly when the carbon sublayer thickness is smaller than 2.5 nm. This value corresponds to the proposed dimension ofsp2bonded carbon islands withina‐C and we find that our results can be understood in terms of the carbon forming these islands, but the islands becoming disconnected as the nominal carbon layer thickness decreases. The conductivity shows a percolation behavior for the thinnest layers.

ISSN:0021-8979    

DOI:10.1063/1.345599

出版商:AIP    

年代:1990

数据来源: AIP