摘要:

The properties of a coherent x‐ray point source in the water window spectral region generated using a small commercially available KrF laser system focused onto a Mylar (essentially carbon) target have been measured. By operating the source in a low‐pressure (approximately 20 Torr) nitrogen environment, the degree of monochromaticity was improved due to the nitrogen acting as an x‐ray filter and relatively enhancing the radiation at a wavelength of 3.37 nm (C vi1s‐2p). X‐ray pinhole camera images show a minimum source size of 12 &mgr;m. A Young’s double slit coherence measurement gave fringe visibilities of approximately 62% for a slit separation of 10.5 &mgr;m at a distance of 31.7 cm from the source. To demonstrate the viability of the laser plasma as a source for coherent imaging applications a Gabor (in‐line) hologram of two carbon fibers, of different sizes, was produced. The exposure time and the repetition rate was 2 min and 10 Hz, respectively.

ISSN:0021-8979    

DOI:10.1063/1.353924

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Undoped, thin silicon films have been deposited at different temperatures on fused quartz substrates by low‐pressure chemical vapor deposition. The heating of these films by continuous wave, argon‐ion laser beam irradiation has been studied.Insitu, normal incidence reflectivity measurements have been obtained at specified locations in the semiconductor films. Melting and recrystallization phenomena have been probed by transient measurements. The static film reflectivity at elevated temperatures, up to about 1400 K, has also been measured. The temperature field has been analyzed numerically, using a modified enthalpy model. Thin‐film optics were used to calculate the argon‐ion laser light absorption in the polysilicon layer and the transient reflectivity response to the probing laser light. The predicted and experimentally measured reflectivity histories have been compared. The initial stages of the phase change process have been captured by high‐speed photography.

ISSN:0021-8979    

DOI:10.1063/1.353925

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

It has been shown experimentally that electron current densities of more than 30 A/cm2can be achieved from a cathode made of ferroelectric ceramic, when applying a field of order 0.1 MV/m. This current exceeds the Child–Langmuir current by two orders of magnitude. The current in the diode varies linearly with the applied voltage, provided that the latter is positive. In this theoretical study we show that the ferroelectric material plays a crucial role in the emission process. When a voltage is applied to the ferroelectric, the internal polarization field varies and the amount of screening charge required decreases. As a result, the electrons distribution near the cathode changes, forming a cloud which fills part of the diode gap. If now a positive voltage is applied to the anode, electrons are readily transferred through the diode gap. The qualitative and quantitative results of the theory are in good accordance with the experiment.

ISSN:0021-8979    

DOI:10.1063/1.353926

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Anode sheaths are explored in detail for collisional, identical‐specie‐temperature plasmas where sheath dimensions are in the micron range. The selected approach involves postulation of a specific electric field distribution with three shape factors. Previous research using this approach is verified and expanded upon using greater parameter ranges.z, a dimensionless quantity specifying plasma composition and condition, groups diverse plasmas into families exhibiting similar sheath characteristics. &eegr;, a nondimensional ratio of electrical energy to thermal energy in the sheath, allows temperature effects to be studied. This investigation focuses on plasma families that span azrange of 1.1729 to 2.1493 at &eegr; values defined by discrete plasma temperatures of 6000, 3000, and 300 K. Results indicate that at the lower temperatures, charge production in the outer sheath is somewhat generic to the electric field distribution, and that the sheaths themselves are nearly unaffected by substantial changes in temperature (i.e., &eegr;). Conversely, sheath density and extent are shown to vary significantly for differingzvalues. Equations governing cylindrical anodes generate sheaths that are virtually identical to corresponding planar cases. It is shown that only those anodes whose radii are comparable to the plasma’s characteristic radius (&ggr;) must be treated with the cylindrical formulation; low‐ and moderate‐pressure plasmas would require micron‐diameter anodes to be thus affected. Finally, an analytical approach shown yields solutions that confirm the numerical results, and offers an algebraic approximation for high‐&eegr; plasmas.  

ISSN:0021-8979    

DOI:10.1063/1.353927

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Our previous calculation of the excited atom population [T. Fujimoto, K. Sawada, and K. Takahata, J. Appl. Phys.66, 2315 (1989)] is revised on the basis of new assessment of the cross sections for excitation from atomic hydrogen and dissociative excitation from molecular hydrogen. The effective ionization rate of molecular hydrogen is also calculated by the method of the collisional‐radiative model. Its ratio to the Balmer &agr; photon emission rate is higher than that for the atomic hydrogen case by more than one order of magnitude.

ISSN:0021-8979    

DOI:10.1063/1.353930

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Electron densities were measured in continuous and pulse‐modulated, 13.56‐MHz, helium and argon discharges. These measurements were made in a symmetrically driven Gaseous Electronics Conference Reference Cell. Pulse modulation of the applied potential allowed observation of the time required for the electron density to achieve steady state. In general, helium discharges reached steady state in approximately 1.5 ms, taking three to ten times longer than argon discharges under similar operating conditions. As much as a threefold increase of the electron density was observed in the afterglow of a pulse‐modulated helium discharge, indicative of large metastable densities. Absorption spectroscopy was used to measure the helium singlet and triplet metastable densities throughout the volume of the discharge. For a 1.0‐Torr discharge, helium triplet metastable densities at the center of the discharge were as large as 2.5×1011cm−3while the peak singlet density was 3.0×1010cm−3. The steady‐state electron density varied from 3.8×1010cm−3at 50 V to 2.3×1011cm−3at 200 V for the 1.0‐Torr helium discharge.

ISSN:0021-8979    

DOI:10.1063/1.353451

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

We report results from experiments performed to measure and characterize the intenseK‐shell radiation from aluminumx‐pinch plasmas at peak driving currents ranging from 280 kA to 1.0 MA. Single pulse aluminumK‐shell (predominantly line radiation at 1.6–2 keV) x‐ray yields ranged from 7.6 J at 290 kA to 240 J at 1.0 MA. In the range from 280 to 470 kA, the yield scales with current to the power of 3.6, whereas nonoptimizedK‐shell yields at 800 kA and 1.0 MA indicate a power of about 3 or higher.

ISSN:0021-8979    

DOI:10.1063/1.353452

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

This paper reports on experimental and theoretical studies of a fundamental mode, large‐orbit gyrotron oscillator designed to produce high‐power radiation in the rectangular TE10mode at about 650 MHz. A basic linear theory for design and operation is presented and experiments confirm its applicability. Results of initial experiments have demonstrated 40±4.2 MW of microwave radiation in pulses equivalent in duration to the electron beam, corresponding to an efficiency of about 16%. In some cases, much greater power levels of 100±20 MW were observed at the expected frequency but in pulses of very short duration. In both cases, the observed frequency and mode pattern were consistent with operation in the fundamental mode. Experiments also demonstrated that the observed radiation was linearly polarized, as expected.

ISSN:0021-8979    

DOI:10.1063/1.353453

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

We report surface cleaning of C‐dopedp+‐GaAs epilayers with hydrogen electron cyclotron resonance plasma. Native oxides on the surface of thep+‐GaAs layer can be removed at a very low substrate temperature of 150 °C. In addition, carrier concentrations decrease after cleaning at about 300 °C, which is attributed to the hydrogenation of carbon acceptors caused by hydrogen plasma exposure. A cleaning temperature of about 400 °C, a cleaning time of about 5 min, and microwave power of about 30 W appear to be optimum for the regrowth process.

ISSN:0021-8979    

DOI:10.1063/1.353454

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

An analysis of the temporal evolution of the electron energy distribution function (EEDF) and the electron swarm parameters in oxygen and chlorine gases is presented. The spatially homogeneous time‐dependent Boltzmann equation is solved for dc and radio‐frequency ac electric fields by a finite‐element method. A comparison is made of the swarm parameters obtained for the following three cases: (a) under the actual ac field; (b) assuming that the EEDF follows faithfully the applied ac field [quasi‐steady‐state (QSS) approximation]; and (c) using an ‘‘effective’’ dc field (effective dc approximation). It is shown that the effective dc approximation is not applicable to either oxygen or chlorine for frequencies <10 MHz; however, the QSS approximation is justified for chlorine discharges at <13.56 MHz. This has important implications for reducing the computation time in modeling the bulk plasma of glow discharge reactors. It is also shown that atomic chlorine resulting from molecular dissociation has a significant effect on the swarm parameters, especially for large degrees of gas dissociation.  

ISSN:0021-8979    

DOI:10.1063/1.353455

出版商:AIP    

年代:1993

数据来源: AIP