摘要:

A discrete, time‐dependent energy deposition model is developed and applied to the study of high‐energy electron‐beam (100 eV–10 MeV) deposition in atomic oxygen. Secondary electron distributions are computed and observed to relax to steady‐state results. Characteristic relaxation times are shown. The loss function, mean energies per electron‐ion pair production, production efficiencies, and distribution functions are presented for a wide range of energies. The model uses the latest experimental and theoretical cross sections as input.

ISSN:0021-8979    

DOI:10.1063/1.340491

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

We have studied methods of varying the ion species generated by a lithium‐fluoride overcoated anode in a 0.5‐MV magnetically insulated ion diode. We found that cleaning the anode surface with a 13.6‐MHz rf glow discharge or illuminating the anode with a pulsed soft x‐ray, vacuum‐ultraviolet (XUV) radiation source just before the accelerator pulse significantly altered the ion species of the ion beam produced by the diode. The glow‐discharge plasma removed adsorbates (carbon, hydrogen, and oxygen) from the surface of the LiF flashover source. The ions seen were lithium and hydrogen. Unfortunately, the diode impedance with a lithium‐fluoride anode was high and the ion efficiency was low; however, XUV irradiation of the surface dramatically lowered the impedance by desorbing neutrals from the ion source via photon‐stimulated desorption. Current densities of ten times the Child–Langmuir space‐charge limit were achieved under XUV irradiation. In particular, ion currents increased by over a factor of 3 when 12 mJ/cm2of XUV radiation was used. However, with XUV irradiation the largest fraction of ions were fluorine, oxygen, carbon, and hydrogen, not lithium.

ISSN:0021-8979    

DOI:10.1063/1.340501

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

Current spreading and current concentrating effects of constricted double‐heterojunction lasers (CDH) have been studied. A quantitative formula of the current profile for CDH structure has been obtained theoretically and verified experimentally. In addition, we have compared the influence of these two effects on the threshold current of CDH lasers. From this study, we proposed a scheme to reduce threshold current and fabricated lasers with typical threshold currents of 30–50 mA. The lowest threshold current is 27.5 mA (L=125 &mgr;m,s=10 &mgr;m, 21 °C, cw). Our results also indicated that the temperature coefficientT0of CDH lasers will be improved only through increasing side leakage current.

ISSN:0021-8979    

DOI:10.1063/1.340457

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

Measurements have been made of the preionization electron number density produced by x‐ray irradiation of typical rare‐gas fluoride laser gas mixtures. The results obtained show significant deviations from predictions made by using the mass absorption coefficients of the individual laser gases. Those deviations are shown to be caused by interactions between the constituent gases of the laser mixture and ionization produced from areas of the laser chamber exposed to the x rays. Measurements of the decay of the ion species produced have also been made, indicating a rate in close agreement with diffusion‐limited ion‐ion recombination. An unexplained dependence of this rate on initial ion number density has been observed.

ISSN:0021-8979    

DOI:10.1063/1.340458

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

An experimental system for studying acoustic signals generated by a modulated MeV ion beam in metals is described. For detection, a closed cell on the rear side of a copper or aluminum sample, a half‐inch condenser microphone, and a lock‐in amplifier were employed. The pressure signal was found to be proportional to beam current and particle energy, and inversely proportional to cell length. Decrease of the signal magnitude and increase of the phase delay with increasing modulation frequency and sample thickness were also observed.

ISSN:0021-8979    

DOI:10.1063/1.340459

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

A theoretical relation is derived for the variance of a 1/fnoise signal as a function of the sample durationT. With the help of this relation the 1/fcharacter of a noise signal in the microhertz range is verified by measuring the variance as a function ofT, withT≤3×105s. Atf=3×10−6Hz the spectral density of the noise signal was still found to be inversely proportional to the frequency.

ISSN:0021-8979    

DOI:10.1063/1.340460

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

Laser light from aQ‐switched Nd:yttrium‐aluminum‐garnet laser (&lgr;=1064 nm; pulse duration=20 ns; pulse energies up to 150 mJ) focused into water creates shock waves by rapidly expanding microplasmas. Using piezoelectric, thin‐film polyvinylidenefluoride (PVDF) as a transducer, a broadband hydrophone (100‐MHz bandwidth) was developed to investigate underwater shock waves. The electrical signal is analyzed with respect to reflections of the shock wave within the transducer and the input impedance of the measuring device. The shock waveform is determined, its peak pressure ranging to kbars (108Pa), decreasing withr−1.12and increases by the square root of the laser pulse energy. The time resolution of the hydrophone (4 ns) is sufficient to determine the plasma dimensions and the number of shock waves generated by a single laserpulse. Both vary statistically, primarily because of contaminations in the fluid. Because of the length of the region containing plasmas, different peak pressures are found in the direction of the laser beam and perpendicular to it.

ISSN:0021-8979    

DOI:10.1063/1.340461

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

In general, the electron attachment rate constantka(⟨&egr;⟩,T), as a function of the mean electron energy ⟨&egr;⟩ and temperatureTfor electronegative gases which attach electronsnondissociativeily, decreases greatly withTfrom room temperature toT≲600 K, while theka(⟨&egr;⟩,T) of electronegative gases which attach electronsdissociativelyincreases with increasingT. Based on recent studies at our laboratory onka(⟨&egr;⟩,T), we investigated the variation withT(∼295–575 K) of the uniform field breakdown strength, (E/N)lim, for three classes of electronegative gases: (a) gases such asc‐C4F8(andc‐C4F6, 1−C3F6) which attach strongly low‐energy (≲1 eV) electrons nondissociatively and for whichka(⟨&egr;⟩,T) decreases precipitously withTabove ambient: (b) gases such as C2F6and CF3Cl which attach electrons exclusively dissociatively and whoseka(⟨&egr;⟩,T) increases withT; and (c) gases such as C3F8andn‐C4F10which attach electrons both nondissociatively and dissociatively over a common low‐energy range and whoseka(⟨&egr;⟩,T) first decreases and then increases withTabove ambient. The (E/N)lim(T) has been found todecreasesignificantly withTfor (a), todecreaseslowly withTfor (c), and toincreaseslightly withTfor (b). These changes in (E/N)limfollow those inka(⟨&egr;⟩,T). A similar behavior is expected for other electronegative gaseous dielectrics in the respective three groups.

ISSN:0021-8979    

DOI:10.1063/1.340462

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

The axial distribution of the plasma parameters in the Faraday dark space (FDS) was measured in the direction moving from the negative glow to the positive column (PC). The axial electric field and the electron temperature increases, whereas the concentration of electrons decreases. The flow of electrons in the FDS consists of a drift flow, diffusion flow, and thermal diffusion flow. Both the diffusion and the thermal diffusion are individually large but as they are in opposite direction their influence on the current is small. The ratio of the anysotropic partf1in the velocity distribution function of the electrons to the isotropic partf0is much higher in the FDS than in the PC. This is due to the gradients of the concentration and temperature of electrons in the FDS.

ISSN:0021-8979    

DOI:10.1063/1.340463

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

A comprehensive study of Si implants (5×1013cm−2, 150 keV) in GaAs and Al0.3Ga0.7As is made with the use of Hall‐effect measurements. A significant annealing‐temperature dependence of the electrical activation is observed. For GaAs, a clear Arrhenius plot is implied by the data, and it gives an activation energy of ∼0.79 eV. For Al0.3Ga0.7As, the electrical activation occurs dramatically from 800 °C and saturates at approximately 875 °C. Temperature‐scanned Hall‐effect measurements indicate that for GaAs the electron concentration is nearly temperature independent, which suggests highly degenerate electron statistics. For Al0.3Ga0.7As, the freezeout of electrons is an exponential function of temperature between 170 and 300 K, which provides a donor ionization energy of ∼110 meV. Low‐temperature (4.2 K) photoluminescence measurements reveal that Si‐ion implantation produces a new emission band at 1.46 eV in GaAs and strongly enhances the intensity of the 1.84‐eV band in Al0.3Ga0.7As. These bands may be due to defect‐impurity (Si) related transitions in GaAs and Si‐donor–Si‐acceptor pair recombination in Al0.3Ga0.7As.

ISSN:0021-8979    

DOI:10.1063/1.341157

出版商:AIP    

年代:1988

数据来源: AIP