ALBERT

Notes: The dedicated VUV radiation source Super-ACO is part of the Orsay synchrotron radiation complex. The construction of this 800-MeV positron storage ring was started in 1982, and the first beam was stored on March 18, 1987. Super-ACO is expected to accommodate up to six undulators. The first, a 3.2-m-long optical klystron has already been operated between 500 and 800 MeV. A 1.3-m-long undulator has been recently installed. The experience gained during the three periods of operation is presented. From March to October 1987 measurements were taken of the machine properties in the low emittance regime, as well as performances in single and multibunch operation. In March 1988 the optical klystron was tested together with the commissioning of two beamlines out of a bending magnet. Starting from May 1988 two undulators and six beamlines progressively operated.

Notes: A 20-channel far-infrared imaging interferometer system has been used to obtain single-shot density profiles in the UCLA Microtor tokamak. This system differs from conventional multichannel interferometers in that the phase distribution produced by the plasma is imaged onto a single, monolithic, integrated microbolometer linear detector array and provides significantly more channels than previous far-infrared interferometers. The system has been demonstrated to provide diffraction-limited phase images of dielectric targets.

Notes: The effect of substrate misorientation on {111} ordering in GaxIn1−xP epilayers grown on (001) GaAs substrates by metalorganic vapor-phase epitaxy has been studied by transmission electron microscopy. Among the four possible variants, only the { 1/2 1/2 1/2 }B are observed, and the substrate misorientation toward (11¯0) can disymmetrize the two B variants to a point such that only one remains. We show that the diffuse intensity observed along (001) originates from a modulation of the degree of order. When the domain size is comparable to the characteristic length of the modulation, domain boundaries and antiphase boundaries contribute to the diffuse scattering, giving rise to the reported wavy diffuse intensity. We propose a few simple rules controlling the growth process at the atomistic level, from which the absence of the { 1/2 1/2 1/2 }A variants, as well as the selection effect of the [11¯0] steps on the two remaining variants may be explained. From existing literature, the validity of these rules seems to be rather general. Finally, considering reports that the ordered structure observed here is not at equilibrium, we suggest that the latter is growth induced. The necessary conditions for such an ordering mechanism to operate are identified.

Notes: 350-keV 209Bi+ was implanted into an Al (1000 A(ring))/V bilayer system. The Bi depth distribution measured by Rutherford backscattering agrees well with predictions obtained via the Monte-Carlo simulation method (trim code). Diffusion coefficients for Bi in both the V substrate of the Al/V system and the pure V foil are extracted after thermal annealings in a temperature range between 200 and 700 °C. The results show that the Bi ions follow a hindered diffusion at the Al film of the Al/V bilayer and for temperatures higher than 580 °C diffuse regularly in the V bulk.

Notes: A series of experiments was performed with an Applied-B ion diode on the Particle Beam Fusion Accelerator-I, with peak voltage, current, and power of approximately 1.8 MV, 6 MA, and 6 TW, respectively. The purpose of these experiments was to explore issues of scaling of Applied-B diode operation from the sub-TW level on single module accelerators to the multi-TW level on a low impedance, self-magnetically insulated, multimodule accelerator. This is an essential step in the development of the 100-TW level light ion beam driver required for inertial confinement fusion. The accelerator and the diode are viewed as a whole because the power pulse delivered by the 36 imperfectly synchronized magnetically insulated transmission lines to the single diode affects module addition, diode operation, and ion beam focusability. We studied electrical coupling between the accelerator and the diode, power flow symmetry, the ionic composition of the beam, and the focusability of the proton component of the beam. Scaling of the diode impedance behavior and beam quality with electrical drive power is obtained from comparison with lower-power experiments.The diode impedance lifetime was about 10 ns, several times shorter than for lower-power experiments. Azimuthal and top-to-bottom variations of the diode and ion currents were found to be approximately ±10%, compared with an estimated requirement of 5%–7% uniformity to avoid focal blurring by self-magnetic field effects. The ion production efficiency was 80%–90%. However, only 50%±10% of the ion current was carried by protons; the balance was carried by multiply charged carbon and oxygen ions. Activation measurements showed a proton beam energy of approximately 50 kJ. A gas cell filled with 5 Torr of argon was used for beam transport. The macroscopic divergence was 15±10 mrad and the microscopic divergence was 20±15 mrad, values that are similar to those from lower-power experiments. A model of beam focusing is formulated that predicts the proton charge focused onto 0.47-cm radius lithium targets, taking into account beam purity, magnetic bending, small-angle multiple scattering, and intrinsic divergence. The model results and activation measurements of the number of protons focused onto targets agree, and indicate that the spatially averaged (over about 3 cm2) peak focal power was about 0.5 TW/cm.2 The most important limitations on power concentration were found to be the low proton content of the beam, the short impedance lifetime of the diode, and the asymmetric current feed of the accelerator. The short impedance lifetime limited the power coupled to the diode, and caused the voltage at peak ion power to be low, which exacerbates the small-angle scattering problem. The asymmetric feed caused focal blurring through nonuniform self-magnetic bending. At least partly because of the experience gained with low impedance beams during these experiments, the next generation accelerator, the 100-TW Particle Beam Fusion Accelerator-II, has been configured to produce a 25–30-MV Li+ beam rather than a 5-MV proton beam. off

Notes: GaInP films grown by metalorganic vapor phase epitaxy on GaAs substrates are observed by transmission electron microscopy in cross sections. A 1/2 (111) (CuPt type) ordering is observed, for the first time in this system, with only two orientation variants occurring. A layered structure (layer thickness 2 nm) develops parallel to the substrate and extra diffuse scattering is also observed at 2/9 (220). The observation of the 1/2 (111) ordering is not predicted by the current first-principle phase diagram calculations. Substrate effects on the orientation and number of ordering variants are identified.

Notes: The term schemes of the doubly charged ions CO2+2, OCS2+, and CS2+2 have been calculated by the CIPSI method using a new and economical basis set of polarized atomic orbitals and difference orbitals. The calculated energies are compared with existing and new experimental data obtained from Auger spectra, double charge transfer, and photoionization including the PIPICO technique. A complete assignment of the manifolds of singlet states is achieved with good agreement between theory and experiment. Only the lowest doubly charged ion states are well described by simple two-hole configurations, while three-hole one-particle configurations are important at higher energy. It is confirmed that the triplet ground states of these ions are well populated by photoionization, while the excited singlet states are revealed most clearly by the double charge transfer technique.

Notes: Vibronic spectra of three band systems of 40Ar2 and 36Ar40Ar at 108, 107, and 105 nm were recorded and analyzed. Isotope shifts of bandheads were used to establish vibrational numbering of the observed bands. Rovibronic structure was examined at resolving powers up to 5×105, yielding band centers, rotational constants, and providing evidence of Hund's case (c) coupling for Ar2. From band system II at 107 nm, with bands involving v'=20–27 and v‘=0–5, improved constants for the ground X 0+g state were obtained: ω‘e=30.68(8), ωex‘e=2.42(5), ωey‘e=−0.062(13), ωez‘e=0.010(2), and D‘e=99.2(10) cm−1, B‘e=0.059 65(8) cm−1, and Re=3.761(3) A(ring). Similarly, constants for v'=20–27 of the B 0+u were evaluated, with estimates of T'e=88 210(400) and De=5640(400) cm−1. Band system I at 108 nm, led to constants for the v'=23–30 levels of the A1u state and to estimates of T'e=87 458(500) and De=5786(500) cm−1. Analysis of band system III at 105 nm yielded new and improved data for levels v'=0–9 of the C 0+u state, including T'e=95 033.6(3), De=465.8(4), ω'e=68.16, ωexe=4.631, ωey'e=0.0479, ωeze=0.0070, and B'e=0.065 24(20) cm−1, the latter resulting in Re=3.596(5) A(ring). Potential energy curves based on these data were calculated for the ground and three excited states.

Notes: The A˜←X˜ absorption spectra of NH3 and ND3, recorded in a cold molecular jet, are presented. Vibrational band progressions resolvable up to v'2=14 appear. No other vibrations are present, either alone or in combinations. Relative band intensities for v2 progressions are recorded, and the homogeneous lifetime broadenings of vibrational levels of the A˜ state are reported. The FWHM linewidths span 34–293 cm−1 over all bands of NH3 and 30–135 cm−1 over the v'2=2 through 14 bands of ND3. In general, the rate of dissociation increases nonlinearly with vibrational energy. The band intensity alternation, previously observed only in matrix spectra below 15 K, has been observed in these very cold gas phase samples.

Notes: Potential energy, electric dipole moment, and electronic transition moment surfaces have been calculated for the A and X states of NH3 from CASSCF and CEPA electronic wave functions. Anharmonic vibrational term values, Franck–Condon factors, and A–X radiative transition probabilities for the symmetric stretching and bending modes of NH3 and ND3 have been evaluated. The theoretical absorption spectra at room and low temperatures agree well with experimental data. The symmetric stretching mode in the A state has only small intensities in the A–X absorption spectrum. Emission rates from various initial vibronic levels of the A state are given. The ab initio electric dipole moment surfaces for the ground state of NH3 have been used to compute transition moments, which are in good agreement with experimental data.