Search Results - (Author, Cooperation:S. Alberti)

2018-03-09

American Physical Society (APS)

0031-9007

1079-7114

Physics

Plasma and Beam Physics

2013-01-22

Nature Publishing Group (NPG)

0028-0836

1476-4687

Biology

Chemistry and Pharmacology

Medicine

Natural Sciences in General

Physics

*Mass Spectrometry ; Peptide Library ; Polymorphism, Genetic ; Proteome/*analysis/genetics ; Proteomics/*methods ; Quantitative Trait Loci/*genetics ; Reference Values ; Saccharomyces cerevisiae/*chemistry/*genetics ; Saccharomyces cerevisiae Proteins/*analysis/genetics ; Selection, Genetic

2018-01-05

American Association for the Advancement of Science (AAAS)

0036-8075

1095-9203

Biology

Chemistry and Pharmacology

Geosciences

Computer Science

Medicine

Natural Sciences in General

Physics

Cell Biology, Online Only

2018-08-10

American Association for the Advancement of Science (AAAS)

0036-8075

1095-9203

Biology

Chemistry and Pharmacology

Geosciences

Computer Science

Medicine

Natural Sciences in General

Physics

Cell Biology, Neuroscience

2018-05-25

American Association for the Advancement of Science (AAAS)

0036-8075

1095-9203

Biology

Chemistry and Pharmacology

Geosciences

Computer Science

Medicine

Natural Sciences in General

Physics

Cell Biology

1089-7674

AIP Digital Archive

Physics

Fully noninductive, steady-state electron cyclotron current drive (ECCD) has been demonstrated for the first time in experiments carried out in the tokamak à configuration variable (TCV) [O. Sauter et al., Phys. Rev. Lett. 84, 3322 (2000)]. By appropriately distributing six 0.45 MW ECCD sources over the discharge cross section, fully noninductive, stable, and stationary plasmas with Ip up to 210 kA were obtained for the full discharge duration of 1.9 s, corresponding to more than 900 energy confinement times and more than 10 current redistribution times at an average current drive efficiency η20CD=0.01[1020 A W−1 m−2]. These experiments have also demonstrated for the first time the steady recharging of the ohmic transformer using ECCD only. The effect of localized off-axis electron cyclotron heating (ECH) and EC current drive (ECCD) (co- and counter-) is investigated showing that locally driven currents amounting to only 1% of Ip significantly alter sawtooth periods and crash amplitudes. An improved quasi-stationary core confinement regime, with little or no sawtooth activity, has been obtained by a combination of off-axis ECH and on-axis CNTR–ECCD. © 2001 American Institute of Physics.

Electronic Resource

1089-7674

AIP Digital Archive

Physics

Electron beam instabilities occurring in a gyrotron electron beam can induce an energy spread which might significantly deteriorate the gyrotron efficiency. Three types of instabilities are considered to explain the important discrepancy found between the theoretical and experimental efficiency in the case of quasi-optical gyrotrons (QOG): the electron cyclotron maser instability (ECMI), the electrostatic Bernstein instability (BI) and the Langmuir instability (LI). When the magnetic field gradient in drift tubes of QOG is low, the ECMI can develop in the drift tube at very low electron beam currents. Experimental measurements show that with a proper choice of absorbing structures in the beam tunnel, this instability can be suppressed. At high beam currents, the BI can induce a significant energy spread at the entrance of the interaction region. The induced energy spread scales approximately linearly with the electron beam density and for QOG one observes that the beam density is significantly higher than the beam density of an equivalent cylindrical cavity gyrotron. © 1998 American Institute of Physics.

Electronic Resource

1089-7674

AIP Digital Archive

Physics

Extensive simulations are performed to investigate effects of electron cyclotron instabilities on the gyrotron beam quality, using two-dimensional axisymmetric particle-in-cell (PIC) codes. Both electrostatic and electromagnetic models, as well as realistic geometries of the gyrotron, are considered. It is found that a large beam density can lead to an electrostatic-instability-induced energy spread which substantially degrades the gyrotron efficiency. © 1997 American Institute of Physics.

Electronic Resource

1089-7674

AIP Digital Archive

Physics

The parallel velocity distribution function of the weakly relativistic electron beam of a quasi-optical gyrotron has been determined by measuring the Doppler-shifted Electron Cyclotron Emission (ECE) at an angle θ=15° with respect to the external magnetic dc-field. Due to the Doppler shift, the frequency of the spontaneous cyclotron emission at the fundamental (ν0=100 GHz) is upshifted to 140 GHz. A broadening of the spectrum up to 10 GHz [full width at half maximium (FWHM)] was measured. The measured mean frequency agrees well with the theoretical predictions, but the observed line-width, and hence the parallel velocity distribution function, is 2–3 times larger than expected. Considerations on ECE-measurements of the electron beam energy spread, performed at larger angles θ, are also discussed. © 1996 American Institute of Physics.

Electronic Resource

1089-7623

AIP Digital Archive

Physics

Electrical Engineering, Measurement and Control Technology

A silicon-based temperature stabilized axial Hall probe with an absolute accuracy of ±40 ppm in the full range between 0 and 6 T is presented. The absolute calibration of the probe is performed against a nuclear magnetic resonance (NMR) probe. The application of the probe to a magnetic field profile of a 5 T superconducting magnet system, with field gradients as high as 30 T/m, reaching an overall accuracy of better than ±100 ppm, is demonstrated. In addition to the high absolute accuracy, this Hall probe allows high spatial resolution measurements of inhomogeneous fields in configurations where present NMR probes are not usable. © 1999 American Institute of Physics.

Electronic Resource

1089-7666

AIP Digital Archive

Physics

A quasi-optical gyrotron (QOG) designed for operation at the fundamental ( fce(approximately-equal-to)100 GHz) exhibits simultaneous emission at fce and 2fce (second harmonic). For a beam current of 4 A, 20% of the total rf power is emitted at the second harmonic. The experimental measurements show that the excitation of the second harmonic is only possible when the fundamental is present. The frequency of the second harmonic is locked by the frequency of the fundamental. Experimental evidence shows that when the second harmonic is not excited, total efficiency is enhanced.

Electronic Resource

1749-6632

Blackwell Publishing Journal Backfiles 1879-2005

Natural Sciences in General

Electronic Resource

1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

Intramolecular vibrational energy redistribution and fragmentation dynamics in I2(B,v=22) (centered ellipsis) Nen (n=2–6) and I2(B,v=21) (centered ellipsis) Nen (n=2–5) clusters is studied by hybrid quantum/classical techniques and the results are compared with experiments. A vibrational version of the molecular dynamics with quantum transitions (MDQT) treatment is used in which the vibrational degree of freedom of I2 is treated quantum mechanically while all the other degrees of freedom are treated classically. The potential energy surface is represented as a sum of pairwise interactions with parameters taken from the literature. The calculated product state distributions are in very good agreement with the experiments. Fragmentation lifetimes were also calculated and agree reasonably well with those measured in time-dependent experiments. Fragmentation proceeds via sequential ejection of Ne monomers through three different mechanisms: (i) sequential intramolecular vibrational redistribution plus vibrational predissociation (in which the I2 molecule loses more than one quantum of vibration); (ii) direct vibrational predissociation (in which the I2 molecule loses only one quantum of vibration); (iii) evaporation (in which the I2 molecule remains in the same vibrational state). © 1999 American Institute of Physics.

Electronic Resource

1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

The A˜ continuum photoexcitation of ICN in an Ar matrix is studied using an implementation of the molecular dynamics with quantum transitions method of Tully. Five excited electronic potential energy surfaces of the ICN molecule, 3Π0+, 1Π1(A′,A″), 3Π1(A′,A″), as well as its ground state, are included in these calculations. The couplings between electronic states at large I–CN internuclear distances are modeled using a diatomic in molecules treatment of the mixing of the different spin-orbit states of iodine induced by the Ar atoms. The electronic motion, as well as the I–CN distance and the corresponding bending angle, are treated quantum mechanically using wave-packet techniques. The rotation and translation of the ICN molecule in the Ar cage are treated classically, as well as the motion of the Ar atoms. In contrast with previous calculations, in which all nuclear degrees of freedom were treated classically, we found a 2% of CN cage exit during the first 0.5 ps of the dynamics. © 2000 American Institute of Physics.

Electronic Resource

1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

Photodissociation of ICN in an Ar matrix is studied by molecular dynamics with quantum transitions (MDQT) with the motion of the nuclei treated classically and the electronic motion quantum mechanically. Four electronic surfaces and their corresponding couplings are included in the calculations. The coupling between electronic states at large I-CN internuclear distances is modeled using a diatomic in molecules (DIM) treatment of the mixing of the different spin-orbit states of iodine induced by the Ar atoms. For a total propagation time of 3 ps, no cage exit is found and 44% of the trajectories recombine to the ground electronic state. The principal mechanism for geminate recombination involves the reaction path 3Π0+→1Π1→1Σ0++. © 1998 American Institute of Physics.

Electronic Resource

1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

The ultrafast nonradiative relaxation processes after photon excitation at 266 nm of Hg2 trapped in a cryogenic argon matrix are studied by molecular dynamics with quantum transitions. Fourteen electronic surfaces and their corresponding couplings are included in the simulations. The couplings between electronic states are modeled using a diatomics-in-molecules treatment of the mixing of the different states of Hg2 induced by the Ar atoms. While the initially excited electronic state is the D1u state, we observe that after 10 ps of dynamics most of the electronic population (90%) is in the A0g± states. The majority of nonadiabatic jumps take place in the first 2 ps of the dynamics and at large Hg–Hg nuclear distances close to the Franck–Condon region of excitation. These results confirm predictions from previous experimental steady-state spectroscopic studies. Finally, we also demonstrate the conservation of vibrational coherence of the Hg2 stretching mode during the first few picoseconds, despite several nonadiabatic crossing events during this time. © 2002 American Institute of Physics.

Electronic Resource

1089-7666

AIP Digital Archive

Physics

Recent experiments on a 100 GHz quasioptical gyrotron have shown that for a large resonator setup [Phys. Fluids B 2, 1654 (1990)] the observed frequency up-shift between the starting current and a current of 10 A corresponds to a shift of 4–5 longitudinal modes. In this Letter it is shown that the interpretation of this frequency up-shift should involve the current-dependent electron beam voltage depression in the beam tunnel and the interaction region for both the single-mode and multimode time evolution codes.

Electronic Resource

1089-7666

AIP Digital Archive

Physics

Experiments on a 100 GHz quasioptical (QO) gyrotron operating at the fundamental (ω=Ωce) are described. Powers larger than 90 kW at an efficiency of about 12% were achieved. Depending on the electron beam parameters, the frequency spectrum of the output can be either single moded or multimoded. One of the main advantages of the QO gyrotron over the conventional gyrotron is its continuous frequency tunability. Various techniques to tune the output frequency have been tested, such as changing the mirror separation, the beam voltage, or the main magnetic field. Within the limitations of the present setup, 5% tunability was achieved. The QO gyrotron designed for operation at the fundamental frequency exhibits simultaneous emission at 100 GHz (fundamental) and 200 GHz (second harmonic). For a beam current of 4 A, 20% of the total rf power is emitted at the second harmonic.

Electronic Resource

0378-1119

Recombinant DNA ; alkaline phosphatase ; cassettes ; cell surface ; cloning ; envelope proteins ; secretion ; vectors

Elsevier Journal Backfiles on ScienceDirect 1907 - 2002

Biology

Electronic Resource

0006-291X

Elsevier Journal Backfiles on ScienceDirect 1907 - 2002

Biology

Chemistry and Pharmacology

Physics

Electronic Resource