Acceleration Section Research Articles

Gold-thiolate cluster emission from SAMs under keV ion bombardment: Experiments and molecular dynamics simulations

In this contribution the emission of gold-molecule cluster ions from self-assembled monolayers (SAMs) of alkanethiols on gold is investigated using time-of-flight secondary ion mass spectrometry (ToF-SIMS). Layers of alkanethiols [CH 3(CH 2) n SH] with various chain lengths ( n=8, 12, 16) have been chosen because they form well-ordered molecular monolayers on gold. First, we compare and interpret the yields and energy spectra of gold-thiolate cluster ions, obtained for different thiol sizes. Our results show that the unimolecular dissociation of larger aggregates in the acceleration section of the spectrometer constitutes a significant formation channel for gold-molecule clusters. Second, we present preliminary results of molecular dynamics simulations performed in order to improve our understanding of the cluster emission processes. These calculations have been conducted using 8 keV projectiles and a long-range term in the hydrocarbon potential in order to account for the van der Waals forces between the thiol chains.

Modeling Accident Occurrence at Signalized Tee Intersections with Special Emphasis on Excess Zeros

In implementing effective remedial treatments at hazardous intersections, it often is necessary to identify the geometric and traffic factors that lead to accident occurrence. However, one particular problem frequently encountered in accident studies is how to distinguish virtually safe intersections with little likelihood of accident occurrence from those that have happened to have no accident due to the random process. To deal with this problem, the "excess" records of zero accident, the zero-inflated negative binomial was used to assign the probability to the accident outcome. Accident data at 104 signalized tee intersections in Singapore over a period of 9 years were employed for model development. The model indicates that uncontrolled left-turn slip road, permissive right-turn phase, existence of a horizontal curve, short sight distances, large number of signal phases, total approach volume, and left-turn volume may increase accident occurrence. On the other hand, right-turn channelization, acceleration section on the left-turn lane, median railings, and more than 5% approach gradient may reduce accident occurrence. Moreover, there is a trend of reducing accidents over the years.

High Ram Acceleration Using Open-Base Projectile

High-acceleration operation has been realized in a 25-mm-bore ram accelerator by employing an open-base projectile. The projectile comprises a centerbody and four e ns like a conventional ram accelerator projectile, but is manufactured as a single piece. The base of the centerbody is perforated so that pressures inside and outside are almost balanced, thereby reducing the required thickness of the centerbody wall and the total mass. Reliable ram acceleration could be started only when the entrance velocity was increased by 0.1 km/s from that set for the conventional projectile. Although the propellant e ll pressure was as modest as 3.5 MPa, an average acceleration of 4:4£ £10 4 g and a velocity increment of 1.0 km/s have been achieved through a 4.1-m-long, three-stage ram acceleration section.

Motorcycle crash prediction model for signalised intersections

A mathematical model to predict the occurrence of motorcycle crashes at four-arm signalized intersections is presented in this paper. Using data of motorcycle crashes at fifty-two intersections in the Southwester part of Singapore over the period from 1992 to 1999; the relationship between the likelihood of occurrence of these crashes and the various geometric elements as well as traffic factors of the intersections is also investigated. The Poisson regression model and the negative binomial regression model are explored. The negative binomial model was found to more suitable. An evaluation of model parameters showed that heavy approach traffic volumes, the presence of uncontrolled left-turn lane, larger approach widths, existence of surveillance cameras, number of phase per cycle and high imposed approach speed limits will increase the likelihood of motorcycle crashes. On the other hand, a higher number of bus bays, the presence of an acceleration section or exclusive right-turn lane and the average cycle time and the adaptive signal control will decrease the likelihood of crashes.

Hydrodynamics in downflow fluidized beds (2): Particle velocity and solids flux profiles

In a 9.3 m high and 0.10 m i.d. gas–solids downflow fluidized bed (downer), the radial distributions of the local particle velocities along the downer column were measured so as to study the hydrodynamics of the gas–solids flow, using a fiber-optic particle velocity probe. Local solids fluxes were also calculated from the local particle velocities and solids hold-ups. Results show that the radial distributions of particle velocity and solids flux in a downer reactor are very uniform compared to those in a riser although they vary significantly with the operating conditions. For most of the conditions tested, the particle velocity profile reaches its fully developed state within 3–8 m from the top entrance of the downer. On the other hand, particles are normally fully accelerated within 1–4 m, although the development zone and/or the acceleration section under certain conditions can extend to as long as the downer itself.

Metastable decay of molecular fragment ions sputtered from hydrocarbon polymers under keV ion bombardment

To investigate the metastable decay processes for molecular ions sputtered from hydrocarbon polymers with different degrees of unsaturation, polyisobutylene, polybutadiene, and polystyrene thin films were bombarded by 15 keV, Ga + ions and the secondary ions were mass and energy analyzed by means of a time-of-flight spectrometer. In general, the kinetic energy distributions show that an important fraction of the secondary ions is detected with an energy deficit, due to the dissociation of their metastable parents in the linear part of the spectrometer. The analysis of the energy spectra leads to propose two types of metastable decay reactions: fast, unindentified dissociation in the acceleration section of the spectrometer and delayed H and H 2 losses in the drift section of the spectrometer. The interpretation of the results in the frame of the unimolecular reactions theory indicates that the decay rates of these reactions are in the range 10 7–10 8 s −1 (fast decay) and 10 4–10 5 s −1 (H and H 2 loss), which corresponds to half lives of 10 ns to 0.1 μs and 10 to 100 μs, respectively. On average, the fraction of ions produced in the vacuum increases with the mass of the daughter ions. When comparing the three polymers, the metastable decay for the lowest mass range (0 < m/ z < 100) is increasing when decreasing unsaturation (polyisobutylene > polybutadiene > polystyrene). The important analytical issue of these unimolecular decomposition reactions is addressed, too.

SLAC's Asymmetric B Factory Will Study CP Violation

Showing off an acceleration section of PEP-II, the brand new “asymmetic B factory” at the Stanford Linear Accelerator Center (SLAC), are Jonathan Dorfan (rear), the project's director, and Pier Oddone (Lawrence Berkeley National Laboratory), who first suggested ten years ago that panicle physicists seeking to find and study CP symmetry violation in B meson systems ought to build such asymmetric electron-positron colliders. B mesons, first observed in the early 1980s, carry the heavy b (bottom) quark.

Influence of the beam-forming conditions on the development of space-charge oscillations in a long-pulse relativistic electron beam

The characteristics of the space-charge oscillations of a long-pulse relativistic electron beam in magnetically insulated diodes are determined for different geometries of the electron acceleration section and for explosive-emission cathodes of different materials. The important role of the stream of electrons having high transverse velocities in the evolution of the oscillations is demonstrated, and the laws governing the generation of this stream are determined. Possible mechanisms of the space-charge oscillations are described, taking into account the interaction of the electron stream in the beam halo with the main electron stream, the development of diocotron instability in the stream of electrons emitted by the outer lateral surface of the plasma emitter, and the instability of the space charge of “long-lived” electrons in the beam transport channel.

Influence of chemical structure and beam degradation on the kinetic energy of molecular secondary ions in keV ion sputtering of polymers

The secondary ions sputtered from thin polymer films under 15 keV, Ga + bombardment were mass- and energy-analysed by means of a Time-of-Flight Secondary Ion Mass Spectrometer. The comparison between aliphatic (polyisobutylene) and aromatic polyolefins (polystyrene) allows to propose a process of ion emission in which the mean kinetic energy of the secondary ions is related the degree of fragmentation with respect to the polymer target structure. The important yield decrease and the broadening of the kinetic energy distributions (KEDs) observed for the C x H 2 x ions sputtered from pre-bombarded polyisobutylene (10 12–10 15 ions/cm 2) are explained in terms of physico-chemical modification of the surface. The fast metastable decay of molecular ions in the acceleration section of the spectrometer is also investigated.

Wavebeam Multiplication Phenomena to RF Power Distribution Systems of High-Energy Linear Accelerators

A new quasi-optical concept of electrically proof Delay Line Distribution System (DLDS) for high-energy electron/positron accelerators is proposed. In order to increase microwave power, used for particle acceleration, the concept assumes summing up of a few wavebeams in multimode rectangular waveguide. Operating by mutual phases of the wavebeams, to be produced by coherent sources, the system allows also a distribution of long multiplied RF pulses on a set of acceleration sections. Calculations of key components are fulfilled at 34 GHz frequency.

Proposal for High-Current Bunched-Beam Acceleration in Electron Linac

A butterfly ring, in which a high current of a bunched beam is accumulated after a buncher system in an electron linac, is proposed. This beam is extracted from the ring, and can be bunched and accelerated to high energy at a succeeding buncher system and acceleration section. This high-energy beam is applicable as a high-current beam of nanosecond pulse duration.

ToF-SIMS analyses of polystyrene and dibenzanthracene: evidence for fragmentation and metastable decay processes in molecular secondary ion emission

To understand the sputtering processes in unsaturated polymers, dibenzanthracene and polystyrene samples were bombarded by 15 keV, Ga + ions and the secondary ions were mass- and energy-analyzed by means of a time-of-flight spectrometer. The influence of various phenomena, which may play an important role in the secondary ion emission from polystyrene surfaces, is investigated: (1) emission of “original” fragments, reflecting the polymer structure; (2) emission with fragmentation, rearrangements or hydrogen losses; (3) formation of molecular ions by hydrogen or proton capture; and (4) unimolecular dissociation of the secondary ions. The dibenzanthracene molecule (C 22H 14) was chosen as a model compound for the study of the emission and metastable decay of polyaromatic hydrocarbon (PAH) ions under static SIMS analysis conditions. The kinetic energy distribution (KED) of most of the secondary ions sputtered from dibenzanthracene exhibits up to three peaks. The peaks corresponding to a negative apparent energy are due to the metastable decomposition of larger ions in the acceleration section of the spectrometer. Their origin, i.e. delayed H and H 2 loss reactions, is discussed in detail. The kinetic energy distributions of the dibenzanthracene secondary ions are compared with those of the PAH ions emitted from polystyrene, especially in the region m z = 100–200 of the PS mass spectrum. It turns out that similar interpretations can be proposed for polystyrene. To confirm the hydrogen loss reaction in the case of polystyrene, the comparison is also made with the KED of similar but deuterated PAH ions sputtered from deuterated PS. On the basis of these hydrogen loss reactions, the formation times of the daughter ions are calculated.

Progress in the injector for FEL at CIAE

An intense current RF-linac for the far-infrared FEL is now under construction at CIAE. The normalized brightness of 3.4 x 10{sup 9} A/(m-rad) was obtained from the injector of the linac. An acceleration section with 9 cells will be connected with the injector to provide an electron beam for the 200 {mu}m FEL oscillator. In this paper, the late results from the injector beam test will be reported. The physical design and research progress in the acceleration section, beam transport, undulator as well as optical cavity will be introduced respectively.

A VUV free electron laser at the TESLA test facility at DESY

Abstract We present the layout of a single pass free electron laser (FEL) to be driven by the TESLA Test Facility (TTF) currently under construction at DESY. The TTF is a test-bed for high-gradient, high efficiency superconducting acceleration sections for a future linear collider. Due to its unrivaled ability to sustain high beam quality during acceleration, a superconducting rf linac is considered the optimum choice to drive a FEL. We aim at a photon wavelength of λ = 6 nm utilizing the TTF after it has been extended to 1 GeV beam energy. Due to lack of mirrors and seed-lasers in this wavelength regime, a single pass FEL and self-amplified spontaneous emission (SASE) is considered. A first test is foreseen at a larger photon wavelength. The overall design as well as both electron and photon beam properties are discussed.

On the shape of impactor efficiency curves

The effect of impactor stage geometry on impactor collection efficiency curves is explored numerically. It is found that the geometry of the impaction stage where the gas is accelerated does not affect the 50% cut size but has a strong effect on the sharpness and shape of the efficiency curve. This phenomenon is explained from the aerodynamic focusing effect of the particles in the nozzle (gas acceleration) section. The flat-plate orifice configuration, commonly used in cascade impactors, tends to focus particles closer to the centerline than the angled nozzle. When the real nozzle configuration is used, the prediction shows a characteristic S shape, in good agreement with the experimental results in the literature.

A Magnetically Guided Slow Positron Beam for Defect Studies

The design and construction of a compact, magnetically guided slow positron beam is discussed. The system uses a 30 mCi 22 Νa source. It consists of three main parts: (i) the source chamber with a tungsten foil transmission moderator and the extraction optics, (ii) the beam line with magnetic beam guidance, a bent tube for the separation of the fast positrons and the accelerator stage, (iii) the target chamber with the sample holder and the detector electronics. The energy of the incident positrons can be varied from 30 eV up to 50 keV. Furthermore source geometries, pre-acceleration, main acceleration sections and various magnetic induction profiles have been considered, such as (i) rectangular, conical and bent Wehnelt electrodes, (ű) pre-accelerator voltage shared over several electrodes, (iii) weak, strong, constant and z-dependent B-profiles, (iv) geometric options in the main accelerator region, (v) purely electrostatic and combined electric/magnetic fields. The beam is mainly designed for defect profile studies in ultra high vacuum conditions. PACS numbers: 06.90.+v, 07.90.+c

Macrokinetics of an autowave mode of synthesis of organic powders in a condensed phase. 2. Structural transformations in a wave of self-propagating synthesis of organic powders

Structural transformations in the front of an autowave mode of synthesis of organic powders in a condensed phase are studied. A mode of the front stopping and an initial acceleration section are considered. The dynamics of structural transformations and the effect of the heat exchange rate on the autowave mode are established.

Evidence from SIMS of Doubly-Charged Boron Contamination During Singly Charged-Ion Implantation

When a dopant is introduced into a semiconductor material by ion implantation, it is sometimes desirable to accelerate and implant the ion in a multiply-charged state. This has the effect of increasing the energy and range of the ion without increasing the accelerating potential. Most modern ion implanters are of the pre-analysis type. In this design the ions are first accelerated through a modest extraction potential, e.g., 25 keV. This is followed by deflection for mass-to-charge selection in an analyzer magnet, after which the selected ions undergo final acceleration. Charge-exchange reactions between the doubly-charged ions and residual gas have been found to occur between the analyzing magnet and the final acceleration section. These reactions produce singly-charged ions that receive only half of the energy of the doubly-charged ions during final acceleration. For the case of B++ implantation the resulting implant profile shows a shallow-depth shoulder due to B+, the amplitude of which may be greater than 50% of the main peak.

Linear-accelerator-based high energy implanter with milliampere capability

The need for moderate and high dose implants at high energies has led to the use of r.f. acceleration techniques for ion implantation. The Eaton NV-1000 implanter is based on the independently phased r.f. linear accelerator (linac), injected with a conventional 80 keV machine. The use of r.f. acceleration allows NV-1000 to produce beams in excess of 500 μA over a wide range of ion species. The final beam energy is 1 MeV for singly charged ions. The injection of multiply charged ions results in proportionally higher energy beams with no charge state contamination. A number of design changes have been incorporated into several new machines based on the NV-1000. The three new models, NV-1001, NV-1002 and NV-1003, differ in the number of r.f. acceleration sections, and therefore final energy. These implanters have been designed to take advantage of the high beam current limits of the r.f. linac. The NV-1002 will provide beam currents up to 2 mA, with final energies of 1.4 MeV for singly charged ions. A test bed has been constructed for the new design. Results indicate that the design specifications for the individual components will be met or exceeded. As a result of several technical changes, the NV-1002 series of implanters will be much more compact than the NV-1000. The Eaton advanced control system and robot-based end station will be incorporated into the three models of the new series.

On the free expansion of three-dimensional streams of perfect gas

The asymptotic behavior of solutions of gasdynamics equations describing the distant regions of three-dimensional streams of inviscid and non-heat-conducting gas freely flowing into vacuum is studied. The same equations define the flows at initial secttions of streams flowing into region of small, but finite, pressure or in the acceleration sections of nozzles. The principal term of coordinate expansion of inciated solutions at large distances was obtained in /1–5/.