Deflection Plates Research Articles

A new LEED instrument for quantitative spot profile analysis

Abstract A new instrument for spot profile analysis of electron diffraction (SPA-LEED) has been set up. The instrument works either with a transparent phosphor screen for visual inspection of the pattern or in its main mode with a channeltron for the measurement of the intensity. The diffraction pattern is recorded with a fixed channeltron position by scanning the beam over the channeltron aperture using two sets of electrostatic deflection plates. The scanning range covers about 30°. The intensity may vary over five orders of magnitude. The SPA-LEED system was checked with the Si(111) 7 × 7 surface. A full width at half maximum of 0.3% of the normal reflex distance corresponding to a transfer width of 110 nm is reproducibly obtained. Under optimum conditions the transfer width rose up to about 200 nm. Initial high resolution measurements have been performed on the system Pb on Cu(111). The results demonstrate the possibilities of the new instrument for qualitative and quantitative analysis.

The involvement of neutral species in the NO− → O+ charge inversion reaction

AbstractIntermediates in the overall charge inversion reaction NO− → O+˙ (la), have been found to be O and NO˙ neutrals, and NO+ ions. The intermediates involved have been characterized by various mass spectrometric techniques; for example, inducing the steps of the reaction in separate field‐free regions allows the peaks for the consecutive reactions NO− → NO+ → O+˙ (II) to be recorded free from interference. However, to observe the involvement of a neutral intermediate, techniques such as floating the collision cell and applying a voltage to a deflector plate in front of the collision cell have been employed. Utilizing these methods allows a peak to be obtained due to one or both of the processes, NO− → NO˙ → O+˙ (IIIa) and NO− → O+˙ (IIIb), which cannot be separated. It is shown that the peak for the overall reaction NO− → O+˙ can be modelled and reconstructed by addition of peaks, in a ratio of 1:3, produced by reactions II and III. This provides a reasonable account of the peak shape, in particular, of the tails of the peak which correspond to fragmentations involving large translational energy releases of up to 10 eV.

Trachealkanüle mit auswechselbarem Sekretbehälter*

The author gives an account of the development of a new-type tracheal cannula. The article discusses the mechanism of a modified tracheal cannula with press-stud, attached to a mucus container. This container prevents mucus coughed accidentally on surrounding objects or people; makes it possible for its wearer to breath moist, body heated air. The apparatus can be concealed under a buttoned-up shirt, which makes it aesthetically advantageous. According to first clinical experiences, this mucus container, together with the built-in deflector plates and ventilation holes gives--apart from the above benefits--unimpaired breathing. Its application may help those patients who--due to total removal of larynx or other reasons--are compelled to wear a tracheal cannula, to adapt to society.

Production of high-energy pulsed electron beam of nanosecond direction

Describes a simple technique for producing a high-energy electron beam and sweeping it across a narrow slit in a very short time so as to obtain a pulsed electron beam. X and Y plates similar to those employed in a cathode-ray oscilloscope are incorporated here to deflect the beam. Transistors working in the avalanche mode produce fast rising pulses to be impressed on the deflection plates. A 4 keV DC beam of current 0.9 mA when swept across a 1 mm wide slit produces a charge pulse of 1.5 pC with a duration of 1.7 ns.

Cell sorting by continuous flow cytometry

Cell sorters have been commercially avail- able for several years, and the principles on which they work have been with us for over 200 years. They allow rapid and accurate determination of cellular and subcellular characteristics, and can accurately sort cells with required features into purified popula- tions. Cell sorters are often known as FAtS (fluorescent activated cell sorter), which is the trade mark of a particular instrument; other makes are available, under the general description of flow cytometers. In flow cytometry, cells (or subcellular components such as isolated chromo- somes) flowing at high speeds in aqueous solution are illuminated briefly and give off optical signals detected by photosensors. Cells are often stained with a fluorescent dye which emits a fluorescent signal when illuminated by the incident light. Scattering of the incident light beam by each cell allows determination of cell size, shape, granularity and surface topography. Signals are analysed by a computer which can then control electrostatic deflection plates enabling desired populations of cells to be physically isolated according to any combi- nation of the measured parameters. A typical flow cytometer is shown in Fig. I. Under nitrogen pressure a suspension of cells stained with a fluorochrome is in- jected into the flow chamber through a small needle and encounters a much larger reservoir of faster flowing sheath fluid (usually physiological saline). The cells are confined to the centre of

Synchronous time-resolved optical and x-ray emission from simultaneous optical and x-ray streak cameras driven by a master ramp generator

An optical and an x-ray streak camera have been synchronized by driving the deflection plates of both cameras from the same ramp generator. The relative timing of the two cameras was calibrated by running UV light onto the x-ray streak camera. The x-ray streak camera was then used to measure the time of the x-ray emission from a laser plasma with respect to the laser pulse.

An experimental variable shaped electron beam lithography system

An experimental variable shaped electron beam lithography system has been built to verify the theoretical design considerations and fundamental control method. The electron beam column consists of four magnetic lenses, two square apertures, two aligners, an angular aperture, a set of blanking plates, and a position deflection yoke. The size of the beam spot is varied by a set of electrostatic deflection plates. In order to maintain the current density in the spot constant, as the size of the beam spot varies, the center of the shaped deflection plates is placed to coincide with the image of the gun crossover, and the shaped deflection plates position can be electrically adjusted. The acceleration voltage is 20 kV. The field size is 2.5 mm square. The maximum spot size is 10 μm2. The maximum current density is 1 A/cm2. The system is controlled by an EG 3003 computer. The beam blanking switching time is 50 nS.

Modelling of the advanced limiter test facility (ALT-I), a module for TEXTOR

The molecular and atomic neutral gas build up within the vacuum vessel of ALT I in general and particularly in the hood of ALT-I-FG and in the pump duct of ALT-I-VG are investigated by means of the 3-D Monte-Carlo code EIR-ENE. The “scrape-off”-module of the code SIMTOK is employed to describe the plasma in the throat of ALT-I-VG. Presupposing a neutralized flux of 100 A at the deflector plate(s), the maximum atomic and molecular pressures in case of ALT-I-FG are 1.5×10−3mbar and 10−4 mbar, respectively. The analogous values for ALT-I-VG in the duct near the deflector plate are 1.9×10−3mbar and 10−3 mbar. Because of the recycling processes within the throat the plasma flux neutralized at the deflector plate turns out to be about four times higher than the net flux entering the throat; this numerical factor indicates the effectiveness of the plasma plugging.

Initial alt-i pump limiter studies on TEXTOR

The ALT-I pump limiter has been used to control the fueling and recycling characteristics of TEXTOR during stable, reproducible tokamak discharges. The module has been operated in three modes: (1) normal limiter, with no particle collection, (2) particle scoop, with a maximum ~ 2×10−3 Torr pressure rise in the 700 1 unpumped collection chamber, and (3) pump limiter, with up to ~10000 1/s pumping speed and particle removal rates of up to 6×1020/s. In a comparison of operation in modes (1) and (3) using identical gas fueling programs, the total core electron number decreased by as much as 50%. The effective TEXTOR particle confinement time, τ∗p = τp/(l - R), was decreased by a similar ratio. Within the throat region, during typical operation, electron densities and temperatures were 6×1011−2×1012/cm3 and 15–30 eV. These conditions are representative of an operating regime in which there is high reionization of neutrals, but no change in incoming plasma parameters in the throat region. Energetic particles near the deflector plate were observed. During a gradual insertion of TiC-coated ALT-I beyond the stainless steel TEXTOR main limiters, the density of Ti in the plasma increased to a level similar to those of Cr and Fe. The gas injection fueling efficiency while puffing hydrogen directly into the plasma at the pump limiter tangency point was measured to be > 0.9. These results are discussed in conjuction with measurements of particle flow within ALT-I and other plasma diagnostics to characterize pump limiter operation on TEXTOR.

Tokamak pump limiters

Experiments with pump limiters on several operating tokamaks have established them as efficient collectors of particles. The gas pressure rise within the chamber behind the limiters has been as high as 50 mTorr when there is no internal chamber pumping. Observations of the plasma power distribution over the front face of these limiter modules yield estimates for the scale length of radial power decay consistent with predictions of relatively simple theory. Interaction of the in-flowing plasma with recycling neutral gas near the limiter deflector plate is predicted to become important when the effective ionization mean free path is comparable to or less than the neutral atom mean path length within the throat structure of the limiter.Recent experiments with a scoop limiter without active internal pumping have been carried out in the PDX tokamak with up to 6 MW of auxiliary neutral beam heating. Experiments have also been performed with a rotating head pump limiter in the PLT tokamak in conjunction with RF plasma heating. Extensive experiments have been done in the ISX-B tokamak and first experiments have been completed with the ALT-I limiter in TEXTOR. The pump limiter modules in these latter two machines have internal getter pumping. Experiments in ISX-B are with ohmic and auxiliary neutral beam heating. The results in ISX-B and TEXTOR show that active density control and particle removal is achieved with pump limiters. In ISX-B, the boundary layer (or scape-off layer) plasma partially screens the core plasma from gas injection. In both ISX-B and TEXTOR, the pressure internal to the module scales linearly with plasma density but in ISX-B, with neutral beam injection, a nonlinear increase is observed at the highest densities studied. Plasma plugging is the suspected cause. Results from PDX suggest that a regime may exist in which core plasma energy confinement improves using a pump limiter during neutral beam injection. Asymmetric radial profiles and an increased edge electron temperature are observed in discharges with improved confinement. The injection of small amounts of neon into ISX-B has more clearly shown an improved electron core energy confinement during neutral beam injection. While carried out with a regular limiter, this “Z-mode” of operation is ideal for use with pump limiters and should be a way to achieve energy confinement times similar to values for H-mode tokamak plasmas. The implication of all these results for the design of a reactor pump limiter is described.

Time-of-flight spectroscopy of sub-MeV cluster ions in the mass range of 1–106 atoms per charge

A time-of-flight spectrometer has been developed for ions in the mass and energy range of 1–106 atoms per charge and 100 keV–1 MeV, respectively. The ion beam is chopped by electrostatic deflection plates, the gate function being step-like. The instrument has been used to measure velocity spectra of intense energetic cluster-ion beams made of hydrogen.

Suppression of anticlastic curvature in isotropic and composite plates

This paper discusses the large deflection bending behavior or plates subjected to bending moments along two opposite edges and free on the other two opposite edges. Isotropic and laminated fiber-reinforced composite plates with small initial curvatures are discussed. It is shown that when the applied moments become large, the classical anticlastic saddle shape disappears and the plate flattens laterally to become more cylindrical in nature. This phenomenon is due to nonlinear geometric effects. The theory governing plate behavior and numerical studies illustrating the sensitivity of the plate's flattening response to anisotropy and initial imperfections are presented. Then an experimental apparatus designed to load plates is briefly described. Experimental results are compared with theoretical predictions for a [(± 45)3]g graphite-epoxy plate, a [902/02/± 45]g, graphite-epoxy plate, and an aluminum plate. Comparison between experiment and theory is shown to be good. Several aspects of the comparison are discussed.

Scanning MeV-ion microprobe for light and heavy ions

The University of Oregon scanning ion microprobe uses a 65 cm focal length plasma lens to form 8.65 × demagnified image of an object aperture. The plasma lens focuses a positive ion beam using the self-electric field of a trapped cylindrical column of electrons of density 3−9 × 10 9 cm −3 and length 13–18 cm. Since the focusing field is electric, the focal length depends only on ion accelerating voltage and not on ion mass or charge state. Our 5 MV Van de Graaff accelerator illuminates the object aperture with a current density of ∼ 0.5 pA/mu;m 2. The lens aperture is defined by a set of slits 3.35 m beyond the object aperture slits and 2.79 m from the lens. Four pairs of deflection plates are located between the intermediate aperture and the lens. Two pairs of plates are used for each scanning direction so the beam always passes through the lens center during rastering. The 1 kV operational amplifiers that drive these plates combine three sets of signals. Computer generated voltages raster the beam. Individual dc offset voltages align the beam with the lens axis. A small 60 Hz signal cancels the effects of background 60 Hz magnetic fields along the beam line. With 1 kV rastering voltage the rastered field at the focal plane is 3 mm square for 3 MeV ions. Focal spot size is now 10 μm with a 2 mm diameter lens aperture and 5 μm with a 0.5 mm lens aperture.

Electron-acoustic microscopy-An overview

Electron-acoustic microscopy is a new way of to look at materials with a scanning electron microscope. Acoustic, or more properly ultrasonic, signals are detected instead of secondary or backscattered electrons. A SEM’s electron beam which is chopped at kHz or MHz rates serves as a localized periodic heat source for the specimen being examined, as illustrated in Fig. 1. Periodic thermal expansion in the specimen generates ultrasonic waves, which are detected by a piezoelectric transducer. Contrast in electron-acoustic images results mainly from spatial variations of thermal, elastic, and dimensional properties of the specimen, rather than from variations in surface topography or average atomic number.Additions to a conventional SEM for doing electron-acoustic microscopy consist of a beam chopping system, a well shielded transducer assembly on which samples are mounted, and electronics for using the transducer output to control the brightness of the SEM’s cathode ray tube (CRT) display, as shown in Fig. 2. Squarewave or sinewave voltages applied to beam deflector plates have been used for beam chopping. PZT transducers and quartz transducers have been used for detecting the acoustic signals. The transducer output, typically several microvolts, feeds a low-noise preamplifier, which drives either an rf amplifier-rectifier or a phase-sensitive (lock-in) amplifier/detector, and a video amplifier. Output of the video amplifier is used to control the CRT display brightness. The beam chopping signal is used as reference for the lock-in. Phasesensitive detection with a lock-in amplifier allows variations of acoustic signal phase as well as acoustic signal amplitude to be used in forming images.

Three-electrode electrostatic deflection plates for low-energy charged beams

A simple system of electrostatic deflection plates with three electrodes is described.

A simple, digitally controlled, automatic, hysteresis free, high precision energy scanning system for Van de Graaff type accelerators: Part I: Principle, results and applications

A digitally controlled energy scanning system is described which may be coupled in a simple and inexpensive way to any accelerator which is high voltage stabilized by a beam position sensing “slit control” system like all single ended or tandem Van de Graaff type machines. The basic idea is to “fool” the existing stabilization system by deflecting the beam at two well chosen points through small angles using electrostatic deflection plates biased to variable voltages U. It is shown that the ensuing terminal potential variation ΔV is linearly related to U, the system acting as a U → ΔV closed loop feedback amplifier with a gain G 0 set only by the beam trajectory in the analyzing magnet and between the plates. G 0 is hence independent of acceleration potential, beam particle mass or change state, the long term stability of G 0 being set only by mechanical components. The calibration can thus be made once and for all. The system is strictly hysteresis free and provides means of varying the bombarding energy in small precisely equal steps in a fast and reproducible way. Sweeping spans up to a ΔV V = ± 8% , may be achieved with maximum differential non-linearities in the percent range. The latter may be easily corrected for. With our 2 MV Van de Graaff steps as small as 30 eV at MeV energies were routinely used with comparable mean energy stabilities and narrow, proton induced, resonances could be registered near 1 MeV with 100 eV fwhm beam energy spreads. The excitation functions may be recorded automatically with a multichannel analyzer with sweeping speeds down to 0.1 to 1 s per channel. Stability, reproducibility, residual non-linearity, transient behaviour, etc. of the device are discussed in depth. It is shown that in systems containing several magnets, beam tracking is achieved simply by fitting them with well chosen deflection plates connected to the scanning plates. Numerous applications are shown in nuclear physics, depth profiling ion beam analysis and particle solid interaction studies.

An Emittance Scanner for Intense Low-Energy Ion Beams

An emittance scanner has been developed for use with low-energy H- ion beams to satisfy the following requirements: (1) angular resolution of ± 1/2 mrad, (2) small errors from beam space charge, and (3) compact and simple design. The scanner consists of a 10-cm-long analyzer containing two slits and a pair of electric deflection plates driven by a ±500-V linear ramp generator. As the analyzer is mechanically driven across the beam, the front slit passes a thin ribbon of beam through the plates. The ion transit time is short compared with the ramp speed; therefore, the initial angle of the ions that pass through the rear slit is proportional to the instantaneous ramp voltage. The current through the rear slit then is proportional to the phase-space density d2i/dxdx. The data are computer-analyzed to give, for example, rms emittance and phase-space density contours. Comparison of measured data with those calculated from a prepared (collimated) phase space is in good agreement.

A high-frequency discharge in an image converter tube

A glow discharge is described which has been observed between the rf excited deflection plates of a high vacuum imaging tube developed at THORN EMI, Electron Tube Division. Light emitted from the glow leads to serious deterioration of image quality. Possible causes of the discharge have been investigated by applying voltages up to 9 kV dc and 300 V, 100 MHz to the deflection plates and making observations of light output and dc pre-breakdown currents. The results have been interpreted using a model based on secondary electron resonance (multi-pacting) between the surfaces of the deflection plates; this produces a population of oscillating electrons. These electrons have long free paths and a significant probability of exciting low work function residual gas molecules. Methods of suppressing the discharge are discussed and include reducing the level of residual cesium, electrode coatings to suppress secondary electron emission and the application of electrostatic or magnetostatic fields.

High precision automatic energy scanning of slit stabilized electrostatic accelerators

An atomic, hysteresis free, energy scanning system is presented which may be doubled in a simple and inexpensive way to slit stabilized accelerators, like those of the Van de Graaf type. Electrostatic deflection plates biased to a digitally controlled voltage U are used to deflect the beam; this induces a terminal a potential variation △V around the initial potential V 0. The system acts as a U→ ΔV closed loop feedback amplifier with a gain G=G 0(1+p 1( ΔV V 0 )) , with G 0 and of p 1 are around mass or charge state and of V 0. Typical values of G 0 are around 100 and of p 1 are around 0.5. Sweeping spans up to ΔV V 0 = ± 8% and steps as small as 30 eV at MeV energies were routinely used with our 2 MV Van de Graaff machine. The principle and main characteristics of the system are discussed and its application in nuclear physics and in depth profiling ion beam analysis are are illustrated.

Design and calibration of the fast ion diagnostic experiment detector on the poloidal divertor experiment

A special purpose charge-exchange analyzer was constructed to measure the spatial distribution of hot-plasma ions, as a function of energy and time, in the poloidal divertor experiment (PDX). The fast neutrals produced by charge exchange within the tokamak are reionized as they pass through a helium stripping cell in the detector. The energies of these ions are determined by the trajectories they follow between cylindrical deflection plates which are set at known electrostatic potentials. We describe the technique used to calibrate the response of this system as it depends on the energies and the masses of the particles which are being detected.