Volume 70, Issue 7, July 1999
Index of content:
- OPTICS; ATOMS and MOLECULES; SPECTROSCOPY
70(1999); http://dx.doi.org/10.1063/1.1149846View Description Hide Description
The instrumentation and performance of the new infrared beamline U12IR at the National Synchrotron Light Source of Brookhaven National Laboratory is described. This beamline utilizes infrared synchrotron radiation from a bending magnet. A combination of beamline design features and spectroscopic instrumentation allows the facility to reach the extremely low frequency limit of ∼2 (i.e., 60 GHz or a photon energy of 250 μeV). The infrared light from the synchrotron emission at U12IR is compared to standard thermal sources and reveals substantial benefits for the study of small samples. In particular, the intensity of the synchrotron radiation in the far infrared can be as much as 200 times greater than that from a blackbody when millimeter-sized samples are measured. The effects of diffraction and noise on beamline performance are also discussed.
70(1999); http://dx.doi.org/10.1063/1.1149847View Description Hide Description
We have characterized the spectra and performance of an ensemble of 11 fiber-coupled laser diode arrays (LDAs) manufactured by Opto Power Inc. These high-power LDAs operate near 795 nm and are of a type commonly used for spin-exchange optical pumping of noble gases. We find the Opto Power LDAs to vary significantly in output power, spectral width, and other important characteristics, in a manner not correlated with age, operating lifetime, or information supplied by the manufacturer. In addition we have developed a two-loop feedback technique for use with LDAs that stabilizes the Rb magnetization in an optical pumping cell to better than one part in a thousand.
New fiber-optic attenuator calibration technique using linear intensity modulation characteristics of a laser diode70(1999); http://dx.doi.org/10.1063/1.1149848View Description Hide Description
A new fiber-optic attenuator calibration technique has been developed using linear intensity modulation characteristics of a laser diode. It does not require detector linearity to be known. The uncertainty of attenuation calibration for a typical 10 dB attenuator is 0.020 dB at two standard deviation levels. The resolution of the system is 0.001 dB and the dynamic range is 25 dB in a single step.
70(1999); http://dx.doi.org/10.1063/1.1149849View Description Hide Description
We describe a siliconanode with integrated electronics for use in photon-counting microchannel-plate (MCP) imaging detectors. Very-large-scale integrated techniques using a 2 μm complementary metal–oxide–semiconductor (CMOS) process allow a passive-anode region, which collects charge from the MCPs, to be surrounded by an active event-processing region. The anode region is made from a rectangular array of pads that are formed using the metal interconnect layers of the CMOS process. Individual pads are electrically connected to form isolated arrays of rows and columns; each row terminates at a well of one charge-coupled device(CCD) register, and each column terminates at a well of a second orthogonal CCD register. The distribution of charge within each register is used to encode the charge-cloud coordinates. A two-dimensional prototype anode was constructed with 128×80 pixels spaced at 50 μm intervals; the anode readout rate is 31 250 Hz. Subpixel centroiding techniques can be employed to reduce the number of pixels that must be read for a given resolution. We envision a rugged, compact, low-power, and low-mass single-substrate imaging anode with a direct digital interface. The design offers large array formats with inherent pixel linearity, orthogonality, and stability. An identified upgrade path promises orders-of-magnitude increases in speed (up to and dynamic range, while maintaining large pixel count (>4000×4000) and MCP pore-limited resolution (<8 μm).
Group-phase refractive index method for improving the accuracy in two-color interferometric length measurements70(1999); http://dx.doi.org/10.1063/1.1149850View Description Hide Description
A group-phase refractive index method was developed for improving the accuracy of a conventional two-color method used in an interferometer for length measurements. This method uses two closer wavelengths to reduce the errors caused by air turbulence and the chromatic aberration. We describe the new method and demonstrate preliminarily its effectiveness by measuring the group refractive index of air using a vacuum cell. We then compared the results with those calculated using the Edlén equation. The mean difference between the measured and calculated values was with a standard deviation of for the vacuum cell with a length of 184.5 mm.
An interferometric determination of the refractive part of optical constants for carbon and silver across soft x-ray absorption edges70(1999); http://dx.doi.org/10.1063/1.1149851View Description Hide Description
Interferometric, direct determinations of the scattering factor near absorption edges in the soft x-ray range is demonstrated. The interferometric system, which is based on wave front division (no beam splitter) with plane mirrors only, produces a linear fringe pattern. The principle consists in direct measuring of the fringe shift occurring upon insertion of a sample into one interferometer arm, by means of a dedicated detection system. This provides the optical thickness, which in turn gives the factor, knowing either the sample mass per unit surface, or the sample thickness and density. With the sample being probed in transmission under near normal incidence, the determination of is not perturbed by the absorption part of the complex scattering factor. Therefore, data obtained here can be said new and independent with respect to those obtained previously, in the sense that they are obtained from a new, purely experimental technique, and are neither deduced from nor perturbed by absorption. The interferometer design used can be implemented in a very large spectral range. For demonstrating the ability of this interferometric system to provide such new data, a thin free-standing carbon foil near the K edge (from 4.1 to 4.65 nm, 302–267 eV), and a membrane-supported silver layer near an M edge (from 2.6 to 3.8 nm, 477–326 eV) were used as test objects.
70(1999); http://dx.doi.org/10.1063/1.1149852View Description Hide Description
Calibration procedures are described for use with electronic x-ray detectors, with an emphasis on detectors based on fiber-optically coupled charge-coupled devices. Methods are detailed for removing spurious events, pixel pedestals, dark-current, spatial distortion, and intensity response variations for both small-angle and wide-angle applications. The accuracy of the calibration procedures is discussed.
70(1999); http://dx.doi.org/10.1063/1.1149889View Description Hide Description
In this article a desktop x-raymicrotomographyXCT) instrument is presented, which utilizes conventional diffraction tubes and tunable focusing optics. The beamline is based on an elastically bent cylindrical multilayermirror, given by a coated and rhombic shaped Si wafer, which is placed on bearings with two of its tips and is actuated by a single transverse center force to a desired curvature and herewith focal length. This optical element is used in a grazing incidence reflection geometry, demagnifying the tube focal spot into an image line with width (w) and position following from a classical imaging equation (magnification ratio M). While the tube and the image (“focal”) position are kept fixed, the curvature and axial position of the mirror are adaptively controlled for different M values and Bragg angles (i.e., pass-energies ), which results in a one-dimensional zoom-optical system. The specimen is placed in the high-depth focal region of the condensed beam for the CT-scanning procedure with the slice orientation given by the focusing direction. Minor modifications of the fundamental rhombic mirror shape also enable the establishment of imaging geometries with elliptical and parabolic cylindrical-type optical figures. Multilayer reflection inherently results in a small bandpass of photon energies Pass-energy is preferably tuned to characteristic lines of the tubes in use (Cr, Cu, Mo target) with the option of also using the white x-ray spectrum. Numerical values of beamline specifications are characterized by: μm, keV, Photon intensity along the focal line is given by depending on the type of tube, mirror reflectivity and M setting. The fundamental principles, the experimental setup and major components of the beamline are described and the theoretical and experimental performance in terms of photon flux, pass-energy bandwidth and beam geometry are evaluated. Examples of XCT scanning are also given. In the current configuration, a fast scintillation counter behind an object slice collimator is used for photondetection, although the sheet-like geometry of the focused x-ray beam can be further used for parallel projection data acquisition along the nonfocusing direction of the optical system (i.e., for different object slices) by application of a suitable charge coupled device-type detector.
70(1999); http://dx.doi.org/10.1063/1.1149853View Description Hide Description
Status of investigations on use of Kumakhov optics in x-ray fluorescence analysis (XRFA) is reported. At present the achieved level of Kumakhov lensesmanufacturing technology makes it possible to increase the concentration of x-ray radiation on a sample under investigation by two orders of magnitude or more. Several schemes of the lens used in XRFA are considered. Results of experiments in scheme “x-ray tube–lens–sample detector” are represented. Minimal detection limit in this scheme by using a small 2 W source was on the order of 0.1 pg. Parameters of the lenses with focal spots of micron size (from several tens to several hundreds of microns) used in the experiments are also considered. A brief description of the portable x-ray spectrometer based on Kumakhov optics is given. The next perspectives of use of Kumakhov optics in XRFA are considered.
70(1999); http://dx.doi.org/10.1063/1.1149890View Description Hide Description
A simplified modification of an optical assembly for single-molecule spectroscopy with the sample located at the joint focus of a parabolic mirror and a focusing lens is described. The optical cell fits a small helium cryostat with a 33 mm diam neck and can be used for experiments with thin films of frozen solutions as well as with other types of thin films. Described is the procedure of preadjustment of the position of a fixed focusing lens for work in different environments with the known index of refraction, such as superfluidliquid helium,helium gas, or vacuum; no adjustment of the lens during the experiment is required. A similar design of an optical cell could be used in a high-pressure cryostat with solid helium where the position of the lens cannot be adjusted. A signal-to-noise ratio of 15:1 was achieved for single-molecule spectra of terrylene in the Shpol’skii matrix -decane.
70(1999); http://dx.doi.org/10.1063/1.1149854View Description Hide Description
Calculations of focal properties and third-order aberration coefficients are presented for atom mirrors based on elastically deformedsingle-crystal surfaces. Biaxial loading of the mirrors is achieved by a combination of a uniform applied pressure and rectangular boundary conditions. The results are calculated for readily available single-crystal wafers of Si(111) with a thickness of 50 μm and a usable free standing diameter of 18 mm. Focusing of beams at non-normal incidence requires the principal radii of curvature, in planes normal to the mirror plane, to be different. The ratio of these principal radii of curvature is shown to be insensitive to the applied pressure and can be varied by a factor greater than 10 by changing the boundary conditions from square (1:1) to rectangular (1:2).
- CHARGED PARTICLE SOURCES, OPTICS and ACCELERATION
70(1999); http://dx.doi.org/10.1063/1.1149855View Description Hide Description
A detector system for the study of the dissociationdynamics of molecular ions is described. The measurements of the time of flight and of the hit coordinates of the charged fragments on the detector are transformed into the three momentum components along Cartesian coordinates. The detector uses 2×32 independent channels which makes possible the simultaneous detection of several particles and a spatial resolution of 0.65 mm on a 40 mm diameter channel plate. The low dead time of this detector (≃1 ns) enables us to study the dissociation of homonuclear molecules and also the angular correlation and energy distribution between two electrons of very low kinetic energies. As an illustration of its possibilities, results obtained in the dissociation of charged molecular ions will be considered to illustrate the capacity of this detector for the identification of dissociation channels, the accurate kinetic-energy distribution, and the investigation of the dissociationkinematics.
70(1999); http://dx.doi.org/10.1063/1.1149856View Description Hide Description
The characteristics, performance, and design feature of a filamentless plasma-cathode electron gun for beamgeneration in the forepump gas pressure range are presented. The plasmacathode is based on a hollow-cathode direct current (dc) discharge. Using the method of “grid stabilization” it was possible to generate an e beam at a background gas pressure as high as about This pressure can be easily obtained by using mechanical pump only. The operation of the gun with a magnetic field up to 0.1 T was investigated. The presence of a magnetic field (B field) is often required, for instance in plasma chemistry and surface treatment processes. The effect of the B field both on discharge and emission parameters of the gun are observed. The results obtained can be explained based on the concept of electron confinement and motion across the B field. With the accelerating voltage up to 8 kV, the gun is able to generate an electron beam of about 0.7 A dc.
The effect of microwave frequency and grad B on the energy of electrons in an electron cyclotron resonance ion source70(1999); http://dx.doi.org/10.1063/1.1149857View Description Hide Description
In an electron cyclotron resonanceion source the energy of the electrons is increased by the electric field of the microwaves. The gyrofrequency of the electrons has to be in resonance with this field. The production of highly charged ions requires very energetic electrons. As a consequence, the acceleration of electrons by the microwaves has to be efficient. In this article the energy gain of the electrons due to the microwaves has been studied. The gradient of the magnetic field and the frequency of the microwaves were the main variables. The simulations show that more efficient heating is achieved with higher microwave frequency and with smaller gradient of the magnetic field.
70(1999); http://dx.doi.org/10.1063/1.1149858View Description Hide Description
The data acquisition and control system for the electron linear accelerator ALID-7 of 5.5 MeV and 0.7 kW, built in Romania, used on pilot-scale radiation processing, is discussed. The system provides: personnel and sensitive device protection against dangerous events; programmed interlocking and warning signals during accelerator operation; single electron pulses or electron pulse trains with small variations in pulse dose;control of electron pulse length and repetition rate, electron beam intensity, magnetron frequency, high voltage level on magnetron and electron-gun modulators, sweeping amplitude and frequency, conveyor velocity, irradiation time, and electron pulses number; simultaneous electron beam and microwave treatment. An important feature of the installation is an original control technique for obtaining programmed beam single shots and pulse trains with programmed pulse number, pulse repetition frequency, and pulse duration, from a diode gun linear accelerator, by discrete pulse temporal position modulation of the gun electron pulses and the magnetron microwave pulses. It is particularly useful for automatic control of absorbed dose rate level, irradiation process control as well as in pulse radiolysis studies, single pulse dose measurement or for research experiments where pulse-to-pulse reproducibility is required.
70(1999); http://dx.doi.org/10.1063/1.1149859View Description Hide Description
We designed an impedance tuner consisting of an inductive material, FINEMET, to cancel the space charge impedance in the longitudinal direction. It was installed in the KEK ProtonSynchrotron (PS) main ring. We observed the frequency shift of the coherent quadrupole oscillations and inferred the shift of the incoherent synchrotron oscillation. The total reactive impedance can be estimated as the coefficient between the shift and the beam intensity. The measured impedance is reduced from to by the impedance tuner which consists of 12 pieces of FINEMET cores. We demonstrated that the space charge impedance is compensated by the impedance tuner. This is the first time for FINEMET to be equipped with an accelerator component. One may think that the characteristics of FINEMET deteriorate under an environment with strong radiation generated by unavoidable beam loss. We have proved that the radiation dose not affect FINEMET even with a total neutron dose of , which is considered to be the highest dose of the main ring.
- NUCLEAR PHYSICS, FUSION and PLASMAS
70(1999); http://dx.doi.org/10.1063/1.1149860View Description Hide Description
An annular Malmberg–Penning trap is described for studies of plasma confinement. A plasma of electrons is contained in the annular region between coaxial cylindrical conductors and is confined radially by an axial magnetic field and axially by an electrostatic field. An azimuthal magnetic field created by a current-carrying center conductor causes gradient, curvature, and additional electric drifts thus allowing new types of transport studies. An initial electron density of is obtained with axial and azimuthal fields of ∼10 mT and fill pressures of to of helium or argon. The electric mobility drift arising from electron collisions with neutral gas determines the density decay time of ∼2 ms.
70(1999); http://dx.doi.org/10.1063/1.1149888View Description Hide Description
A detailed study of the harmonic technique, which exploits the generation of harmonics resulting from excitation of the nonlinearity of the single Langmuir probe characteristic, is presented. The technique is used to measure electron temperature and its fluctuations in tokamak plasmas and the technical issues relevant to extending the technique to high bandwidth (200 kHz) are discussed. The technique has been implemented in a fast reciprocating probe in the TEXTOR tokamak, gaining the ability to study denser and hotter plasmas than previously possible. A corrected analytical expression is derived for the harmonic currents. Measurement of the probe current by inductive pickup is introduced to improve electrical isolation and bandwidth. The temperature profiles in the boundary plasma of TEXTOR have been measured with high spatial (∼2 mm) and temporal (200 kHz) resolution and compared to those obtained with a double probe. The exact expansion of the probe characteristic in terms of Bessel functions is compared to a computationally efficient power series. Various aspects of the interpretation of the measurement are discussed such as the influence of plasma potential and density fluctuations. The technique is well suited to study fast phenomena such as transient plasma discharges or turbulence and turbulent transport in plasmas.
70(1999); http://dx.doi.org/10.1063/1.1149861View Description Hide Description
An improved sniffer probe was constructed for measurements of the hydrogen isotope ratio and impurities in the plasma edge of the W7-AS stellarator. Details of the new design and the probe performance are presented. The new design allows changing the head without breaking the vacuum in the torus. It has a high mechanical stability, effective screening of the magnetic field and high sensitivity. The gas dynamic properties of the probe are analyzed using transmission line calculus, resulting in a rise time of 114 ms for hydrogen. During the 1997 spring measurement campaign, H/D isotope ratio measurements were carried through showing considerable outgassing of the walls during and after the discharge. He glow discharges reduce the isotope ratio drastically. Results from a typical experiment day are presented together with the analytic procedure for determining the isotope ratio in both the plasma edge and in the neutral gas region between the plasma and the vessel walls.
70(1999); http://dx.doi.org/10.1063/1.1149862View Description Hide Description
Disturbance of the Maxwellianplasma may occur in the vicinity of a spacecraft due to photoemission, interactions between the spacecraft and thermospheric gases, or electron emissions from other devices on the spacecraft. Significant non-Maxwellian plasma distributions may also occur in nature as a mixture of ionospheric and magnetospheric plasmas or secondaries produced by photoionization in the thermosphere or auroral precipitation. The general formulas for current collection (volt–ampere curves) by planar, cylindrical, and spherical Langmuir probes in isotropic and anisotropic non-Maxwellian plasmas are examined. Examples are given of how one may identify and remove the non-Maxwellian components in the Langmuir probe current to permit the ionospheric parameters to be determined. Theoretical volt–ampere curves presented for typical examples of non-Maxwellian distributions include: two-temperature plasmas and a thermal plasma with an energetic electron beam. If the nonionospheric electrons are Maxwellian at a temperature distinct from that of the ionosphere electrons, the volt–ampere curves can be fitted directly to obtain the temperatures and densities of both electron components without resorting to techniques that attempt to derive the plasma distribution from the current by taking derivatives. For an arbitrary isotropic distribution, the current for retarded particles is shown to be identical for the three geometries. For anisotropic distributions, the three probe geometries are not equally suited for measuring the ionospheric electron temperature and density or for determining the distribution function in the presence of non-Maxwellian background electrons.