Index of content:
Volume 71, Issue 9, September 2000
- OPTICS; ATOMS and MOLECULES; SPECTROSCOPY
71(2000); http://dx.doi.org/10.1063/1.1288262View Description Hide Description
A prototype monolithic capillary lens for focusing neutrons produced by thermally drawing straight multicapillary bundles has been characterized with cold neutrons, and gives an intensity gain of a factor of 25 at a focal distance of 8 mm, over the focal spot area of width 87 μm. This is over an order of magnitude smaller in area than for the multifiber capillary lens. The spatial resolution available with the lens has been tested with prompt gamma measurements on slivers of dysprosium. Background problems that can affect the spatial resolution of measurements taken at the focal position of the lens are addressed. The boron glass of the tapered monolithic lens provides good shielding from unfocused neutrons in the vicinity of the lens focus.
Performance of a very high resolution soft x-ray beamline BL25SU with a twin-helical undulator at SPring-871(2000); http://dx.doi.org/10.1063/1.1287626View Description Hide Description
We report on the excellent performance of a newly constructed soft x-ray helical undulator beamline BL25SU of SPring-8 for photon energies 500–1800 eV. The full beamline was designed to perform very high resolution soft x-rayspectroscopy of solids with using high brilliance, highly circularly polarized undulator radiation. The grazing incidence monochromator employs varied-line-spacing plane gratings which operate in convergent light from a spherical mirror and focuses monochromatic light onto the exit slit. A resolving power in excess of 15 000 was measured at 540 and 870 eV for a grating with a central groove density of 600 lines/mm from the photoemissionspectra of Au. A resolving power of more than 20 000 is estimated near 870 eV for a grating with a central groove density of 1000 lines/mm. A photon flux of more than mA/0.02% b.w. is supplied onto the sample between 500 and 1800 eV with very low amount of higher-order light. The low heat load from the twin-helical undulator gives high stability to all optical components, which is essential to obtain high resolution in a wide energy region. Three experimental stations are installed in tandem on this beamline for various high resolution spectroscopy measurements.
Apparatus for in situ x-ray absorption fine structure studies on catalytic systems in the energy range 1000 eV71(2000); http://dx.doi.org/10.1063/1.1287634View Description Hide Description
A new apparatus for in situ x-ray absoprtion fine structure measurements in the medium energy range of 1000–3500 eV has been developed. Measurements can be performed in a gaseous environment (max. pressure 1 bar) at temperatures ranging from 80 to 750 K. Pre-treatments can be performed at 5 bar and 750 K in the same cell, after which XAFS measurements can be done without exposing the sample to ambient air. In a modular set-up several detector systems can be used: fluorescence detection using a gas proportional counter, a photodiode or a microstrip detector. All detectors are highly integrated into the cell, gaining solid angle for detection. Electron yield detection can be used simultaneously using conversion electron yield or total electron yield. The performance of the new apparatus is demonstrated by a study of the K edge of Al in Zeolite Beta. The Al content is as low as 2 wt%. It will be shown that octahedral framework Al is formed while adding gaseous water at room temperature after ammonia removal (at 450 °C) of an
Log spiral of revolution highly oriented pyrolytic graphite monochromator for fluorescence x-ray absorption edge fine structure71(2000); http://dx.doi.org/10.1063/1.1287753View Description Hide Description
We have constructed an x-raymonochromator based on a log spiral of revolution covered with highly oriented pyrolytic graphite. Such a monochromator is used for obtaining x-rayabsorption edge fine structure by the fluorescence method, and is particularly useful for measuring the fine structure of dilute element A in a concentrated matrix of element B, where B is to the left of A in the Periodic Table. Using the log spiral monochromator, we measure good Cr x-ray fine structure in an alloy of 1% Cr in a V matrix, whereas the corresponding spectrum is severely distorted by the V background if nonmonochromatized fluorescence is used. We also obtain excellent rejection of Mn fluorescence relative to Cr fluorescence in a alloy, and can tune the monochromator such that the entire Mn step height is significantly smaller than the Cr x-rayabsorption edge fine structure oscillations for this system.
Area detector based photon correlation in the regime of short data batches: Data reduction for dynamic x-ray scattering71(2000); http://dx.doi.org/10.1063/1.1287637View Description Hide Description
A method for reducing time sequences of raw scatteringimages to intensity time-autocorrelation functions is presented. The procedure is based on the use of a charge coupled device(CCD) area detector, and optimized for operating in the regime of short data batches. Its application to x-rayphotoncorrelation spectroscopy (XPCS) measurements is described in detail. Using a slow-scan CCD, we explain how to achieve data acquisition on a 30 ms or faster time scale, while simultaneously acquiring data from many coherence areas in parallel. The statistical uncertainties of the acquired XPCS data are quantified experimentally, and compared to the theoretically expected noise levels of the correlation functions.
71(2000); http://dx.doi.org/10.1063/1.1288256View Description Hide Description
We describe two electro-optical systems for adaptive focusing of linearly polarized light. The aperture size is 5 mm and the focal length can be varied from 1 to 4 m for wavelengths from 0.663 to 0.85 μm. The first is a commercially available system including a PC compatible control unit and software, and an adaptive liquid crystal lens. The other is an experimental system consisting of a self-contained unit with an autonomous power supply and an adaptive lens. The safe operating limit in the visible region is 10 W/cm2 with a transmission of 70% without antireflection coating. The switching speed of focus variation from 2 to 1 m and from 1 to 2 m is 780 and 860 ms, respectively. The operating principles of the spherical adaptive lenses and their control units are described. Phase aberrations of the lenses were measured by a Zygo phase shifting interferometer, and the results are presented.
71(2000); http://dx.doi.org/10.1063/1.1288263View Description Hide Description
Two lasers are phase locked together by injection locking, where radiation between the two lasers is exchanged using a copperprism as a beam folding device in the resonators. Extraction of the output radiation is achieved by a common output coupler. As this method is only utilizing reflective optics (except for the output coupler), it is also practical as a technique for phase-locking lasers with powers in the multikilowatt range. Single frequency oscillation is only achieved if the lengths of both resonators are within a so-called locking range. This can only be fulfilled if the length of one resonator is actively controlled. The method of detection of the phase difference between the lasers and actively controlling the length of one resonator is presented as well as its implementation into a digital signal processor. Stable phase locked operation is achieved and proved through detection of twice the intensity in the far field in the phase-locked regime.
71(2000); http://dx.doi.org/10.1063/1.1287633View Description Hide Description
We have devised an all-optical setup for the generation of two phase-locked laser fields with a frequency difference of 3 GHz using only standard optics and two acousto-optical frequency shifters, that are operated at 253 MHz in sixtupel pass. The spectral width of the beat frequency is measured to be 300 Hz (full width at half maximum) limited by the resolution bandwidth of the spectrum analyzer. We routinely obtain an overall efficiency of more than 15% and demonstrate that the frequency shifted light can be further amplified by injecting it into additional “slave” lasers. This setup provides a low-cost alternative over conventional methods to generate laser fields with difference frequencies in the GHz domain, as for example, used in laser spectroscopy,laser cooling and trapping, and coherent manipulation of atomic quantum states.
71(2000); http://dx.doi.org/10.1063/1.1288265View Description Hide Description
We have constructed a cw dye master oscillator/dye pulse-amplifier system that generates 4 ns, 100 mJ pulses with a repetition rate of 30 Hz. The output pulse has a bandwidth of 275 MHz. Backward-stimulated Brillouin scattering is used to control the growth of amplified spontaneous emission(ASE). The content of ASE in the final output is under our detection limit for the entire tuning range.
71(2000); http://dx.doi.org/10.1063/1.1287049View Description Hide Description
For the generation of a well defined prepulse in high intensity laser plasma interactions, we implemented a newly developed prepulse device into a multiterawatt titanium:sapphire amplifier system. The temporal delay between the prepulse and main pulse is variable between −350 ps and + 100 ps. If the laser pulse is focused on a target, the spatial overlap of the two pulses in the focal point is corrected automatically over the entire range. It is possible to attenuate the main pulse as well as the prepulse independently of each other by more than one order of magnitude. We investigated the dependence of the emission of the laser produced plasma of a silicon target on the delay between the prepulse and main pulse. An increase of the yield of the emission by a factor of 2 for a temporal delay of several tens of picoseconds between the prepulse and main pulse was measured.
71(2000); http://dx.doi.org/10.1063/1.1287043View Description Hide Description
We have measuredoptical Kerr effect responses, transmission spectra, and emission properties due to two photon absorption for 17 kinds of optical glass materials and have evaluated their capabilities as an optical Kerr shutter (OKS) for time-resolved luminescencespectroscopy. There is a tendency that the larger the refractive index and dispersion are, the higher the electronic response of optical Kerr effect becomes. Transmission efficiency as high as several percents with the time resolution of 350 fs is obtained using a highly glass plate under 200 kHz repetitive pulse excitation. The time-resolved luminescence spectrum of Zn-tetraphenylporphyrin dye solution is demonstrated using the combination of the OKS and a monochromator equipped with a charge coupled device detector.
- PARTICLE SOURCES, OPTICS and ACCELERATION
71(2000); http://dx.doi.org/10.1063/1.1287044View Description Hide Description
A novel coil system is described that enables the magnetic angle-changing technique to be used at higher energies. The system consists of coils of wire that are combined with specially shaped iron cores in such a way as to preserve localization of the magnetic field. The system extends the operational energy range of previous systems consisting of solenoids alone by a factor of 25. Measurements of the electron impact excitation of the states of helium are presented as an example of the use of the device.
71(2000); http://dx.doi.org/10.1063/1.1287635View Description Hide Description
We have performed measurements of the spatial distribution of current in various alkali and reactive ion beams over the energy range 5–600 eV using a Faraday cup. Ion beamcurrent densities have been extracted from these measurements using a simple deconvolution procedure. Our results reveal that the beams are Gaussian in shape with a constant width, σ, for energies greater than approximately 75 eV and for all ion species investigated. This width is consistent with that determined from the distribution of oxygen on a Cu(001) crystal after an ion beam deposition,measured using Auger electron spectroscopy. Using the measurement technique outlined in this article, together with the linear relationship between current density and Faraday cup current, it is possible to determine the beam current density using a single current measurement.
71(2000); http://dx.doi.org/10.1063/1.1288264View Description Hide Description
We report on the design, construction, and magnetic field measurements of a system of high field sextupole magnets made from NdFeB compounds. The magnets are utilized as a focusing system for neutral hydrogen (or deuterium) atoms in a polarized atomic beamsource based on Stern–Gerlach spin separation. Each magnet consists of 24 segments of permanently magnetizedmaterial differing in remanence and coercivity to reduce demagnetization. According to quadratic extrapolation to the pole tip the magnetic flux density reaches values of up to T. Three-dimensional field calculations using the MAFIA code were carried out to optimize the magnet performance and to avoid demagnetization by selecting appropriate materials for the individual segments. Measurements of the radial, azimuthal, and longitudinal magnetic flux density distributions were carried out by means of a small Hall probe The measurements with the small probe permitted to extract experimentally higher order multipole components very close (∼100 μm) to the inner surface. Experimental values obtained are compared to predictions based on MAFIA calculations and on the Halbach formalism.
71(2000); http://dx.doi.org/10.1063/1.1288261View Description Hide Description
We describe a long-lived, bright and intense rubidium atomic beamsource based on a previously published recirculating candlestick design for sodium, with several modifications and enhancements. The device operates for thousands of hours without maintenance, with brightness of
- NUCLEAR PHYSICS, FUSION and PLASMAS
71(2000); http://dx.doi.org/10.1063/1.1288258View Description Hide Description
The design of a flow cell that is applicable to pulse radiolysis/transient absorption experiments on supercritical water is described. The cell is designed to minimize dead volume and prevent the accumulation of radiolytic products. It is also necessary to minimize emission and absorption of sapphire windows from high energy electron beam irradiation. To obtain an optical throughput of the inner diameter is 6 mm, and distance between windows is 25 mm. The effective optical path length is 20 mm for irradiation from the side through a thin Hastelloy wall. Belleville spring washers were used to keep a constant force on the 3 mm sapphire windows, which were sealed to the Hastelloy body with copper gaskets. An application of this cell to measurements of solvated electrons in supercritical water is demonstrated.
71(2000); http://dx.doi.org/10.1063/1.1287340View Description Hide Description
The first measurements of plasma parameters in a discharge created by electron cyclotron resonance heating in a toroidal cusp configuration are discussed along with the predictions of particle orbits and confinement from a guiding center model. The configuration appears to be suitable for basic investigations of the physics of plasmaflows and instabilities around a magnetic null, hence of magnetic reconnection.
71(2000); http://dx.doi.org/10.1063/1.1287045View Description Hide Description
We describe a technique for producing a Maxwell–Boltzmann electron energy distribution using an electron beamion trap (EBIT). The technique was implemented on the Lawrence Livermore EBIT to simulate Maxwellian plasmas. We discuss technical and experimental issues related to these simulations. To verify the fidelity of the quasi-Maxwellian, we have measured line emission due to dielectronic recombination (DR) and electron impact excitation (EIE) of heliumlike neon, magnesium, and argon for a range of simulated electron temperatures. The ratio of DR to EIE lines in heliumlike ions is a well understood electron temperature diagnostic. The spectroscopically inferred quasi-Maxwellian temperatures are in excellent agreement with the simulated temperatures.
71(2000); http://dx.doi.org/10.1063/1.1287751View Description Hide Description
Magnetic multipole plasma confinement geometries employing permanent magnets are commonly used to confine laboratory plasmas and improve uniformity. Here we describe the design and operation of a particularly simple and low-cost kind of current-driven multipole system that makes use of a similar magnetic field structure but does not require permanent magnets. The magnetic field is generated by current carrying wires attached to the inside wall of a cylindrical supporting structure to provide a magnetic field resembling that of permanent multipole magnetic homogenizers. We demonstrate the use of the current-driven homogenizer to improve the uniformity of the radial density profile of a vacuum arc generated metalplasma. Our configuration of metalplasma gun, macroparticle filter, and current-driven homogenizer was used for thin film deposition, and was able to produce a relatively homogeneous film thickness profile while maintaining a deposition rate comparable to that at the peak point of the freely expanded beam at the same location.
Simultaneous measurements of plasma parameters in the Saha Institute of Nuclear Physics Tokamak using a new low cost optoisolator71(2000); http://dx.doi.org/10.1063/1.1287042View Description Hide Description
In the Saha Institute of Nuclear PhysicsTokamak (SINP-TOKAMAK), simultaneous measurement of signals from some diagnostics, like, electrostatic probes, etc., is not possible because pickup noises due to the formation of ground and other loops affect them quite severely. To avoid these loops and also to isolate some diagnostics from the ground, we developed a new type of optical isolator using a very low cost optical isolator chip (MC2TE made by Motorola) in a novel configuration. We continuously kept the light emitting diode on and used a feedback arrangement at the receiving side of the photoemitted signal for achieving good isolation for very low signals with reasonably high frequency response and good linearity. The method is highly cost effective in low budget tokamak experiments.