Volume 71, Issue 7, July 2000
Index of content:
- OPTICS; ATOMS and MOLECULES; SPECTROSCOPY
71(2000); http://dx.doi.org/10.1063/1.1150667View Description Hide Description
Focusing characteristics of asymmetrically bent single crystal mirrors are discussed in the context of fabricating an optic element for an helium atom microscope. We demonstrate the principle that deforming a clamped, elliptical, single crystal under electrostatic pressure can produce submicron focusing of an helium beam. We present a systematic procedure that may be used to fabricate high precision mirrors close to the Cartesian ideal of any chosen optical configuration. In particular, imaging systems with asymmetric mirror profiles are discussed. Results are independent of crystal characteristics and can be adapted to fit a range of experimental geometries. The calculations indicate that mirror-induced aberrations can be eliminated to fourth order by use of a single actuation electrode in an ideal system.
71(2000); http://dx.doi.org/10.1063/1.1150668View Description Hide Description
A differential coating method is described for fabricating high-performance x-ray microfocusing mirrors. With this method, the figure of ultrasmooth spherical mirrors can be modified to produce elliptical surfaces with low roughness and low figure errors. Submicron focusing is demonstrated with prototype mirrors. The differential deposition method creates stiff monolithic mirrors which are compact, robust, and easy to cool and align. Prototype mirrors have demonstrated gains of more than in beam intensity while maintaining submilliradian divergence on the sample. This method of producing elliptical mirrors is well matched to the requirements of an x-ray microdiffraction Kirkpatrick–Baez focusing system.
71(2000); http://dx.doi.org/10.1063/1.1150669View Description Hide Description
Enhancement of the quality of images or maps obtained using micro x-ray fluorescence spectrometry requires knowledge of the flux distribution profile of the primary x-ray beam. A simple procedure based on a wire scan was developed for the determination of an x-ray microbeam profile. This technique is simple and can be used for a given excitation/detection geometry without any change in spectrometer configuration. However, the result of such a scan is a convolution of the beam profile with the shape of the wire. To restore the actual distribution of photon flux density, a nonlinear least-squares procedure was adapted to model the profile using a parametric function to describe the flux distribution. Among the algorithms tested, a sigmoid function was found to be the best for describing the beam profile. The results of calculations were verified by measurements using a charge coupled devicecamera. Reasonably good agreement was obtained between the two different techniques. The results of this work confirmed that a wire scan combined with a mathematical restoration procedure is useful for describing the actual profile of an x-ray microbeam created using a pinhole collimator. An advantage is that the microbeam profile is calculated at the location of the measured samples. The results indicated that the value could be used to select the optimum function to describe the microbeam profile. The procedure can also be used to evaluate the quality of a pinhole collimator. We plan to use measured microbeam profiles in combination with mathematical procedures to improve the spatial resolution of linear scans of patterned samples.
A simple high-resolution on-line x-ray imaging crystal spectrograph for laser–plasma interaction studies71(2000); http://dx.doi.org/10.1063/1.1150670View Description Hide Description
A simple, high-resolution x-ray imaging crystal spectrograph is described for on-line measurements and analysis of x-ray emission from laser-produced plasmas in the wavelength range of ∼5–12 Å. The spectrograph uses a planar TAP crystal for dispersion and a phosphor-coated optical fiber plate followed by an image intensifier and a charge-coupled device camera for detection. The spectrograph covers a wavelength range of ∼1.4 Å in a single shot about a desired wavelength setting with a spectral resolution of ∼0.012 Å. It also has a design flexibility to cover the full spectral range of 5–12 Å in a single shot with a spectral resolution of ∼0.040 Å. The spectrograph is particularly suited for studies involving low vacuum environment, e.g., plasma interaction with a background gas, laser interaction with a gas jet, etc., which preclude the use of other on-line detectors such as open-ended microchannel plates. An example of space resolved x-ray emission from laser-produced magnesium plasma expanding in a helium gas is also presented.
71(2000); http://dx.doi.org/10.1063/1.1150671View Description Hide Description
A new detection system for extreme ultraviolet (XUV) and vacuum ultraviolet (VUV)spectroscopic measurements, in the energy range of 7–2500 eV, is demonstrated. A photonstimulated luminescence mechanism in previously used in image plates (IPs) for hard x rays and particle beam detection, is used for detecting the XUV–VUV spectra. The IP detection system proved to be one to two orders of magnitude more sensitive than conventional photographic glass plates. The measured dynamic range of the IP system (four to five orders of magnitude) is wider than any other available XUV recording media. The internal resolution of the IP system (50 μm) is slightly lower than photographic glass plate resolution, restricted by scattering processes in the IP during their readout procedure. Theoretically, much higher resolution (∼5 μm) could be obtained in the XUV–VUV region by using thinner IPs with better scanning systems.Image plates are flexible, easy to handle, and reusable. No dark room loading and developing conditions are needed and good reproducibility is obtained. All these characteristics make the image plate a very suitable recording medium for XUV–VUV spectroscopy.
Doppler-free modulation transfer spectroscopy of rubidium transitions using a frequency-doubled diode laser blue-light source71(2000); http://dx.doi.org/10.1063/1.1150672View Description Hide Description
A frequency-doubled diode laserspectrometer in the blue-wavelength region with a continuous-frequency tuning range of up to ∼7 GHz has been developed, comprising a powerful fundamental light source based on a master/slave laser combination coupled with a crystal for second-harmonic generation in a power-enhancement cavity. It has been employed for the observation of saturated absorption spectra in hyperfine transitions of atomic rubidium with the aid of a modulation transfer spectroscopic technique, and also the frequency stabilization to one of them has been demonstrated, suggesting the possibility of the establishment of a precision optical frequency marker in the blue range. A relative stability of better than is estimated based on the locked error signal in the preliminary stabilization. With a further study of the frequency instability and uncertainty, in addition to supplementary improvements of apparatus, one may expect a realization of a stabilized laser in the blue-spectral domain. Details of the developed blue-light source are described and the demonstrated results of the rubidiumspectroscopy are also described and discussed.
71(2000); http://dx.doi.org/10.1063/1.1150673View Description Hide Description
For wave front distribution measurements of excimer laser radiation, a Hartmann–Shack wave front sensor with subapertures and a field of view adapted to the lateral laser beam profile was utilized. The peak-to-valley wave front resolution is λ/12 and the dynamic range 100λ per subaperture at 248 nm. Time-averaged propagation characteristics were determined for several combinations of resonator configuration and laser medium from the ns down to the fs pulse length regime. Above that, time-resolved measurements were carried out on a KrF excimer laser, showing a reduction of the beam divergence during the laser pulse by a factor of 1.6. This transient behavior is discussed on the basis of a simple geometrical model. The theoretical results are in qualitative agreement with the experiment.
71(2000); http://dx.doi.org/10.1063/1.1150715View Description Hide Description
We demonstrate a new heterodyneMichelson interferometer design for displacement measurements capable of fringe interpolation accuracy of one part in 36 000. Key to this level of accuracy are the use of two acousto-opticmodulators for heterodyne frequency generation and digital signal processing demodulation electronics. We make a direct comparison of our interferometer to a commercial interferometer based on a Zeeman-stabilized laser, and show that the residual periodic errors in ours are two orders of magnitude lower than those in the commercial unit. We discuss electronically induced optical cross talk and optical feedback as sources of periodic error. Our new interferometer is simple, robust, and readily implemented.
New design of the variable angle infrared spectroscopic ellipsometer using double Fourier transforms71(2000); http://dx.doi.org/10.1063/1.1150674View Description Hide Description
A new type of incidence-angle-variable infrared spectroscopicellipsometer working in the 2100–10 000 wave number range has been designed and constructed. For the system, the analyzer and polarizer were driven by two microstepping motors having hollow shafts and rotating synchronously with a speed ratio of 2:1, i.e., The incidence angle can be varied from 30° to 90° with an accuracy of 0.01°. The doubled Fourier transforms as both functions of the wave number and the azimuthal angles of the polarizer and analyzer were carried out and integrated in the system. Two sets of ellipsometric parameters measured in the experiment have been used to test the data self-consistency of the system. In data reduction, the error arising from the slight anisotropy of the stray light was corrected. In application, the complex dielectric function of the Au film was measured with a data accuracy better that 1% in the entire spectral range.
71(2000); http://dx.doi.org/10.1063/1.1150675View Description Hide Description
A microinterferometric backscatterdetector (MIBD) has been developed to perform subnanoliter volume refractive index measurements using a simple, folded optical train based on the interaction of a laser beam and a fused silica capillary tube. Positional changes of the interference pattern extrema (fringes) allow for the determination of at the level, corresponding to 5.3 pmole or 0.48 ng of solute, when thermal noise is controlled at MIBD is relatively path-length insensitive for capillaries ranging in inner diameter from 75 to 775 μm, allowing a large range of detection volumes, from 350 pL to 40 nL, to be produced. A theoretical model of the microinterferometric backscatterdetector has also been developed and evaluated and has been found to be in agreement with experimental data. This model indicates increased sensitivity of the instrument as the wavelength of the probe beam and the wall thickness of the capillary tube are reduced.
- PARTICLE SOURCES, OPTICS and ACCELERATION
71(2000); http://dx.doi.org/10.1063/1.1150676View Description Hide Description
A facility is described to record the blocking pattern of a single crystal produced by 400 keV protons. By this we aim at a reliable, fast, and careful procedure to align single crystals to the incoming ion beam. The two-dimensional position sensitive detector (2D-PSD) system from Roentdek GmbH Frankfurt/M, Germany, is based on three microchannel plates arranged in a Z stack. The essential shortcoming of this detector is its low sensitivity to high energy (>200 keV) projectiles. To reproduce the pattern formed by 400 keV protons on the 2D-PSD we had to decrease their energy to a few tens of kiloelectron-volts by a stopping foil of 3.5 μm Mylar®. This foil has the additional advantage of absorbing slow projectiles and sputtered atoms emerging from the target. On their way to the detector, these slow particles, in case they are charged, would be strongly influenced by electric fields and would thus obscure the true pattern. At present, our facility allows us to routinely align crystals for channeling measurements accurately, fast and, for most applications, with an acceptable amount of radiation damage.
71(2000); http://dx.doi.org/10.1063/1.1150677View Description Hide Description
The development of a high-energy neutral particle measurement system for ion temperature measurements and high-energy particle confinement analysis during neutral beam injection and ion cyclotron resonance frequency heating experiments in the large helical device (LHD) is described here. We have been improving the time-of-flight neutral particle analyzer for the LHD which had been developed in ENEA Frascati. The control and data acquisition systems were designed to be suitable for long discharges in the LHD. The horizontal and vertical movable stage is prepared to investigate pitch-angle distribution and loss cone. We have resolved many difficulties at installation, for example, the strong leakage magnetic field from the LHD and the limitations of magnetized materials near the LHD, the quench of the superconducting magnetic field, the narrow viewing port, and the fully remote control system. The preliminary results in plasma experiments are also described here.
71(2000); http://dx.doi.org/10.1063/1.1150678View Description Hide Description
The absolute detection efficiency of a continuous channel electron multiplier (DeTech 407-EIC) has been measured for the noble gas ions and with energies up to 4.9 keV. For all ions, the detection efficiency initially rises linearly as a function of velocity. Our results and a reanalysis of literature data show that the threshold velocity below which a multiplier does not respond to impinging ions cannot be regarded as independent of the ion’s atomic number, Z, as previously reported. Instead, the threshold velocity is proportional to The experimentally obtained efficiency-versus-velocity curves for our detector have been analyzed in the region of linear increase. The resulting expression for the detector efficiency, ε, is a function of the ion’s atomic number and velocity, and can be written as with a, b, c, and n being constants.
A sensitive dispersion interferometer with high temporal resolution for electron density measurements71(2000); http://dx.doi.org/10.1063/1.1150679View Description Hide Description
A dispersion interferometer with a sensitivity of up to fringe shifts and a time resolution of about 1 ns was developed. This time resolution exceeds previous systems by three orders. The operation of the dispersion interferometer and its characteristic properties are discussed and the experimental setup of the installation is explained in detail. The stability of the interferometer was tested and it was shown that its operation point drifts less than 0.25 rad/h. The interferometer was used to investigate the high power ion diodes at the pulse generator KALIF, where focused protonbeams with power densities of up to 1 TW/cm2 are generated. For the very first time the electron line density inside the diode gap, where the protons are accelerated, could be directly measured. The line density was on the order of several cm−2. A straightforward estimation of the line density using a simplified diode model proved to be in excellent agreement with the results. The dispersion interferometer can be used for sensitive electron density measurements on nanosecond time scales. Further developments to improve the sensitivity are discussed.
- NUCLEAR PHYSICS, FUSION and PLASMAS
Efficient multichannel Thomson scattering measurement system for diagnostics of low-temperature plasmas71(2000); http://dx.doi.org/10.1063/1.1150680View Description Hide Description
An efficient multichannel Thomson scattering measurement system for measuring the electron energy distribution function in low-temperature processing plasmas was constructed. It consists of a specially designed triple-grating spectrometer, equipped with a spatial filter for eliminating the interference due to Rayleigh scattering, and an image-intensified charge-coupled-device (ICCD) camera working in the photon counting mode. The spectrometer produces a dispersed Thomson spectrum with the Rayleigh interference highly suppressed, thereby enabling us to operate the ICCD camera in the photon-counting mode giving a high signal-to-noise ratio; the measured Rayleigh light suppression ratio was at the center wavelength. Proper operation of the system was demonstrated in the measurements for a low-pressure (20–100 mTorr) inductively coupled Ar plasma.
71(2000); http://dx.doi.org/10.1063/1.1150681View Description Hide Description
This article describes how to extract accurate information about a plasma from a capacitively coupled planar probe that is biased using pulsed radio-frequency excitation. The conditions necessary to observe correct saturation of the probe current are investigated, particularly the use of correct geometry and biasing for the guard ring. With these precautions the probe is an effective diagnostic for electron tail temperature at energies beyond those probed by conventional cylindrical probes. The dynamic response of the probe is investigated using conventional sweep voltages and shows the onset of displacement current and inertial effects associated with ions and electrons. In addition the effect of insulating films on the probe surface is examined, showing how the probe continues to operate even when it is coated. Characteristic changes caused by the presence of an insulating film give information about its electrical properties and its thickness.
71(2000); http://dx.doi.org/10.1063/1.1150682View Description Hide Description
A simple economical 2.45 GHz microwave system has been developed and utilized for preionization on the Korea Advanced Institute of Science and Technology (KAIST)-TOKAMAK. The magnetron microwave source was obtained from a widely used, household microwave oven. Since ac operation of the magnetron is not suitable for tokamak application, the magnetron cathode bias circuit was modified to obtain continuous and stable operation of the magnetron for several hundred milliseconds. Application of the developed microwave system to KAIST-TOKAMAK resulted in a reduction of ohmic flux consumption.
71(2000); http://dx.doi.org/10.1063/1.1150683View Description Hide Description
Observations on steady state ion-rich sheath around a negatively biased grid in a double plasma device have been made in multicomponent plasma with negative ions. A hump in the potential profile is found to occur at the sheath edge. The characteristics of the hump depend on the grid voltage, source biasing voltage, density difference in the two chambers, and the negative to positive ion concentration ratio. It is argued that resonance which is coupled with the beam ions and background ions is associated in the excitation mechanism to create the nonsymmetrical structures in the plasma potential profile. Below the critical concentration ratio, the hump grows in height and above the critical ratio, the width of the hump increases but the height decreases and ultimately it vanishes. The variation in the hump potential profile due to the injection of negative ions can be explained by the ion momentum exchange and by their dynamics.
- BASIC PHENOMENA
71(2000); http://dx.doi.org/10.1063/1.1150684View Description Hide Description
The operation of a novel monolithic sapphire transducer from room temperature to cryogenic temperature is presented. The transducer is a microwave sapphire resonator that senses the motion of internal acoustic resonances through the interaction between electric and acoustic fields. The system is noncontacting and has low mechanical and electrical losses. Also, the microwave characteristics of sapphire are used as an inherent temperature sensor. High mechanical quality factors of and were attained at 300 and 4 K, respectively.
- MICROSCOPY and IMAGING
Subresolution axial distance measurements in far-field fluorescence microscopy with precision of 1 nanometer71(2000); http://dx.doi.org/10.1063/1.1150685View Description Hide Description
Fluorescent objects closer than the diffraction resolution limit can be distinguished in far-field microscopy provided they feature different emission spectra. Utilizing the superior axial resolution of 4Pi-confocal microscopy of 100–150 nm, we investigate the precision with which fluorescence objects with subdiffraction axial distance can be measured in the far field. At a wavelength of 820 nm distances on the order of 60 nm between beads and a monomolecular Langmuir–Blodgett layer were determined with a precision of 1.2 nm within 3.2 s. The reduced spatial extent of the 4Pi-confocal point-spread-function improves the precision of colocalization measurements in double stained specimens and opens up the prospect on far-field fluorescence profilometry with (sub) nanometer height resolution.