Volume 71, Issue 6, June 2000
Index of content:
- OPTICS; ATOMS and MOLECULES; SPECTROSCOPY
71(2000); http://dx.doi.org/10.1063/1.1150661View Description Hide Description
Integrated diffractive/refractive microlens with dual focus point is introduced in detail in the view of design, microfabrication, and testing. Two manufacturing methods—milling and deposition directly by focused ion beam technology is discussed and compared. It was shown by testing results that the deposition method is more suitable to form the refractive spherical/aspherical lens and milling is suitable for microfabrication of diffractive optical elements. Focusing spot size (full width at half maximum) are 0.55 and for refractive and diffractive lens, respectively, with NA 0.35 and 0.5.
71(2000); http://dx.doi.org/10.1063/1.1150439View Description Hide Description
The effect of strain upon both the fluorescence intensity ratio and the fluorescence decay-time–temperature-sensing techniques has been studied using Yb-doped fiber. The measured variation with strain of the former method is consistent with no strain sensitivity, while the latter method demonstrates a linear dependence on strain. A single optical sensor element using both these fluorescence techniques thereby offers an attractive way of providing temperature compensation in a simultaneous strain and temperature monitoring system.
Reflectance and transmittance measurements of anisotropically scattering samples in focusing Coblentz spheres71(2000); http://dx.doi.org/10.1063/1.1150440View Description Hide Description
A procedure for obtaining the correct hemispherical reflectance and transmittance of anisotropicallyscattering samples in focusing Coblentz spheres are presented. Consideration is taken not only to the angle-dependent detector efficiency and to multiple reflections between sample and detector, but a separation of the scattered light into a low and a high angle fraction is also introduced. To validate the formalism, the correction procedure is applied to six samples with different scattering characteristics in the visible and near-infrared region. The agreement with results from a double beam spectrophotometer, equipped with an integrating sphere, was found to be good. Without making the separation into high and low angle scattering, it was not possible to reach agreement between the results from the Coblentz and integrating spheres, and errors of the order of 10% could be obtained.
71(2000); http://dx.doi.org/10.1063/1.1150441View Description Hide Description
A novel method to measure low absorption of pulsed laser radiation by materials with high optical transparency is demonstrated. The absorbed energy generates a temperature gradient and thus a refractive index profile. A probe laser beam is deflected by the refractive index profile and its deflection is a measure of pure volume absorption without interference from surface effects. Numerical calculations were carried out to estimate the sensitivity of the discussed laser induced deflection (LID) arrangement and to optimize the path of the probe beam in relation to the irradiated field within the sample. The experimental results agree sufficiently with numerical calculations by finite element method combined with raytracing procedures. Because the transparency is not measured directly a calibration is necessary. To provide this, a sample with known bulk absorption coefficient and thermal properties was used. In order to compensate probe beam fluctuations in air outside the sample, a double beam arrangement was designed. The LID method is applied to investigate fused silica with high ultraviolet (UV) transparency under KrF excimer laser irradiation A He/Ne laser serves as probe beam source. Using sample dimensions of and UV beam cross sections of absorbed power down to 1 mW is detected. At typical irradiation conditions (50 s−1, 300 mJ/cm2), this value corresponds with an absorption coefficient of around The limitation of the absorption measurement depends on the input power of the pump laser.
A simple instrument for ultraviolet-visible absorption spectrophotometry in high temperature molten salt media71(2000); http://dx.doi.org/10.1063/1.1150442View Description Hide Description
An inexpensive (<US$5000) spectrophotometer of simple design was constructed from commercially available components for absorptionspectroscopic studies in molten chloride salts. The spectrometer consists of a replaceable fused silica cell of square exterior cross-section housed in a temperature-controlled ceramic tubular furnace of 13 mm inner diameter. Light communication to and from the cell occurs via a pair of 1 mm core silica-on-silica optical fibers from diametrically opposite sides. The light source is a pulsed xenon flash lamp and detection is accomplished by a photodiode array spectrometer card housed in a personal computer. The system has been operated up to temperatures of 950 °C. The effective spectral range is 280–650 nm at 900 °C; higher wavelengths can be accessed at lower operating temperatures. A spectral snapshot can be acquired in as little as 1 ms. With 1 s integration time, the intrinsic system noise level is absorbance units. The system is compact and energy efficient. Applications of the system are demonstrated with spectral studies of some metal chloride systems in an equimolar sodium and potassium chloride eutectic.
71(2000); http://dx.doi.org/10.1063/1.1150443View Description Hide Description
Experimental investigation of laser output characteristics of an elemental copper vapor laser using a cylindrical grid electrode has been carried out. The grid electrode, which is placed at the inside of a cathode and separated 2 cm away from a cathode, is used for increasing the output power by improving the discharge characteristics. When the grid electrode is coupled with a peaking capacitor, it is observed that the peak voltage (voltage hold-off) in the pulsed discharge wave forms increases by about 20%. Also, the initial part of the discharge current pulse is reduced. The laser average output power increased from 16 to 21 W. Additionally, when applying dc voltage (0–100 V) to the grid electrode, the output power increased by around 6%. The effect of employing the grid electrode and the mechanisms responsible for the improved characteristics are discussed.
71(2000); http://dx.doi.org/10.1063/1.1150444View Description Hide Description
We describe the implementation of a crossed graded parabolic x-ray mirror system that is suitable for incorporation into both conventional and high resolution x-ray diffractionsystems. A novel optical alignment system has been devised that permits independent adjustment of the positions and the angular settings of the mirrors. This alignment system is simple and inexpensive to construct, yet is mechanically stable and easy to use. The crossed mirror system has been successfully introduced into both proteincrystallography and high resolution x-ray diffractionsystems.
71(2000); http://dx.doi.org/10.1063/1.1150445View Description Hide Description
A new hardware compensation method reducing displacement measurement errors, caused by tilt of the index scale in a moiré linear encoder, has been developed. In conventional moiré linear encoders, the detectors are aligned perpendicular to the line of the moiré fringes and this structure is very sensitive to an unwanted tilt of the gratings. In this article, a newly designed grating, called a phase-modulated grating, is developed to compensate for nonorthogonal errors. By using the phase-modulated grating instead of a conventional index, it is possible to reduce nonorthogonal errors of the moiré linear encoder.
- PARTICLE SOURCES, OPTICS and ACCELERATION
71(2000); http://dx.doi.org/10.1063/1.1150446View Description Hide Description
We report on the design, numerical modeling, and experimental testing of a cylindrical dielectric loaded traveling wave structure for charged particlebeam acceleration. This type of structure has similar accelerating properties to disk-loaded metal slow wave structures but with some distinct advantages in terms of simplicity of fabrication and suppression of parasitic wakefield effects. Efficient coupling of external rf power to the cylindrical dielectric waveguide is a technical challenge, particularly with structures of very high dielectric constant ε. We have designed and constructed an X-band structure loaded with a permittivitydielectric to be powered by an external rf power source. We have attained high efficiency broadband rf coupling by using a combination of a tapered dielectric end section and a carefully adjusted coupling slot. Bench testing using a network analyzer has demonstrated a power coupling efficiency in excess of 95% with bandwidth of 30 MHz, thus providing a necessary basis for construction of an accelerator using this device. We have also simulated the parameters of this structure using a finite difference time domain electromagnetic solver. Within the limits of the approximations used, the results are in reasonable agreement with the bench measurements.
Pulse radiolysis of liquid water using picosecond electron pulses produced by a table-top terawatt laser system71(2000); http://dx.doi.org/10.1063/1.1150659View Description Hide Description
A laser based electron generator is shown, for the first time, to produce sufficient charge to conduct time resolved investigations of radiation induced chemical events. Electron pulses generated by focussing terawatt laser pulses into a supersonic helium gas jet are used to ionize liquidwater. The decay of the hydrated electrons produced by the ionizing electron pulses is monitored with 0.3 μs time resolution. Hydrated electron concentrations as high as 22 μM were generated. The results show that terawatt lasers offer both an alternative to linear accelerators and a means to achieve subpicosecond time resolution for pulse radiolysis studies.
Fast impedance spectroscopy: General aspects and performance study for single ion channel measurements71(2000); http://dx.doi.org/10.1063/1.1150447View Description Hide Description
In the present work, we propose a novel technical approach in time domain impedance spectroscopy enabling both enhanced time and current resolution by an improved data analysis and hardware setup. We introduce nonstationary time-to-frequency conversion methods such as short-time Fourier transform and wavelet transform resulting in an improved time resolution. The combination of a time domain impedance spectrometer with a patch-clamp amplifier enables the resolution of gigaohm impedance at low perturbation signal amplitudes. A fast impedance spectroscopy (FIS) setup is presented which is optimized for biophysical application of single ion channelmeasurements in supported biomembranes. The applicability and performance of the technique is first evaluated by simulations. It is then verified by measurements on model circuits which exhibit the characteristic key properties of single ion channelmeasurements. Here, FIS improves the time resolution by about three orders of magnitude down to milliseconds. The general aspects derived in this work are also valid in other fields, where the spectral information of a perturbation applied on a system and its response is analyzed, e.g., mechanical impedance spectroscopy and microrheology.
71(2000); http://dx.doi.org/10.1063/1.1150618View Description Hide Description
In this work the possibility of using corona discharge as an ionizationsource in ion mobility spectrometry has been investigated. The results for both positive and negative polarity in air are consistent with the Townsend formula which states that is a linear function of V. The distribution profile of the corona ions has also been investigated and compared with that of the ionizationsource. Generally, the total ion current obtained from the corona ionizationsource was greater than that of the source by about an order of magnitude, which results in a better sensitivity and a higher signal-to-noise ratio. The positive and negative spectra of air were recorded and the positive spectra of acetone and dimethylmethyl-phosphonate were compared with that of In the negative mode, a number of new peaks, apart from those of reactant ions, were observed which are mainly due to nitrogen oxides, formed by corona discharge. This problem was minimized by increasing the interdistance of the electrodes.
Characterization of neutral density profile in a wide range of pressure of cylindrical pulsed gas jets71(2000); http://dx.doi.org/10.1063/1.1150619View Description Hide Description
The neutral density profile of cylindrical gas jets is measured with a Mach–Zehnder interferometer under a wide range of backing pressures. The sensitivity of this diagnostic together with the mathematical treatment of the data allows us to measure neutral densities for argon gas as low as for a 5 mm diam gas jet.
- NUCLEAR PHYSICS, FUSION and PLASMAS
Doppler-free two-photon excitation of Lyman-α fluorescence for the diagnostics of magnetically confined fusion plasmas71(2000); http://dx.doi.org/10.1063/1.1150620View Description Hide Description
Doppler-free two-photon excitation of hydrogen Lyman-α fluorescence is investigated as a possible laser-induced fluorescence(LIF) technique for the diagnosis of magnetically confined fusion (MCF) plasmas. A formal analysis is presented of the underlying atomic and plasma physics as well as of various practical aspects, such as parameter optimization and experimental precision. The latter is analyzed with regard to the photon noise and to the sensitivity of the fluorescence signals to the plasma and laser parameters. The diagnostic potential of the LIF technique described relies on its high spectral resolution. Thus, the absorption lines of the hydrogen isotopes H, D, and T are clearly separated from each other and can serve for isotope-selective density measurements. In addition, using a tunable laser system with small bandwidth, various plasma parameters can be inferred from the spectral line shapes, such as the neutrals’ temperatures or the effective charge number The polarization of the fluorescence can, under favorable circumstances, be exploited for magnetic field measurements. The photon statistics impose neutral densities above and thus make the diagnostic suitable primarily for the plasma edge. However, previous work has shown that it is applicable even in the plasma bulk of large machines if a neutral beam is used that generates sufficient neutral densities by charge exchange with the plasma ions. Variations or insufficient knowledge of the neutrals’ temperatures are found to seriously affect the precision of absolute isotopedensity measurements. They are rather uncritical, however, for the determination of the H/D/T density ratios, which are of prime importance for the burn control of large MCF devices. A notable sensitivity is also found to variations of the laser frequency detuning, which should be known and stable to better than 100 MHz. The perspectives demonstrated in the present study and the success of a first experiment on the plasma generator PSI 1 at IPP Berlin are considered to be sufficient motivation for testing the diagnostic on a tokamak or other MCF device.
Development of a novel fast frequency modulation scheme for the Joint European Torus multichannel reflectometer71(2000); http://dx.doi.org/10.1063/1.1150660View Description Hide Description
A novel frequency modulation scheme has been developed for the multichannel reflectometer used to measure density profiles and density fluctuations on the Joint European Torus (JET) tokamak. This reflectometer normally uses slow frequency sweeping (6 ms), combined with fixed-frequency operation, to measure the group delay, as well as plasma fluctuations, at ten different microwave frequencies. The novel scheme uses continuous frequency modulation on a time scale much shorter (1/65 μs) than that of plasma fluctuations, the main aim being to make the group delay measurement more robust against plasma fluctuations. This article discusses the theoretical background of the scheme, gives a detailed description of the system, and presents results from plasma measurements. Although the implementation used suffered from an excessive power-dependent group delay distortion (resulting in a positional error of 10 cm), the main objectives have been achieved.
Measurements of poloidal rotation velocity using charge exchange spectroscopy in a large helical device71(2000); http://dx.doi.org/10.1063/1.1150621View Description Hide Description
Absolute measurements of poloidal rotation velocity with the accuracy up to 1 km/s (2 pm in wavelength) were done using charge exchangespectroscopy in a large helical device. Radial profiles of the absolute Doppler shift of charge exchange emission with a beam are obtained from spectra measured with four sets of optical fiber arrays that view downward and upward at the poloidal cross section with and without neutral beam injection. By arranging the optical fiber from four arrays close to each other at the entrance slit, the apparent Doppler shift due to aberrations of the spectrometer and due to interference of the cold component (the charge exchange between He-like oxygen and thermal neutrals 8 pm from the charge exchange emission with a beam) can be eliminated from the measurements. The measured poloidal rotation velocity is 1–3 km/s in the electron diamagnetic direction at half of the plasma minor radius.
71(2000); http://dx.doi.org/10.1063/1.1150622View Description Hide Description
α particles emitted from an radioactive source at energies of 5.4 MeV generate bursts of about ten electrons when passing through an aluminized Mylar foil. Besides the typical surface barrier electrons, the energy spectra of the secondary electrons clearly reveal two additional peaks. One at 66 eV is ascribed to aluminum Auger electrons and another at 10.5 eV is attributed to the decay of aluminum volume plasmons. The well-resolved angular and energy distributions of these secondary electrons are used to calibrate the relative detection efficiencies of a large set of individual detectors of a complex multicoincidence system.
71(2000); http://dx.doi.org/10.1063/1.1150623View Description Hide Description
A new type of microstructure device for a gas radiation detector is proposed. This microstructure, the microhole and strip plate structure, merges the structures of a gas electron multiplier and a microstrip plate in one single plate. This design allows two-multiplication stages and a separation of the sensitive and the detection regions, with full optical positive feedback suppression. Simulations for gas gain and electron transparency of the microstructure are presented. Different applications are discussed.
71(2000); http://dx.doi.org/10.1063/1.1150662View Description Hide Description
The influence of secondary electrons released by gamma radiation from the cathode of a nitrogen-filled tube, on probability of electrical breakdown and memory curve, has been investigated. The results show that the nitrogen-filled tube is sensitive to gamma radiation only at longer afterglow periods in the case of overvoltage values grater than 50%. For shorter afterglow periods, the lower values of overvoltages can be used to separate the contribution of gamma rays.
71(2000); http://dx.doi.org/10.1063/1.1150624View Description Hide Description
The prospects of a miniature photoconducting capacitor array as a source for THz radiation are discussed. The device consists of an alternately biased capacitor array built on a semiconductor substrate and illuminated by a short laser pulse. The laser pulse creates a propagating plasma front inside the crystal volume by side illumination, and triggers the discharging of the capacitor by shorting out the photoconducting material between each capacitor plate. The sequential discharge of the capacitor array inside the expending plasma region can be considered as an interaction between a static wave and a superluminous ionizing front. In the present article we have demonstrated production of two cycle pulses with a center frequency of up to 1.5 THz. The device combines elements from the well-known “frozen-wave” generator and from the photoconducting switches array. The underlying physics of this radiation mechanism is very similar to the dc-to-ac conversion mechanism inside a gas-filled capacitor array which generates coherent microwave radiation.