Volume 70, Issue 12, December 1999
Index of content:
- OPTICS; ATOMS and MOLECULES; SPECTROSCOPY
70(1999); http://dx.doi.org/10.1063/1.1150096View Description Hide Description
Bending magnet beamlines at third-generation synchrotron sources combined with well-designed optics offer unique capabilities for providing high x-ray fluxes into relatively small focal spots. This article provides a description of the x-ray optics used in the Advanced Photon Source 1-BM beamline. The performance of these optics in terms of the delivered flux ( ph/s/100 mA at 10 keV), energy resolution with Si(111)], and focusing properties (spot size ≈0.25×0.60 mm) is compared with that expected from ideally reflecting and shaped optics.
70(1999); http://dx.doi.org/10.1063/1.1150097View Description Hide Description
In this research we have taken advantage of near-field scanning optical microscopy, a recently developed technique, to test the optical nature of GaAlAs semiconductor laserdiodes working at 780 nm. With this method, both the images of the topographic and the near-field intensity of the laser diodes can be simultaneously obtained. With the obtained results, we can analyze the variety of the geometric structure, the local near-field optical intensity, the propagating modes, and the near-field mode-field diameter at different working states of the laser diodes. Hereby, we can find the factors that affected the radiation cavity of the laser diode and explore its alive state.
70(1999); http://dx.doi.org/10.1063/1.1150098View Description Hide Description
An all-solid-state, low-cost device for fluorescent wavelength-ratiometric detection is described. Ultrabright light-emitting diodes were used as light sources. This allowed electronic modulation of the light, simple optical configuration, and miniaturization of the instrument. Narrow-bandpass interference filters were used for wavelength separation. Detection was accomplished by high-sensitivity, large-surface PINphotodiodes. An integrating double-ramp technique with fixed upper and lower thresholds was employed for conversion of the light intensities into time intervals. The duty ratio of the output signal was a function of the fluorescence intensity ratio. Additionally, the concentration of the fluorophore could be measured. The device could be easily designed as a battery-operated version. It could be used for a variety of ratiometric fluorescence measurements.
70(1999); http://dx.doi.org/10.1063/1.1150099View Description Hide Description
An in situ comparison of spectrophotometer scales of wavelength, spectral bandwidth, and regular spectral transmittance was made in 1997/98 towards the harmonization of spectrocolorimetric scales for the measurements in transmittance mode. It used a set of three neutral density glass filter standards and a crystal with sharp absorption bands as wavelength and spectral bandwidth standard. Measurements by 22 laboratories with a great diversity of instrumentation constituted a good testing reference data to examine the aspects in scale harmonization. The intercomparison reflects a widespread misunderstanding about levels of accuracy achieved and a generalized lack of use of service standard coverage in checking uncertainty levels along the range of the concerned metrological scales. The spectral bandwidth scale is usually not taken in appropriated attention. It is demonstrated that relative small discrepancies in the spectral bandwidth setting lead to large uncertainties in the measurements that may overexceed computed uncertainty values derived from wavelength and spectral transmittance scale uncertainties.
70(1999); http://dx.doi.org/10.1063/1.1150100View Description Hide Description
We describe our research on the employment of an infrared upconversion screen made of electron trapping material (ETM) in combination with the high sensitivity of the S-20 photocathode responsive to visible radiation to produce a streak camera arrangement capable of viewing and recording infrared incident pulses. The ETM-based upconversion screen converts 800–1600 nm infrared radiation to visible light which is viewed or recorded by the S-20 photocathode. The peak values of the upconversion efficiency are located at 1165 nm for CaS:Eu, Sm and 1060 nm for CaS:Ce, Sm. The present experiment showed time resolution was 12.3 ps for a CaS:Eu, Sm screen and 8.4 ps for a CaS:Ce, Sm screen. The minimum detectability is (minimum detectability of the coupled visible streak camera is Other parameters, such as spatial resolution and dynamic range, have also been measured and analyzed. The results show ETM can be used in the measurement of infrared ultrafast phenomena up to picosecond time domain. In consideration of the limited number of trapped electrons in ETM, the infrared-sensitive streak camera consisting of an ETM-based upconversion screen is suitable to operate in the single shot mode.
Systematic and statistical errors in correlation estimators obtained from various digital correlators70(1999); http://dx.doi.org/10.1063/1.1150101View Description Hide Description
A number of commercially available digital correlators is discussed with respect to algorithms, systematic errors, and statistical errors. Special attention has been paid to the determination of an influence of the temporal integration of measured signals on autocorrelation functions measured in digital correlators. A calculation of the standard deviation for statistical errors in correlation estimators has been performed. The result is compared with the experiment. Favorable noise properties of the multiple-tau correlator which have been reported in the literature are not confirmed.
70(1999); http://dx.doi.org/10.1063/1.1150102View Description Hide Description
A new mass selection technique has been developed, which allows one to size-select charged particles from atoms to nanoparticles of almost unlimited size. It provides a mass resolution of and a transmission of about 50% for the selected size, both independent of mass. The technique is based on the time-of-flight principle, but differs fundamentally from time-of-flight mass selection normally used. The basic idea is to use time-limited high voltage pulses to displace laterally a preaccelerated ion beam, without changing its direction or shape. As the movement of the ions perpendicular to their original beam direction is independent of their forward velocity, mass resolution and calibration does not depend on the ion beam energy. A mass selector of this type has been implemented successfully into a cluster deposition experiment and has proven to be reliable and simple to operate.
- CHARGED PARTICLE SOURCES, OPTICS and ACCELERATION
Self-excitation and operational characteristics of the crossed-field secondary emission electron source70(1999); http://dx.doi.org/10.1063/1.1150103View Description Hide Description
We have investigated the crossed-field secondary emission (CFSE) electron source which is of a magnetron type with smooth cylindrical electrodes and axial applied magnetic field. It initiates at the negative slope of the high voltage pulse but further current production is maintained by a self-sustained secondary electron emission regardless to the voltage pulse shape. The output electron beam is tubular with a thin ∼1 mm wall. This article is concerned mainly with the identification of the mechanisms governing the excitation and generation of the electron beam and with the determination of the principles upon which the “optimal” CFSE electron source should be designed. We have demonstrated that the CFSE diode starts operation in a self-excitation regime (i.e., without application of the primary current) provided there is a partial trapping of the multiplying electrons inside the diode boundaries. The required axial decelerating force can be established with the use of either axial electric or nonuniform magnetic fields. Amongst all of the practical methods tested (shifting of the anode with respect to the cathode, double diode, diodes with ferromagnetic parts, use of the nonuniform external magnetic field), the diode with a ferromagnetic ring insert inside the cathode cylinder has been shown to be the most successful. It has generated an ∼240 A electron beam with a perveance of ∼85 μA/V3/2. The operating range of the CFSE diode is limited by both low and high magnetic fields. The lower limit arises from a necessity to comply with a Hull cutoff condition. The upper limit is determined by the time required for development of an electron avalanche. A secondary electron emission mechanism of current production in the CFSE diode allows the diode to operate in an oscillating regime when the applied magnetic field is higher but close to the Hull cutoff value. It has thus been possible to generate 100% density modulated electron beams at a modulation frequency of in our present experiments with the possibility of further increases up to A geometrical scaling law for the CFSE diodes has been deduced empirically. It states that the perveance of the output electron beam is proportional to the geometrical factor where is the cathode diameter, is an effective diode gap, and is the diode length. The scaling law provides a tool for designing the CFSE diodes and predicting the ultimate beam currents. For a practical size of device, this electron current could be as high as ∼1 kA.
Simultaneous energy distribution and ion fraction measurements using a linear time-of-flight analyzer with a floatable drift tube70(1999); http://dx.doi.org/10.1063/1.1150104View Description Hide Description
A technique for simultaneous energy distribution and ion fraction measurements using a linear time-of-flight analyzer with a floatable drift tube is described. Analytical expressions for the relative collection efficiency and viewing region of the apparatus are developed as functions of the analyzed particle reduced energy and dimensionless apparatus parameters. The method was applied to studies of large-angle scattering of singly charged oxygen ions incident on Au(110), and carried out at the Oak Ridge National Laboratory’s Multicharged Ion Research Facility. Energy distributions of the scattered projectiles and the negative ion fraction are presented as a function of scattered projectile energy. As a by-product of the measurements, the relative ion detection efficiency of the particle detector was reconstructed as a function of ion impact energy on the detector.
70(1999); http://dx.doi.org/10.1063/1.1150105View Description Hide Description
A new type of variable-frequency radio-frequency quadrupole (RFQ) linac has been constructed as a new preinjector for the RIKEN heavy-ion linac (RILAC). The RFQ resonator, based on a folded-coaxial structure with a movable shorting plate, is compact even in a low frequency region below 20 MHz. It accelerates ions with mass-to-charge ratios of 5.3 to 26.4 in the energy range up to 450 keV per charge, by varying its resonant frequency from 17.4 to 39.0 MHz. Moreover, the power loss is small in the low frequency region; the rf power consumption in cw mode is 7 kW at 17.4 MHz, and it increases to 30 kW at 39.0 MHz at the maximum intervane voltage of 36.8 kV. We initiated this project in 1991, and installed the new preinjector consisting of an electron cyclotron resonance ion source of 18 GHz and this RFQ in 1996. Since the installation, the beam intensity has become larger by more than 1 order of magnitude than that formerly obtained with the Cockcroft–Walton preinjector. Recently, the maximum power of the beam extracted from the RILAC and that from the ring cyclotron has reached 560 W and 2 kW, respectively.
70(1999); http://dx.doi.org/10.1063/1.1150106View Description Hide Description
A power supply for “triggerless,” repetitively pulsed cathodic arcs has been developed. It is based on a thyristor-switched, high-voltage, high-current, pulse-forming network (PFN). It can provide high pulsed currents (up to 2 kA), with duration of 600 μs, and pulse repetition rate of up to 10 Hz. Higher repetition rates are possible at lower current. The rectangular pulse shape and amplitude are reproducible to within a few percent. Cathodic arc initiation is extremely reliable because the charging voltage is much higher than the minimum starting voltage for the triggerless arc initiation method. The energy utilization efficiency is very high by intentionally mismatching load and PFN impedances and by using an efficiency-enhancing diode; the stored energy is dissipated primarily in the arc.
70(1999); http://dx.doi.org/10.1063/1.1150107View Description Hide Description
We have developed a technique for detecting single-ions that uses CR-39 plastic. Chemical etching of the plastic enables us to visualize both the incident sites and the existence of the single-ion incidences as etch pits. Using this technique, we obtained a singularity rate of 88.3%.
- NUCLEAR PHYSICS, FUSION and PLASMAS
70(1999); http://dx.doi.org/10.1063/1.1150108View Description Hide Description
Magnetized electron columns are a valuable experimental tool used to study two-dimensional (2D) fluid phenomena. Traditionally, the electrons have been generated with thermionic sources, typically limiting the initial electron distribution to one filled circle and thereby restricting the range of accessible fluid phenomena. Here, we describe a new electron source based on a cesium antimonide photocathode that can generate more complicated initial electron distributions. Experiments so far have focused on the stability of 2D vortex patterns.
70(1999); http://dx.doi.org/10.1063/1.1150109View Description Hide Description
The concept of a modular large size ion source is under investigation in our laboratory for the development of the very intense (tens of amperes) negative ion beams needed for neutral beam injection in thermonuclear fusion research. The basic idea is to develop a compact small ion source producing the required ion flux (20–30 mA/cm2) over a total surface of about 200 cm2 and designed in such a way that it can easily be juxtaposed with other identical modules. A large negative ion source of any size (up to several m2) and shape could be realized as a set of several modules. The anticipated advantage of this concept is the minimization of the risk inherent in a large extrapolation in size (e.g., ≈1.0 m2 for an ignition device to test engineering concepts) of the present ion sources. In this context, we have developed and tested a source module, called the DRIFT source, whose main properties are presented in this article. The particular magnetic field configuration of this module ensures, in a simplified way, a very good plasma confinement allowing operation of the source at very low filling pressures. Up to now, a current of 1 A (20 mA/cm2) 50 keV energy, 1 s pulse length, was obtained with cesium vapor seeding at 0.15 Pa sourcepressure with an arc power of 2.5 kW/l (12 kW).
70(1999); http://dx.doi.org/10.1063/1.1150110View Description Hide Description
Rotating magnetic islands produce fluctuations on a variety of diagnostics in magnetic fusion energy plasmas. The analysis of these fluctuations requires the calculation of the amplitude, phase, and frequency of the oscillations. These three spectral quantities generally evolve in time, necessitating nonstationary signal analysis techniques. The Hilbert transform offers an efficient and accurate method of calculating these three quantities from one diagnostic signal. This feature allows the Hilbert transform to determine the success of the active rotation control of magnetic islands, and to calculate the profile of the diagnosticmeasurements in a frame of reference co-rotating with the magnetic island. Comparisons to quadrature and spectrogram techniques demonstrate the accuracy of the Hilbert transform method.
70(1999); http://dx.doi.org/10.1063/1.1150111View Description Hide Description
The design and first results are presented from a bolometric diagnostic with high temporal resolution recently installed on the TCV tokamak. The system consists of two pinhole cameras viewing the plasma from above and below at the same toroidal location. Each camera is equipped with an AXUV-16ELO linear array of 16 junction photodiodes, characterized by a flat spectral sensitivity from ultraviolet to x-ray energies, a high temporal response (<0.5 μs), and insensitivity to low-energy neutral particles emitted by the plasma. This high temporal resolution allows the study of transient phenomena such as fast magnetohydrodynamic(MHD) activity hitherto inaccessible with standard bolometry. In the case of purely electromagnetic radiation, good agreement has been found when comparing results from the new diagnostic with those from a standard metal foil bolometer system. This comparison has also revealed that the contribution of neutrals to the foil bolometermeasurements can be extremely important under certain operating conditions, precluding the application of tomographic techniques for reconstruction of the radiation distribution.
70(1999); http://dx.doi.org/10.1063/1.1150112View Description Hide Description
An experimental setup for studying the influence of the radial electric field on very low plasma on the Saha Institute of Nuclear Physicstokamak is presented. A high current, high voltage pulsed power supply, using a semiconductor controlled rectifier (SCR) as a dc switch is developed and used to bias a tungstenelectrode inserted inside the plasma. The electrode’s exposed length and its position inside the plasma are controlled by a double bellows assembly to optimize the electrode-exposed length. We show that using the force commutation method to turn the SCR off to get the power pulse desired has good potential for carrying out similar kinds of studies, especially in a low budget small tokamak.
- BASIC PHENOMENA
70(1999); http://dx.doi.org/10.1063/1.1150113View Description Hide Description
We investigate the shielding of superconducting and μ-metal forms in axial and transverse directed background magnetic noise fields. Analytical expressions are obtained for the improvement in signal-to-noise ratio obtained by placing a superconducting disk in the presence of a dipole source and a uniform noise field. Axial and transverse shielding factors are then compared for identical superconducting and μ-metal cylinders. The signal-to-noise ratio is found to be infinite at certain points inside a superconducting cylinder as well as a superconducting cylinder with a central partition. Shielding factors obtained here are relevant to SQUID measurements of small dipole source fields in the presence of large background noise fields such as those encountered in biomagnetism and nondestructive evaluation.
70(1999); http://dx.doi.org/10.1063/1.1150114View Description Hide Description
Theoretical and experimental investigations aimed at optimizing the detection performance in nonresonant pulsed photoacoustic (PA) studies in the gas phase by selecting microphones with appropriate characteristics are presented. An analytical function is introduced that allows the simulation of the measured signal-to-noise-ratio (SNR) for given microphone specifications. The crucial parameters for maximum SNR are the microphone responsivity, its bandwidth, and noise characteristics. Good agreement between calculated and measured PA signal shapes and their fast Fourier transform spectra is obtained. Comparisons of experimental data recorded in a gas mixture of 100 ppm ethylene buffered in synthetic air with various condenser and electretmicrophones with different characteristics confirm our theoretical predictions. The peak amplitude of the recorded microphone signal increases with decreasing laser beam radius However, for standard microphones with a bandwidth of ≈20 kHz, there is no reason to use because the spectral composition of the signal shape for frequencies <20 kHz remains unchanged. A large microphone bandwidth of a few 100 kHz is advantageous for resolving the temporal evolution of the PA signal but it can be disadvantageous for trace gas monitoring due to an enhanced noise level. Our simulation permits the selection of the most appropriate microphone for maximum SNR under given experimental conditions. Finally, an analysis on the pressure dependence of the responsivity of a selected microphone is presented that is of interest when performing trace gas measurements at reduced pressure to enhance the detection selectivity.
Direct scanning tunneling microscope detection of laser induced ultrasonic pulses with nanometer resolution70(1999); http://dx.doi.org/10.1063/1.1150115View Description Hide Description
We show the use of scanning tunneling microscope(STM) with a self-designed wide-bandwidth current preamplifier (50 MHz) for direct registration of ultrasonic pulses induced by nanosecond laser pulses in substrate. The changes in tip–sample separation caused by ultrasonic vibrations are seen as perturbations in tunneling current. At moderate incident laser pulse energy of 1–2 mJ ultrasonic waves in glass rod and silica wafer were observed. This wide-band signal, low noise registration in addition with nanometer spatial resolution of a STM opens new possibilities in investigations of ultrasonic near-field distribution, elasticmaterial properties on a nanometer scale, and measurements of the ultrasound velocity in laser ultrasonics.