Volume 72, Issue 11, November 2001
Index of content:
- OPTICS; ATOMS and MOLECULES; SPECTROSCOPY
72(2001); http://dx.doi.org/10.1063/1.1412857View Description Hide Description
A novel concept of capillary magneto-electrostatic (CAMEL) optics is presented. The concept is based on the use of thin liquid films pressed through a porous substrate and quickly pulled back after the optical pulse, thereby allowing one to create optical elements anew after every pulse. Possible optical elements include planar and focusing mirrors, reflecting diffraction gratings, and reflecting zone plates. A set of engineering equations required for design of the CAMEL optics is presented.
72(2001); http://dx.doi.org/10.1063/1.1408935View Description Hide Description
Magneto-optic traps which employ current windings to generate pulsed magnetic fields require rapid switch-off times for many applications. Practical difficulties in attaining rapid switch-off of the magnetic field, including the generation of induced currents, are addressed. Several methods for minimizing the switch-off time are presented which do not require complex feedback mechanisms involving direct measurement of the magnetic field.
72(2001); http://dx.doi.org/10.1063/1.1406921View Description Hide Description
A compact optical tweezer package has been developed for use on a microscope to be flown on the International Space Station as part of a series of experiments in colloidcrystallization. A brief introduction to the principles of single-beam optical tweezer operation will be presented, after which a detailed system layout will be shown. Special design requirements due to the spaceflight nature of the hardware will also be discussed. The tweezer apparatus is capable of trapping many particles through use of a two-axis acousto-optical deflector. The trap strength is sufficient to perform the required science (50 pN at The trap beam behaves approximately as a diffraction limited single mode Gaussian beam of numerical aperture, NA=1.4, as shown through spot size measurements and confocal-type images of the focal region. This is the first time optical tweezers will be deployed in a microgravity environment.
72(2001); http://dx.doi.org/10.1063/1.1412261View Description Hide Description
This article describes a method for assessing the accuracy of a new linear birefringence measurement system based on photoelastic modulator technology. We used a Soleil–Babinet compensator in the experiment and observed that its retardation varies significantly across the optical aperture. To use this compensator as a meaningful retardation standard, we fixed the beam position relative to the Soleil–Babinet compensator before and after its calibration. Our results show that the birefringence measurement system is capable of providing accurate measurements for linear retardation below 125 nm with a relative uncertainty below 1%. Experimental results support the fact that this birefringence measurement system is self-calibrating for measuring linear retardation.
Ultrashort pulse characterization using a scanning Fabry–Pérot étalon enabling rapid acquisition and retrieval of a sonogram at rates up to 1.52 Hz72(2001); http://dx.doi.org/10.1063/1.1406932View Description Hide Description
Rapid characterization of ultrashort pulses has been demonstrated by measuring a two-photon absorption sonogram trace. The use of a scanning Fabry–Pérot frequency filter has made it possible to acquire and retrieve a sonogram at rates as high as 1.52 Hz. We discuss details of the experimental setup, including the synchronization electronics and the acquisition/retrieval software. Excellent agreement is obtained between retrieved pulse data and independent experimental pulse measurements.
72(2001); http://dx.doi.org/10.1063/1.1406925View Description Hide Description
An ultrahigh-resolution x-ray crystal monochromator providing a 120 μeV bandwidth at 14.41 keV is presented. The design, which uses four independent silicon crystals and has an output beam parallel to the incident beam, may be generalized to arbitrary energies. Fluxes of photons/s photons/s) in bandwidths of 120±15 μeV (140±15 μeV) were measured. The performance of the monochromator, including the preservation of coherence through it, was verified by measurement of a 9.6±2.0% enhancement in the coincidence rate (i.e., in an intensity correlation experiment.
- PARTICLE SOURCES, OPTICS and ACCELERATION, DETECTORS
72(2001); http://dx.doi.org/10.1063/1.1406923View Description Hide Description
We present an iterative method for the extraction of velocity and angular distributions from two-dimensional (2D) ion/photoelectron imaging experiments. This method is based on the close relationship which exists between the initial 3D angular and velocity distribution and the measured 2D angular and radial distributions, and gives significantly better results than other inversion procedures which are commonly used today. Particularly, the procedure gets rid of the center-line noise which is one of the main artifacts in many current ion/photoelectron imaging experiments.
72(2001); http://dx.doi.org/10.1063/1.1405786View Description Hide Description
The SERSE source [P. Ludwig et al., Rev. Sci. Instrum. 69, 4082 (1998), and references therein] is a superconducting electron cyclotron resonance (ECR) ion source, operating at the Laboratori Nazionali del Sud in Catania since 1998; it is currently used as the main injector for the K-800 superconducting cyclotron. Its high magnetic field provides a high plasma confinement and large currents of highly charged ions, as compared to conventional sources. It can efficiently operate at the microwave frequency of 14 and 18 GHz [S. Gammino and G. Ciavola, Rev. Sci. Instrum. 71, 631 (2000); S. Gammino et al., ibid. 70, 3577 (1999)] and has been used as a test bench for injection at 28 GHz. High-frequency operation is expected to create a higher plasma density, thus resulting in larger currents of multiply charged ions. In this article, we report the first operation of an ECR ion source at 28 GHz by using a gyrotron. The gyrotron itself and the waveguide line are described, along with the operational results (in xenon gas for the sake of simplicity). Given the limited amount of power (about 4 kW), which can be injected in the plasma chamber during dc-mode operation, the results are less outstanding than in the pulsed mode (up to 6.5 kW). However, in both cases the beam intensities are far better than the ones obtained by the other ECR ion sources operating at lower frequencies.
72(2001); http://dx.doi.org/10.1063/1.1412861View Description Hide Description
The results of experimental and theoretical studies, aiming at a quantitative characterization of photoion-induced energy broadening effects in a laser photoelectron source, are reported. The electron source is based on two-step cw laser photoionization of potassium atoms in a collimated beam. In the experimental studies, the attachment spectra for the formation of cluster ions through a narrow vibrational Feshbach resonance (full width at half maximum 2.3 meV) were measured as a function of the photocurrent. The theoretical studies involved Monte Carlo simulations of the broadening effects and were based on potential distributions caused by realistic spatial distributions of the photoions. Using the corresponding electric field distribution, trajectories were calculated for a representative ensemble of electrons, and effective electron energy distributions were obtained from averages over the electron trajectories in the volume relevant for electron attachment. Furthermore, the effects of additional weak electric fields, applied along the atomic beam direction, have been simulated. For our geometry (ionization volume about 2 the effective space charge related energy width is found to be about 16 μeV/pA.
- NUCLEAR PHYSICS, FUSION and PLASMAS
Reducing influence of ion current on measurements of electron energy distributions in collisional plasmas72(2001); http://dx.doi.org/10.1063/1.1408933View Description Hide Description
A method for reducing the influence of ion current on probe measurements of electron velocity distributions in plasmas has been developed and tested in noble gas afterglow plasmas. It is valid for diffusive transport of ions to the probe. Experiments have demonstrated high accuracy of the proposed method.
72(2001); http://dx.doi.org/10.1063/1.1409567View Description Hide Description
Construction and operation of a novel fiber opticcatalytic probe is presented. The probe is intended for measurements of atomic oxygen density in plasma postglows. The operation of the probe is based on a catalytic recombination of oxygen atoms and remote temperature sensing via optical fiber. Compared to the classical catalytic probes, the new approach results in smaller dimensions, better sensitivity, and higher immunity to the electromagnetic interference. Comparative measurements of atomic oxygen density performed simultaneously with both types of probes demonstrated a superior performance of the novel probes.
72(2001); http://dx.doi.org/10.1063/1.1405792View Description Hide Description
A two channel multicrystal spectrometer of Bragg geometry was constructed for measurements of absolute photon fluxes of ASDEX Upgrade plasmas in the wavelength range from 0.5 up to 26 Å. The calibration of the spectrometer was done by an ab initio calculation using the absolute reflection data from literature of four crystals. The calibration was validated by the direct comparison with other absolute calibrated diagnostics at the ASDEX Upgrade as well as by using the impurity transport code STRAHL. The absolute impurity concentrations of elements from N up to Cu have been extracted. O and F appear to be main impurity components after C, which is routinely measured by an independent diagnostic. Despite the significantly lower concentration of Ar, it contributes measurably to and can account for main plasma impurities as well. Other impurities either have never been seen or appear only sporadically (like Fe and Cu) and normally do not contribute significantly to the plasma radiation in the soft x-ray range.
72(2001); http://dx.doi.org/10.1063/1.1412262View Description Hide Description
This article describes the realization and testing of the electronic system which forms part of the multichannel far-infrared (FIR) polarimeter for the RFX machine, a plasma confinement experiment with Reversed Field Pinch(RFP) configuration. The electronic system, which comprises the detectors, the signal-processing electronics, and the motion electronics for the half-wave plate movement, is described. Emphasis is placed in the analysis of the polarimeter signals, which permits an in-depth understanding of the performance of the data processing electronics and the role of the various sources of noise in the system. After a brief outline of the basic principle of the measurement, the choice of detectors and their characteristics are described in order to achieve the best performances at the FIR wavelength of interest. Various tests, which are described, confirmed the need for a specifically designed pyroelectric detector capable of operating in the hostile magnetic environment near the machine. The processing of the raw polarimeter signals to produce the required sum and difference signals and to convert them into dc signals with 3 ms time constant is presented. These signals are synchronous with a chopper signal on the FIR beam and are subsequently fed to a lock-in amplifier. An accurate analysis of the data processing procedure is described, which helps to clarify the understanding of the output signals that are eventually recorded in the data acquisition system. In particular, various sources of noise, such as thermal noise of the detectors, laser fluctuations, spurious signals at harmonics of the chopper frequency, and phase jitter of the chopper, are evaluated, discussed, and compared with the observed signals. Finally, the control circuitry for the movement of the half-wave plates, both for manual control and for the programmed sequences of zero-search and calibration performed by a PLC control system, is described. Calibration curves obtained during experiments are also reported.
72(2001); http://dx.doi.org/10.1063/1.1409570View Description Hide Description
An analytical solution of a time-dependent two-dimensional (2D) full-wave equation is obtained for the case of microwave propagation in a plasma with axial symmetry. The 2D structure of the electromagnetic wave in a nonmagnetized plasma (or an ordinary wave in a magnetized plasma) is studied for the general case of arbitrary time dependence of the incident wave emitted by the horn. The solutions cover the cases of conventional microwavereflectometry as well as the ultrashort pulse reflectometry of fusion and processing plasma. Analytical expressions can be further applied to study plasma density fluctuations as well as effects of plasma curvature and multidimensionality. The 2D plasma cylinder model is deemed by the authors to be more realistic as compared to the one-dimensional plasma slab model previously employed in all the analytical and most numerical treatments since the plasma in the fusion toroidal devices, mirror machines, and plasma processing chambers can be considered axially symmetric on the scale relevant to microwavereflectometry.
- MICROSCOPY and IMAGING
72(2001); http://dx.doi.org/10.1063/1.1406926View Description Hide Description
A confocal laser-scanning microscope for ultrasensitive fluorescence lifetime imaging on surfaces is presented. The system employs a compact electronics for time-correlated single-photon counting (TCSPC), allowing for measuring fluorescence lifetime with 40 ps time resolution, and for continuously recording photon arrival times with 100 ns time resolution. Additionally developed driver electronics serve for synchronization of scanning and data acquisition, which is significant for achieving high spatialimage resolution. The capabilities of the measurement system are demonstrated on imaging single molecules immobilized on glass substrates. Finally, it is shown how the TCSPC capabilities of the system can be used not only for lifetime imaging but also for multichannel measurements.
72(2001); http://dx.doi.org/10.1063/1.1406931View Description Hide Description
We report on a scanning superconducting quantum interference device microscope operating at temperatures down to 20 mK in a dilution refrigerator. The instrument is designed for studying quantum mechanical coherenceeffects in mesoscopic systems and investigating magnetic effects on a mesoscopic length scale in novel materials. We have demonstrated the low-temperature operating capabilities of the instrument by studying superconducting tin disks and the superconducting transition of a thin-film tungsten sample and vortices in the same film. Looking forward, we discuss the applicability of the instrument to measurements of persistent currents in normal-metal rings.
72(2001); http://dx.doi.org/10.1063/1.1406933View Description Hide Description
To investigate the surface morphologies of biological systems in a controlled gaseous environment (e.g., the temperature, humidity and composition), most commercial atomic force microscopes require modification. We have designed a double-jacketed environmental chamber specifically for a Nanoscope IIIa (Digital Instruments, Santa Barbara, CA) force microscope. We use cold nitrogen and thermoelectric devices to control the temperature in the chamber; the nitrogen simultaneously serves to create an inert environment. We have also designed a temperature controlled sample stage utilizing thermoelectric devices for fine temperature regulation. A variation of this sample stage allows us to image samples in fluids at cold temperatures with an O-ringless configuration. The relative humidity within the chamber is also measured with commercially available relative humidity sensors. We investigate the surface morphology of ice Ih in its pure phase and shall extend the study to ice in the presence of biological molecules, such as antifreeze proteins. We present a detailed description of our design and our first images of polycrystallineice and single crystals of ice grown in situ from the vapor.
72(2001); http://dx.doi.org/10.1063/1.1406934View Description Hide Description
Interparticle and surface forces are of great importance in many fields of pure and applied science. We present an apparatus to measure the normal and friction forces acting between a particle (radius of 0.5–20 μm) and another solid surface. The apparatus is based on the principle of an atomic force microscope. For quantitative friction measurements we propose a method to determine the lateral spring constants of atomic force microscope cantilevers with attached spherical particles.
New two-dimensional friction force apparatus design for measuring shear forces at the nanometer scale72(2001); http://dx.doi.org/10.1063/1.1412860View Description Hide Description
A device to study the friction of two molecularly smooth surfaces separated by an ultrathin liquid film is presented along with its design, calibration, and performance. The apparatus can move one of the surfaces and measure the friction force on the other one bidimensionally for both processes. A high mechanical impedance system measures continuous friction forces where only stick–slip was previously observed. The frequency and travel distance of the movement can be varied over a wide range (frequency from to 7 Hz and distance from 1 to 800 μm) to provide variations of the shear rate over seven orders of magnitude. The actual movement provided by piezoelectric bimorph drive can be affected by the friction forces and is measured by strain gauges. The friction forces are measured with an accuracy of ±2μN with a capacitance sensor. The mechanical design prevents the surfaces from rolling under force. The apparatus is tested with hexadecane. The potential applications of this apparatus and its limitations are discussed.
72(2001); http://dx.doi.org/10.1063/1.1406922View Description Hide Description
We show that the use of fiber overtone resonance modes up to 3 MHz for shear-force distance control has two advantages for near-field microscopy. The higher dither frequency allows a shorter feedback loop time delay, which in turn allows scanning speeds of 100 μm/s on a sample with, for example, a 150 nm height variation over a 1 μm distance. Experiments on a hard semiconductor device and on a soft polymer sample demonstrate a factor of 5 improvement in scanning speed when the dither frequency is increased by a factor of 20. The second advantage is a reduction of the minimum lateral force required for height regulation, which is important for soft samples. Modeling the piezoelectric detection system as a harmonic oscillator indicates a factor of 33 increase in lateral force sensitivity when using the third overtone resonance of a typical fiber tip. This result is confirmed experimentally by immersing the tip into water.