Volume 80, Issue 12, December 2009
- optics; atoms and molecules; spectroscopy; photon detectors
- particle sources, optics and acceleration; particle detectors
- nuclear physics, fusion and plasmas
- microscopy and imaging
- condensed matter; materials
- biology and medicine
- gravity; geophysics; astronomy and astrophysics
- electronics; electromagnetic technology; microwaves
- thermometry; thermal diffusivity; acoustic; photothermal and photoacoustic
- general instruments
Index of content:
- OPTICS; ATOMS AND MOLECULES; SPECTROSCOPY; PHOTON DETECTORS
Application of a simple asynchronous mechanical light chopper to multielectron coincidence spectroscopy80(2009); http://dx.doi.org/10.1063/1.3258200View Description Hide Description
A simple asynchronous mechanical light chopper, based on modification of a turbo-molecular pump, has been developed to extend the interval between light pulses in single bunch operation at the Photon Factory storage ring. A pulse repetition rate of 80 kHz was achieved using a cylinder rotating at 48000 rpm, with 100 slits of width. This allows absolute timing of particles up to instead of the single-bunch period of 624 ns. We have applied the chopper together with a light pulse monitor to measure multielectron coincidence spectra using a magnetic bottle time-of-flight electron spectrometer. With such a system, the electron energies are determined without any ambiguity, the folding of coincidence spectra disappears and the effect of false coincidences is drastically reduced.
Solid and liquid spectroscopic analysis (SALSA)–a soft x-ray spectroscopy endstation with a novel flow-through liquid cell80(2009); http://dx.doi.org/10.1063/1.3257926View Description Hide Description
We present a novel synchrotron endstation with a flow-through liquid cell designed to study the electronic structure of liquids using soft x-rayspectroscopies. In this cell, the liquid under study is separated from the vacuum by a thin window membrane, such that the sample liquid can be investigated at ambient pressure. The temperature of the probing volume can be varied in a broad range and with a fast temperature response. The optimized design of the cell significantly reduces the amount of required sample liquid and allows the use of different window membrane types necessary to cover a broad energy range. The liquid cell is integrated into the solid and liquid spectroscopic analysis (SALSA) endstation that includes a high-resolution, high-transmission x-ray spectrometer and a state-of-the-art electron analyzer. The modular design of SALSA also allows the measurement of solid-state samples. The capabilities of the liquid cell and the x-ray spectrometer are demonstrated using a resonant inelastic x-ray scattering map of a 25 wt % NaOD solution.
80(2009); http://dx.doi.org/10.1063/1.3266974View Description Hide Description
A new approach to high temperature gas analysis by means of photoacoustic (PA) spectroscopy is presented. The transverse modes of the resonant PA cell were excited with a pulsed laser and detected with a microphone. Changes in the properties of the PA cell resulting from a varying temperature are discussed and considered when processing the PA signal. The feasibility of the proposed method was demonstrated by studying PA response from saturated vapor of potassium chloride (KCl) in the temperature range extending from 410 to . The PA spectrum, the detection limit, and the signal saturation of KCl vapor are discussed. At 245 nm excitation wavelength and pulse energy, the achieved detection limit for KCl is 15 ppb.
80(2009); http://dx.doi.org/10.1063/1.3271039View Description Hide Description
We demonstrate a frequency-agile terahertz wave parametric oscillator (TPO) in a ring-cavity configuration (ring-TPO). The TPO consists of three mirrors and a crystal under noncollinear phase-matching conditions. A novel, fast frequency-tuning method was realized by controlling a mirror of the three-mirror ring cavity. The wide tuning range between 0.93 and 2.7 THz was accomplished. For first demonstration using the ring-TPO, terahertz spectroscopy was performed as the verification of the frequency-agile performance, measuring the transmission spectrum of the monosaccharide glucose. The spectrum was obtained within about 8 s in good comparison to those of Fourier transform infrared spectrometer.
80(2009); http://dx.doi.org/10.1063/1.3264079View Description Hide Description
We describe an approach for the absolute density measurement of rotationally symmetric molecular beams via multiphoton ionization. This simple single-projection tomographic technique requires only knowledge of the spatial intensity profile and ionization characteristics of the focused laser beam that probes the pulsed molecular jet. Multiphoton ionization (MPI) of a xenon beam allowed tomographic reconstruction of a two-dimensional density profile with a peak density of , which was compared with the theoretical predictions of the sudden freeze model. An analytic solution to the Abel transform is derived for Gaussian projected density profiles which greatly simplifies the reconstruction of the absolute radial density. MPI is sufficiently general that this technique can be readily applied to atomic beams with a broad range of chemistries.
Frequency-resolved optical gating system with a tellurium crystal for characterizing free-electron lasers in the wavelength range of80(2009); http://dx.doi.org/10.1063/1.3265318View Description Hide Description
A second-harmonic generation frequency-resolved optical gating (SHG-FROG) system has been developed for the complete characterization of laser pulses in the wavelength range of . A tellurium crystal is used so that spectrally resolved autocorrelation signals with a good signal-to-noise ratio are obtained. Pulses (wavelength ) generated from a free-electron laser are measured by the SHG-FROG system. The SHG intensity profile and the spectrum obtained by FROG measurements are well consistent with those of independent measurements of the pulse length and spectrum. The pulse duration and spectral width determined from the FROG trace are 0.6 ps and 5.2 THz at full width half maximum, respectively.
Interferometric polarization pulse shaper stabilized by an external laser diode for arbitrary vector field shaping80(2009); http://dx.doi.org/10.1063/1.3270254View Description Hide Description
We achieved reliable and stable generation of pulses with all possible polarization states by a Mach–Zehnder pulse shaper. This was realized by incorporating a stabilization mechanism using an external laser diode in the interferometric pulse shaper. This stabilization mechanism has overcome an inherent instability in the Mach–Zehnder interferometer, which caused serious distortion of shaped pulses. For a demonstration of polarization shaping, we generated and measured chiral pulses with a rotating major axis of polarizing orientations at arbitrary frequencies. We expect these chiral pulses enables us to study on new chirality-related light-matter interactions.
Collinear laser spectroscopy of francium using online rubidium vapor neutralization and amplitude modulated lasers80(2009); http://dx.doi.org/10.1063/1.3271037View Description Hide Description
Performing collinear laser spectroscopy on low intensity radioactive beams requires sensitive detection techniques. We explain our apparatus to detect atomic resonances in neutralized ion beams at beam energies of 5 keV and intensities of . Efficient neutralization is accomplished by passing the beam through a dense Rb vapor. Increased detection efficiency is achieved by amplitude modulating the exciting laser to decrease the scattered light background, allowing fluorescence detection only when the laser is near its minimum in the modulation cycle. Using this technique in a collinear geometry we achieve a background reduction by a factor of 180 and a signal-to-noise increase of 2.2, with the lifetime of the atomic state playing a role in the efficiency of this process. Such laser modulation will also produce sidebands on the atomic spectra which we illustrate.
Time-delay compensated monochromator for the spectral selection of extreme-ultraviolet high-order laser harmonics80(2009); http://dx.doi.org/10.1063/1.3273964View Description Hide Description
The design and the characterization of a monochromator for the spectral selection of ultrashort high-order laser harmonics in the extreme ultraviolet are presented. The instrument adopts the double-grating configuration to preserve the length of the optical paths of different diffracted rays, without altering the extremely short duration of the pulse. The gratings are used in the off-plane mount to have high efficiency. The performances of the monochromator have been characterized in terms of spectral response, efficiency, photon flux, imaging properties, and temporal response. In particular, the temporal characterization of the harmonic pulses has been obtained using a cross-correlation method: Pulses as short as 8 fs have been measured at the output of the monochromators, confirming the effectiveness of the time-delay compensated configuration.
- PARTICLE SOURCES, OPTICS AND ACCELERATION; PARTICLE DETECTORS
80(2009); http://dx.doi.org/10.1063/1.3262501View Description Hide Description
We report a multipurpose furnace designed for studies using synchrotron radiation on polycrystalline materials, namely, metals, ceramics, and (semi)crystalline polymers. The furnace has been designed to carry out three-dimensional (3D) x-ray diffractionmeasurements but can also be used for other types of synchrotron radiation research. The furnace has a very low thermal gradient across the specimen . Accurate determination of the temperature can be carried out by welding a thermocouple to the specimen. The furnace can be rotated over an angle of 90° in order to determine the crystallographic orientation of each individual grain. It is possible to follow growth kinetics of all grains in the illuminated volume of the specimen. The specimen environment can be controlled varying from vacuum (up to ) to gas or air filled. The maximum temperature of operation is , with the possibility of achieving high heating (up to ) and cooling rates (up to without quenching gas). 3D maps of the microstructure of the specimen can be generated at elevated temperatures by bringing the high-resolution detector close to the specimen. We show an example of a simulation of the heat affected zone during the thermal cycle of a weld in a transformation-induced plasticity steel carried out using the furnace. The unique characteristics of the furnace open possibility of new fields in materials research using synchrotron radiation.
80(2009); http://dx.doi.org/10.1063/1.3271537View Description Hide Description
The quality factor of a superconducting NbTi resonator at 1.6 MHz in a magnetic field up to 1.2 T as well as its temperature dependence is investigated. A hysteresis effect in the superconductingsurfaceresistance as a function of the magnetic field is observed. An unloaded -value of the resonator of 40 500 is achieved at 3.9 K. It is shown that this -value is limited by dielectric losses in the FORMVAR insulation of the coils wire. The details of the -value optimization are discussed. In the temperature dependence of the -value a steep decrease is observed above . Finally, the implications of these measurements for real trap experiments are discussed in detail.
80(2009); http://dx.doi.org/10.1063/1.3272784View Description Hide Description
A new particle sizer which integrates the dispersion and detection parts is presented. Particles are dispersed based on charging between two parallel plates connected to a high voltage power source. The charged particles bounce between the two plates and escape into a measuring area where the size is determined by light scattering. The instrument is calibrated using standard powders. The data obtained from the new instrument are in good agreement with those obtained from a commercial particle size analyzer. The sizer works both for insulator and conductive powders.
80(2009); http://dx.doi.org/10.1063/1.3272789View Description Hide Description
X-ray generation based on laser-electron Compton scattering is one attractive method to achieve a compact laboratory-sized high-brightness x-ray source. We have designed, built, and tested such a source; it combines a 50 MeV multibunch electron linac with a mode-locked 1064 nm laser stored and amplified in a Fabry–Pérot optical cavity. We directly observed trains of pulsed x rays using a microchannel plate detector; the resultant yield was found to be in good agreement with prediction. We believe that the result has demonstrated good feasibility of linac-based compact x-ray sources via laser-electron Compton scatterings.
High intensity electron cyclotron resonance proton source for low energy high intensity proton accelerator80(2009); http://dx.doi.org/10.1063/1.3272786View Description Hide Description
Electron cyclotron resonance (ECR) protonsource at 50 keV, 50 mA has been designed, developed, and commissioned for the low energy high intensity proton accelerator (LEHIPA). Plasma characterization of this source has been performed. ECR plasma was generated with 400–1100 W of microwave power at 2.45 GHz, with hydrogen as working gas. Microwave was fed in the plasmachamber through quartz window. Plasma density and temperature was studied under various operating conditions, such as microwave power and gas pressure. Langmuir probe was used for plasma characterization using current voltage variation. The typical hydrogen plasma density and electron temperature measured were and 6 eV, respectively. The total ion beam current of 42 mA was extracted, with three-electrode extraction geometry, at 40 keV of beam energy. The extracted ion current was studied as a function of microwave power and gas pressure. Depending on source pressure and discharge power, more than 30% total gas efficiency was achieved. The optimization of the source is under progress to meet the requirement of long time operation. The source will be used as an injector for continuous wave radio frequency quadrupole, a part of 20 MeV LEHIPA. The required rms normalized emittance of this source is less than . The simulated value of normalized emittance is well within this limit and will be measured shortly. This paper presents the study of plasma parameters, first beam results, and the status of ECR protonsource.
- NUCLEAR PHYSICS, FUSION AND PLASMAS
80(2009); http://dx.doi.org/10.1063/1.3263820View Description Hide Description
The magnetic confinement of plasmas in fusion experiments can significantly degrade due to perturbations of the magnetic field. A precise analysis of the magnetic field in a stellarator-type experiment utilizes electrons as test particles following the magnetic fieldline. The usual fluorescent detector for this electron beam limits the provided information to two-dimensional cut views at certain toroidal positions. However, the technique described in this article allows measuring the three-dimensional structure of the magnetic field by means of close-range photogrammetry. After testing and optimizing the main diagnostic components, measurements of the magnetic fieldlines were accomplished with a spatial resolution of 5 mm. The results agree with numeric calculations, qualifying this technique as an additional tool to investigate magnetic field configurations in a stellarator. For a possible future application, ways are indicated on how to reduce experimental error sources.
80(2009); http://dx.doi.org/10.1063/1.3263911View Description Hide Description
Two large area multistep position sensitive (two dimensional) multiwire proportional counters have been developed for experiments involving study of fission dynamics using general purpose scattering chamber facility at IUAC. Both detectors have an active area of and provide position signals in horizontal (X) and vertical (Y) planes, timing signal for time of flight measurements and energy signal giving the differential energy loss in the active volume. The design features are optimized for the detection of low energy heavy ions at very low gas pressures. Special care was taken in setting up the readout electronics, constant fraction discriminators for position signals in particular, to get optimum position and timing resolutions along with high count rate handling capability of low energy heavy ions. A custom made charge sensitive preamplifier, having lower gain and shorter decay time, has been developed for extracting the differential energy loss signal. The position and time resolutions of the detectors were determined to be 1.1 mm full width at half maximum (FWHM) and 1.7 ns FWHM, respectively. The detector could handle heavy ion count rates exceeding 20 kHz without any breakdown. Time of flight signal in combination with differential energy loss signal gives a clean separation of fission fragments from projectile and target like particles. The timing and position signals of the detectors are used for fission coincidence measurements and subsequent extraction of their mass, angular, and total kinetic energy distributions. This article describes systematic study of these fission counters in terms of efficiency, time resolution, count rate handling capability, position resolution, and the readout electronics. The detector has been operated with both five electrode geometry and four electrode geometry, and a comparison has been made in their performances.
80(2009); http://dx.doi.org/10.1063/1.3270257View Description Hide Description
A laser system with flexible parameters was developed for fundamental research related to an extreme ultraviolet(EUV) lithography source. The laser is a master oscillator and power amplifier (MOPA) system, consisting of a master oscillator, an externally triggered plasma switch, a preamplifier, a main amplifier, and electronic synchronization units. The laser pulse duration can be varied easily from 10 to 110 ns, with a constant peak power for pulse durations from 25 to 110 ns. The MOPA laser system can also be operated in dual-oscillator mode to produce laser pulse with pulse duration as long as 200ns and a train of laser pulses with flexible interval. The divergence of the laser beam is 1.3 times the diffraction limit. The laser intensity on the target surface can be up to . Utilizing this MOPA laser system, high conversion efficiency from laser to in-band (2% bandwidth) 13.5 nm EUV emission has been demonstrated over a wide range of laser pulse durations.
Hybrid radio-frequency/direct-current plasma-enhanced chemical vapor deposition system for deposition on inner surfaces of polyethylene terephthalate bottles80(2009); http://dx.doi.org/10.1063/1.3273944View Description Hide Description
A hybrid radio-frequency (rf)/direct-current (dc) system has been developed to control the biasing effects during deposition of diamondlike carbon (DLC) films onto the inner wall of polyethylene terephthalate (PET) bottles. An additional dc bias is coupled to the rf electrode to produce the effect of equivalent rf self-biasing. This allows more flexible control of the deposition of the DLC films which are intended to improve the gas barrier characteristics. The experimental results demonstrate that the additional dc bias improves the adhesion strength between the DLC film and PET, although the enhancement in the gas barrier properties is not significantly larger compared to the one without dc bias. The apparatus and methodology have practical importance in the food and beverage industry.
80(2009); http://dx.doi.org/10.1063/1.3271388View Description Hide Description
For the first time, a geometry has been developed to allow for an axial imaging system for wire-array Z-pinch experiments that produce high-resolution x-rayimages. The new geometry required a significant redesign of the electrode hardware. Calibrated areal density measurements of the Z-pinchplasma including wire cores, coronal plasma, streaming plasma, and the precursor were obtained. The system used eight-wire molybdenum(Mo) X pinches in series with and directly below the Z-pinch axis to provide micron-scale x-rayssources for point-projection radiography. The images formed on the x-ray sensitive film had a 15 mm diameter field of view at the center height of the array and a magnification of about 7.5:1. Titanium (Ti) filters in front of the film transmitted radiation in the spectral range of 3–5 keV. For calibration, a separate film with the same thickness Ti filter was placed the same distance from the X pinch. This film had an unobstructed path that bypasses the Z-pinch but included step wedges for calibration of the Z-pinchplasma. The step wedges had thicknesses of tungsten (W) ranging from 0.015 to to obtain areal density measurements of the W plasma from the wire-array. Images had subnanosecond temporal resolution and about spatial resolution.
80(2009); http://dx.doi.org/10.1063/1.3266065View Description Hide Description
We describe a fast reciprocating Langmuir probe and drive system, which has four main new features: (1) use of high-temperature, vacuum, circuit boards instead of cables to reduce weight and increase to 21 the number of possible connections, (2) rotatable and removable shaft, (3) 10 tip construction with designed hardware bandwidth up to 10 MHz, and (4) a detachable and modular tip assembly for easy maintenance. The probe is mounted in a fast pneumatic drive capable of speeds and ’s acceleration in order to reach the scrape-off layer (SOL) and pedestal regions and remain inserted long enough to obtain good statistics while minimizing the heat deposition to the tips and head in a power density environment of . The National Spherical Torus Experiment SOL features electron temperature, , and electron density, while the pedestal features and . The probe described here has ten tips which obtain a wide spectrum of plasma parameters: electron temperature profile , electron density profile and Mach number profile , floating potential , poloidal and radial electric field profiles and , saturation current profile , and their fluctuations up to 3 MHz. We describe the probe and show representative radial profiles of various parameters.