Volume 81, Issue 7, July 2010
Index of content:
- Optics; Atoms and Molecules; Spectroscopy; Photon Detectors
81(2010); http://dx.doi.org/10.1063/1.3455213View Description Hide Description
The in-plane component of the wavefield provides valuable information about media properties from seismology to nondestructive testing. A new compact scanning laser ultrasonicinterferometer collects light scattered away from the angle of incidence to provide the absolute ultrasonic displacement for both the out-of-plane and an in-plane components. This new system is tested by measuring the radial and vertical polarization of a Rayleigh wave in an aluminum half-space. The estimated amplitude ratio of the horizontal and vertical displacement agrees well with the theoretical value. The phase difference exhibits a small bias between the two components due to a slightly different frequency response between the two processing channels of the prototype electronic circuitry.
81(2010); http://dx.doi.org/10.1063/1.3455203View Description Hide Description
We present an analysis of the time evolution of a highly excited silicon substrate after partial absorption of a femtosecond soft x-ray pulse. The detailed time-dependent thermoelastic behavior of the substrate in terms of the displacements is derived for time delays for which the usual local thermodynamic variables, temperature and density , become well-defined, namely, a few hundred femtoseconds after x-ray pulse absorption. For practical optical components under present conditions of operation with trains of pulses, we find that in a worst case scenario, already the second pulse in the train could be adversely affected by dynamic thermal distortion induced by the preceding pulse.
Stoichiometric analysis of compositionally graded combinatorial amorphous thin film oxides using laser-induced breakdown spectroscopy81(2010); http://dx.doi.org/10.1063/1.3455218View Description Hide Description
Laser-induced breakdown spectroscopy(LIBS) is a recently developed locally destructive elemental analysis technique that can be used to analyze solid, liquid, and gaseous samples. In the system explored here, a neodymium-doped yttrium aluminum garnet laser ablates a small amount of the sample and spectral emission from the plume is analyzed using a set of synchronized spectrometers. We explore the use of LIBS to map the stoichiometry of compositionally graded amorphous indiumzinc oxide thin-film libraries. After optimization of the experimental parameters (distance between lens and samples, spot size on the samples, etc.), the LIBS system was calibrated against inductively coupled plasma atomic emission spectroscopy which resulted in a very consistent LIBS-based elemental analysis. Various parameters that need to be watched closely in order to produce consistent results are discussed. We also compare LIBS and x-ray fluorescence as techniques for the compositional mapping of libraries.
A simple, compact, and efficient diode-side-pumped linear intracavity frequency doubled Nd:YAG rod laser with 50 ns pulse width and 124 W green output power81(2010); http://dx.doi.org/10.1063/1.3457000View Description Hide Description
We have developed an efficient and high power repetitively -switched diode-pumped intracavity frequency doubled Nd:YAG/ based green laser capable of generating 124 W of average green power with 50 ns pulse duration in a highly compact and robust linear cavity configuration. The pump to green beam conversion efficiency is 16.8% and the overall wall-plug efficiency is 8.3%. The long term power stability is excellent with ±0.4 W variation at the maximum output power and ±2% amplitude fluctuation with ±2.9 ns timing jitter. The parameter of the green beam was measured to be . This, combined with the short pulse duration and the high average power, makes this laser ideal for pumping ultrafast Ti:sapphire laser amplifier systems and for micromachining applications.
Wideband and high-gain frequency stabilization of a 100-W injection-locked Nd:YAG laser for second-generation gravitational wave detectors81(2010); http://dx.doi.org/10.1063/1.3458007View Description Hide Description
Second-generation gravitational wave detectors require a highly stable laser with an output power greater than 100 W to attain their target sensitivity. We have developed a frequency stabilization system for a 100-W injection-locked Nd:YAG (yttrium aluminum garnet) laser. By placing an external wideband electro-opticmodulator used as a fast-frequency actuator in the optical path of the slave output, we can circumvent a phase delay in the frequency control loop originating from the pole of an injection-locked slave cavity. Thus, we have developed an electro-opticmodulator made of a MgO-doped stoichiometric crystal. Using this modulator, we achieve a frequency control bandwidth of 800 kHz and a control gain of 180 dB at 1 kHz. These values satisfy the requirement for a laser frequency control loop in second-generation gravitational wave detectors.
81(2010); http://dx.doi.org/10.1063/1.3455198View Description Hide Description
We report on the design of an electric tube furnace that can be used for the fabrication of dopedglass microsphere lasers. The tube furnace has a short hot zone of length 133 mm and is based on a quartz tube design. Dopedlaser glass particles, specifically Er:Yb phosphate glass (IOG-2), of diameter are blown into the furnace using a 60 ml syringe and microspheres ranging in size from 10 to are collected at the output of the tube furnace in a Petri dish. The furnace operates at a wall temperature of and is capable of making microspheres from glasses with glass transition temperatures of at least . High quality whispering gallery modes have been excited within the microspheres by optically pumping at 978 nm via a tapered optical fiber.
81(2010); http://dx.doi.org/10.1063/1.3458014View Description Hide Description
We present here the design of a new tandem mass spectrometer that combines an electrospray ion source with a cryogenically cooled ion trap for spectroscopic studies of cold, gas-phase ions. The ability to generate large ions in the gas phase without fragmentation, cool them to in an ion trap, and perform photofragment spectroscopy opens up new possibilities for spectroscopic characterization of large biomolecular ions. The incorporation of an ion funnel, together with a number of small enhancements, significantly improves the sensitivity, signal stability, and ease of use compared with the previous instrument built in our laboratory.
Tunable ultrafast extreme ultraviolet source for time- and angle-resolved photoemission spectroscopy81(2010); http://dx.doi.org/10.1063/1.3460267View Description Hide Description
We present a laser-based apparatus suitable for visible pump/extreme UV (XUV) probe time-, energy-, and angle-resolved photoemissionspectroscopy utilizing high-harmonic generation from a noble gas. Tunability in a wide range of energies (currently 20–36 eV) is achieved by using a time-delay compensated monochromator, which also preserves the ultrashort duration of the XUV pulses. Using an amplified laser system at 10 kHz repetition rate, approximately per harmonic are made available for photoelectron spectroscopy. Parallel energy and momentum detection is carried out in a hemispherical electron analyzer coupled with an imaging detector. First applications demonstrate the capabilities of the instrument to easily select the probe wavelength of choice, to obtain angle-resolved photoemission maps (GaAs and ), and to trace ultrafast electron dynamics in an optically excited semiconductor (Ge).
81(2010); http://dx.doi.org/10.1063/1.3458015View Description Hide Description
A new monochromator setup for quick scanning x-ray absorption spectroscopy in the subsecond time regime is presented. Novel driving mechanics allow changing the energy range of the acquired spectra by remote control during data acquisition for the first time, thus dramatically increasing the flexibility and convenience of this method. Completely new experiments are feasible due to the fact that time resolution, edge energy, and energy range of the acquired spectra can be changed continuously within seconds without breaking the vacuum of the monochromator vessel and even without interrupting the measurements. The advanced mechanics are explained in detail and the performance is characterized with x-ray absorption spectra of pure metal foils. The energy scale was determined by a fast and accurate angular encoder system measuring the Bragg angle of the monochromator crystal with subarcsecond resolution. The Bragg angle range covered by the oscillating crystal can currently be changed from 0° to 3.0° within 20 s, while the mechanics are capable to move with frequencies of up to ca. 35 Hz, leading to ca. 14 ms/spectrum time resolution. A new software package allows performing programmed scan sequences, which enable the user to measure stepwise with alternating parameters in predefined time segments. Thus, e.g., switching between edges scanned with the same energy range is possible within one in situ experiment, while also the time resolution can be varied simultaneously. This progress makes the new system extremely user friendly and efficient to use for time resolvedx-ray absorption spectroscopy at synchrotron radiation beamlines.
81(2010); http://dx.doi.org/10.1063/1.3457847View Description Hide Description
We report for the first time the performance of axicon based conical resonators (ABCRs) in a copper vapor laser, with novel results. The unstable conical resonator comprising of conical mirror (reflecting axicon) with axicon angle , cone angle , and a convex mirror of 60 cm radius of curvature was effective in reducing the average beam divergence to ( fold reduction compared to standard multimode plane-plane cavity) with output power of . Extraction efficiency of and beam divergence of was achieved in other stable ABCR configurations using flat and concave mirrors with the axicon. This is a significant improvement compared to 4–5 mrad normally observed in conventional stable resonators in copper vapor lasers. The conical resonators with copper vapor laser provide high misalignment tolerance where is the tilt angle of the conical mirror from optimum position responsible for decline in laser power. The depth of focus was times larger in case of conical resonator as compared to that of standard spherical unstable resonator under similar beam divergence and focusing conditions.
81(2010); http://dx.doi.org/10.1063/1.3462978View Description Hide Description
We have electro-optically sliced the output light of a commercial Coherent Evolution Nd:YLF laser to pump a kilohertz repetition rate nanosecond dye laser system. Simple and highly adjustable, this laser system can easily be used for initial state preparation for ultrafast systems as well as high resolution spectroscopy.
Determining time resolution of microchannel plate detectors for electron time-of-flight spectrometers81(2010); http://dx.doi.org/10.1063/1.3463690View Description Hide Description
The temporal resolution of a 40 mm diameter chevron microchannel plate (MCP) detector followed by a constant fraction discriminator and a time-to-digital converter was determined by using the third order harmonic of 25 fs Ti:sapphire laser pulses. The resolution was found to deteriorate from 200 to 300 ps as the total voltage applied on the two MCPs increased from 1600 to 2000 V. This was likely due to a partial saturation of the MCP and/or the constant fraction discriminator working with signals beyond its optimum range of pulse width and shape.
- Particle Sources, Optics and Acceleration; Particle Detectors
81(2010); http://dx.doi.org/10.1063/1.3458010View Description Hide Description
In this paper, two designs of pulsed neutron counter structure are introduced. To increase the activation counter efficiency, BC-400 plastic scintillator plates along with silver foils are utilized. Rectangular cubic and cylindrical geometries for activation counter cell are modeled using MCNP4C code. Eventually, an optimum length of 14 cm is calculated for the detector cell and optimum numbers of 20 silver foils for rectangular cubic geometry and ten foils for cylindrical geometry have been acquired. Due to the high cost of cutting, polishing of plastics, and etc., the rectangular cubic design is found to be more economical than the other design. In order to examine the functionality and ensure the detector output and corresponding designing, neutron yield of a 2.48 kJ plasma focusdevice (SBUPF1) in 8 mbar pressure with removal source method for calibration was measured neutrons per shot.
81(2010); http://dx.doi.org/10.1063/1.3458012View Description Hide Description
An open-walled ionization chamber is developed to monitor the tritium concentration in gloveboxes in tritium processing systems. Two open walls are used to replace the sealed wall in common ionization chambers, through which the tritium gas can diffuse into the chamber without the aid of pumps and pipelines. Some basic properties of the chamber are examined to evaluate its performance. Results turn out that an open-walled chamber of 1 l in volume shows a considerably flat plateau over 700 V for a range of tritium concentration. The chamber also gives a good linear response to gamma fields over 4 decades under a pressure condition of 1 atm. The pressure dependence characteristics show that the ionization current is only sensitive at low pressures. The pressure influence becomes weaker as the pressure increases mainly due to the decrease in the mean free path of particles produced by tritium decay. The minimum detection limit of the chamber is .
81(2010); http://dx.doi.org/10.1063/1.3436636View Description Hide Description
A difficult task with many particle detectors focusing on interactions below is to perform a calibration in the appropriate energy range that adequately probes all regions of the detector. Because detector response can vary greatly in various locations within the device, a spatially uniform calibration is important. We present a new method for calibration of liquid xenon (LXe) detectors, using the short-lived . This source has transitions at 9.4 and 32.1 keV, and as a noble gas like Xe, it disperses uniformly in all regions of the detector. Even for low source activities, the existence of the two transitions provides a method of identifying the decays that is free of background. We find that at decreasing energies, the LXe light yield increases, while the amount of electric field quenching is diminished. Additionally, we show that if any long-lived radioactive backgrounds are introduced by this method, they will present less than in the next generation of LXe dark matter direct detection searches.
A high resolution, broad energy acceptance spectrometer for laser wakefield acceleration experiments81(2010); http://dx.doi.org/10.1063/1.3458013View Description Hide Description
Laser wakefield experiments present a unique challenge in measuring the resulting electron energy properties due to the large energy range of interest, typically several 100 MeV, and the large electron beam divergence and pointing jitter . In many experiments the energy resolution and accuracy are limited by the convolved transverse spot size and pointing jitter of the beam. In this paper we present an electron energy spectrometer consisting of two magnets designed specifically for laser wakefield experiments. In the primary magnet the field is produced by permanent magnets. A second optional electromagnet can be used to obtain better resolution for electron energies above 75 MeV. The spectrometer has an acceptance of 2.5–400 MeV with a resolution of better than 1% rms for electron energies above 25 MeV. This high resolution is achieved by refocusing electrons in the energy plane and without any postprocessing image deconvolution. Finally, the spectrometer employs two complimentary detection mechanisms: (1) absolutely calibrated scintillation screens imaged by cameras outside the vacuum chamber and (2) an array of scintillating fibers coupled to a low-noise charge-coupled device.
A system for measuring bubble voidage and frequency around tubes immersed in a fluidized bed of particles81(2010); http://dx.doi.org/10.1063/1.3462967View Description Hide Description
Gas-solid fluidized beds are common in chemical processing and energy production industries. These types of reactors frequently have banks of tubes immersed within the bed to provide heating or cooling, and it is important that the fluid dynamics within these bundles is efficient and uniform. This paper presents a simple, low-cost method for quantitatively analyzing the behavior of gas bubbles within banks of tubes in a fluidized bed cold flow model. Two probes, one containing an infrared emitter and one containing an infrared (IR) detector, are placed into adjacent glass tubes such that the emitter and detector face each other. As bubbles pass through the IR beam, the detector signal increases due to less solid material blocking the path between the emitter and detector. By calibrating the signal response to known voidage of the material, one can measure the bubble voidage at various locations within the tube bundle. The rate and size of bubbles passing through the beam can also be determined by high frequency data collection and subsequent analysis. This technique allows one to develop a map of bubble voidage within a fluidized bed, which can be useful for model validation and system optimization.
- Nuclear Physics, Fusion and Plasmas
81(2010); http://dx.doi.org/10.1063/1.3455186View Description Hide Description
A new digital time differential perturbed angular correlation spectrometer, designed to measure the energy of and coincidence time between correlated detector signals, here correlated photons, is presented. The system overcomes limitations of earlier digital approaches and features improved performance and handling. By consequently separating the data recording and evaluation, it permits the simultaneous measurement of decays with several -ray cascades at once and avoids the necessity of premeasurement configuration. Tests showed that the spectrometer reaches a time resolution of 460 ps [using a sample and (LYSO) scintillators, otherwise better than 100 ps], an energy resolution that is equivalent to the limit of the used scintillation material, and a processing capability of more than 200 000 quanta per detector and second. Other possible applications of the presented methods include nuclear spectroscopy, positron emission tomography, time of flight studies, lidar, and radar.
81(2010); http://dx.doi.org/10.1063/1.3455216View Description Hide Description
Tangential soft-x ray (SXR) imaging diagnostic has been developed and three-dimensional (3D) structure of the internal magnetic surface has been deduced by comparing the experimental and calculated two-dimensional SXR images in a reversed field pinch. The SXR imaging system, consisting of a MCP, a fluorescent plate, and an intensified charge coupled device camera, has been installed in REversed field pinch of Low-Aspect-ratio eXperiment (RELAX) machine. Major characteristics of an experimental SXR image could be reproduced by numerical calculations of the image using a single island model, suggesting a helical hot core in RELAX. The SXR imaging system could be useful for 3D structural studies when tangential and vertical simultaneous imaging systems would be installed, with appropriate numerical modeling of 3D structure of the magnetic surfaces.
81(2010); http://dx.doi.org/10.1063/1.3455201View Description Hide Description
In an effort to temporally resolve the electron density, electron temperature, and plasma potential for turbulent plasma discharges, a unique high-speed dual Langmuir probe (HDLP) has been developed. A traditional single Langmuir probe of cylindrical geometry (exposed to the plasma) is swept simultaneously with a nearby capacitance and noise compensating null probe (fully insulated from the plasma) to enable bias sweep rates on a microsecond timescale. Traditional thin-sheath Langmuir probe theory is applied for interpretation of the collected probe data. Data at a sweep rate of 100 kHz are presented; however the developed system is capable of running at 1 MHz—near the upper limit of the applied electrostaticLangmuir probe theory for the investigated plasma conditions. Large sets (100 000 sweeps at each of 352 spatial locations) of contiguous turbulent plasma properties are collected using simple electronics for probe bias driving and current measurement attaining 80 dB signal-to-noise measurements with dc to 1 MHz bandwidth. Near- and far-field plume measurements with the HDLP system are performed downstream from a modern Hall effect thruster where the time-averaged plasma properties exhibit the approximate ranges: electron density from , electron temperature from 1 to 3.5 eV, and plasma potential from 5 to 15 V. The thruster discharge of 200 V (constant anode potential) and 2 A (average discharge current) displays strong, 2.2 A peak-to-peak, current oscillations at 19 kHz, characteristic of the thruster “breathing mode” ionization instability. Large amplitude discharge current fluctuations are typical for most Hall thrusters, yet the HDLP system reveals the presence of the same 19 kHz fluctuations in , , and throughout the entire plume with peak-to-peak divided by mean plasma properties that average 94%. The propagation delays between the discharge current fluctuations and the corresponding plasma density fluctuations agree well with expected ion transit-times observed with distinct plasma waves traveling away from the thruster at velocities .