Volume 81, Issue 1, January 2010
Index of content:
- Optics; Atoms and Molecules; Spectroscopy; Photon Detectors
Applying genetic algorithm optimization to a folded geometry acousto-optic modulated spatial pulse shaper81(2010); http://dx.doi.org/10.1063/1.3276682View Description Hide Description
A folded geometry acousto-optic modulator spatial pulse shaper has been designed for shaping individual pulses from a high power amplified laser. The design preserves the capability of computer programmable amplitude and phase modulation of femtosecond laser pulses. An additional application of genetic algorithm optimization approach for compressing a stretched pulse is also demonstrated for such a pulse shaper. Spectrally and temporally resolved optical gating technique is used to characterize the shaped pulses.
Velocity distribution function of sputtered Cu atoms obtained by time resolved optical absorption spectroscopy81(2010); http://dx.doi.org/10.1063/1.3284528View Description Hide Description
A new method based on time resolvedoptical absorptionspectroscopy is proposed to determine the velocitydistribution function of sputteredCu atoms in a magnetron plasma discharge. The method consists of applying a short pulse of and of recording time variations in copper atom density in off pulse at different positions (1, 2, and 3 cm) from target surface under 3–30 mTorr. The time evolution of the density is then converted into velocity distribution. We estimate that only sputtered atoms with radial velocity component lower than 0.5 km/s are detected. The average velocity of Cu atoms is evaluated as the first order moment of the velocitydistribution functions. The velocitydistribution functions become the more dispersive the farther from target surface. The average velocities vary in the range of 2.5–3 km/s at the vicinity of target surface whereas at 3 cm a decrease from 2.5 to 1.2 km/s is observed at 30 mTorr.
Construction, figures of merit, and testing of a single-cell fluorescence excitation spectroscopy system81(2010); http://dx.doi.org/10.1063/1.3270251View Description Hide Description
Characterization of phytoplankton community composition is critical to understanding the ecology and biogeochemistry of the oceans. One approach to taxonomic characterization takes advantage of differing pigmentation between algal taxa and thus differences in fluorescence excitation spectra. Analyses of bulk water samples, however, may be confounded by interference from chromophoric dissolved organic matter or suspended particulate matter. Here, we describe an instrument that uses a laser trap based on a Nikon TE2000-U microscope to position individual phytoplankton cells for confocal fluorescence excitation spectroscopy, thus avoiding interference from the surrounding medium. Quantitative measurements of optical power give data in the form of photons emitted per photon of exposure for an individual phytoplankton cell. Residence times for individual phytoplankton in the instrument can be as long as several minutes with no substantial change in their fluorescence excitation spectra. The laser trap was found to generate two-photon fluorescence from the organisms so a modification was made to release the trap momentarily during data acquisition. Typical signal levels for an individual cell are in the range of of fluorescence using a monochromated 75 W Xe arc lamp excitation source with a 2% transmission neutral density filter.
Development of a new rectangular NaI(Tl) scintillator and spectroscopy of low-energy charged particles81(2010); http://dx.doi.org/10.1063/1.3276691View Description Hide Description
Standard NaI(Tl) scintillators have only one optical window and are housed in airtight protective enclosures to protect against hygroscopicity from moisture in the air. For these reasons, NaI(Tl) scintillators are unsuitable for the collection of scintillation photons or for the detection of low-energy charged particles. To overcome these disadvantages, a rectangular NaI(Tl) scintillator has been newly developed. In this paper, we estimate the photon response of this new scintillator. The energy resolution was for 662 keV gamma rays from a standard source. The number of photoelectrons was 3,686 for the photoelectric peak. Good energy resolution and photon collection were therefore confirmed for this specialized form of NaI(Tl) scintillator. The ability of this scintillator to detect low-energy particles was confirmed by successful measurement of low-energy charged particles (250–700 keV region) from a thin film radioisotope source.
A slow atom source using a collimated effusive oven and a single-layer variable pitch coil Zeeman slower81(2010); http://dx.doi.org/10.1063/1.3276712View Description Hide Description
We describe a simple slow atom source for loading a rubidiummagneto-optical trap. The source includes an effusive oven with a long heated collimation tube. Almost all components are standard vacuum parts. The heating elements and thermocouples are external to the vacuum, protecting them from the hostile hot alkali environment and allowing repair without breaking vacuum. The thermal source is followed by a Zeeman slower with a single-layer coil of variable winding pitch. The single-layer design is simple to construct and has low inductance which allows for rapid switching of the magnetic field. The coil pitch was determined by fitting the analytic form of the magnetic field for a variable winding pitch to the desired magnetic field profile required to slow atoms. The measured magnetic field for the constructed coil is in excellent agreement with the desired field. The source produces atoms at 35 m/s with a flux up to at .
81(2010); http://dx.doi.org/10.1063/1.3290418View Description Hide Description
We demonstrate a concept to study transient liquids with picosecond time-resolved x-ray scattering in a high-repetition-rate configuration. Femtosecond laser excitation of crystalline indium antimonide (InSb) induces ultrafast melting, which leads to a loss of the long-range order. The remaining local correlations of the liquid result in broad x-ray diffraction rings, which are measured as a function of delay time. After 2 ns the liquid structure factor shows close agreement with that of equilibrated liquid InSb. The measured decay of the liquidscattering intensity corresponds to the resolidification rate of 1 m/s in InSb.
81(2010); http://dx.doi.org/10.1063/1.3276686View Description Hide Description
A new method for ion extraction from an anharmonic electrostatic trap is introduced. Anharmonicity is a common feature of electrostatic traps which can be used for small scale spatial confinement of ions, and this feature is also necessary for autoresonant ion extraction. With the aid of ion trajectory simulations, novel autoresonant trap mass spectrometers (ART-MSs) have been designed based on these very simple principles. A mass resolution is demonstrated for the prototypes discussed here. We report also on the pressure dependencies, and the (mV) rf field strength dependencies of the ART-MS sensitivity. Importantly the new MSdesigns do not require heavy magnets, tight manufacturing tolerances, introduction of buffer gases, high power rf sources, nor complicated electronics. The designs described here are very inexpensive to implement relative to other instruments, and can be easily miniaturized. Possible applications are discussed.
Broadband 308 nm vibrational Raman spectroscopy of gaseous species using a potassium hydrogen phthalate liquid filter and polarization fluorescence suppression81(2010); http://dx.doi.org/10.1063/1.3276683View Description Hide Description
Broadband XeCl excimer lasers operating at 308 nm are not currently used in the field of gas phase vibrational Raman spectroscopy (VRS). An explanation as to why alternative wavelengths, and in particular tuneable, narrowband lasers are currently preferred for gas phase VRS is presented in addition to demonstrating a setup which makes the XeCl laser a viable alternative when considering excitation sources for VRS. A solution of potassium hydrogen phthalate is shown to be a practical low-pass liquid filter and to reduce substantially the effects of Rayleigh scattering on collected Raman spectra. The use of a commercial beam polarizer is also shown to be effective in suppressing background fluorescence that otherwise necessitates the use of expensive tuneable, narrowband lasers when performing VRS with sources of background fluorescence. Finally, an unconventional excitation beam arrangement is shown to produce viable Raman spectra from which species concentrations and distributions can be determined.
81(2010); http://dx.doi.org/10.1063/1.3274813View Description Hide Description
We describe new techniques in the construction of optical lattices to realize a coherent atom-based microscope, comprised of two atomic species used as target and probe atoms, each in an independently controlled optical lattice. Precise and dynamic translation of the lattices allows atoms to be brought into spatial overlap to induce atomic interactions. For this purpose, we have fabricated two highly stable, hexagonal optical lattices, with widely separated wavelengths but identical lattice constants using diffractive optics. The relative translational stability of 12 nm permits controlled interactions and even entanglement operations with high fidelity. Translation of the lattices is realized through a monolithic electro-optic modulator array, capable of moving the lattice smoothly over one lattice site in , or rapidly on the order of 100 ns.
Subwavenumber charge-coupled device spectrometer calibration using molecular iodine laser-induced fluorescence81(2010); http://dx.doi.org/10.1063/1.3287951View Description Hide Description
Spectrometers configured with charge-coupled devices(CCD) or other array-based detectors require calibration to convert from the pixel coordinate to a spectral coordinate. A CCDcalibration method well suited for Raman spectroscopy has been developed based on the 514.5 nm laser-induced fluorescence(LIF)spectrum of room-temperature molecular iodine vapor. Over 360 primary and secondary LIFcalibration lines spanning 510–645 nm were identified as calibrant peaks using an instrumental resolution of . Two instrument calibration functions were evaluated with these peaks: a second-order polynomial and a function derived from simple optomechanical considerations. The latter function provided better fitting characteristics. Calibration using LIF was tested with measurements of both laser light scattering and Raman spectra. The LIF reference spectra and the signal spectra were recorded simultaneously, with no cross talk, by separating the two signals spatially along the vertical axis of the CCDimager. In this way, every CCDimage could be independently calibrated. An accuracy and a precision of were achieved with this calibration technique.
81(2010); http://dx.doi.org/10.1063/1.3276699View Description Hide Description
We report on the design and testing of an array of Penning ion traps made from printed circuit board. The system enables fast shuttling of ions from one trapping zone to another, which could be of use in quantum information processing. We describe simulations carried out to determine the optimal potentials to be applied to the trap electrodes for enabling this movement. The results of a preliminary experiment with a cloud of laser cooled calcium ions demonstrate a round-trip shuttling efficiency of up to 75%.
81(2010); http://dx.doi.org/10.1063/1.3280227View Description Hide Description
We have developed a novel laboratory instrument for studying gas phase, anion-neutral chemistry. To the best of our knowledge, this is the first such apparatus which uses fast merged beams to investigate anion-neutral chemical reactions. As proof-of-principle we have detected the associative detachment reaction. Here we describe the apparatus in detail and discuss related technical and experimental issues.
- Particle Sources, Optics and Acceleration; Particle Detectors
81(2010); http://dx.doi.org/10.1063/1.3276685View Description Hide Description
The interest to produce negative osmium ions is manifold in the realm of high-accuracy ion trap experiments: high-resolution nearly Doppler-free laser spectroscopy, antihydrogen formation in its ground state, and contributions to neutrino mass spectrometry. Production of these ions is generally accomplished by sputtering an Os sample with ions at tens of keV. Though this is a well-established method commonly used at accelerators, these kind of sources are quite demanding and tricky to operate. Therefore, the development of a more straightforward and cost effective production scheme will be of benefit for ion trap and other experiments. Such a scheme makes use of desorption and ionization with pulsed lasers and identification of the ions by time-of-flight mass spectrometry. First investigations of negative osmium ion production using a pulsed laser for desorption and ionization and a commercial matrix-assisted laser desorption/ionization time-of-flight system for identification has demonstrated the suitability of this technique. More than negative osmium ions per shot were registered after bombarding pure osmiumpowder with a 5 ns pulse width Nd:yttrium aluminum garnet laser. The limitation in the ion number was imposed by the detection limit of the microchannel plate detector.
81(2010); http://dx.doi.org/10.1063/1.3271539View Description Hide Description
Design analysis of a high power indirectly heated solidcathode (for a 200 kW, 45 kV, and 270° bent strip type electron gun) has been presented. The design approach consists of simulation followed by extensive experimentation with different cathode configurations. The preferred cathode is of trapezoidal section with an emitting area of made up of tantalum operating at about 2500 K. The solidcathode at the operating temperature of 2500 K generated a well defined electron beam. Electromagnetic and thermomechanical simulation is used to optimize the shape of the beam. Thermal modeling has also been used to analyze the temperature and stress distribution on the electrodes. The simulation results are validated by experimental measurement.
Highly efficient pulsed power supply system with a two-stage LC generator and a step-up transformer for fast capillary discharge soft x-ray laser at shorter wavelength81(2010); http://dx.doi.org/10.1063/1.3276705View Description Hide Description
Highly efficient and compact pulsed power supply system for a capillary discharge soft x-ray laser (SXRL) has been developed. The system consists of a two-stage LC inversion generator, a 2:54 step-up transformer, a 3 nF water capacitor, and a discharge section with a few tens of centimeter length capillary. Adoption of the pulsed transformer in combination with the LC inversion generator enables us to use only one gap switch in the circuit for charging the water capacitor up to about 0.5 MV. Furthermore, step-up ratio of a water capacitor voltage to a LC inversion generator initial charging voltage is about 40 with energy transfer efficiency of about 50%. It also leads to good reproducibility of a capillary discharge which is necessary for lasing a SXRL stably. For the study of the possibility of lasing a SXRL at shorter wavelength in a small laboratory scale, high-density and high-temperature plasma column suitable for the laser can be generated relatively easily with this system.
A laser-Compton scattering prototype experiment at 100 MeV linac of Shanghai Institute of Applied Physics81(2010); http://dx.doi.org/10.1063/1.3282445View Description Hide Description
As a prototype of the Shanghai Laser Electron Gamma Source in the Shanghai Synchrotron Radiation Facility, an x-raysource based on laser-Compton scattering (LCS) has been installed at the terminal of the 100 MeV linac of the Shanghai Institute of Applied Physics. LCSx-rays are generated by interactions between Q-switched Nd:yttrium aluminum garnet laser pulses [with wavelength of 1064 nm and pulse width of 21 ns (full width at half maximum)] and electron bunches [with energy of 108 MeV and pulse width of 0.95 ns (rms)] at an angle of 42° between laser and electron beam. In order to measure the energy spectrum of LCSx-rays, a Si(Li) detector along the electron beam line axis is positioned at 9.8 m away from a LCS chamber. After background subtraction, the LCSx-rayspectrum with the peak energy of and the peak width (rms) of is observed. Normally the 100 MeV linac operates with the electron macropulse charge of 1.0 nC/pulse, and the electron and laser collision repetition rate of 20 Hz. Therefore, the total LCSx-ray flux of can be achieved.
Physics design of a 100 keV acceleration grid system for the diagnostic neutral beam for international tokamak experimental reactor81(2010); http://dx.doi.org/10.1063/1.3271535View Description Hide Description
This paper describes the physics design of a 100 keV, 60 A accelerator for the diagnostic neutral beam (DNB) for international tokamak experimental reactor (ITER). The accelerator is a three grid system comprising of 1280 apertures, grouped in 16 groups with 80 apertures per beam group. Several computer codes have been used to optimize the design which follows the same philosophy as the ITER Design Description Document (DDD) 5.3 and the 1 MeV heating and current drive beam line [R. Hemsworth, H. Decamps, J. Graceffa, B. Schunke, M. Tanaka, M. Dremel, A. Tanga, H. P. L. De Esch, F. Geli, J. Milnes, T. Inoue, D. Marcuzzi, P. Sonato, and P. Zaccaria, Nucl. Fusion49, 045006 (2009)]. The aperture shapes, intergrid distances, and the extractor voltage have been optimized to minimize the beamlet divergence. To suppress the acceleration of coextracted electrons,permanent magnets have been incorporated in the extraction grid, downstream of the cooling water channels. The electron power loads on the extractor and the grounded grids have been calculated assuming 1 coextracted electron per ion. The beamlet divergence is calculated to be 4 mrad. At present the design for the filter field of the RF based ion sources for ITER is not fixed, therefore a few configurations of the same have been considered. Their effect on the transmission of the electrons and beams through the accelerator has been studied. The OPERA-3D code has been used to estimate the aperture offset steering constant of the grounded grid and the extraction grid, the space charge interaction between the beamlets and the kerb design required to compensate for this interaction. All beamlets in the DNB must be focused to a single point in the duct, 20.665 m from the grounded grid, and the required geometrical aimings and aperture offsets have been calculated.
High quality single shot diffraction patterns using ultrashort megaelectron volt electron beams from a radio frequency photoinjector81(2010); http://dx.doi.org/10.1063/1.3292683View Description Hide Description
Single shot diffraction patterns using a 250-fs-long electron beam have been obtained at the UCLA Pegasus laboratory. High quality images with spatial resolution sufficient to distinguish closely spaced peaks in the Debye–Scherrer ring pattern have been recorded by scattering the 1.6 pC 3.5 MeV electron beamgenerated in the rf photoinjector off a 100-nm-thick Au foil. Dark current and high emittance particles are removed from the beam before sending it onto the diffraction target using a 1 mm diameter collimating hole. These results open the door to the study of irreversible phase transformations by single shot MeV electron diffraction.
81(2010); http://dx.doi.org/10.1063/1.3284524View Description Hide Description
The characterization of the absolute number of electrons generated by laser wakefield acceleration often relies on absolutely calibrated FUJI imaging plates (IP), although their validity in the regime of extreme peak currents is untested. Here, we present an extensive study on the dependence of the sensitivity of BAS-SR and BAS-MS IP to picosecond electron bunches of varying charge of up to 60 pC, performed at the electron accelerator ELBE, making use of about three orders of magnitude of higher peak intensity than in prior studies. We demonstrate that the response of the IPs shows no saturation effect and that the BAS-SR IP sensitivity of 0.0081 photostimulated luminescence per electron number confirms surprisingly well data from previous works. However, the use of the identical readout system and handling procedures turned out to be crucial and, if unnoticed, may be an important error source.
- Nuclear Physics, Fusion and Plasmas
81(2010); http://dx.doi.org/10.1063/1.3280161View Description Hide Description
A new time-delay estimation (TDE) technique based on dynamic programming is developed to measure the time-varying time-delay between two signals. The dynamic programming based TDE technique provides a frequency response five to ten times better than previously known TDE techniques, namely, those based on time-lag cross-correlation or waveletanalysis. Effects of frequency spectrum, signal-to-noise ratio, and amplitude of time-delay on response of the TDE technique (represented as transfer function) are studied using simulated data signals. The transfer function for the technique decreases with increase in noise in signal; however it is independent of signal spectrum shape. The dynamic programming based TDE technique is applied to the beam emission spectroscopy diagnostic data to measure poloidal velocity fluctuations, which led to the observation of theoretically predicted zonal flows in high-temperature tokamakplasmas.