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Dec 2011

Volume 82, Issue 12, Articles (12xxxx)

Issue Cover Spotlight Figure

Rev. Sci. Instrum. 82, 121101 (2011); http://dx.doi.org/10.1063/1.3663970 (22 pages)

U. Zammit, M. Marinelli, F. Mercuri, S. Paoloni, and F. Scudieri

Processed image of integrated photopyroelectric calorimetry set-up. Background shows heating and transmitted red beams and white illuminating beam for imaging. Superimposed along the diagonals are semitransparent deformed images of liquid crystal textures, respectively, at the smectic-nematic transition together with typical specific heat profile and at the nematic isotropic transition.

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Perspective: Photopyroelectric effects and pyroelectric measurements: “Invited Review Article: Photopyroelectric calorimeter for the simultaneous thermal, optical, and structural characterization of samples over phase transitions” [Rev. Sci. Instrum. 82, 121101 (2011)]

Andreas Mandelis

Rev. Sci. Instrum. 82, 120901 (2011); http://dx.doi.org/10.1063/1.3669517 (4 pages)

Online Publication Date: 23 December 2011

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Abstract Unavailable
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77.70.+a Pyroelectric and electrocaloric effects
07.20.Fw Calorimeters
72.40.+w Photoconduction and photovoltaic effects
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Invited Review Article: Photopyroelectric calorimeter for the simultaneous thermal, optical, and structural characterization of samples over phase transitions

U. Zammit, M. Marinelli, F. Mercuri, S. Paoloni, and F. Scudieri

Rev. Sci. Instrum. 82, 121101 (2011); http://dx.doi.org/10.1063/1.3663970 (22 pages) | Cited 8 times

Online Publication Date: 23 December 2011

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The study of thermophysical properties is of great importance in several scientific fields. Among them, the heat capacity, for example, is related to the microscopic structure of condensed matter and plays an important role in monitoring the changes in the energy content of a system. Calorimetric techniques are thus of fundamental importance for characterizing physical systems, particularly in the vicinity of phase transitions where energy fluctuations can play an important role. In this work, the ability of the Photopyroelctric calorimetry to study the versus temperature behaviour of the specific heat and of the other thermal parameters in the vicinity of phase transitions is outlined. The working principle, the theoretical basis, the experimental configurations, and the advantages of this technique, with respect to the more conventional ones, have been described and discussed in detail. The integrations in the calorimetric setup giving the possibility to perform, simultaneously with the calorimetric studies, complementary kind of characterizations of optical, structural, and electrical properties are also described. A review of the results obtained with this technique, in all its possible configurations, for the high temperature resolution studies of the thermal parameters over several kinds of phase transitions occurring in different systems is presented and discussed.
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07.20.Fw Calorimeters
85.60.-q Optoelectronic devices
85.50.-n Dielectric, ferroelectric, and piezoelectric devices
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back to top Optics; Atoms and Molecules; Spectroscopy; Photon Detectors

Junction temperature measurement of light emitting diode by electroluminescence

S. M. He, X. D. Luo, B. Zhang, L. Fu, L. W. Cheng, J. B. Wang, and W. Lu

Rev. Sci. Instrum. 82, 123101 (2011); http://dx.doi.org/10.1063/1.3664619 (4 pages) | Cited 3 times

Online Publication Date: 1 December 2011

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Junction temperature (JT) is a key parameter of the performance and lifetime of light emitting diodes (LEDs). In this paper, a mobile instrument system has been developed for the non-contact measurement of JTs of LED under LabVIEW control. The electroluminescence (EL) peak shift of the LED is explored to measure the JT. Commercially available high power blue LEDs are measured. A linear relation between emission peak shift and JT is found. The accuracy of the JT is about 1 °C determined by the precision of the emission peak shift, ±0.03 nm, at 3σ standard deviation for blue LED. Using this system, on-line temperature rise curves of LED lamps are determined.
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85.60.Jb Light-emitting devices

Scanning absorption nanoscopy with supercontinuum light sources based on photonic crystal fiber

Noeul Park, Kyoung-Duck Park, Youngjoo Chung, and Mun Seok Jeong

Rev. Sci. Instrum. 82, 123102 (2011); http://dx.doi.org/10.1063/1.3665206 (4 pages) | Cited 1 time

Online Publication Date: 7 December 2011

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We have experimentally demonstrated a scanning absorption nanoscopy system combining a near-field scanning optical microscope with an absorption spectroscope using supercontinuum radiation generated by coupling a mode-locked Ti:sapphire pulse laser to a nonlinear photonic crystal fiber as a light source. For the performance test of the system, the absorption spectrum and near-field absorption image of Rhodamine 6G were observed. As this system allows us to investigate the absorption properties and distribution of materials with high spatial resolution, it is expected to be effectively applied in various research areas.
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07.79.Fc Near-field scanning optical microscopes
42.25.Bs Wave propagation, transmission and absorption
42.55.Tv Photonic crystal lasers and coherent effects
42.60.Fc Modulation, tuning, and mode locking
42.65.-k Nonlinear optics
42.72.-g Optical sources and standards

Measurement of axial neutral density profiles in a microwave discharge ion thruster by laser absorption spectroscopy with optical fiber probes

Ryudo Tsukizaki, Hiroyuki Koizumi, Kazutaka Nishiyama, and Hitoshi Kuninaka

Rev. Sci. Instrum. 82, 123103 (2011); http://dx.doi.org/10.1063/1.3665954 (6 pages) | Cited 1 time

Online Publication Date: 12 December 2011

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In order to reveal the physical processes taking place within the “μ10” microwave discharge ion thruster, internal plasma diagnosis is indispensable. However, the ability of metallic probes to access microwave plasmas biased at a high voltage is limited from the standpoints of the disturbance created in the electric field and electrical isolation. In this study, the axial density profiles of excited neutral xenon were successfully measured under ion beam acceleration by using a novel laser absorption spectroscopy system. The target of the measurement was metastable Xe I 5p5(2P03/2)6s[3/2]02 which absorbed a wavelength of 823.16 nm. Signals from laser absorption spectroscopy that swept a single-mode optical fiber probe along the line of sight were differentiated and converted into axial number densities of the metastable neutral particles in the plasma source. These measurements revealed a 1018 m−3 order of metastable neutral particles situated in the waveguide, which caused two different modes during the operation of the μ10 thruster. This paper reports a novel spectroscopic measurement system with axial resolution for microwave plasma sources utilizing optical fiber probes.
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52.80.Pi High-frequency and RF discharges
42.62.Fi Laser spectroscopy
52.50.Dg Plasma sources

High coherent bi-chromatic laser with gigahertz splitting produced by the high diffraction orders of acousto-optic modulator used for coherent population trapping experiments

Peter Yun, Bozhong Tan, Wei Deng, and Sihong Gu

Rev. Sci. Instrum. 82, 123104 (2011); http://dx.doi.org/10.1063/1.3665986 (5 pages) | Cited 5 times

Online Publication Date: 12 December 2011

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To prepare the coherent population trapping (CPT) states with rubidium and cesium, the commonly used atoms in CPT studies, a coherent bi-chromatic light field with frequency difference of several GHz is a basic requirement. With a 200 MHz center frequency acousto-optic modulator (AOM), we have realized bi-chromatic laser fields with several GHz frequency splits through high diffraction orders. We have experimentally studied the coherence between two frequency components of a bi-chromatic laser beam, which is composed of ±6 orders with frequency split of 3 GHz diffracted from the same laser beam, and the measured residual phase noise is Δφ2<0.019 rad2. The bi-chromatic laser fields were used to prepare CPT states with 85Rb and 87Rb atoms, and high contrast CPT signals were obtained. For CPT states preparation, our study result shows that it is a feasible approach to generate the bi-chromatic light field with larger frequency splits through high diffraction orders of AOM.
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42.60.Da Resonators, cavities, amplifiers, arrays, and rings
42.60.Mi Dynamical laser instabilities; noisy laser behavior
42.79.Fm Reflectors, beam splitters, and deflectors
42.79.Hp Optical processors, correlators, and modulators
42.79.Jq Acousto-optical devices
42.50.Wk Mechanical effects of light on material media, microstructures and particles
42.55.Px Semiconductor lasers; laser diodes

Apparatus for laser-assisted electron scattering in femtosecond intense laser fields

Reika Kanya, Yuya Morimoto, and Kaoru Yamanouchi

Rev. Sci. Instrum. 82, 123105 (2011); http://dx.doi.org/10.1063/1.3665926 (9 pages) | Cited 2 times

Online Publication Date: 13 December 2011

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An apparatus for observation of laser-assisted electron scattering (LAES) in femtosecond intense laser fields was developed. The unique apparatus has three essential components, i.e., a photocathode-type ultrashort pulsed-electron gun, a toroidal-type electron energy analyzer enabling simultaneous detection of energy and angular distributions of scattered electrons with high efficiency, and a high repetition-rate data acquisition system combined with a high power 5 kHz Ti:sapphire laser system. These advantages make extremely weak femtosecond-LAES signals distinguishable from the huge elastic scattering signals. A precise method for securing a spatial overlap between three beams, that is, an atomic beam, an electron beam, and a laser beam, and synchronization between the electron and laser pulses is described. As a demonstration of this apparatus, an electron energy spectrum of the LAES signals with 1.4 × 1012 W/cm2, 795 nm, 50 fs laser pulses was observed, and the detection limit and further improvements of the apparatus are examined.
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34.80.Bm Elastic scattering
34.80.Dp Atomic excitation and ionization

Versatile multispectral microscope based on light emitting diodes

Mikkel Brydegaard, Aboma Merdasa, Hiran Jayaweera, Jens Ålebring, and Sune Svanberg

Rev. Sci. Instrum. 82, 123106 (2011); http://dx.doi.org/10.1063/1.3660810 (13 pages) | Cited 1 time

Online Publication Date: 13 December 2011

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We describe the development of a novel multispectral microscope, based on light-emitting diodes, capable of acquiring megapixel images in thirteen spectral bands from the ultraviolet to the near infrared. The system captures images and spectra in transmittance, reflectance, and scattering modes. We present as examples of applications ground truth measurements for remote sensing and parasitology diagnostics. The system is a general purpose scientific instrument that could be used to develop dedicated simplified instruments with optimal bands and mode selection.
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07.60.Pb Conventional optical microscopes
42.30.-d Imaging and optical processing
85.60.Jb Light-emitting devices

Multimode laser beam analyzer instrument using electrically programmable optics

Philip J. Marraccini and Nabeel A. Riza

Rev. Sci. Instrum. 82, 123107 (2011); http://dx.doi.org/10.1063/1.3669535 (6 pages) | Cited 1 time

Online Publication Date: 15 December 2011

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Presented is a novel design of a multimode laser beam analyzer using a digital micromirror device (DMD) and an electronically controlled variable focus lens (ECVFL) that serve as the digital and analog agile optics, respectively. The proposed analyzer is a broadband laser characterization instrument that uses the agile optics to smartly direct light to the required point photodetectors to enable beam measurements of minimum beam waist size, minimum waist location, divergence, and the beam propagation parameter M2. Experimental results successfully demonstrate these measurements for a 500 mW multimode test laser beam with a wavelength of 532 nm. The minimum beam waist, divergence, and M2 experimental results for the test laser are found to be 257.61 μm, 2.103 mrad, 1.600 and 326.67 μm, 2.682 mrad, 2.587 for the vertical and horizontal directions, respectively. These measurements are compared to a traditional scan method and the results of the beam waist are found to be within error tolerance of the demonstrated instrument.
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42.60.Jf Beam characteristics: profile, intensity, and power; spatial pattern formation
42.79.Bh Lenses, prisms and mirrors
07.60.-j Optical instruments and equipment

Pulsed laser noise analysis and pump-probe signal detection with a data acquisition card

Christopher A. Werley, Stephanie M. Teo, and Keith A. Nelson

Rev. Sci. Instrum. 82, 123108 (2011); http://dx.doi.org/10.1063/1.3669783 (6 pages) | Cited 2 times

Online Publication Date: 16 December 2011

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A photodiode and data acquisition card whose sampling clock is synchronized to the repetition rate of a laser are used to measure the energy of each laser pulse. Simple analysis of the data yields the noise spectrum from very low frequencies up to half the repetition rate and quantifies the pulse energy distribution. When two photodiodes for balanced detection are used in combination with an optical modulator, the technique is capable of detecting very weak pump-probe signals (ΔI/I0 ∼ 10−5 at 1 kHz), with a sensitivity that is competitive with a lock-in amplifier. Detection with the data acquisition card is versatile and offers many advantages including full quantification of noise during each stage of signal processing, arbitrary digital filtering in silico after data collection is complete, direct readout of percent signal modulation, and easy adaptation for fast scanning of delay between pump and probe.
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42.62.Eh Metrological applications; optical frequency synthesizers for precision spectroscopy
85.60.Dw Photodiodes; phototransistors; photoresistors
42.79.Hp Optical processors, correlators, and modulators
32.80.Xx Level crossing and optical pumping
85.60.Gz Photodetectors (including infrared and CCD detectors)
84.40.Ua Telecommunications: signal transmission and processing; communication satellites

Time-resolved soft x-ray absorption setup using multi-bunch operation modes at synchrotrons

L. Stebel, M. Malvestuto, V. Capogrosso, P. Sigalotti, B. Ressel, F. Bondino, E. Magnano, G. Cautero, and F. Parmigiani

Rev. Sci. Instrum. 82, 123109 (2011); http://dx.doi.org/10.1063/1.3669787 (6 pages) | Cited 1 time

Online Publication Date: 16 December 2011

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Here, we report on a novel experimental apparatus for performing time-resolved soft x-ray absorption spectroscopy in the sub-ns time scale using non-hybrid multi-bunch mode synchrotron radiation. The present setup is based on a variable repetition rate Ti:sapphire laser (pump pulse) synchronized with the ∼500 MHz x-ray synchrotron radiation bunches and on a detection system that discriminates and singles out the significant x-ray photon pulses by means of a custom made photon counting unit. The whole setup has been validated by measuring the time evolution of the L3 absorption edge during the melting and the solidification of a Ge single crystal irradiated by an intense ultrafast laser pulse. These results pave the way for performing synchrotron time-resolved experiments in the sub-ns time domain with variable repetition rate exploiting the full flux of the synchrotron radiation.
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29.27.Eg Beam handling; beam transport
29.20.dk Synchrotrons
29.30.Kv X- and γ-ray spectroscopy
07.85.Nc X-ray and γ-ray spectrometers

Long-term frequency stabilization system for external cavity diode laser based on mode boundary detection

Zhouxiang Xu, Kaikai Huang, Yunfeng Jiang, and Xuanhui Lu

Rev. Sci. Instrum. 82, 123110 (2011); http://dx.doi.org/10.1063/1.3669529 (5 pages)

Online Publication Date: 19 December 2011

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We have realized a long-term frequency stabilization system for external cavity diode laser (ECDL) based on mode boundary detection method. In this system, the saturated absorption spectroscopy was used. The current and the grating of the ECDL were controlled by a computer-based feedback control system. By checking if there are mode boundaries in the spectrum, the control system determined how to adjust current to avoid mode hopping. This procedure was executed periodically to ensure the long-term stabilization of ECDL in the absence of mode hops. This diode laser system with non-antireflection coating had operated in the condition of long-term mode-hop-free stabilization for almost 400 h, which is a significant improvement of ECDL frequency stabilization system.
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42.55.Px Semiconductor lasers; laser diodes
42.60.By Design of specific laser systems
42.60.Da Resonators, cavities, amplifiers, arrays, and rings
42.79.Wc Optical coatings

Dual beat-frequencies laser Doppler interferometer

A. Takita, H. Ebara, and Y. Fujii

Rev. Sci. Instrum. 82, 123111 (2011); http://dx.doi.org/10.1063/1.3669785 (3 pages)

Online Publication Date: 21 December 2011

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A dual beat-frequencies laser Doppler interferometer which can measure high speed back and forth motion is proposed and demonstrated. In this interferometer, the two frequencies, f1 and f2, emitted by a Zeeman laser are introduced into the signal beams. They are changed to f1′ and f2′ by Doppler shift fDoppler, which is proportional to the velocity of the object. The use of the two beat signals of |f1′ − f2| and |f1 − f2′| appearing at two photodetectors enables to measure the velocity over the limitation vc = λ(f1 − f2)/2.
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07.60.Ly Interferometers
85.60.Gz Photodetectors (including infrared and CCD detectors)

Feedback-controlled laser fabrication of micromirror substrates

Benjamin Petrak, Kumarasiri Konthasinghe, Sonia Perez, and Andreas Muller

Rev. Sci. Instrum. 82, 123112 (2011); http://dx.doi.org/10.1063/1.3671291 (6 pages)

Online Publication Date: 21 December 2011

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Short (40–200 μs) single focused CO2 laser pulses of energy ≳100 μJ were used to fabricate high quality concave micromirror templates on silica and fluoride glass. The ablated features have diameters of ≈20–100 μm and average root-mean-square (RMS) surface microroughness near their center of less than 0.2 nm. Temporally monitoring the fabrication process revealed that it proceeds on a time scale shorter than the laser pulse duration. We implement a fast feedback control loop (≈20 kHz bandwidth) based on the light emitted by the sample that ensures an RMS size dispersion of less than 5% in arrays on chips or in individually fabricated features on an optical fiber tip, a significant improvement over previous approaches using longer pulses and open loop operation.
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42.62.-b Laser applications
42.55.Lt Gas lasers including excimer and metal-vapor lasers
06.60.Vz Workshop procedures (welding, machining, lubrication, bearings, etc.)
42.60.-v Laser optical systems: design and operation
42.79.Bh Lenses, prisms and mirrors

Limitations of the terahertz photomixer

Kacper Nowak, Przemyslaw P. Jarzab, and Edward F. Plinski

Rev. Sci. Instrum. 82, 123113 (2011); http://dx.doi.org/10.1063/1.3665103 (4 pages) | Cited 1 time

Online Publication Date: 23 December 2011

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In the paper, Fourier transform has been used for calculations of a refractive index of dielectric samples measured in the terahertz photomixer arrangement. We considered measurement limitations caused by a sampling frequency and a photomixer bandwidth.
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84.40.-x Radiowave and microwave (including millimeter wave) technology
84.30.Qi Modulators and demodulators; discriminators, comparators, mixers, limiters, and compressors

Optic-microwave mixing velocimeter for superhigh velocity measurement

Jidong Weng, Xiang Wang, Tianjiong Tao, Cangli Liu, and Hua Tan

Rev. Sci. Instrum. 82, 123114 (2011); http://dx.doi.org/10.1063/1.3670403 (3 pages) | Cited 2 times

Online Publication Date: 27 December 2011

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The phenomenon that a light beam reflected off a moving object experiences a Doppler shift in its frequency underlies practical interferometric techniques for remote velocity measurements, such as velocity interferometer system for any reflector (VISAR), displacement interferometer system for any reflector (DISAR), and photonic Doppler velocimetry (PDV). While VISAR velocimeters are often bewildered by the fringe loss upon high-acceleration dynamic process diagnosis, the optic-fiber velocimeters such as DISAR and PDV, on the other hand, are puzzled by high velocity measurement over 10 km/s, due to the demand for the high bandwidth digitizer. Here, we describe a new optic-microwave mixing velocimeter (OMV) for super-high velocity measurements. By using currently available commercial microwave products, we have constructed a simple, compact, and reliable OMV device, and have successfully obtained, with a digitizer of bandwidth 6 GH only, the precise velocity history of an aluminum flyer plate being accelerated up to 11.2 km/s in a three stage gas-gun experiment.
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06.30.Gv Velocity, acceleration, and rotation
07.60.-j Optical instruments and equipment
07.60.Ly Interferometers
07.57.-c Infrared, submillimeter wave, microwave and radiowave instruments and equipment

Mapping the magnetic field vector in a fountain clock

Marina Gertsvolf and Louis Marmet

Rev. Sci. Instrum. 82, 123115 (2011); http://dx.doi.org/10.1063/1.3671633 (5 pages)

Online Publication Date: 27 December 2011

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We show how the mapping of the magnetic field vector components can be achieved in a fountain clock by measuring the Larmor transition frequency in atoms that are used as a spatial probe. We control two vector components of the magnetic field and apply audio frequency magnetic pulses to localize and measure the field vector through Zeeman spectroscopy.
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06.30.Ft Time and frequency
07.55.Ge Magnetometers for magnetic field measurements

Frequency discriminators for the characterization of narrow-spectrum heterodyne beat signals: Application to the measurement of a sub-hertz carrier-envelope-offset beat in an optical frequency comb

Stéphane Schilt, Nikola Bucalovic, Lionel Tombez, Vladimir Dolgovskiy, Christian Schori, Gianni Di Domenico, Michele Zaffalon, and Pierre Thomann

Rev. Sci. Instrum. 82, 123116 (2011); http://dx.doi.org/10.1063/1.3670357 (11 pages) | Cited 5 times

Online Publication Date: 28 December 2011

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We describe a radio-frequency (RF) discriminator, or frequency-to-voltage converter, based on a voltage-controlled oscillator phase-locked to the signal under test, which has been developed to analyze the frequency noise properties of an RF signal, e.g., a heterodyne optical beat signal between two lasers or between a laser and an optical frequency comb. We present a detailed characterization of the properties of this discriminator and we compare it to three other commercially available discriminators. Owing to its large linear frequency range of 7 MHz, its bandwidth of 200 kHz and its noise floor below 0.01 Hz2/Hz in a significant part of the spectrum, our frequency discriminator is able to fully characterize the frequency noise of a beat signal with a linewidth ranging from a couple of megahertz down to a few hertz. As an example of application, we present measurements of the frequency noise of the carrier envelope offset beat in a low-noise optical frequency comb.
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84.40.Ua Telecommunications: signal transmission and processing; communication satellites
84.30.Ng Oscillators, pulse generators, and function generators
42.79.Nv Optical frequency converters
back to top Particle Sources, Optics and Acceleration; Particle Detectors

High-precision soft x-ray polarimeter at Diamond Light Source

H. Wang, S. S. Dhesi, F. Maccherozzi, S. Cavill, E. Shepherd, F. Yuan, R. Deshmukh, S. Scott, G. van der Laan, and K. J. S. Sawhney

Rev. Sci. Instrum. 82, 123301 (2011); http://dx.doi.org/10.1063/1.3665928 (6 pages) | Cited 3 times

Online Publication Date: 8 December 2011

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The development and performance of a high-precision polarimeter for the polarization analysis in the soft x-ray region is presented. This versatile, high-vacuum compatible instrument is supported on a hexapod to simplify the alignment with a resolution less than 5 μrad, and can be moved with its own independent control system easily between different beamlines and synchrotron facilities. The polarimeter can also be used for the characterization of reflection and transmission properties of optical elements. A W/B4C multilayer phase retarder was used to characterize the polarization state up to 1200 eV. A fast and accurate alignment procedure was developed, and complete polarization analysis of the APPLE II undulator at 712 eV has been performed.
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07.60.Fs Polarimeters and ellipsometers

Influence of frequency tuning and double-frequency heating on ions extracted from an electron cyclotron resonance ion source

F. Maimone, L. Celona, R. Lang, J. Mäder, J. Roßbach, P. Spädtke, and K. Tinschert

Rev. Sci. Instrum. 82, 123302 (2011); http://dx.doi.org/10.1063/1.3665673 (12 pages) | Cited 1 time

Online Publication Date: 15 December 2011

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The electromagnetic field within the plasma chamber of an electron cyclotron resonance ion source (ECRIS) and the properties of the plasma waves affect the plasma properties and ion beam production. We have experimentally investigated the “frequency tuning effect” and “double frequency heating” on the CAPRICE ECRIS device. A traveling wave tube amplifier, two microwave sweep generators, and a dedicated experimental set-up were used to carry out experiments in the 12.5–16.5 GHz frequency range. During the frequency sweeps the evolution of the intensity and shape of the extracted argon beam were measured together with the microwave reflection coefficient. A range of different ion source parameter settings was used. Here we describe these experiments and the resultant improved understanding of these operational modes of the ECR ion source.
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76.40.+b Diamagnetic and cyclotron resonances
29.25.Ni Ion sources: positive and negative
52.35.-g Waves, oscillations, and instabilities in plasmas and intense beams

Characterization of plasma ion source utilizing anode spot with positively biased electrode for stable and high-current ion beam extraction

Yeong-Shin Park, Yuna Lee, Kyoung-Jae Chung, and Y. S. Hwang

Rev. Sci. Instrum. 82, 123303 (2011); http://dx.doi.org/10.1063/1.3664616 (6 pages) | Cited 1 time

Online Publication Date: 20 December 2011

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The operating conditions of a rf plasma ion source utilizing a positively biased electrode have been investigated to develop a stably operating, high-current ion source. Ion beam characteristics such as currents and energies are measured and compared with bias currents by varying the bias voltages on the electrode immersed in the ambient rf plasma. Current-voltage curves of the bias electrode and photographs confirm that a small and dense plasma, so-called anode spot, is formed near an extraction aperture and plays a key role to enhance the performance of the plasma ion source. The ion beam currents from the anode spot are observed to be maximized at the optimum bias voltage near the knee of the characteristic current-voltage curve of the anode spot. Increased potential barrier to obstruct beam extraction is the reason for the reduction of the ion beam current in spite of the increased bias current indicating the density of the anode spot. The optimum bias voltage is measured to be lower at higher operating pressure, which is favorable for stable operation without severe sputtering damage on the electrode. The ion beam current can be further enhanced by increasing the power for the ambient plasma without increasing the bias voltage. In the same manner, noble gases with higher atomic number as a feedstock gas are preferable for extracting higher beam current more stably. Therefore, performance of the plasma ion source with a positively biased electrode can be enhanced by controlling the operating conditions of the anode spot in various manners.
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52.50.Dg Plasma sources

A novel scaling law relating the geometrical dimensions of a photocathode radio frequency gun to its radio frequency properties

Shankar Lal, K. K. Pant, and S. Krishnagopal

Rev. Sci. Instrum. 82, 123304 (2011); http://dx.doi.org/10.1063/1.3671289 (10 pages) | Cited 1 time

Online Publication Date: 27 December 2011

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Developing a photocathode RF gun with the desired RF properties of the π-mode, such as field balance (eb) ∼1, resonant frequency fπ = 2856 MHz, and waveguide-to-cavity coupling coefficient βπ ∼1, requires precise tuning of the resonant frequencies of the independent full- and half-cells (ff and fh), and of the waveguide-to-full-cell coupling coefficient (βf). While contemporary electromagnetic codes and precision machining capability have made it possible to design and tune independent cells of a photocathode RF gun for desired RF properties, thereby eliminating the need for tuning, access to such computational resources and quality of machining is not very widespread. Therefore, many such structures require tuning after machining by employing conventional tuning techniques that are iterative in nature. Any procedure that improves understanding of the tuning process and consequently reduces the number of iterations and the associated risks in tuning a photocathode gun would, therefore, be useful. In this paper, we discuss a method devised by us to tune a photocathode RF gun for desired RF properties under operating conditions. We develop and employ a simple scaling law that accounts for inter-dependence between frequency of independent cells and waveguide-to-cavity coupling coefficient, and the effect of brazing clearance for joining of the two cells. The method has been employed to successfully develop multiple 1.6 cell BNL/SLAC/UCLA type S-band photocathode RF guns with the desired RF properties, without the need to tune them by a tiresome cut-and-measure process. Our analysis also provides a physical insight into how the geometrical dimensions affect the RF properties of the photo-cathode RF gun.
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85.60.Ha Photomultipliers; phototubes and photocathodes
84.40.Az Waveguides, transmission lines, striplines
02.60.-x Numerical approximation and analysis
back to top Nuclear Physics, Fusion and Plasmas

Variable dual-frequency electrostatic wave launcher for plasma applications

Benjamin Jorns, Robert Sorenson, and Edgar Choueiri

Rev. Sci. Instrum. 82, 123501 (2011); http://dx.doi.org/10.1063/1.3664785 (9 pages)

Online Publication Date: 7 December 2011

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A variable tuning system is presented for launching two electrostatic waves concurrently in a magnetized plasma. The purpose of this system is to satisfy the wave launching requirements for plasma applications where maximal power must be coupled into two carefully tuned electrostatic waves while minimizing erosion to the launching antenna. Two parallel LC traps with fixed inductors and variable capacitors are used to provide an impedance match between a two-wave source and a loop antenna placed outside the plasma. Equivalent circuit analysis is then employed to derive an analytical expression for the normalized, average magnetic flux density produced by the antenna in this system as a function of capacitance and frequency. It is found with this metric that the wave launcher can couple to electrostatic modes at two variable frequencies concurrently while attenuating noise from the source signal at undesired frequencies. An example based on an experiment for plasma heating with two electrostatic waves is used to demonstrate a procedure for tailoring the wave launcher to accommodate the frequency range and flux densities of a specific two-wave application. This example is also used to illustrate a method based on averaging over wave frequencies for evaluating the overall efficacy of the system. The wave launcher is shown to be particularly effective for the illustrative example—generating magnetic flux densities in excess of 50% of the ideal case at two variable frequencies concurrently—with a high adaptability to a number of plasma dynamics and heating applications.
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52.35.Fp Electrostatic waves and oscillations (e.g., ion-acoustic waves)
52.40.Fd Plasma interactions with antennas; plasma-filled waveguides
52.50.-b Plasma production and heating
52.25.Xz Magnetized plasmas

Simple method of determining plasma impedance of streamer discharge in atmospheric air

Daisuke Okano

Rev. Sci. Instrum. 82, 123502 (2011); http://dx.doi.org/10.1063/1.3665098 (5 pages) | Cited 1 time

Online Publication Date: 9 December 2011

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For atmospheric streamer discharges using a lightning impulse generator, we demonstrate a method of determining the plasma impedance in a streamer region by analyzing the periodic attenuated discharge waveforms having high-frequency components. When the streamer region in the plasma can be treated as an equivalent series circuit model including resistance and inductance elements, the regression waveforms obtained by reducing and smoothing the discharge waveforms are analyzed in the equivalent circuit. We found that the streamer resistance increased exponentially with time after the discharge, whereas the streamer inductance and series impedance were constant at 4.0 Ω for longer than the first period of the discharge waveforms. Moreover, the slope of the regression curve increases more rapidly for the positive streamer resistance than for the negative resistance. Finally, the absolute values of the streamer impedance versus time were 3.3–19 Ω and 3.5–9.0 Ω for positive and negative discharges, respectively.
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52.80.Pi High-frequency and RF discharges
52.25.Fi Transport properties

A novel zirconium Kα imager for high energy density physics research

K. U. Akli, M. Sanchez del Rio, S. Jiang, M. S. Storm, A. Krygier, R. B. Stephens, N. R. Pereira, E. O. Baronova, W. Theobald, Y. Ping, H. S. McLean, P. K. Patel, M. H. Key, and R. R. Freeman

Rev. Sci. Instrum. 82, 123503 (2011); http://dx.doi.org/10.1063/1.3665931 (6 pages) | Cited 2 times

Online Publication Date: 12 December 2011

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We report on the development and characterization of a zirconium Kα imager for high energy density physics research. The imager consists of a spherically bent quartz crystal operating at 15.7 keV photon energy. We compare the performance of the imager in terms of integrated reflectivity (Rint) and temperature dependent collection efficiency (ηTe) to that of the widely used Cu Kα imager. Our collisional-radiative simulations show that the new imager can be reliably used up to 250 eV plasma temperature. Monte Carlo simulations show that for a 25 μm thick tracer layer of zirconium, the contribution to Kα production from photo-pumping is only 2%. We present, for the first time, 2D spatially resolved images of zirconium plasmas generated by a high intensity short pulse laser interacting with Zr solid targets.
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52.70.-m Plasma diagnostic techniques and instrumentation
52.20.-j Elementary processes in plasmas
52.25.-b Plasma properties
52.40.Db Electromagnetic (nonlaser) radiation interactions with plasma
52.65.-y Plasma simulation
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