Index of content:
Volume 82, Issue 4, April 2011
We present a compact modular apparatus with a flexible design that will be operated at the DiProI beamline of the Fermi@Elettra free electron laser(FEL) for performing static and time-resolved coherent diffractionimaging experiments, taking advantage of the full coherence and variable polarization of the short seeded FEL pulses. The apparatus has been assembled and the potential of the experimental setup is demonstrated by commissioning tests with coherent synchrotron radiation. This multipurpose experimental station will be open to general users after installation at the Fermi@Elettra free electron laser in 2011.
- Optics; Atoms and Molecules; Spectroscopy; Photon Detectors
82(2011); http://dx.doi.org/10.1063/1.3574218View Description Hide Description
The power measurement of high-power continuous-wave laser beams typically calls for the use of water-cooled thermopile power meters. Large thermopile meters have slow response times that can prove insufficient to conduct certain tests, such as determining the influence of atmospheric turbulence on transmitted beam power. To achieve faster response times, we calibrated a digital camera to measure the power level as the optical beam is projected onto a white surface. This scattered-light radiometric power meter saves the expense of purchasing a large area power meter and the required water cooling. In addition, the system can report the power distribution, changes in the position, and the spot size of the beam. This paper presents the theory of the scattered-light radiometric power meter and demonstrates its use during a field test at a 2.2 km optical range.
A system and methodologies for absolute quantum efficiency measurements from the vacuum ultraviolet through the near infrared82(2011); http://dx.doi.org/10.1063/1.3574220View Description Hide Description
In this paper we present our system design and methodology for making absolute quantum efficiency (QE) measurements through the vacuum ultraviolet (VUV) and verify the system with delta-doped siliconCCDs. Delta-doped detectors provide an excellent platform to validate measurements through the VUV due to their enhanced UV response. The requirements for measuring QE through the VUV are more strenuous than measurements in the near UV and necessitate, among other things, the use of a vacuum monochromator, good dewar chamber vacuum to prevent on-chip condensation, and more stringent handling requirements.
82(2011); http://dx.doi.org/10.1063/1.3572331View Description Hide Description
In this paper an algorithm for extracting spectral information from signals containing a series of narrow periodic impulses is presented. Such signals can typically be acquired by pickup detectors from the image-charge of ion bunches oscillating in a linear electrostaticion trap, where frequency analysis provides a scheme for high-resolution mass spectrometry. To provide an improved technique for such frequency analysis, we introduce the CHIMERA algorithm (Comb-sampling for High-resolution IMpulse-train frequency ExtRAaction). This algorithm utilizes a comb function to generate frequency coefficients, rather than using sinusoids via a Fourier transform, since the comb provides a superior match to the data. This new technique is developed theoretically, applied to synthetic data, and then used to perform high resolution mass spectrometry on real data from an ion trap. If the ions are generated at a localized point in time and space, and the data is simultaneously acquired with multiple pickup rings, the method is shown to be a significant improvement on Fourier analysis. The mass spectra generated typically have an order of magnitude higher resolution compared with that obtained from fundamental Fourier frequencies, and are absent of large contributions from harmonic frequency components.
Self-eliminating instrumental frequency response from free carrier absorption signals for silicon wafer characterization82(2011); http://dx.doi.org/10.1063/1.3577043View Description Hide Description
Accurate determination of electronic transport properties of semiconductor wafers with modulated free carrier absorption (MFCA) and multiparameter fitting requires the total elimination of instrumental response from the MFCA signals. In this paper, an approach to eliminate the effect of instrumental response on the frequency dependence of MFCA amplitude and phase is developed both theoretically and experimentally to simultaneously determine the transport properties (minority-carrier lifetime, carrier diffusion coefficient, and front surface recombination velocity) of silicon wafers. Experimental results showed that with the proposed method the instrumental frequency response was fully eliminated from the experimental MFCA data and had no impact on the multiparameter fitting, while with conventional methods the accuracy of the fitted transport parameters was influenced detrimentally by the errors of the measured instrumental frequency responses, in particular for the minority-carrier lifetime and the front surface recombination velocity.
Polarization-preserving confocal microscope for optical experiments in a dilution refrigerator with high magnetic field82(2011); http://dx.doi.org/10.1063/1.3574217View Description Hide Description
We present the design and operation of a fiber-based cryogenic confocal microscope. It is designed as a compact cold-finger that fits inside the bore of a superconducting magnet, and which is a modular unit that can be easily swapped between use in a dilution refrigerator and other cryostats. We aimed at application in quantum optical experiments with electron spins in semiconductors and the design has been optimized for driving with and detection of optical fields with well-defined polarizations. This was implemented with optical access via a polarization maintaining fiber together with Voigt geometry at the cold finger, which circumvents Faraday rotations in the optical components in high magnetic fields. Our unit is versatile for use in experiments that measure photoluminescence, reflection, or transmission, as we demonstrate with a quantum optical experiment with an ensemble of donor-bound electrons in a thin GaAs film.
The hyperspectral irradiometer, a new instrument for long-term and unattended field spectroscopy measurements82(2011); http://dx.doi.org/10.1063/1.3574360View Description Hide Description
Reliable time series of vegetation optical properties are needed to improve the modeling of the terrestrial carbon budget with remote sensing data. This paper describes the development of an automatic spectral system able to collect continuous long-term in-field spectral measurements of spectral down-welling and surface reflected irradiance. The paper addresses the development of the system, named hyperspectral irradiometer (HSI), describes its optical design, the acquisition, and processing operations. Measurements gathered on a vegetated surface by the HSI are shown, discussed and compared with experimental outcomes with independent instruments.
82(2011); http://dx.doi.org/10.1063/1.3568824View Description Hide Description
Two, lightweight diode laser frequency stabilization systems designed for experiments in the field are described. A significant reduction in size and weight in both models supports the further miniaturization of measurement devices in the field. Similar to a previous design,magnetic field lines are contained within a magnetic shield enclosing permanent magnets and a Rb cell, so that these dichroic atomic vapor laser lock (DAVLL) systems may be used for magnetically sensitive instruments. The mini-DAVLL system (49 mm long) uses a vapor cell (20 mm long) and does not require cell heaters. An even smaller micro-DAVLL system (9 mm long) uses a microfabricated cell (3 mm square) and requires heaters. These new systems show no degradation in performance with regard to previous designs while considerably reducing dimensions.
An integrated laser Raman optical sensor for fast detection of nitrogen and oxygen in a cryogenic mixture82(2011); http://dx.doi.org/10.1063/1.3581336View Description Hide Description
An integrated fiber optic Raman sensor was designed for real-time, nonintrusive detection of liquid nitrogen (LN2) in liquid oxygen (LO2) at high pressures and high flow rates. This was intended to monitor the quality of LO2 in oxidizer feed lines during the ground testing of rocket engines. Various issues related to optical diagnosis of cryogenic fluids (LN2/LO2) in supercritical environment of rocket engine test facility, such as fluorescence from impurity in optical window of feed line, signal-noise ratio, and fast data acquisition time, etc., are well addressed. The integrated sensor employed a frequency doubled 532-nm continuous wave Nd:YAG laser as an excitation light source. The other optical components included were InPhotonics Raman probes, spectrometers, and photomultiplier tubes(PMTs). The spectrometer was used to collect the Raman spectrum of LN2 and LO2. The PMT detection unit was integrated with home-built LABVIEW software for fast monitoring of concentration ratios LN2 and LO2. Prior to designing an integrated sensor system, its optical components were also tested with gaseous nitrogen (GN2) and oxygen (GO2).
One-dimensional space resolving flat-field holographic grating soft x-ray framing camera spectrograph for laser plasma diagnostics82(2011); http://dx.doi.org/10.1063/1.3579494View Description Hide Description
A 1D space resolving x-rayspectrumdiagnostic system has been developed to study the radiation opacity of hot plasma on SG-II laser facility. The diagnostic system consists of a 2400 lines/mm flat-field holographic grating and a gated microchannel plate coupled with an optical CCD and covers the wavelength range of 5–50 Å. The holographic grating was compared with a ruled one by measuring the emission spectra from a laser-produced molybdenum plasma. The results indicate that the holographic grating possesses better sensitivity than the ruled grating having nearly similar spectral resolution. The spectrograph has been used in radiative opacitymeasurement of Fe plasma. Simultaneous measurements of the backlight source and the transmission spectrum in appointed time range in one shot have been accomplished successfully with the holographic gratingspectrometer. The 2p–3d transition absorption of Fe plasma near 15.5Å in has been observed clearly.
82(2011); http://dx.doi.org/10.1063/1.3581217View Description Hide Description
Microlens-ended fibers, which have found tremendous interest in the recent past, find potential biomedical applications, in particular, in endoscopic imaging. The work presented in this paper focuses on the stand-alone microlenses along with custom-fabricated specialty optical fiber, such as imaging fiber, for probe imaging applications. Stand-alone self-aligned microlenses have been fabricated employing microcompression molding and then attached at the end facet of imaging fiber. A detailed characterization of the fabricated microlens is carried and it demonstrates appropriate focusing ability, high fluorescence collection efficiency and imaging ability for biomedical applications. The surface roughness of the microlens is found to be 25 nm with a minimum spot size of 38 μm. The probe imaging system is found to be able to image the fluorescence microspheres of 10 μm size. The collection efficiency of the fiber probe with lens found to be enhanced by three times approximately.
- Particle Sources, Optics and Acceleration; Particle Detectors
82(2011); http://dx.doi.org/10.1063/1.3575581View Description Hide Description
A novel ion wide angle spectrometer (iWASP) has been developed, which is capable of measuring angularly resolved energy distributions of protons and a second ion species, such as carbon C6 +, simultaneously. The energy resolution for protons and carbon ions is better than 10% at ∼50 MeV/nucleon and thus suitable for the study of novel laser-ion acceleration schemes aiming for ultrahigh particle energies. A wedged magnet design enables an acceptance angle of 30°(∼524 mrad) and high angular accuracy in the μrad range. First, results obtained at the LANL Trident laser facility are presented demonstrating high energy and angular resolution of this novel iWASP.
82(2011); http://dx.doi.org/10.1063/1.3581334View Description Hide Description
The design of optical fiber based heads offering high accuracy and bandwidth for use in VISAR (velocity interferometer system for any reflector) experiments measuring ballistic velocities is described. A new, expanded, model for predicting the distance-dependent collection efficiency of the heads is presented. The model is shown to agree very well with experimental results, both within and outside the “depth of field”. Various optical heads are demonstrated, to suit different experimental setups and conditions. Designs offering options for high bandwidths, accurate prealignment, and large stand-off distances are discussed. Results from a typical VISAR experiment are presented, verifying that our designs yield high-quality data.
82(2011); http://dx.doi.org/10.1063/1.3581207View Description Hide Description
A novel design of calibration equipment has been developed for static and dynamic calibrations of gas and thermal sensors. This system is cheap, compact, and easily adjustable, which is also combined with a plasmasurface modificationsource for tailoring the surface of sensors to ensure the sensitivity and selectivity. The main advantage of this equipment is that the operating temperature, bias voltage, types of plasma source (for surface modification), types of feeding gases, and gas flow rate (for calibrations), etc., can be independently controlled. This novel system provides a highly reliable, reproducible, and economical method of calibrations for various gas and thermal sensors.
82(2011); http://dx.doi.org/10.1063/1.3585862View Description Hide Description
The laser wakefield acceleration electron beams can essentially deviate from the axis of the system, which distinguishes them greatly from beams of conventional accelerators. In case of energy measurements by means of a permanent-magnet electron spectrometer, the deviation angle can affect accuracy, especially for high energies. A two-screen single-shot electron spectrometer that correctly allows for variations of the angle of entry is considered. The spectrometer design enables enhancing accuracy of measuring narrow electron beams significantly as compared to a one-screen spectrometer with analogous magnetic field, size, and angular acceptance.
- Nuclear Physics, Fusion and Plasmas
82(2011); http://dx.doi.org/10.1063/1.3571299View Description Hide Description
An electric-field sensor consisting of thin copper plates is designed to measure an oscillating electric field produced by charge separations on a plasma column. The sensor installed in a vacuum region around plasma detects charges induced by the electric field on the copper plates. The value of the induced charges depends not only on the strength of the electric field, but also on the design of the sensor. To obtain the correct strength of the electric field, a correction factor arising from the design of the sensor must be known. The factor is calculated numerically using Laplace's equation and compared with a value measured using a uniform electric field in the frequency range of 10–500 kHz. When an external circuit is connected to the sensor to measure the induced charges, the electric field around the sensor is disturbed. Therefore, a double-sensor method for excluding a disturbed component in the measuredelectric field is proposed. The reliability of the double-sensor method is confirmed by measuring dipole-like and quadrupole-like electric fields.
Evaluation of the Faraday angle by numerical methods and comparison with the Tore Supra and JET polarimeter electronics82(2011); http://dx.doi.org/10.1063/1.3570621View Description Hide Description
On the Tore Supra tokamak, a far infrared polarimeter diagnostic has been routinely used for diagnosing the current density by measuring the Faraday rotation angle. A high precision of measurement is needed to correctly reconstruct the current profile. To reach this precision, electronics used to compute the phase and the amplitude of the detected signals must have a good resilience to the noise in the measurement. In this article, the analogue card's response to the noise coming from the detectors and their impact on the Faraday angle measurements are analyzed, and we present numerical methods to calculate the phase and the amplitude. These validations have been done using real signals acquired by Tore Supra and JET experiments. These methods have been developed to be used in real-time in the future numerical cards that will replace the Tore Supra present analogue ones
82(2011); http://dx.doi.org/10.1063/1.3572271View Description Hide Description
Several measurements of the radon concentration are performed by RAD7 in the University of South China. We find that 30–40 min is needed for RAD7 for tracing the concentration of the standard radon chamber. There are two reasons. The first is that the sufficient time of air cycle is needed for the radon concentration in internal cell of RAD7 equal to that of the environment; and the second is that the sufficient decay time is needed for the 218Po concentration in internal cell of RAD7 equal to that of the radon. We used a zeroth order approximation to describe the evolution of the environment radon concentration, and obtained a novel algorithm for quick and continuous tracing the change of radon concentration. The corrected radon concentration obtained through this method is in good agreement with the reference value. This method can be applied to develop and improve the instruments for tracing the change of radon concentration quickly.
82(2011); http://dx.doi.org/10.1063/1.3572265View Description Hide Description
Uniform and stable dischargeplasma requires very short duration pulses with fast rise times. A repetitive high-voltage nanosecond pulse generator for the application of gas discharge is presented in this paper. It is constructed with all solid-state components. Two-stage magnetic compression is used to generate a short duration pulse. Unlike in some reported studies, common commercial fast recovery diodes instead of a semiconductor opening switch (SOS) are used in our experiment that plays the role of SOS. The SOS-like effects of four different kinds of diodes are studied experimentally to optimize the output performance. It is found that the output pulse voltage is higher with a shorter reverse recovery time, and the rise time of pulse becomes faster when the falling time of reverse recovery current is shorter. The SOS-like effect of the diodes can be adjusted by changing the external circuit parameters. Through optimization the pulse generator can provide a pulsed voltage of 40 kV with a 40 ns duration, 10 ns rise time, and pulse repetition frequency of up to 5 kHz. Diffuse plasma can be formed in air at standard atmospheric pressure using the developed pulse generator. With a light weight and small packaging the pulse generator is suitable for gas discharge application.
82(2011); http://dx.doi.org/10.1063/1.3581219View Description Hide Description
The retarding field analyzer (RFA) is a widely used diagnostic tool for the ion temperature measurement in the scrape-off-layer (SOL) of the thermonuclear plasma devices. However, the temporal resolution in the standard RFA application is restricted to the ms timescale. In this paper, a dc operation of the RFA is considered, which allows for the measurement of the plasma ion temperaturefluctuations. The method is based on the relation for the RFA current–voltage (I–V) characteristic resulted from a common RFA model of shifted Maxwellian distribution of the analyzed ions, and the measurements of two points on the exponentially decaying region of the I–Vcharacteristic with two differently dc biased RFAelectrodes. The method has been tested and compared with conventional RFAmeasurements of the ion temperature in the tokamak ISTTOK SOLplasma. An ion temperature of T i = 17 eV is obtained near the limiter position. The agreement between the results of the two methods is within ∼25%. The amplitude of the ion temperaturefluctuations is found to be around 5 eV at this location. The method has been validated by taking into account the effect of fluctuations in the plasma potential and the noise contamination, proving the reliability of the results obtained. Finally, constrains to the method application are discussed that include a negligible electron emission from the RFA grids and the restriction to operate in the exponentially decaying region of the I–Vcharacteristic.
- Microscopy and Imaging
82(2011); http://dx.doi.org/10.1063/1.3569765View Description Hide Description
We expand the range of applications of a tuning fork probe (TFP) in frequency-modulation atomic force microscopy (FM-AFM) by attaching a long metal tip at a certain angle. By the combined flexure of the metal tip and the tuning fork prong, this TFP can change the direction of the detectable force by switching the resonance frequency, which has not been realized with conventional TFPs with short tips. The oscillatory behavior of the tip apex of the TFP is predicted by computer simulations and is experimentally confirmed with scanning electron microscope. FM-AFM operations using this TFP are performed in various environments, i.e., in ultrahigh vacuum, air, and water. FM-AFM images obtained at an atomic step of highly oriented pyrolytic graphite in air show a clear difference depending on the excitation frequency. It is also revealed that the higher order flexural modes of this TFP are advantageous for FM-AFM in water due to the reduction in the degree of hydrodynamic damping.