Design and study of an enhanced Faraday cup detector

Faraday cups (FCs) have been used to measure charged particle beams currents for a wide variety of experiments since the early days of particle accelerators. The major error source in FC performance is the escape of ejected electrons produced by energetic particles striking with interior metal surfaces of the FC. To avoid this problem, electrostatic and magnetic guard rings are used. Some kinds of non-symmetrical electrostatic FC with superior performance than traditional cylindrically symmetric FC have been reported. Here, we propose a novel non-symmetrical electrostatic FC structure, and a method to compare different non-symmetrical FCs. The purpose of this design is to recapture energetic electrons with lower guard ring bias. Performance simulations of the proposed FC show considerable improvement over other reported FCs.     

Performance enhancement study of an electrostatic Faraday cup detector

Faraday cups (FCs) have long been used to measure charged particle beam currents in experiments that seek to determine cross-sections in energetic particle collisions. The reliable operation of a FC as a detector depends on the ability of the device to recapture the electrons ejected when energetic particles strike its interior metal surfaces. We have conducted comparative performance studies of a traditional cylindrically-symmetric, electrostatic-based FC versus an alternative design in which the cylindrical symmetry is broken. The purpose of the alternative design is to generate a transverse electric field to recapture the ejected (secondary and tertiary) electrons. The alternate FC design is shown to be superior in its ability to recapture these electrons, including those having kinetic energies greater than the potential energy barrier determined by the repeller voltage applied to the FC.     

An improved design of Faraday cup detector to reduce the escape of secondary electrons in plasma focus device by COMSOL

Faraday cup (FC) is a beamline diagnostic which is used to measure the total charge of pulsed beams. The reliable operation of an FC depends on its ability to recapture the ejected electrons when energetic particles strike with interior cup(IC) of an FC. In this paper COMSOL multi-physics software is used to investigate the performance of two new proposed FCs. The FC with A = 40 mm and B = 98 mm had better efficiency in contrast with traditional FC geometries.     

An improved design of Faraday cup detector to reduce the escape of secondary electrons in plasma focus device by COMSOL

Faraday cup (FC) is a beamline diagnostic which is used to measure the total charge of pulsed beams. The reliable operation of an FC depends on its ability to recapture the ejected electrons when energetic particles strike with interior cup(IC) of an FC. In this paper COMSOL multi-physics software is used to investigate the performance of two new proposed FCs. The FC with A = 40 mm and B = 98 mm had better efficiency in contrast with traditional FC geometries.     

Faraday cup for measuring the electron beams of TWT guns

A compact Faraday cup reported in this paper is designed to investigate the current distributions of the electron beams of Traveling Wave Tube’s guns. It consists of a 0.06 mm thick molybdenum aperture plate and a copper shield with a graphite collector inside. There are plates with different laser-cut aperture holes that were 100 μm, 50 μm, 20 μm and 10 μm in diameter for measuring electron beams with different sizes. The thermal analysis of the Faraday cup with pulse beam heating was performed and discussed in this paper. The pulse test shows that this device has fast response and small dissipation. A 0.58 beam perveance (μP) electron gun with expected minimum beam radius 1.0 mm was measured with the Faraday cup and the three-dimensional current density distribution and beam envelope were presented. The experiment results show that the design is reasonable for measuring the electron beam with a high resolution.     

Faraday rotation density measurements of optically thick alkali metal vapors

We investigate the measurement of alkali number densities using the Faraday rotation of linearly polarized light. We find that the alkali number density may be reliably extracted even in regimes of very high buffer gas pressure, and very high alkali number density. We have directly verified our results in potassium using absorption spectroscopy on the second resonance (4²S→5²P).     

Unambiguous coherence retrieval from intensity measurements

It is demonstrated that the members of different classes of two-dimensional fields that have the same intensity distributions everywhere in free space but different coherence properties can be identified by measuring the fields' intensities after passing the fields through an anamorphic optical system, such as a cylindrical lens. In this way the ambiguity in coherence determination from intensity measurements alone, present for the case of two-dimensional fields, is removed.     

Preparation of short submicron-fiber yarn by an annular collector through electrospinning

Electrospinning is known to be a unique method to prepare continuous nanofibers. Its brevity and extensive application in obtaining non-woven membrane, nanofiber bundle or yarn have attracted increasing attention throughout the world. Especially, how to obtain aligned nanofiber array by a modified electrospinning process has been a hot topic in recent years. An annular collector with the cross-section of circle to obtain fibrous yarn through electrospinning, pre-drafting, and subsequent twisting was reported in this investigation. Fiber arrangement in bundle and mechanical properties of the yarn were given by scanning electron microscope and strength tester. Results showed that the pre-drafted bundle consisted of highly-aligned submicron fibers with the diameter from 750 to 1000 nm. In addition, it was found that the yarn is unsuitable for clothing, but may be applied in many other fields, such as composites, tissue engineering, and catalyst materials.     

Substrate temperature control from RHEED intensity measurements

Substrate temperature is an importance parameter controlling nucleation and growth dynamics during the thin film epitaxy in MBE system. The most covenant methods able to determine the substrate temperature is based on some calibration procedures, where substrate temperature is calculated as a function of electric power dissipated in heater system. This paper presents possibility of substrate temperature control based on reflection high-energy electron diffraction (RHEED) intensity measurements as in situ technique. Temperature dependence of the specularly reflected electron beam intensity versus the substrate temperature is predicted by dynamical theory of electron scattering. The theoretical RHEED intensity dependence versus Si(1 1 1) substrate temperature was obtained from one-dimensional rocking-curve calculation.     

Macroporous ultramicroelectrodes for improved electroanalytical measurements

Recent work on the preparation of highly organized macroporous electrodes and nanoporous ultramicroelectrodes has been combined and extended to elaborate macroporous ultramicroelectrodes (UMEs) by template synthesis using colloidal crystals and following two different and complementary methods. On the one hand, arched porous UMEs were prepared, and on the other hand, cylindrical porous UMEs were obtained by using cavity UMEs. These macroporous UMEs have an active surface area which is up to 2 orders of magnitude higher compared to that of a classical disk UME as characterized by cyclic voltammetry. To study their analytical performance, the macroporous UMEs have been modified with a redox-active thiol and also a model bioelectrocatalytical system containing a redox mediator, a cofactor, and glucose-dehydrogenase. In both cases the electrochemical signal is amplified by up to 2 orders of magnitude, which increases significantly the analytical performance of such electrodes and therefore opens up new applications for this kind of miniaturized electrochemical system.     

Increasing the output of a Littman-type laser by use of an intracavity Faraday rotator

We present a method of external-cavity diode-laser grating stabilization that combines the high output power of the Littrow design with the fixed output pointing of the Littman-Metcalf design. Our new approach utilizes a Faraday-effect optical isolator inside the external cavity. Experimental testing and a model that describes the tuning range and optimal tuning parameters of the laser are described. Preliminary testing of this design has resulted in a short-term linewidth of 360 KHz and a side-mode suppression of 37 dB. The laser tunes mode hop free over 7 GHz, and we predict that much larger tuning ranges are possible.     

Transverse mode analysis of a laser beam by near- and far-field intensity measurements

A quantitative measurement of laser-beam quality can be performed by determination of the presence of multiple transverse modes of the laser oscillator and by calculation of their power content. Along this line of argument, we discuss a new approach that, starting from near-field and far-field intensity measurements, can evaluate the complex excitation coefficients of the transverse modes in a laser beam. The exploitation of near-field measurements sharply improves the performances of the technique in those cases in which only far-field measurements are used. The validity of the method is confirmed by several accurate numerical simulations and by some experimental results relative to a multimode Q-switched Nd:YAG laser.     

Lensless imaging from diffraction intensity measurements by use of a noniterative phase-retrieval method

A method of reconstructing the complex amplitude of an object that is illuminated by a coherent wave from its Fresnel diffraction patterns is proposed for high-frequency wave phenomena such as x-rays and electron waves. A noniterative phase-retrieval method that uses a Gaussian filter is employed here, and it is shown that the object's illumination with amplitude distribution in the Fraunhofer diffraction pattern of a circular aperture can be used as a substitute for the Gaussian filter. This method has an advantage over other noniterative phase-retrieval methods in that it can retrieve phase vortices.     

Phase reconstruction from intensity measurements in linear systems

The phase of a signal at a plane is reconstructed from the intensity profiles at two close parallel screens connected by a small abcd canonical transform; this applies to propagation along harmonic and repulsive fibers and in free media. We analyze the relationship between the local spatial frequency (the signal phase derivative) and the derivative of the squared modulus of the signal under a one-parameter canonical transform with respect to the parameter. We thus generalize to all linear systems the results that have been obtained separately for Fresnel and fractional Fourier transforms.     

Current measurement by Faraday effect on GEPOPU

The design of an optical current sensor to be used in a pulsed power generator is presented. The current sensor is based on the polarization rotation by the Faraday effect. GEPOPU is a pulsed power generator, 110 kA, 120 ns double transit time, 1.5 Ω coaxial geometry, and current rise time of 50 ns. Two different optical geometries surrounding the conductor were tried, using Amici roof prism and pentaprism to go around the current once, as a way to preserve the state of polarization along the optical path by means of complementary reflections within the sensing element. We believe this to be the first time that such large and rapidly varying currents have been measured with this configuration. The values obtained for both geometries agree with the values obtained with a Rogowski coil. The traces obtained are completely noise-free and no significant time lag has been observed between the current determined from the Faraday rotation and the current measured using a Rogowski coil.     

Luminous-flux measurements by an absolute integrating sphere

We present an original implementation of the absolute-sphere method recently proposed by Ohno. The luminous-flux unit, the lumen, is realized by means of an integrating sphere with an opening calibrated by a luminous-intensity standard placed outside. The adapted experimental setup permits one to measure luminous-flux values between 5 and 2500 lm with a significant improvement with respect to the simulated performances reported in the literature. Traditionally, the luminous-flux unit, the lumen, is realized by goniophotometric techniques in which the luminous-intensity distribution is measured and integrated over the whole solid angle. Thus sphere results are compared with those obtained with the Istituto Elettrotecnico Nazionale goniophotometer. In particular, a set of standards, characterized by luminous-flux values of ~2000 lm, has been calibrated with both techniques. We highlight some of the problems encountered. Experimental results show that the agreement between the two methods is within the estimated uncertainty and suggest promising areas for future research.     

Detection of biological particles by the use of circular dichroism measurements improved by scattering theory

Light scattered from optically active spheres was theoretically analyzed for biodetection. The circularly polarized signal of near-forward scattering from circularly dichroic spheres was calculated. Both remote and point biodetection were considered. The analysis included the effect of a circular aperture and beam block at the detector. If the incident light is linearly polarized, a false signal would limit the sensitivity of the biodetector. If the incident light is randomly polarized, shot noise would limit the sensitivity. Suggested improvements to current techniques include a beam block, precise angular measurements, randomly polarized light, index-matching fluid, and larger apertures for large particles.     

Self-referencing intensity measurements based on square-wave gated phase-modulation fluorimetry

An adaptation of square-wave gated phase-modulation (GPM) fluorimetry allows for self-referenced intensity measurements without the complexity of dual excitation or dual emission wavelengths. This AC technique utilizes square-wave excitation, gated detection, a reference emitter, and a sensor molecule. The theory and experimental data demonstrating the effectiveness and advantages of the adapted GPM scheme are presented. One component must have an extremely short lifetime relative to the other. Both components are affected identically by changes in intensity of the excitation source, but the sensor intensity also depends on the concentration of the analyte. The fluctuations of the excitation source and any optical transmission changes are eliminated by ratioing the sensor emission to the reference emission. As the concentration of the analyte changes, the corresponding sensor intensity changes can be quantified through several schemes including digitization of the signal and digital integration or AC methods. To measure pH, digital methods are used with Na3[Tb(dpa)3] (dpa = 2,6-pyridinedicarboxylic acid) as the long-lived reference molecule and fluorescein as the short-lived sensor molecule. Measurements from the adapted GPM scheme are directly compared to conventional ratiometric measurements. Good agreement between the data collection methods is demonstrated through the apparent pKa. For the adapted GPM measurements, conventional measurements, and a global fit the apparent pKa values agree within less than 2%. A key element of the adapted GPM method is its insensitivity to fluctuations in the source intensity. For a roughly 8-fold change in the excitation intensity, the signal ratio changes by less than 3%.