Multipass Systems Research Articles

Radiant fluence from ray tracing in optical multipass systems

Optical multipass cells are used in photochemical reactors and laser excitation of weak transitions. In these applications, estimation of the radiation dose in a volume of interest allows us to assess the performance and optimize the design of the cell. We adopt radiant fluence as the figure of merit and employ the radiative transfer equation to derive analytical expressions for average radiant fluence in a given volume of interest. These expressions involve quantities that are either obtained with Monte Carlo ray tracing methods or approximated on the grounds of geometry arguments. Furthermore, we compute the spatial distribution of fluence by applying either of the two methods to individual volume elements in a rectangular voxel grid. Ray tracing is performed with Zemax OpticsStudio 18.9.

General Method to Obtain Collision Cross-Section Values in Multipass High-Resolution Cyclic Ion Mobility Separations.

In recent years, ion mobility spectrometry-mass spectrometry (IMS-MS) has advanced the field of omics-based research, especially with the development of high-resolution platforms; however, these separations have generally been qualitative in nature. The rotationally averaged ion neutral collision cross section (CCS) is one of the only quantitative metrics available for aiding in characterizing biomolecules in IMS-MS. However, determining the CCS of an ion for multipass IMS systems, such as in cyclic ion mobility-mass spectrometry (cIMS-MS) and structures for lossless ion manipulations, has been challenging due to the lack of methods available for calculating CCS when more than a single pass is required for separation as well as the laborious nature of requiring calibrants and unknown compounds to be subjected to identical number of passes, which may not be possible in certain instances because of peak splitting, high levels of diffusion, etc. Herein, we present a general method that uses average ion velocities for calculating CCS values in cIMS-MS-based separations. Initially, we developed calibration curves using common CCS calibrants [i.e., tetra-alkylammonium salts, polyalanine, and hexakis(fluoroalkoxy)phosphazines] at different traveling wave (TW) conditions and the calculated cIMS CCS values were within ∼1% error or less compared to previously established drift tube IMS CCS measurements. Since it has been established that glycans can split into their α/β anomers, we utilized this method for two glycan species, 2α-mannobiose and melibiose. Both glycans were analyzed at the same TW conditions as the calibrants, and we observed anomer splitting at pathlengths of 20 m for 2α-mannobiose and 40 m for melibiose and thus assigned two unique CCS values for each glycan, which is the first time this has ever been done. We have demonstrated that the use of average ion velocities is a robust approach for obtaining CCS values with good agreement to CCS measurements from the previous literature and anticipate that this methodology can be applied to any IMS-MS platform that utilizes multipass separations. Our future work aims to incorporate this methodology for the development of a high-resolution CCS database to aid in the characterization of human milk oligosaccharides.

Multi-foci metalens for spectra and polarization ellipticity recognition and reconstruction

Multispectral and polarized focusing and imaging are key functions that are vitally important for a broad range of optical applications. Conventional techniques generally require multiple shots to unveil desired optical information and are implemented via bulky multi-pass systems or mechanically moving parts that are difficult to integrate into compact and integrated optical systems. Here, a design of ultra-compact transversely dispersive metalens capable of both spectrum and polarization ellipticity recognition and reconstruction in just a single shot is demonstrated with both coherent and incoherent light. Our design is well suited for integrated and high-speed optical information analysis and can significantly reduce the size and weight of conventional devices while simplifying the process of collecting optical information, thereby promising for various applications, including machine vision, minimized spectrometers, material characterization, remote sensing, and other areas which require comprehensive optical analysis.

Modified ray transfer matrix method for accurate non-sequential ray tracing between arbitrary reflective mirrors.

The ray transfer matrix method is widely used for analyzing beam-transmission properties and designing multi-pass systems. In this paper, a modified ray transfer matrix method is proposed for tracing an accurate non-sequential ray in three-dimensional space based on a vector reflection theory. The modified ray transfer matrix method can be utilized to analyze a ray transmitting between arbitrary surfaces, which is not confined to rotational symmetrical structures. There is no need to project rays onto two perpendicular planes, nor to introduce a paraxial approximation in our calculation. Since the length and angle of every ray can be calculated accurately, almost no deviation is accumulated in multi-pass system. The modified ray transfer matrix method can be applied in optical design, especially in the design of multi-pass cavities and multi-pass cells.

Generalized method for seeking q-preserving configurations of multi-pass cells.

A method is presented for seeking specified multi-pass cell (MPC) configurations with q-parameters of the input and output Gaussian beams that remain the same (q-preserving configurations). The Frobenius norm (F-norm) is chosen as the quantitative criterion of the deviation between the transfer matrix of the MPC configurations and identity matrix. When the deviation is down near to zero, the corresponding configuration is close to an ideal q-preserving configuration. In contrast to common 2 × 2 transfer matrixes that are only applicable to MPCs with negligible astigmatism, we adopt 4 × 4 transfer matrixes that are also workable for multi-pass systems with astigmatism. To demonstrate the validation of the method, several q-preserving structures of four-objective multi-pass matrix system (FO-MMS) and double-row circular MPC (DR-CMPC), which are two fundamentally different MPC types, are illustrated. The variation tendencies of the quantitative deviation between the transfer matrix and the identity matrix are discussed in detail for each structure as functions of structural parameters. The optimal q-preserving structures are found for FO-MMS and DR-CMPC, with the matrix deviation of 0.0047 and 0.0051, while the corresponding total optical path length (OPL) are 33.6 m and 67.8 m, respectively. The OPL of the optimal DR-CPMC is longer than the traditional CMPC in the state of the art, and the deviation is three orders of magnitude smaller if the similar spherical reflecting surfaces are adopted.

Effect of Single‐Pass and Multipass Milling Systems on Whole Wheat Durum Flour and Whole Wheat Pasta Quality

Single‐pass and multipass milling systems were evaluated for the quality of whole wheat durum flour (WWF) and the subsequent whole wheat (WW) spaghetti they produced. The multipass system used a roller mill with two purifiers to produce semolina and bran/germ and shorts (bran fraction). The single‐pass system used an ultracentrifugal mill with two configurations (fine grind, 15,000 rpm with 250 μm mill screen aperture; and coarse grind, 12,000 rpm with 1,000 μm mill screen aperture) to direct grind durum wheat grain into WWF or to regrind the bran fraction, which was blended with semolina to produce a reconstituted WWF. Particle size, starch damage, and pasting properties were similar for direct finely ground WWF and multipass reconstituted durum flour/fine bran blend and for direct coarsely ground WWF and multipass reconstituted semolina/coarse bran blend. The semolina/fine bran blend had low starch damage and had desirable pasting properties for pasta cooking. WW spaghetti was better when made with WWF produced using the multipass than single‐pass milling system. Mechanical strength was greatest with spaghetti made from the semolina/fine bran or durum flour/fine bran blends. The semolina/fine bran and semolina/coarse bran blends made spaghetti with high cooked firmness and low cooking loss.

Multipass relativistic high-order-harmonic generation for intense attosecond pulses

We demonstrate that the total reflected field produced by the interaction of a moderately relativistic laser with dense plasma is itself an efficient driver of high-order-harmonic generation. A system of two or more successive interactions of an incident laser beam on solid targets may therefore be an experimentally realizable method of optimizing conversion of laser energy to high-order harmonics. Particle-in-cell simulations suggest that attosecond pulse intensity may be increased by up to four orders of magnitude in a multipass system, with decreased duration of the attosecond pulse train. We discuss high-order-harmonic wave-form engineering for enhanced attosecond pulse generation with an electron trajectory model, present the behavior of multipass systems over a range of parameters, and offer possible routes towards experimental implementation of a two-pass system.

High-resolving mass spectrographs and spectrometers

Discussed are different types of high resolving mass spectrographs and spectrometers. In detail outlined are (1) magnetic and electric sector field mass spectrographs, which are the oldest systems, (2) Penning Trap mass spectrographs and spectrometers, which have achieved very high mass-resolving powers, but are technically demanding (3) time-of-flight mass spectrographs using high energy ions passing through accelerator rings, which have also achieved very high mass-resolving powers and are equally technically demanding, (4) linear time-of-flight mass spectrographs, which have become the most versatile mass analyzers for low energy ions, while the even higher performing multi-pass systems have only started to be used, (5) orbitraps, which also have achieved remarkably high mass-resolving powers for low energy ions.

Modeling of Natural Convection Non Reversible Single Pass Pressurized and Non Pressurized Solar Hot Water Systems for Domestic Applications

This paper mainly deals with the problem of reverse flow and explicit expressions have been presented which shows that the placement of hot water storage tank above the collector helps to eliminates reverse flow and it was observed that if tank is kept very high above the collector then there is always some flow of liquid through the collector especially if the outlet of the collector is connected near the middle of the storage tank. This is due to considerable heat losses resulting poor thermal efficiency of the system. The new design based on single and multi pass systems for domestic hot water systems which completely eliminate reverse flow problems have been forwarded and studies in details. The single pass systems besides avoiding reverse flow has two distinct features: (i) Allows the place the main water storage tank at any desirable place (even below the collector) (ii) Water is obtained at pre-determined temperature irrespective of insolation levels. The numerical calculations for single pass natural convection pressurized solar hot water systems meander tube fluid flow channel absorber, serpentine tube collectors and for non pressurized thermosyphonic solar hot water systems using parallel tube absorber were performed. The experimental measurements on such modified systems were performed manually by noting thermosyphonic flow rate and temperatures at every half hours time interval. The theoretical results matches well with experimental results. It was found that the non pressurized solar hot water systems using parallel tube absorber have average thermal efficiency of 41 % in the single pass and 27. 3 % in the multi-pass modes. The system efficiency of pressurized solar hot water systems using meander tube absorber are 30 % in single pass mode and 20 % in the multi pass mode due to higher temperature of absorber and higher heat losses from absorber in pressurized solar hot water systems.

Design and optimization of a highly efficient optical multipass system forγ-ray beam production from electron laser beam Compton scattering

A new kind of nonresonant optical recirculator, dedicated to the production of $\ensuremath{\gamma}$ rays by means of Compton backscattering, is described. This novel instrument, inspired by optical multipass systems, has its design focused on high flux and very small spectral bandwidth of the $\ensuremath{\gamma}$-ray beam. It has been developed to fulfill the project specifications of the European Extreme Light Infrastructure ``Nuclear Pillar,'' i.e., the Gamma Beam System. Our system allows a single high power laser pulse to recirculate 32 times synchronized on the radio frequency driving accelerating cavities for the electron beam. Namely, the polarization of the laser beam and crossing angle between laser and electrons are preserved all along the 32 passes. Moreover, optical aberrations are kept at a negligible level. The general tools developed for designing, optimizing, and aligning the system are described. A detailed simulation demonstrates the high efficiency of the device.

Analyzing natural gas by spontaneous Raman scattering spectroscopy

The use of multipass optical systems as well as the compression of the gaseous test medium to increase the signal intensity of the spontaneous Raman scattering of light has been experimentally studied. A description of a prototype developed for a natural-gas analyzer based on spontaneous Raman scattering spectroscopy is presented. A technique is described for calculating the composition of natural gas from its spectrum. The results are compared with chromatographic-analysis data.

Feasibility study for a new high resolution Thomson scattering system for the ASDEX Upgrade pedestal

A new Thomson scattering diagnostic is proposed for the study of fast plasma dynamics in the pedestal of ASDEX Upgrade. The diagnostic will measure electron temperature and density profiles over a ∼ 3 cm wide area in the edge transport barrier region, with ∼ 1–2 mm spatial resolution and ∼ 10 kHz sampling rate. A challenging goal of the project is the study of the bootstrap current in the plasma pedestal by measuring the distortion and shift of the electron distribution along the toroidal direction. Expected spatial and time resolutions of the current density measurements are ∼ 3 mm and ∼ 1 ms correspondingly. The new diagnostic will be used to study the fast dynamic behaviour of the pedestal bootstrap current, where models indicate that it plays a key role in regulating edge stability, e.g. during ELMs. The diagnostic design is based on the intra-cavity multi-pass system currently in operation in TEXTOR, which uses a probing ruby laser, a grating spectrometer and two fast CMOS cameras for scattered light detection, and has achieved measuring accuracies of the order of ∼ 1% for ne and ∼ 2% for Te. Parts of that system will be reused in ASDEX Upgrade (some with significant modifications), but the laser multi-pass and light collection systems are entirely redesigned. Restrictions in space and line-of-sight availability have led to the adoption of a design which uses in-vessel multi-pass mirrors and light collection optics, requiring a number of innovative technical solutions to permit remote laser alignment and light collection. We give an overview of the project, discuss the underlying physics basis and present a number of technical solutions employed.

Optimization of the rib structure inside cooling channels with genetic algorithms

To increase the thermal efficiency of industrial gas turbines, the performance of the internal cooling channels is of utmost importance. These cooling channels are often equipped with ribs on the top and bottom walls to increasetheturbulenceandthemixingoftheflow.Dependingonthepositionofthecoolingchannelintheturbine blade and the external heat load, the ribs have to be designed differently. Until now, the decision regarding the optimal design parameters was mainly based on the knowledge of the engineer. The present work investigates genetic algorithms as an optimization method to find optimal designs for the rib configuration in internal cooling channels. The main optimization targets are the enhancement of the heat transfer with a low increase in pressure drop. Results are presented for simplified 2D as well as realistic 3D cooling channel configurations. In the present study, a numerical method is investigated for the optimization of the rib structure inside cooling channels. Cooling channels are used for the internal convective cooling of gas turbine blades. The channels are mostly designed as multi-pass systems through the blade. In the main part of the blade, these channels have square or rectangular cross sections, while in the leading and trailing edge region the channels have triangular cross sections. Such channels normally have roughness elements, for example ribs, along two walls. Due to the ribs, the turbulence and the mixing of the flow is enhanced.To increase the thermal efficiency, different rib designs are used, as can be seen in the wide rangeofribangles α,ribheights eandinnerpitchdistances pconsideredinmanystudies.Atypicalgas turbine blade with cooling channels is shown in Fig. 1.The present work investigates an optimization method based on genetic algorithms (GAs) to find optimal designs for the rib configuration in an internal cooling channel. Until now, the choice of the rib angle, rib height and inner pitch distance was mainly based on the engineers' knowledge, obtained from experiments and sometimes numerical computations. Finding an adequate design is a time-consuming task due to the numerous parameters influencing the performance of a cooling channel. Even small changes can influence the turbulent flow and the heat transfer and pressure loss in the channel. One can find a large number of experimental investigations about the heat transfer performance and friction factor characteristics in cooling channels. Some of the important experimental works are summarized in (1-3). Numerical calculations offer a cheap alternative to experimental investigations and have shown good results in previous studies that dealt with heat transfer in internal cooling channels (4, 5). In future, when turbulence models are further developed, they might have the potential to replace more and more experimental work. GAs are an abstraction of natural phenomena. They try to simulate the mechanisms of natural selection or evolution copied from biological organisms. Due to the high complexity of the turbulent flow and heat transfer problems appearing in cooling channels, traditional search algorithms are often not applicable. Furthermore, as gradient-based methods, they often result in local minima or maxima. This problem cannot appear in GAs due to the random search procedure. The whole optimization process is based on the automatic coupling of a GA with other software tools (for the geometry generation ProEngineer (6), for the grid generation CENTAUR (7) and for the calculation of the

Advanced evaluation of transient heat transfer experiments using thermochromic liquid crystals

An advanced evaluation method for transient heat transfer experiments using thermochromic liquid crystals (TLCs) combining the advantages of standard hue and maximum intensity methods is presented. In order to obtain a global evaluation of locally correct heat transfer coefficients by using the one-dimensional solution of Fourier's equation, assuming heat conduction in a semi-infinite medium with a convective boundary condition, local input values have to be identified from measurements of the fluid and surface temperatures. For that reason, two different approaches have emerged. First, a two-dimensional numerical method has been adapted to evaluate the transient fluid temperature distributions in multi-pass systems from a few local measurements. Additionally, on the basis of latest calibration and indication experience of TLCs, especially in complex passages, an innovative temporal indication analysis method using a neural network has been implemented in the process of heat transfer evaluation.

Polarization transformers for optical anisotropy: I. Nonreciprocal optical systems and the equivalence theorem

The problem of transformation of anisotropic properties of multipass nonreciprocal systems is considered. The equivalence theorem is generalized to the case of nonreciprocal elements (i.e., to elements that do not satisfy the equivalence theorem). It is shown that any nonreciprocal anisotropic phase element is equivalent to a combination of five elements, namely, a reciprocal linear phase plate, a nonreciprocal linear phase plate, a reciprocal rotator, and two Faraday rotators. The difference between reciprocal and nonreciprocal elements is considered from the viewpoint of their basis states.

An Investigation of High Gradient Dielectrophoretic Filtration

Experimental and modelling studies of particle separation from liquids using a dielectric force show that dielectrophoresis may be an appropriate mechanism for multipass filtration systems if the dielectric properties of the fluid and contaminant are favourable. The general trends in filtration efficiency have been identified through an experimental investigation. Higher filtration efficiency is obtained when the viscosity of the liquid is lowered, when the contaminant particle sizes are larger, when the collector particle sizes are smaller and when the concentration of particles in the feed is lower. Efficiency was almost independent of flow rate over the range studied. A model is developed to calculate filter efficiencies; although the general trends in the variation of filter efficiency with process variables can be predicted, absolute values of the calculated efficiencies are too high; ways to improve the model are suggested. A filter design that can be scaled-up has been used in the experimental work, which has identified the main parameters in the process.

Promising version of the three-objective multipass matrix system

Since they were described in Refs. [1, 2], multipass matrix systems have found extensive application in various areas of science and engineering. They provide the longest optical path lengths within a small volume with linear control of the number of beam passages. In the present paper a modified three-objective multipass matrix system is considered. The system is used equally successfully both in highresolution spectroscopy and in metrology as well-controlled delay lines of pulse optical signals. New potential applications offered by the system that favorably distinguish it from its known analogs are also discussed.

Large scale discriminative training of hidden Markov models for speech recognition

This paper describes, and evaluates on a large scale, the lattice based framework for discriminative training of large vocabulary speech recognition systems based on Gaussian mixture hidden Markov models (HMMs). This paper concentrates on the maximum mutual information estimation (MMIE) criterion which has been used to train HMM systems for conversational telephone speech transcription using up to 265 hours of training data. These experiments represent the largest-scale application of discriminative training techniques for speech recognition of which the authors are aware. Details are given of the MMIE lattice-based implementation used with the extended Baum-Welch algorithm, which makes training of such large systems computationally feasible. Techniques for improving generalization using acoustic scaling and weakened language models are discussed. The overall technique has allowed the estimation of triphone and quinphone HMM parameters which has led to significant reductions in word error rate for the transcription of conversational telephone speech relative to our best systems trained using maximum likelihood estimation (MLE). This is in contrast to some previous studies, which have concluded that there is little benefit in using discriminative training for the most difficult large vocabulary speech recognition tasks. The lattice MMIE-based discriminative training scheme is also shown to out-perform the frame discrimination technique. Various properties of the lattice-based MMIE training scheme are investigated including comparisons of different lattice processing strategies (full search and exact-match) and the effect of lattice size on performance. Furthermore a scheme based on the linear interpolation of the MMIE and MLE objective functions is shown to reduce the danger of over-training. It is shown that HMMs trained with MMIE benefit as much as MLE-trained HMMs from applying model adaptation using maximum likelihood linear regression (MLLR). This has allowed the straightforward integration of MMIE-trained HMMs into complex multi-pass systems for transcription of conversational telephone speech and has contributed to our MMIE-trained systems giving the lowest word error rates in both the 2000 and 2001 NIST Hub5 evaluations.

Development of optical multipass matrix systems

Abstract The basic principles of designing multipass matrix systems (MMSs) are considered. A promising version of the MMS providing complete stability of the exit image is discussed. The new matrix system is versatile and most suitable, due to its potentialities and optical properties, for use in laser spectroscopy. A multipass transferable gas analyser comprising this matrix system and non-cooled diode lasers enabled record (for instruments of this type) sensitivity values to be achieved. In particular, the threshold sensitivity to methane is 0.02 ppm and the long-time stability of the measurements is 0.1 ppm. Designs of small large-aperture four- and six-row matrix systems are depicted.

Development of optical multipass matrix systems

The basic principles of designing multipass matrix systems (MMSs) are considered. A promising version of the MMS providing complete stability of the exit image is discussed. The new matrix system is versatile and most suitable, due to its potentialities and optical properties, for use in laser spectroscopy. A multipass transferable gas analyser comprising this matrix system and non-cooled diode lasers enabled record (for instruments of this type) sensitivity values to be achieved. In particular, the threshold sensitivity to methane is 0.02 ppm and the long-time stability of the measurements is 0.1 ppm. Designs of small large-aperture four- and six-row matrix systems are depicted.