Pulsed Mode Operation Research Articles

A low-power, micromachined, double spiral hotplate for MEMS gas sensors

This paper presents the design, fabrication and reliability testing of a double spiral platinum-based MEMS hotplate for gas sensing applications. The structure of MEMS hotplate consists of a 0.7 µm-thick thermally grown SiO2 membrane of size 120 µm × 120 µm over which a double spiral platinum resistor is laid out. The hotplate membrane is supported by its four arms connected to the Si-substrate. The design and simulation of the hotplate structure was carried out using MEMS-CAD Tool COVENTORWARE. Based on the design, a double spiral platinum resistor of 103 Ω is fabricated on SiO2 membrane using lift-off technique. The platinum deposition is carried out using DC sputtering technique. Bulk micromachining of Si is done from front side of the structure to create the suspended SiO2 membrane. The temperature coefficient of resistance (TCR) of platinum is measured and found to be 2.19 × 10?3/ °C. The TCR value is used for temperature estimation of the hotplate. The test results show that the microhotplate consumes only 20 mW power when heated up to 500 °C. For reliability testing of fabricated structure, the hotplate is continuously operated at 300 °C for 1.8 h. Also, it can sustain at least 61 cycles pulse-mode operation at 530 °C with ultra-low resistance and temperature drifts. The structure can sustain a maximum temperature and current of 611 °C and 11.55 mA respectively without any damage.

High peak power (≥10 mW) quantum cascade superluminescent emitter

We report room temperature and milliwatt range mid-infrared superluminescent emission at 5 μm from Quantum Cascade (QC) devices. To achieve high power superluminescence, we utilize an ultrastrong coupling QC laser design, and employ a cavity formed by the combination of a 17° tilted cleaved facet and a wet etched rounded and sloped facet to introduce additional mirror loss. For pulsed mode operation, a 8 mm long and 15 μm wide device achieves ∼1.3 mW peak power at 300 K and a 25 μm wide device with Si3N4 anti-reflection coated rounded facet achieves ∼10.2 mW peak optical output power at 250 K.

On the use of pulsed Dielectric Barrier Discharges to control the gas-phase composition of atmospheric pressure air plasmas

We presents results obtained from the numerical simulation of the gas-phase chemical kinetics in atmospheric pressure air non-equilibrium plasmas. In particular we have addressed the effect of pulsed operation mode of a plane dielectric barrier discharge. It was conjectured that the large difference in the time scales involved in the fast dissociation of oxygen molecules in plasma and their subsequent reactions to produce ozone and nitrogen oxides, makes the presence of a continuously repeated plasma production unnecessary and a waste of electrical power and thus efficiency. In order to test such suggestion we have performed a numerical study of the composition and the temporal evolution of the gas-phase of atmospheric pressure air non-equilibrium plasmas. Comparison with experimental findings in a dielectric barrier discharge with an electrode configuration symmetrical and almost ideally plane is briefly addressed too, using plasma diagnostics to extract the properties of the single micro-discharges and a sensor to measure the concentration of ozone produced by the plasma.

The Thermal State Computational Research of the Low-Thrust Oxygen-Methane Gaseous-Propellant Rocket Engine in the Pulse Mode of Operation

The Thermal State Computational Research of the Low-Thrust Oxygen-Methane Gaseous-Propellant Rocket Engine in the Pulse Mode of Operation

The problem of reducing a transient process in a damped piezoelectric plate operating in the pulse mode

The pulse operation mode of a piezoelectric plate that irradiates into water is considered. Its back side borders on the air or a damper with different specific acoustic impedances. The electrical excitation of a transducer is carried out by a signal in the form of the basic half-period of a sinusoid that is applied at the initial instant and a compensating one that is applied at a specified instant. The problem of terminating the transient process by applying a compensation pulse at the required instant is solved by a numerical theoretical method, which implies the application of the D’Alembert principle. The amplitudes of the compensating pulses are determined. The possibility of radiation of acoustic pulses of different durations is shown.

Pulsed actuation avoids failure in dielectric elastomer artificial muscles

Dielectric elastomer actuators (DEAs) are a class of artificial muscles capable of large linear strains (well over 100%), and with high energy density, and low cost and weight. One of the most prominent failure modes of a DEA is electrical breakdown, which can damage the device permanently, limiting its deformation capability. Breakdown is also common, since to maximize energy output, devices often operate near the breakdown limit. Elucidating breakdown mechanisms, as well as finding ways to prevent it, are of intense research interest. We show that by applying short electrical pulses, one could minimize the exposure of the DEAs to high leakage current, which is one of the main mechanisms for electrical breakdown. This allows one to operate at significantly higher potentials than the DC breakdown voltage. By applying pulses, we demonstrate up to 81.7% area strain repeatedly, at voltages more than twice the DC breakdown limit, without the risk of failure. The pulsed operation mode of DEAs accommodating higher voltages than possible with DC represents an opportunity for potential applications, safer and simpler device designs, and a technique for further study of DEA breakdown mechanisms.

Hypergolic Propellant Ignition Phenomenon Associated with Two-Phase Oxidizer Flow Injection

During pulse-mode operation of a bipropellant engine that uses mixed oxides of nitrogen and monomethyl hydrazine as a propellant, a titanium alloy injector with an unlike-doublet impinging stream absorbs the heat from the injector face, which is heated to a high temperature during combustion. When the injector’s temperature exceeds the boiling point of the oxidizer under several operating conditions, evaporation will occur at the initial inflow of each pulse. This evaporation phenomenon causes an unsteady two-phase flow that affects ignition and combustion phenomena. In this paper, this abnormal ignition and combustion phenomenon associated with the two-phase flow is investigated through experiments with a single-element injector. Data analysis on the impinging stream instability and the ignition phenomenon reveals that the unsteady oxidizer jet caused by evaporation may act as a trigger for high-frequency instability.

A pulsed supersonic gas jet target for precision spectroscopy at the HITRAP facility at GSI

A pulsed supersonic gas jet target for experiments at the HITRAP facility at the GSI Helmholtzzentrum für Schwerionenforschung in Darmstadt has been designed and built as a multi-purpose installation for key experiments on fundamental atomic physics in strong fields. This setup is currently installed at the Institut für Kernphysik of Goethe-University, Frankfurt am Main (IKF), in order to explore its operation prior to its installation at the HITRAP facility. Design and performance of the target are described. The measured target densities of 5.9×1012atoms/cm3 for helium and 8.1×1012atoms/cm³ for argon at the stagnation pressure of 30bar match the required values. The target-beam diameter of 0.9mm and the pulsed operation mode (jet built-up-time ≤15ms) are well suited for the use at HITRAP.

Impedance spectroscopy of pellets made from yttria stabilized zirconia nanoparticles generated via CW and pulsed mode of laser vaporization method

We report impedance characteristics and ac conductivity response of ~1mm thick pellets made from yttria-stabilized zirconia (YSZ) nanoparticles. Nanoparticles of different average sizes (10–16nm) and of different yttria (6mol%, 10mol% and 16mol%) contents were obtained via CO2 laser based laser vaporization method. We employed both the continuous and the pulsed mode of CO2 laser operation and obtained smallest size nanoparticles under pulsed mode of laser vaporization. Impedance spectroscopic measurements were carried out in frequency range of 10Hz–1MHz at various temperatures ranging from room temperature to 500C. Oxygen ion conductivity response was compared with respect to laser vaporization mode and yttria content in nanoparticles of YSZ. The grain and grain boundary ionic conductivity of pellets made from large sized nanoparticles generated in continuous mode of laser vaporization was two orders of magnitude low compared to pellet made of smallest average size nanoparticles (generated with pulsed mode of laser vaporization). The activation energy of ion hopping in all the samples was estimated using Arrhenius equation and found in the range 0.95–1.34eV. Possible origin of differences between conductivity in pellets made from nanoparticles generated via pulsed mode and CW mode of vaporization is discussed on the basis of association of oxygen vacancy and Y+3 ion.

Analysis and Experimental Setup of a Switched Reluctance Generator for Maximum Output Power

Output power of a switched reluctance generator (SRG) in single pulse mode operation depends on control variables which consist of dc bus voltage, shaft speed, and excitation angles (turn-on and turn-off angles).This paper presents a model using equations to express maximum output power in terms of the control variables. The proposed analytical model is applied from a nonlinear magnetization curve that is used to mathematically demonstrate relationship of the control variables and to find the optimal control variables of the SRG for maximum output power. The validity of the analytical model has been confirmed by experimental results

Electron-leakage-related low-temperature light emission efficiency behavior in GaN-based blue light-emitting diodes

The typical light emission efficiency behaviors of InGaN/GaN multi-quantum well (MQW) blue light-emitting diodes (LEDs) grown on c-plane sapphire substrates are characterized by pulsed current operation mode in the temperature range 40 to 300 K. At temperatures lower than 80 K, the emission efficiency of the LEDs decreases approximately as an inverse square root relationship with drive current. We use an electron leakage model to explain such efficiency droop behavior; that is, the excess electron leakage into the p-side of the LEDs under high forward bias will significantly reduce the injection possibility of holes into the active layer, which in turn leads to a rapid reduction in the radiative recombination efficiency in the MQWs. Combining the electron leakage model and the quasi-neutrality principle in the p-type region, we can readily derive the inverse square root dependent function between the light emission efficiency and the drive current. It appears that the excess electron leakage into the p-type side of the LEDs is primarily responsible for the low-temperature efficiency droop behavior.

Large-Signal RF Circuit Model for a High-Power Laser Diode Module

An electrical large-signal circuit model for a 30-W high-power laser diode module is presented. Such modules are designed primarily for continuous wave (cw) operation but can be pulsed in the sub- μs temporal range for special applications. Our model is thus valid up to 20 MHz in the electrical frequency domain. The elements of the circuit model have been derived from RF impedance measurements using a calibrated vector network analyzer and a high-current dc/RF bias-T. The impedance is dominated by the inductance of the high-current connecting leads from the laser driver to the laser chip. The skin effect has been found to influence considerably both resistive and inductive impedances at high frequencies. For large-signal circuit simulations in the time domain, the current-voltage characteristic of the diode p-n junction is included by an analytic equation. The model is verified by comparison of simulation results with measured currents, voltages, and laser powers in large-signal pulsed-mode operation. This model is well suited for the design of optimized pulsed-current driver circuits.

A study of dynamic Lorentz force detuning of 650 MHz βg=0.9 superconducting radiofrequency cavity

The small bandwidth of superconducting cavities makes the study of dynamic Lorentz force detuning and its compensation indispensable in case of pulsed mode operation of high gradient accelerators. In this paper, we present the study of this detuning and also propose an optimized design for five cell 650MHz βg=0.9 elliptic superconducting cavities, which will be used in the high energy section of the 1GeV H− linear accelerator for the proposed Indian spallation neutron source project, by suitably inserting the inter-cell stiffeners. The paper presents a sequential design methodology which starts with study of static Lorentz force detuning and tunability; and progresses to find out the structural modes and related dynamic detuning values by performing transient structural dynamics calculations. The developed methodology is general in nature and can be used for a three dimensional model of any geometry. The work will be useful for optimizing the design against dynamic Lorentz force detuning of superconducting radiofrequency cavities of any shape.

Investigation on the Fatigue Characteristic in Support Structure of HL-2M Tokamak

In order to obtain enhanced plasma parameters a complete new tokamak HL-2M is now under construction in Southwestern Institute of Physics. To assure the structural safety of the device for the entire operation cycle, one of the most important issues is the lifetime-limiting effects due to the pulsed operation mode. Fatigue is one of the major failure modes to be considered in mechanical design, and pulsed operation imposes stress with significant alternating components on the support structure (SS). Therefore, the reliability of the whole device is strongly affected by the stress and fatigue characteristic of the SS as the interface structure. This article introduces the SS design and details the fatigue life calculation methods based on the different characteristics of the sub-structures. The fatigue life in hazardous areas of the toroidal field coils anti-torque structure (TFCs-ATs) has been determined by non-linear analysis results. And with the stress-time history data of the vacuum vessel & poloidal field coils support structure (VV&PFCs SS), the fatigue analysis of the hot spots has been completed based on rain-flow counting method and linear cumulative damage method. The calculated minimum fatigue life on TFCs-ATs and VV&PFCs SS is 4.743E+05 and 1.805E+06 cycles, respectively. And the calculated fatigue life on sub-structures can meet the required life for HL-2M tokamak: 1.0E+05 cycles.

Pulsed-mode operation and performance of a ferromagnetic shape memory alloy actuator

The actuation capabilities and positioning performance of a single crystal ferromagnetic shape memory alloy (FSMA) operated in pulsed mode are evaluated in a prototype device. It consists of two orthogonal coil pairs that produce the magnetic fields necessary for the non-contact deformation of the material. The position of the top of the crystal after actuation is measured by a capacitive sensor. A specifically designed power module drives the discharge of a set of capacitors through the coils, producing fast current pulses of large amplitudes (about 250 A), the coil pairs are driven independently to control the direction of actuation. Open-loop experiments demonstrate that successive pulses of increasing magnitude successfully produced the desired expansion and contraction of the crystal, depending on the pair of coils that is activated. The deformation achieved is maintained after the pulses, highlighting the advantageous set-and-forget operation of the device. Closed-loop experiments are performed using a double proportional–integral–derivative controller, designed to take advantage of the energy-saving quality of the set-and-forget operation. Despite the nonlinear response and hysteric response of FSMA materials, a reference position can be reached and maintained with a maximum error of 0.5 μm.

Automated system for efficient microwave power coupling in an S-band ECR ion source driven under different operating conditions

This article presents an automated system for optimizing the microwave power coupling to the plasma generated in a proton/deuteron Electron Cyclotron Resonance (ECR) source, based on a specific model of a rectangular waveguide triple-stub tuner and the integrated measurement and control electronics, helping to get stable plasma states. The control and improvement of the RF power absorption into the plasma is a complex process, essential for the ion source development and optimization under different operating conditions. A model and a matching algorithm for the triple-stub tuner have been developed and, besides, different methods to accurately measure the power transfer in a waveguide RF system have been studied and deployed in the ESS-Bilbao ion source system. The different parts have been integrated through a controller, which allows to run an automatic plasma matching system in closed loop. The behavior of the system implemented for low and high power regimes has been tested under different conditions: with several load impedances, with plasma inside the chamber, in continuous wave and pulsed wave operation modes, demonstrating power absorption typically over 90% in all the ion source configurations. The developed system allows to achieve significant improvement in the ECR ion source power absorption efficiency, both in continuous and pulsed mode. The automatic tuning unit enhances the system operation finding an optimum solution much faster than manually, also behaving as an adaptive system able to respond in few pulses to ion source configuration changes to maintain the power coupling as high as possible.

Image denoising techniques applied to glow discharge optical emission spectroscopy elemental mapping

Glow discharge optical emission spectroscopy (GDOES) is becoming a mature technique for depth profiling analysis. The advantages it affords, including fast, quantitative, and multi-elemental analysis, as well as allowing very high depth-resolution, have attracted the attention of the thin film community. Recently, the use of GDOES under pulsed-mode operation and coupled to hyper-spectral imaging techniques has been proposed to perform surface elemental mapping. Several manuscripts have reported on the underlying mechanisms in GDOES pertaining to the spatial resolution, while other manuscripts have reported on elemental mapping applications, for example, regarding separated proteins or thin film combinatorial libraries. Only a couple of studies have reported image processing techniques applied to GDOES elemental mapping and none having to do with image denoising purposes. Herein, image denoising techniques are compared in several scenarios: (a) mapping of homogeneous samples; (b) mapping of heterogeneous samples in two dimensions; (c) mapping of heterogeneous samples in three dimensions. Denoising techniques compared include averaging, median filtering, principal component analysis (PCA), and local pixel grouping-PCA. The peak signal-to-noise ratio is used to show the efficiency of noise removal, while the full-with-half-maximum of emission from sharp features is used to demonstrate the resolution degradation effects of each denoising technique. In general, it is observed that PCA outperforms other techniques albeit with a higher cost of image processing time. Also, it becomes evident that having multiple image slices (a 3rd dimension) affords more efficient noise removal while minimizing losses in spatial resolution.

Desorption/ionization induced by neutral cluster impact as a soft and efficient ionization source for ion trap mass spectrometry of biomolecules

Desorption and ionization induced by neutral clusters (DINeC) using SO2 as cluster constituents was previously shown to produce clear and fragmentation-free spectra with low background from samples prepared with standard oligopeptides. Here we demonstrate a more general applicability of this method based on examples from different classes of (bio-)molecules. In order to make better use of the ions generated during the millisecond cluster-pulse, the DINeC source was combined with an ion trap mass spectrometer. Desorption and ionization was induced by neutral SO2 clusters with a mean size of 10(3) to 10(4) molecules seeded in a pulsed He beam. The desorbed ions were accumulated in an ion trap over the whole pulse duration prior to mass spectrometric analysis. Samples were prepared by simply drop casting the respective aqueous solution of biomolecules on Si/SiO2 substrates. Clear and fragmentation-free spectra of oligopeptides were detected in single pulse operation mode. The very soft nature of the desorption process was demonstrated for phosphopeptides. DINeC spectra from bovine serum albumin samples after tryptic digest led to a clear identification of the original sequence using mass fingerprinting analysis. MS(n) capability was illustrated with two types of rhodamine dyes. Desorption and ionization induced by neutral clusters can efficiently be combined with ion trap mass spectrometry since the pulse width and repetition rate of a typical pulsed cluster beam correspond well to the discontinuous accumulation time as well as the spectral rate of the ion trap. Clear mass spectra were obtained with such a setup for a variety of biosamples demonstrating the wider applicability of the DINeC process.

Quantum cascade laser intracavity absorption spectrometer for trace gas sensing

A mid-infrared intracavity laser absorption spectrometer for trace gas sensing is demonstrated. An external-cavity multi-mode quantum cascade laser with central wavelength 8.0 μm was combined with a scanning Fabry-Perot interferometer, which analyzed the change of the laser emission spectrum caused by introducing an analyte inside the cavity. The detection mechanism is based on monitoring the laser spectrum dynamics at adiabatically changing laser conditions in long pulse operation mode. Fast acquisition and vapor exchange allow nearly real-time analyte detection. Sensitivity at the level of 1 × 10−5 cm−1 was demonstrated based on a weak water vapor absorption line.

Beam dynamics studies and parametric characterization of a standingwave electron linac

This paper presents the results of electron beam trackingsimulations for a 30 MeV standing wave electron linac at Electron BeamCentre Kharghar, Navi Mumbai, India. For the pulsed mode operation of thepresent linac preferential operation parameters have been determined fromthe results of beam dynamics studies. This electron accelerator is a generalpurpose facility for generation of Bremsstrahlung X-rays and neutronscattering experiments. This electron accelerator-based experimental neutronfacility will be used for measurement of neutron cross-section (n,γ), (n, xn) and (n, f) reactions at different energies for various materialsand material irradiation studies.