Showing papers on "Buffer gas published in 2011"

The Buffer Gas Beam: An Intense, Cold, and Slow Source for Atoms and Molecules

Abstract: Beams of atoms and molecules are stalwart tools for spectroscopy and studies of collisional processes. The supersonic expansion technique can create cold beams of many species of atoms and molecules. However, the resulting beam is typically moving at a speed of 300-600 m/s in the lab frame, and for a large class of species has insufficient flux (i.e. brightness) for important applications. In contrast, buffer gas beams can be a superior method in many cases, producing cold and relatively slow molecules in the lab frame with high brightness and great versatility. There are basic differences between supersonic and buffer gas cooled beams regarding particular technological advantages and constraints. At present, it is clear that not all of the possible variations on the buffer gas method have been studied. In this review, we will present a survey of the current state of the art in buffer gas beams, and explore some of the possible future directions that these new methods might take.

Microfabrication of cesium vapor cells with buffer gas for MEMS atomic clocks

Abstract: This paper reports on the Si-glass anodic bonding process to fill micro Cs vapor cells with a buffer gas (Ar or Ne) at a controlled pressure (up to 20 kPa), which is one of the technological key steps to fabricate Cs vapor cells for miniature atomic clocks. In the atmosphere of these gases, the applicable bonding voltage was not high enough to achieve strong bonding because of the electrical breakdown caused by ionization of the gas. To improve the bonding quality, an original two-step anodic bonding method was proposed. The first step of the anodic bonding, which intends to pre-seal the gas in microcells, is carried out in the presence of a buffer gas by applying a voltage lower than the breakdown voltage. Subsequently, the second bonding is performed in air at sufficiently high voltages to improve the sealing quality. By employing optical spectroscopy, it was demonstrated that the cells maintain the buffer gas at an appropriate pressure for atomic clock operation. The accelerated aging tests show that Cs vapor as well as the buffer gas remained in the cells without any significant change in the pressure, which allow us to estimate the lifetime of the cells to be at least 3 years. Further CPT experiments revealed that the buffer-gas pressure change is less than 6.13 × 10−4 kPa throughout the aging test at 125 °C for more than 3 weeks. These results show that these microcells are appropriate for applications to atomic frequency references.

A cold and slow molecular beam

Abstract: Employing a two-stage cryogenic buffer gas cell, we produce a cold, hydrodynamically extracted beam of calcium monohydride molecules with a near effusive velocity distribution. Beam dynamics, thermalization and slowing are studied using laser spectroscopy. The key to this hybrid, effusive-like beam source is a “slowing cell” placed immediately after a hydrodynamic, cryogenic source [Patterson et al., J. Chem. Phys., 2007, 126, 154307]. The resulting CaH beams are created in two regimes. In one regime, a modestly boosted beam has a forward velocity of vf = 65 m s−1, a narrow velocity spread, and a flux of 109 molecules per pulse. In the other regime, our slowest beam has a forward velocity of vf = 40 m s−1, a longitudinal temperature of 3.6 K, and a flux of 5 × 108 molecules per pulse.

A cryogenic beam of refractory, chemically reactive molecules with expansion cooling

Abstract: Cryogenically cooled buffer gas beam sources of the molecule thorium monoxide (ThO) are optimized and characterized. Both helium and neon buffer gas sources are shown to produce ThO beams with high flux, low divergence, low forward velocity, and cold internal temperature for a variety of stagnation densities and nozzle diameters. The beam operates with a buffer gas stagnation density of ∼1015–1016 cm−3 (Reynolds number ∼1–100), resulting in expansion cooling of the internal temperature of the ThO to as low as 2 K. For the neon (helium) based source, this represents cooling by a factor of about 10 (2) from the initial nozzle temperature of about 20 K (4 K). These sources deliver ∼1011ThO molecules in a single quantum state within a 1–3 ms long pulse at 10 Hz repetition rate. Under conditions optimized for a future precision spectroscopy application [A. C. Vutha et al., J. Phys. B: At., Mol. Opt. Phys., 2010, 43, 074007], the neon-based beam has the following characteristics: forward velocity of 170 m s−1, internal temperature of 3.4 K, and brightness of 3 × 1011 ground state molecules per steradian per pulse. Compared to typical supersonic sources, the relatively low stagnation density of this source and the fact that the cooling mechanism relies only on collisions with an inert buffer gas make it widely applicable to many atomic and molecular species, including those which are chemically reactive, such as ThO.

Processes involving clusters and small particles in a buffer gas

Abstract: Processes involving clusters and small particles are considered from the standpoint of interaction of clusters or small particles with atomic particles of a buffer gas. Two opposite interaction regimes are the kinetic (dynamic) and diffusion (hydrodynamic) ones, so that in the first case collisions of a gas atom with a cluster or small particle are analogous to collisions of two atomic particles in a gas, whereas in the diffusion regime a cluster or a small particle strongly interacts simultaneously with many atoms. Criteria and parameters of processes for the kinetic and diffusion regimes are given for transport phenomena in gases involving clusters or small particles, cluster charging in an ionized gas and particle combustion, and also nucleation processes including cluster growth as a result of atom attachment to a growing cluster, the coagulation and coalescence processes.

Optical pumping and spectroscopy of Cs vapor at high magnetic field

Abstract: We have measured changes in the ground-state populations of Cs vapor induced by optical pumping at high magnetic field. The $2.7$-T field of our experiments is strong enough to decouple the nuclear and electronic spins, allowing us to independently measure each population. The spatial dependence of the Cs populations in small amounts of buffer gas obeys a simple coupled diffusion model and the relative populations reveal the details of relaxation within the vapor cell. Optical pumping can produce high nuclear polarization in the Cs vapor due to perturbations of the hyperfine interaction during collisions with buffer-gas particles and depending on the pumping transition, radiation trapping can strongly influence the electronic and nuclear polarizations in the vapor.

Size-controlled formation of Cu nanoclusters in pulsed magnetron sputtering system

Abstract: Size-controlled Cu clusters are formed in a system which combines pulsed magnetron sputtering and gas condensation at room temperature. The discharge repetition frequency (0.1–25 kHz) and the duty cycles (20–90%) of the magnetron sputtering are varied systematically, the influence of discharge current (100–800 mA) and the pressure in the condensation tube (25–90 Pa) is also investigated. The cluster mass is determined by a quadrupole mass filter in the aggregation tube, and the cluster size by atomic force microscopy (AFM) imaging of deposited clusters. For all preparation conditions, the cluster mass shows a log-normal distribution. A non-monotonic frequency dependence with a maximum at 1 kHz and 20% of duty cycle is observed (about 10 5 amu, or cluster diameter 8–10 nm). By adjusting discharge frequency and duty cycle, the cluster mass can be decreased by one order of magnitude. We suggest that this effect is caused by energy dissipated into the aggregation tube; and find a critical buffer gas temperature T g-cr which limits cluster growth. This view is supported by the constant cluster mass flux which does not change on variation of discharge repetition frequency or duty cycle. This feature indicates mass conservation.

Coherent population trapping resonances in Cs–Ne vapor microcells for miniature clocks applications

Abstract: We report the characterization of dark line resonances observed in Cs vapor microcells filled with a unique neon (Ne) buffer gas. The impact on the coherent population trapping (CPT) resonance of some critical external parameters such as laser intensity, cell temperature, and microwave power is studied. We show the suppression of the first-order light shift by proper choice of the microwave power. The temperature dependence of the Cs ground state hyperfine resonance frequency is shown to be canceled in the 77–80 °C range for various Ne buffer gas pressures. The necessity to adjust the Ne buffer gas pressure or the cell dimensions to optimize the CPT signal height at the frequency inversion temperature is pointed out. Based on such Cs–Ne microcells, we preliminary demonstrate a 852 nm vertical cavity surface emitted laser (VCSEL)-modulated based CPT atomic clock exhibiting a short term fractional frequency instability σy(τ)=1.5×10−10τ−1/2 until 30 s. These results, similar to those published in the literature by others groups, prove the potential of our original microcell technology in view of the development of high-performance chip scale atomic clocks.

Diffusion, thermalization, and optical pumping of YbF molecules in a cold buffer-gas cell

Abstract: We produce YbF molecules with a density of ${10}^{18}$ m${}^{\ensuremath{-}3}$ using laser ablation inside a cryogenically cooled cell filled with a helium buffer gas. Using absorption imaging and absorption spectroscopy we study the formation, diffusion, thermalization, and optical pumping of the molecules. The absorption images show an initial rapid expansion of molecules away from the ablation target followed by a much slower diffusion to the cell walls. We study how the time constant for diffusion depends on the helium density and temperature and obtain values for the YbF-He diffusion cross section at two different temperatures. We measure the translational and rotational temperatures of the molecules as a function of time since formation, obtain the characteristic time constant for the molecules to thermalize with the cell walls, and elucidate the process responsible for limiting this thermalization rate. Finally, we make a detailed study of how the absorption of the probe laser saturates as its intensity increases, showing that the saturation intensity is proportional to the helium density. We use this to estimate collision rates and the density of molecules in the cell.

Microfabricated atomic vapor cell arrays for magnetic field measurements.

Abstract: We describe a method for charging atomic vapor cells with cesium and buffer gas. By this, it is possible to adjust the buffer gas pressure in the cells with good accuracy. Furthermore, we present a new design of microfabricated vapor cell arrays, which combine silicon wafer based microfabrication and ultrasonic machining to achieve the arrays of thermally separated cells with 50 mm3 volume. With cells fabricated in the outlined way, intrinsic magnetic field sensitivities down to 300 fT/Hz1/2 are reached.

Kinetics of a single trapped ion in an ultracold buffer gas

Abstract: The immersion of a single ion confined by a radiofrequency (RF) trap in an ultracold atomic gas extends the concept of buffer gas cooling to a new temperature regime. The steady-state energy distribution of the ion is determined by its kinetics in the RF field rather than the temperature of the buffer gas. Moreover, the finite size of the ultracold gas facilitates the observation of back-action of the ion onto the buffer gas. We numerically investigate the system's properties depending on atom–ion mass ratio, trap geometry, differential cross-section and non-uniform neutral atom density distribution. Experimental results are well reproduced by our model considering only elastic collisions. We identify excess micromotion to set the typical scale for the ion energy statistics and explore the applicability of the mobility collision cross-section to the ultracold regime.

Pressure broadening and collisional shift of the Rb D2 absorption line by CH4, C2H6, C3H8, n-C4H10, and He

Abstract: The pressure broadening and shift rates of the rubidium D2 absorption line 52S1/2→52P3/2 (780.24 nm) with CH4, C2H6, C3H8, n-C4H10, and He were measured for pressures ≤80 Torr using high-resolution laser spectroscopy. The broadening rates γB for CH4, C2H6, C3H8, n-C4H10, and He are 28.0, 28.1, 30.5, 31.3, and 20.3 (MHz/Torr), respectively. The corresponding shift rates γS are −8.4, −8.8, −9.7, −10.0, and 0.39 (MHz/Torr), respectively. The measured rates of Rb for the hydrocarbon buffer gas series of this study are also compared to the theoretically calculated rates of a purely attractive van der Waals difference potential. Good agreement is found to exist between measured and theoretical rates.

Using mixed gases for massive gas injection disruption mitigation on Alcator C-Mod

Abstract: Mixed gases are used for massive gas injection disruption mitigation on Alcator C-Mod in order to optimize radiation efficiency, halo current reduction and response time. Gas mixtures of helium and argon (argon fraction 0–50%) are investigated in detail, as well as mixtures of deuterium, argon, krypton and helium. Experiments show that injecting He/Ar mixtures leads to faster thermal and current quenches than with pure helium or argon injection, thus improving the time response of the disruption mitigation system and reducing the halo current. Small fractions of argon (~5–10%) in helium also lead to optimized radiation fractions with large electron density increases in the core plasma. These results are consistent with the expectation that small fractions of argon will be entrained with the faster helium in the early phases of gas flow. The gas mixing allows one to simultaneously exploit the fast particle delivery rate of light helium gas and the large radiation capability of argon.

Prospects for ultracold carbon via charge exchange reactions and laser cooled carbides.

Abstract: Strategies to produce an ultracold sample of carbon atoms are explored and assessed with the help of quantum chemistry. After a brief discussion of the experimental difficulties using conventional methods, two strategies are investigated. The first attempts to exploit charge exchange reactions between ultracold metal atoms and sympathetically cooled C+ ions. Ab initio calculations including electron correlation have been conducted on the molecular ions [LiC]+ and [BeC]+ to determine whether alkali or alkaline earth metals are a suitable buffer gas for the formation of C atoms but strong spontaneous radiative charge exchange ensure they are not ideal. The second technique involves the stimulated production of ultracold C atoms from a gas of laser cooled carbides. Calculations on LiC suggest that the alkali carbides are not suitable but the CH radical is a possible laser cooling candidate thanks to very favourable Frank-Condon factors. A scheme based on a four pulse STIRAP excitation pathway to a Feshbach resonance is outlined for the production of atomic fragments with near zero centre of mass velocity.

Investigating the gas phase emitter effect of caesium and cerium in ceramic metal halide lamps in dependence on the operating frequency

Abstract: The work function and with it the temperature of tungsten electrodes in HID lamps can be lowered and the lifetime of lamps increased by the gas phase emitter effect. A determination of the emitter effect of Cs and Ce is performed by phase resolved measurements of the electrode tip temperature Ttip(), plasma temperature Tpl() and particle densities N() by means of pyrometric, optical emission and broadband absorption spectroscopy in dependence on the operating frequency. The investigated HID lamps are ceramic metal halide lamps with transparent discharge vessels made of YAG, filled with a buffer gas consisting of Ar, Kr and predominantly Hg and seeded with CsI or CeI3. In the YAG lamp seeded with CsI and CeI3 as well as in a YAG lamp seeded with DyI3 (corresponding results can be found in a preceding paper) a gas phase emitter effect is observed in the cathodic phase due to a Cs, Ce or Dy ion current. In the YAG lamp seeded with CsI the phase averaged coverage of the electrode surface with emitter atoms decreases and the electrode temperature rises with increasing frequency, whereas the emitter effect of Ce and Dy is extended to the anodic phase, which leads to a decreased average temperature Ttip() with increasing frequency. This different behaviour of the averaged values of Ttip() for increasing frequency is caused by the differing adsorption energies Ea of the respective emitter materials. In spite of the influence of Ea on the coverage of the electrode with emitter atoms, the cathodic gas phase emitter effect produces in the YAG lamps seeded with CsI, CeI3 and DyI3 a general reduction in the electrode tip temperature Ttip() in comparison with a YAG lamp with Hg filling only.

Collisional redistribution laser cooling of a high-pressure atomic gas

Abstract: We describe measurements demonstrating laser cooling of an atomic gas by means of collisional redistribution of radiation. The experiment uses rubidium atoms in the presence of several hundred bar of argon buffer gas pressure. Frequent collisions in the dense gas transiently shift a far-red detuned optical field into resonance, while spontaneous emission occurs close to the unperturbed atomic transition frequency. Evidence for the cooling is obtained via both thermographic imaging and thermographic deflection spectroscopy. The cooled gas has a density above 1021 atoms/cm3, yielding evidence for the laser cooling of a macroscopic ensemble of gas atoms.

Method and equipment for selectively collecting process effluent

Abstract: An apparatus and process for recovering a desired gas such as xenon difluoride, xenon, argon, helium or neon, from the effluent of a chemical process reactor that utilizes such gases alone or in a gas mixture or in a molecule that becomes decomposed wherein the chemical process reactor uses a sequence of different gas composition not all of which contain the desired gas and the desired gas is captured and recovered substantially only during the time the desired gas is in the effluent.

First-order cancellation of the Cs clock frequency temperature-dependence in Ne-Ar buffer gas mixture.

Abstract: Through the detection of Coherent Population Trapping (CPT) resonances, we demonstrate the temperature-dependence cancellation of the Cs clock frequency in microfabricated vapor cells filled with a mixture of Ne and Ar. The inversion temperature at which the Cs clock frequency temperature sensitivity is greatly reduced only depends on the partial pressure of buffer gases and is measured to be lower than 80°C as expected with simple theoretical calculations. These results are important for the development of state-of-the-art Cs vapor cell clocks with improved long-term frequency stability.

Laser cooling of a potassium–argon gas mixture using collisional redistribution of radiation

Abstract: We study laser cooling of atomic gases by collisional redistribution, a technique applicable to ultradense atomic ensembles at a pressure of a few hundred bars. Frequent collisions of an optically active atom with a buffer gas shift atoms into resonance with a far red detuned laser beam, while spontaneous decay occurs close to the unperturbed resonance frequency. In such an excitation cycle, a kinetic energy of the order of the thermal energy k B T is extracted from the sample. Here we report of recent experiments investigating the cooling of a potassium–argon gas mixture, which compared to a rubidium–argon mixture investigated in earlier experiments has a smaller fine structure of the optically active alkali atoms. We observe a relative cooling of the potassium–argon gas mixture by 120 K.

Collisional Redistribution Laser Cooling of a High Pressure Atomic Gas

Abstract: We describe measurements demonstrating laser cooling of an atomic gas by means of collisional redistribution of radiation. The experiment uses rubidium atoms in the presence of several hundred bar of argon buffer gas pressure. Frequent collisions in the dense gas transiently shift a far red detuned optical field into resonance, while spontaneous emission occurs close to the unperturbed atomic transition frequency. Evidence for the cooling is obtained both via thermographic imaging and via thermographic deflection spectroscopy. The cooled gas has a density above 10$^{21}$ atoms/cm$^3$, yielding evidence for the laser cooling of a macroscopic ensemble of gas atoms.

Influence of inert gas species on the growth of silver and molybdenum films via a magnetron discharge

Abstract: Argon is the process gas of choice for most magnetron sputtering applications due to its large atomic mass, inert chemistry, and relatively low cost. Other inert gases are available for use in sputtering deposition that have varying mass and hence different momentum behaviour during ion bombardment of solid surfaces — affecting sputter yield, particle implantation and incorporation of process gas into deposited films. The plasma discharges generated from these gases vary in terms of the nature and energy of species incident at both target and substrate. In particular, the contribution from energetic neutrals varies as a consequence of the atomic mass of the process gas in comparison to the target material to be sputtered. Magnetron plasma discharges were generated from neon, argon, krypton and xenon gases in DC and mid-frequency pulsed-DC modes with both silver and molybdenum cathodes. The electrical characteristics, such as potential and current at the target and substrate were measured and compared. Thin metallic films were then deposited and analysed in terms of structure; mechanical, optical and electrical properties; and process gas incorporation. The data generated is used to establish the relationship between process gas species, plasma discharge characteristics of those gases, and the subsequent growth and properties of deposited coatings.

Laser cooling of a potassium-argon gas mixture using collisional redistribution of radiation

Abstract: We study laser cooling of atomic gases by collisional redistribution, a technique applicable to ultradense atomic ensembles at a pressure of a few hundred bar. Frequent collisions of an optically active atom with a buffer gas shift atoms into resonance with a far red detuned laser beam, while spontaneous decay occurs close to the unperturbed resonance frequency. In such an excitation cycle, a kinetic energy of the order of the thermal energy kT is extracted from the sample. Here we report of recent experiments investigating the cooling of a potassium-argon gas mixture, which compared to an rubidium-argon mixture investigated in earlier experiments has a smaller fine structure of the optically active alkali atom. We observe a relative cooling of the potassium-argon gas mixture by 120 K.

Temperature Dependence Cancellation of the Cs Clock Frequency in the Presence of Ne Buffer Gas

Abstract: The temperature dependence of the Cs clock transition frequency in a vapor cell filled with Ne buffer gas has been measured. The experimental setup is based on the coherent population trapping technique and a temporal Ramsey interrogation allowing a high resolution. A quadratic dependence of the frequency shift is shown. The temperature of the shift cancellation is evaluated. The actual Ne pressure in the cell is determined from the frequency shift of the 895-nm optical transition. We can then determine the Cs-Ne collisional temperature coefficients of the clock frequency. These results can be useful for vapor cell clocks and, particularly, for future microclocks.

Buffer gas cooling of YbF molecules

Abstract: This thesis reports on the production and characterisation of the first slow and cold beam of YbF molecules using buffer gas cooling. These molecules are being used to measure the electron’s electric dipole moment, and an intense source of slow-moving molecules is desirable for this experiment. The molecules are loaded into a buffer gas cell via laser ablation where they thermalise with cold helium buffer gas. They are then detected inside the cell using laser absorption imaging and spectroscopy on the XΣ→AΠ1/2 transition. The formation, diffusion and thermalisation dynamics of the molecules inside the cell are studied. Measurements of laser absorption versus intensity reveal that saturation of the absorption is due to a competition between optical pumping into dark states and repopulation of the addressed level by inelastic and velocity-changing collisions. A beam of YbF molecules is extracted through an aperture in the buffer gas cell and characterised using laser induced fluorescence detection. Peak fluxes of 1010 molecules per steradian per pulse, in the rotational and vibrational ground state, are obtained. The translational and rotational temperatures are in equilibrium with the cell temperature of 4 K. The forward velocity of the pulses can be varied between 130 m/s and 200 m/s by changing the buffer gas pressure. This source is an order of magnitude brighter and more than three times slower than a supersonic source of YbF molecules and provides an excellent starting point for improving the measurement of the electron’s electric dipole moment and for deceleration and trapping experiments. In order to reduce the helium load on the vacuum system and to shorten the molecular pulses, a second set-up, delivering the buffer gas into an open copper cylinder in pulses rather than in a continuous flow,is characterised and shows promising first results.

Sacrificial microchannel sealing by glass-frit reflow for Chip Scale Atomic Magnetometer

Abstract: This paper reports on a novel and simple packaging technique to enhance the atmosphere controllability of the microfabricated alkali vapor cell used for CSAMs (Chip Scale Atomic Magnetometers), in which sacrificial microchannels at the bonded interface are utilized as gas feedthrough for evacuation and filling, subsequently sealed by glass-frit reflow. By applying the proposed method, a 10 mm3 potassium vapor cell with 0.1 MPa helium buffer gas was fabricated. Sealing of the microchannels by glass-frit reflow were successfully demonstrated adopting the optimized parameters as 460–480 °C at bonding pressure of more than 250 kPa. The leak rate of less than 3.1×10−14 Pa·m3/s, which is required sealing quality for CSAM, was verified through a high resolution helium leak test.

Production of iodine atoms by RF discharge decomposition of CF3I

Abstract: Generation of atomic iodine by dissociation of CF3I in a RF discharge was studied experimentally in a configuration ready for direct use of the method in an oxygen?iodine laser. The discharge was ignited between coaxial electrodes with a radial distance of 3.5?mm in a flowing mixture of 0.1?0.9?mmol?s?1 of CF3I and 0.5?6?mmol?s?1 of buffer gas (Ar, He) at a pressure of 2?3?kPa. The discharge stability was improved by different approaches so that the discharge could be operated up to a RF source limit of 500?W without sparking. The gas leaving the discharge was injected into the subsonic or supersonic flow of N2 and the concentration of generated atomic iodine and gas temperature were measured downstream of the injection. An inhomogeneous distribution of the produced iodine atoms among the injector exit holes was observed, which was attributed to a different gas residence time corresponding to each hole. The dissociation fraction was better with pure argon as a diluting gas than in the mixture of Ar?He, although the variation in the Ar flow rate had no significant effect on CF3I dissociation. The dissociation fraction calculated from the atomic iodine concentration measured several centimetres downstream of the injection was in the range 7?30% when the absorbed electric energy ranged from 200 to 4000?J per 1?mmol of CF3I. The corresponding values of the fraction of power spent on the dissociation decreased from 8% to 2% and the energy cost for one iodine atom increased from 30 to 130?eV. Due to a possible high rate of the atomic iodine loss by recombination after leaving the discharge, these values were considered as lower limits of those achieved in the discharge.

Frequency dependence of amplifying parameters of a copper vapor laser using air as a buffer gas

Abstract: We use a pair of copper vapor lasers in an oscillator–amplifier configuration to investigate amplifying parameters such as the small signal gain and the saturation intensity versus the pulse repetition frequency when two different types of buffer gases are employed. We show that the values of these parameters are not the same if different gas mixtures are used in the gain medium. We show that the values of the parameters are estimated to be higher if a He–Ne buffer gas is used than in the case of air. The laser output power is relatively high and has fairly good stability at some special pulse repetition frequencies when air is used as a buffer gas.

Construction and Operation of a Cryogenic Source for Cold Polar Molecules

Abstract: This thesis reports on the combination of buffer-gas cooling with electric velocity filtering to produce a high-density guided beam of slow and internally cold polar molecules. In the continuous source, warm molecules are first cooled by collisions with a cryogenic helium buffer gas. Cold molecules are then extracted by means of an electric quadrupole guide. For formaldehyde the population of individual rotational states in the guide is monitored by depletion spectroscopy, resulting in single-state populations up to 82 %. This thesis also reports on a technique to produce pulses of slow guided molecules. The pulses exhibit a narrow velocity distribution around a tunable velocity.

Laser-Induced Plasma Ignition Experiments in a Scramjet Inlet

Abstract: An experimental investigation on the behavior of laser-induced plasma (LIP) ignition for scramjets has been conducted. Two different ignition methodologies are examined which are referred to as shear-layer-LIP-ignition and fuel-jet-LIP-ignition. A two-dimensional scramjet model with inlet injection, fueled with hydrogen gas, is used in the study. The experiments were conducted in the T-ADFA shock tunnel using a flow condition with a specific total enthalpy of 2.7 MJ/kg and a Mach 9 freestream. In the shear-layer-LIP-ignition experiment, LIP is formed in the shear-/mixing layers immediately downstream of four transversely orientated port hole injectors. In the fuel-jet-LIP-ignition experiment, LIP is formed inside the sonic throat of a single fuel injector. The influence of using fuel diluted with a plasma buffer gas (8% Ar, 92% H2) to extend plasma lifetimes is also investigated. The planar laser-induced fluorescence technique on the hydroxyl radical (OH-PLIF) is used to yield qualitative concentration images of the hydroxyl combustion species allowing us to compare the two different ignition methodologies in their effectiveness. Time-integrated schlieren images have been obtained and superimposed on the OH-PLIF intensity maps to determine the effect of the flow structure on the hydroxyl concentration. The broadband self-luminescence signal of the LIP in the early stages immediately after initiation is also recorded. Both ignition techniques are found to be effective in terms of post-LIP hydroxyl production. The fuel-jet-LIP-ignition technique allows lower laser energies to be used but requires a plasma buffer to compete with the shear-layer-LIP-ignition technique.