Ultraviolet Preionization Research Articles

A compact 4 GW pulse generator based on pulse forming network-Marx for high-power microwave application.

A compact PFN (pulse forming network)-Marx generator, which is based on mica capacitors, is constructed to generate intense electron beams for the purpose of high-power microwave generation. The key components of the generator, including a PFN-Marx, a trigger source, a multi-stage gas switch, and a load, are introduced in this paper. The PFN-Marx is composed of 19 PFN modules, which are formed by four mica capacitors in parallel. The temperature characteristics and pulse characteristics are tested in this study. The mica capacitor is insensitive to the temperature, and its lifetime is tens of thousands. Considering the influence of the ultraviolet light pre-ionization, gas gap switches are assembled in a nylon house. The trigger source, which is a mini-Marx generator based on the integration into the magnetic transformer and the magnetic switch, is used for increasing the stability of the switches. The generator has been tested on a water load and a relative magnetron. The experimental results show that this generator can deliver electrical pulses with the pulse width of 120ns and the amplitude of 400 kV on a 40-Ω water load at a repetition rate of 50Hz. In addition, utilizing the generator to drive a Ku band magnetron at a voltage level of 330 kV, a 500MW microwave pulse with the pulse width of 50ns at the frequency of 14.4GHz is obtained.

Gas dynamic regimes observed in dual-pulse laser ignition

A numerical study of the gas dynamics induced by a dual-pulse laser pre-ionization plasma used in laser ignition applications is presented herein. Past experimental observations have revealed important differences in the gas dynamics of the dual-pulse plasma, generated by overlapping a pair of ultraviolet (UV) and near-infrared (NIR) pulses, as compared to those of single-pulse (typically near-infrared) laser breakdown that has been more commonly studied. The simulation results reported here show that the dual-pulse pre-ionization scheme can lead to various gas dynamic regimes depending on the axial offset of the focal points of the two beams along the optical axis. If the UV and NIR pulses are perfectly overlapped (no offset) the energy deposition is uniform along the optical axis, leading to the formation of a toroidal structure in the post-discharge cooling phase. Alternatively, if the NIR energy addition pulse is focused with a small axial offset, (i.e., upstream or downstream of the UV pre-ionization pulse by ~0.5 mm) then an asymmetric torus forms that exhibits a third lobe which propagates away from the main kernel (to the side where the NIR was focused). Finally, if the two beams are focused with a larger offset of ~2.5 mm (weaker coupling), then another regime with a fourth lobe can arise. The four main flow regimes revealed from the model are in agreement with OH* experimental chemiluminescence images. These flow regimes influence the development of the early flame kernels, therefore playing an important role in practical laser ignition applications.

Collinear dual-pulse laser optical breakdown and energy deposition

A non-equilibrium model for laser-generated plasmas is used to represent collinear dual nano-second-pulse interactions. In the specific case considered, the breakdown is initiated with an ultraviolet (UV) laser pulse at 266 nm, which pre-ionizes the gas, and it is followed by a near-infrared (NIR) pulse at 1064 nm, which deposits significant energy into the ionized mixture. The model is validated against corresponding experiments, and simulation results are interrogated to understand key features of the plasma-kernel dynamics and the post-discharge hydrodynamics. The hydrodynamics of the non-equilibrium plasma is governed by the two-temperature Navier–Stokes equations accounting for both multiphoton ionization and inverse bremsstrahlung. The interaction between the laser beam and the plasma is modeled based on the radiative transfer equation. The temporal and spatial offsets of the two pulses can generate ionization kernels with different topology and dynamics. It is shown that the UV pre-ionization pulse can tailor the plasma region, leading to a larger ionized volume than would occur for a single-pulse breakdown, and it increases the efficiency of the energy deposition for the following NIR discharge. Vorticity in the early post-discharge phase is generated via baroclinic torque from the misalignment of the radial gradient of density (sudden gas expansion) and the strong pressure gradient (initiated by energy deposition from the laser beam). It is predicted that the collinear dual pulse can be designed to specify the plasma kernel size, shape, and maximum temperature in the breakdown phase, and the initial post-breakdown vorticity and plasma-core decay.

A three-stage blumlein-circuit to generate transversely excited atmospheric nitrogen laser by using three spark gaps

In this research constructed N2 laser system by use developed method of electric discharge. In this method used four step of electric discharge by using four capacitors, three spark gaps, high tension power supply varying in range from 12kV to 24 kV and three resistors, this method called three stage blumlein circuit. The breakdown time delay of these parallel spark gaps cement strong ultraviolet preionization in the laser channel, thus the result of these amendments the laser output is many doubled and is more increasing than that obtained using the one and two stage blumlein circuits. This system has been designed and operated to give pulse laser with wavelength at 337.1 nm. This laser system can operate without mirrors and optical resonator. The best result of energy was about (20.2 mJ) at electrode separation (3.5 mm) with flow rate (13 L/min) and applied voltage (24 kV). With this conditions of electric discharges the pulse duration was (6.65 ns) then the peak power was (3.04 MW), the efficiency of convert the electric energy to optical energy was (0. 93%) and The divergence of laser beam at optimal condition was (0.348 mrad). The results of fabricated system indicate the output characteristics can be significantly improved by improving the discharge characteristics.

A 490 W transversely excited atmospheric CO2 spark gap laser with added H2

In this paper we present a new design for a high pulse repetition rate transversely excited atmospheric CO2 laser with ultraviolet pre-ionization. A new method of fast thyristor capacitor charging and discharging by a spark gap is used. The effect of H2 gas addition on the output and stability of a transversely excited atmospheric laser operating with a basic mixture of CO2, N2 and He is investigated. The output power was increased by adding H2 to the gas mixture ratio of CO2:N2:He:H2 = 1:1:8:0.5 at total pressure of 850 mbar. An average power of 490 W at 110 Hz with 4.5 J per pulse was obtained. The laser efficiency was 11.2% and oxygen gas was used in the spark gap for electron capture to reduce the recovery time and increase the repetition rate.

Triggering breakdown voltage of a high-current, two-electrode graphite gas switch assisted by ultraviolet pre-ionization.

A simple ultraviolet pre-ionization device designed to extend the lifetime of a high current, two-electrode graphite gas switch is described in this paper. The effects of UV-illumination on decreasing the triggering breakdown voltage were studied in theory and experiments. The results showed that UV-illumination can provide extra initial electrons on the cathode surface and make more air molecules in the excited state, which resulted in positive effects in reducing the triggering breakdown voltage and jitter. However, the effects became weak as the pressure increased; the reason of this phenomenon was that the ultraviolet absorption rate of the air molecules increased with the rising pressure, leading to a reduction of the ultraviolet intensity on the discharge gap. The largest value of the triggering breakdown voltage decreased was 13.5 kV under 100 kPa, and the reliable operation gap distance can extend another 1.2 mm, which was equivalent to 500 shots in our experiments.

Control of Early Flame Kernel Growth by Multi-Wavelength Laser Pulses for Enhanced Ignition

The present contribution examines the impact of plasma dynamics and plasma-driven fluid dynamics on the flame growth of laser ignited mixtures and shows that a new dual-pulse scheme can be used to control the kernel formation process in ways that extend the lean ignition limit. We perform a comparative study between (conventional) single-pulse laser ignition (λ = 1064 nm) and a novel dual-pulse method based on combining an ultraviolet (UV) pre-ionization pulse (λ = 266 nm) with an overlapped near-infrared (NIR) energy addition pulse (λ = 1064 nm). We employ OH* chemiluminescence to visualize the evolution of the early flame kernel. For single-pulse laser ignition at lean conditions, the flame kernel separates through third lobe detachment, corresponding to high strain rates that extinguish the flame. In this work, we investigate the capabilities of the dual-pulse to control the plasma-driven fluid dynamics by adjusting the axial offset of the two focal points. In particular, we find there exists a beam waist offset whereby the resulting vorticity suppresses formation of the third lobe, consequently reducing flame stretch. With this approach, we demonstrate that the dual-pulse method enables reduced flame speeds (at early times), an extended lean limit, increased combustion efficiency, and decreased laser energy requirements.

Laser plasma formation assisted by ultraviolet pre-ionization

We present experimental and modeling studies of air pre-ionization using ultraviolet (UV) laser pulses and its effect on laser breakdown of an overlapped near-infrared (NIR) pulse. Experimental studies are conducted with a 266 nm beam (fourth harmonic of Nd:YAG) for UV pre-ionization and an overlapped 1064 nm NIR beam (fundamental of Nd:YAG), both having pulse duration of ∼10 ns. Results show that the UV beam produces a pre-ionized volume which assists in breakdown of the NIR beam, leading to reduction in NIR breakdown threshold by factor of >2. Numerical modeling is performed to examine the ionization and breakdown of both beams. The modeled breakdown threshold of the NIR, including assist by pre-ionization, is in reasonable agreement with the experimental results.

A small size 1–3 atm pulsed CO2 laser with series-connected spark gaps ultraviolet preionization

A small size pulsed CO2 laser with rounded edge flat profiled electrodes and variable gas pressure in 1-3 atm range has been constructed and characterized. The perionization system consists of a pair of 16 series-connected spark gaps located along either side of the main electrodes by which, the number of required preionization capacitors is reduced to 4. Sharpening of the main discharge has been performed using a 300 pF peaking capacitor. A maximum energy of 160 mJ/pulse was obtained for CO2:N2:He≡1:1:8 gas mixture and 33 kV discharge voltage. By increasing the laser gas pressure in 1-3 atm range, the duration of spike and tail parts of the laser pulses have been reduced from 110 ns and 4 μs to 37 ns and 1 μs, respectively.

A 300 Hz TEA CO2 Laser

We present a design and construction of a high pulse repetition rate (PRR) transversely excited atmospheric (TEA) CO2 laser with ultraviolet preionization. We use a new combination of spark and corona preionizations. Under semi-sealed off condition, we obtained 220 W at 300 Hz with 7.7% efficiency in a 735 mJ/pulse. The best results we have obtained in different experiments are a 1.07 J/pulse, 320 Hz PRR, 11 MW peak power, and 10.6% efficiency.

Experimental investigation on large-area dielectric barrier discharge in atmospheric nitrogen and air assisted by the ultraviolet lamp

In this paper, ultraviolet radiation produced by the ultraviolet lamp is employed to supply pre-ionization for the dielectric barrier discharge in N 2 or air at atmospheric pressure. The effect of the ultraviolet pre-ionization on improving the uniformity of the dielectric barrier discharge is investigated experimentally. The atmospheric pressure glow discharge of the large area (270 mm × 120 mm) is obtained successfully via the ultraviolet pre-ionization in atmospheric DBD in N 2 when the gas gap decrease to 3 mm. Based on the emission spectra, the mechanism which ultraviolet pre-ionization improves the uniformity of the dielectric barrier discharge is discussed.

Constraints on First‐Light Ionizing Sources from Optical Depth of the Cosmic Microwave Background

We examine the constraints on high-redshift star formation, ultraviolet and X-ray preionization, and the epoch of reionization at redshift zr, inferred from the recent WMAP-5 measurement, τe = 0.084 ± 0.016, of the electron-scattering optical depth of the cosmic microwave background (CMB). Half of this scattering can be accounted for by the optical depth, τe = 0.04-0.05, of a fully ionized intergalactic medium (IGM) at z ⩽ zGP ≈ 6-7, consistent with Gunn-Peterson absorption in neutral hydrogen. The required additional optical depth, Δ τe = 0.03 ± 0.02 at z > zGP, constrains the ionizing contributions of "first light" sources. WMAP-5 also measured a significant increase in small-scale power, which lowers the required efficiency of star formation and ionization from minihalos. Early massive stars (UV radiation) and black holes (X-rays) can produce a partially ionized IGM, adding to the residual electrons left from incomplete recombination. Inaccuracies in computing the ionization history, xe(z) , and degeneracies in cosmological parameters (Ωm, Ωb, σ8, ns) add systematic uncertainty to the measurement and modeling of τe. From the additional optical depth from sources at z > zGP, we limit the star formation efficiency, the rate of ionizing photon production for Population III and Population II stars, and the photon escape fraction, using standard histories of baryon collapse, minihalo star formation, and black hole X-ray preionization.

Nanosecond volume gas discharge in a flow with gasdynamic discontinuities

Experimental results concerning the interaction of pulsed volume ionization with the supersonic gas flow in a shock tube are described. The spatiotemporal and spectral characteristics of a nanosecond volume discharge plasma with ultraviolet preionization from plasma electrodes are presented. It is shown that the ionization region can be localized using gasdynamic discontinuities. The coincidence of the glow region with the discharge energy release region is discussed.

Experimental Study of Laser-Initiated Radiofrequency-Sustained High-Pressure Plasmas

Experiments are performed using 193-nm ultraviolet laser preionization of a seed gas in atmospheric pressure range argon and nitrogen to initiate a discharge that is sustained by 13.56-MHz radiofrequency (RF) power using efficient inductive wave coupling. High-density (4.5times10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">12</sup> /cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> line average density) large-volume (~500 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> ) 760-torr argon plasma is initiated and maintained for more than 400 ms with 2.2 kW of net RF power coupled to the plasma. Using the same technique, a 50-torr nitrogen plasma with line average electron density of 3.5times10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sup> /cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> is obtained. The nitrogen plasma volume of 1500 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> is initiated by the laser and maintained by a net RF power of 3.5 kW for 350 ms. Measurements of the time-varying plasma impedance and optimization of the RF matching for the transition from laser-initiated to RF-sustained plasma are carried out. Both laser-initiated plasmas provide much larger plasma volumes at lower RF power densities than can be obtained by RF alone. Millimeter wave interferometry is used to determine the electron density and the total electron-neutral collision frequency. A new diagnostic technique based on interferometry is developed to evaluate the electron temperature in high-pressure plasmas with inclusion of the neutral heating. Broadband plasma emission spectroscopy is used to illustrate the changes in the ionized species character immediately after the laser pulse and later during the RF pulse

Uniform glowlike plasma source assisted by preionization of spark in ambient air at atmospheric pressure

The ultraviolet radiation produced by spark discharges is employed to supply preionization for the dielectric barrier discharge in ambient air at atmospheric pressure. The effect of ultraviolet preionization and overvoltage on improving the uniformity of the dielectric barrier discharge is investigated experimentally. Based on the emission spectra and voltage-current wave forms, the optical and electrical characteristics of the discharge are discussed.

Measurements of gas preionization for plasma radiation sources

Azimuthally symmetric ultraviolet (UV) preionization of the outer periphery of a gas puff z pinch, prior to current initiation, may reduce the growth of magnetically driven Rayleigh–Taylor instabilities or other nonuniformities affecting the final implosion stage of the pinch, leading to an improvement in K-shell x-ray yield. We report on measurements of a flashover UV photoionization scheme, capable of ionizing the periphery of argon gas puffs to 1%–10% of the initial gas density. Measurements are made with a two-color, (1064 and 532 nm), high sensitivity (∼10−5λ), Mach–Zehnder type interferometer. Two methods of measuring preionization are investigated. The first uses a single laser wavelength, 1064 nm, to probe a chord of the cylindrical gas puff. The gas density is measured first without preionization and then with the UV flashover discharge. The difference in phase shift determines the free electron density. The second technique uses both wavelengths to simultaneously probe the same line of sight, determining neutral and free electron densities using only one gas puff event. Gas preionization may introduce refractive species that complicate the conversion of phase shift to density. However, single-color interferometry with the IR and green beams indicates that it is sufficient to consider only two species, neural argon and free electrons for times of interest.

Multiwave excimer lamps using XeF/XeCl/KrF/KrCl molecules

Results are presented of an investigation of the spectral and temporal characteristics of an electricdischarge excimer lamp emitting simultaneously on the 351 nm XeF, 308 nm XeCl, 249 nm KrF, and 222 nm KrCl transitions. The He/Kr/Xe/SF6/HCl working mixtures were excited in a transverse discharge with ultraviolet spark preionization at a total pressure of 25–100 kPa. In order to obtain the same brightness the concentration ratio [Kr]/[Xe] for the excimer molecular transitions was 8/0.8 kPa, and that for the halogen-containing molecules was [SF6]/[HCl][=[0.06–0.12]/[0.08–0.16] kPa. The duration of the radiation pulses for the excimer molecular transitions at atmospheric pressure was 100–200 ns and when the total mixture pressure was reduced to 250 kPa, this was doubled or trebled. The service life of the spontaneous radiation for B-X transitions in excimer molecules was 105 pulses.

Use of a transverse nanosecond discharge at the prebreakdown electron ionization multiplication stage for selective excitation of neon atoms

An investigation is made of the characteristics of a transverse nanosecond discharge in Ne/SF6 and He/Ne/SF6 mixtures at the prebreakdown electron ionization multiplication stage. The conditions needed to obtain a stable transverse discharge with ultraviolet spark preionization are studied as well as the spectral and temporal characteristics of the plasma radiation. It is shown that this transverse discharge burning regime may be promising for the selective excitation of neon atoms which may be used to develop an electric-discharge, λ=585.3 nm, Ne(3s–3p) plasma laser.

Characteristics of a submicrosecond transverse discharge in mixtures of helium with N2 and CO molecules and xenon atoms

An investigation is made of the electrical and optical characteristics of a moderate-pressure transverse discharge in typical active media of infrared CO lasers. The discharge was ignited in a system of unprofiled “ grid-plane” electrodes with automatic ultraviolet preionization by a corona discharge and had a 2×3 cm aperture. The pulse repetition frequency was 1–10 Hz, the current pulse length was ≤0.5 μs, and the pressure of the working mixtures was 10–100 kPa. Studies were made of the transverse discharge current voltage pulses, and also the spectral and temporal characteristics of the plasma radiation in N2:CO, He:N2:CO, and He: N2:CO:Xe mixtures in the ultraviolet and visible ranges.

An ultraviolet preionized nitrogen laser switched by parallel spark gaps

We present a simple method of switching a nitrogen laser with three parallel, self-breakdown spark gaps by incorporating them into a two-stage Blumlein circuit. These spark gaps are preionized by ultraviolet radiation from an auxiliary spark which suppresses their breakdown jitters and improves their temporal sychronization. The breakdown time delay of these parallel spark gaps enables strong ultraviolet preionization of the laser channel. As a result of these improvements, the laser output is doubled and is more reproducible than that obtained using the one-stage Blumlein circuit.