Multiple breakdown in helium generated by hybrid 10 μm Co2 laser pulses

In this paper we describe the experimental investigation of the post-breakdown development of a helium plasma generated by a hybrid 10 m CO₂ laser pulse. As a result we found that a long tail and a high energy of the CO₂ laser pulse cause multiple successive breakdown in the gas. We have measured the time to the first breakdown and the time between first and second breakdown as functions of gas pressure and laser pulse peak intensity as well as the spatial intensity distribution of the transmitted laser pulse. Among the relevant mechanisms self-focusing is not observed before the second breakdown.     

The effect of external visible light on the breakdown voltage of a long discharge tube

The breakdown characteristics of a discharge tube with a configuration typical of gas-discharge light sources and electric-discharge lasers (a so-called “long discharge tube”) filled with argon or helium at a pressure of 1 Torr have been investigated. A breakdown has been implemented using positive and negative voltage pulses with a linear leading edge having a slope dU/dt ~ 10–10⁷ V/s. Visible light from an external source (halogen incandescent lamp) is found to affect the breakdown characteristics. The dependences of the dynamic breakdown voltage of the tube on dU/dt and on the incident light intensity are measured. The breakdown voltage is found to decrease under irradiation of the high-voltage anode of the tube in a wide range of dU/dt. A dependence of the effect magnitude on the light intensity and spectrum is obtained. Possible physical mechanisms of this phenomenon are discussed.     

Influence of dissolved gas on optical breakdown and small-angle scattering of light in liquids

It is demonstrated that the small-angle laser beam scattering pattern in liquid depends greatly on the dissolved gas content: the intensity of the scattering steeply drops after degassing doubly distilled water. It was experimentally found that the parameters or small-angle scattering of light correlate strongly with those of optical breakdown in liquid. Namely, the fractal dimension and the gyration radius of scatterers in aqueous KCl solutions behave similar to the optical breakdown probability in those liquids versus salt concentration, while the optical breakdown cannot be induced after degassing. The effect might be associated with the presence of stable gas microbubbles in liquid.     

Photo Ionization of Excited Atoms in Gas-Filled Photodiodes: Improved Detectivity with Microsecond-Order Risetime

Photoionization of excited atoms and resonant absorption effects, which can decrease gas breakdown thresholds by high-power lasers at decreasing wavelengths, also improve low intensity detection sensitivities of gas-filled photodiodes when such devices are biased with dc fields close to gas breakdown. Spectral results indicate very promising potential sensitivity for detecting ultra-violet raidation. D* even in the visible is comparible to that of silicon photodiodes. The "relatively" large dc bias fields employed here improve speed of response significantly. Over moderate (10 -7 - 10-8W) received signal powers response is non-linear with ? varying inversely with wavelength.     

多间隙气体开关光纤诊断

介绍了一套用于多间隙气体开关光学诊断的光纤探测系统,该系统主要由光纤阵列探测器和沉入式日盲型光纤探测器构成。用该系统研究了多间隙气体开关的详细击穿过程,并分析触发电压、气压及紫外光预电离对开关性能的影响。光纤阵列探测器定时精度小于0.6 ns,脉冲响应3.2 ns;日盲型光纤探测器光谱响应为260~320 nm,通过实验可知,气体开关击穿次序为:3#-2#-1#-4#-5#-6#,统计时延和过压击穿时延占主要部分;提高开关的触发电压、降低开关的工作气压,主要影响开关击穿过程的前半部分,从而降低开关的击穿抖动;如果能够产生强度稳定的紫外光,可以改善开关的抖动性能。     

Measurement of laser-induced breakdown threshold intensities of high-pressure gasses and water droplets to determine the number density of an aerosol

The laser-induced breakdown threshold intensities of high-pressure gasses as a function of pressure were measured. The breakdown threshold intensity of N₂ gas was found to be much higher than those of He and Ar gasses at a gas pressure of 2.0 MPa. It was also 7 to 11 times higher than that of aerosol droplets, and is sufficiently high to allow the number density of droplets to be measured in a high-pressure aerosol by laser-induced breakdown.     

Investigation of the threshold intensity versus gas pressure in the breakdown of helium by 248 nm laser radiation

An investigation of the unexpectedly strong dependence of the threshold intensity on the gas pressure in the experimental study on the breakdown of He by short laser wavelength (Turcu et al., in Opt Commun, 134:66–68, 1997) is presented. A modified electron cascade model is applied (Evans and Gamal, in J Phys D Appl Phys, 13:1447–1458, 1980). Computations revealed reasonable agreement between the calculated thresholds and the measured ones. Moreover, the calculated electron energy distribution function and its parameters proved that multiphoton ionization of ground and excited atoms is the main source for the seed electrons, which contributes to the breakdown of helium. The effect of diffusion losses over pressures <1,000 Torr elucidated the origin of the strong dependence of the threshold intensity on the gas pressure. Collisional ionization dominates only at high pressures. No evidence for recombination losses is observed for pressures up to 3,000 Torr.     

Size measurement of nanoparticles using the emission intensity distribution of laser-induced plasma

A two-dimensional optical imaging method is presented for monitoring the laser-induced breakdown events of aqueous nanoparticles. The method is based on measuring the light intensity distribution of plasma from spatially resolved breakdown events. The number of laser breakdown events as a function of the emission intensity follows the Gaussian distribution and its full-width at a half-maximum appears in direct correlation with the particle size. Hence the particle size can be determined by measuring the plasma emission intensity distribution. Calibration of the method is carried out with reference polystyrene particles dispersed in water. Application is demonstrated for measuring bentonite colloidal particles of different sizes in groundwater. PACS 42.62.Eh; 52.25.Rv; 52.70.Nc     

A study of balancing the competing effects of ultrashort laser induced plasma for optimal laser machining

We report that plasma generated during processing of materials with ultrashort pulse lasers and the associated high intensity optical beam have both favourable and unfavourable impact on the machined surface quality. Intensity of the optical beam propagating through ambient air medium enhanced further by self-focusing is sufficiently high to cause gas breakdown forming air plasma. The generated plasma reduces the effect of self-focusing but also distorts the beam profile. Duration of the pulse being too short for thermal equilibrium to establish, ablation occurs largely by direct removal of the material forming another plasma plume. Normally, the scattering effect of plasma results in distortions of the fabricated features. However, for certain parameter ranges, the competing self-focusing and gas plasma plumes supplemented with the material plasma can combine to cause filamentation, eliminating the distortions. Filament of hot plasma also acts as a well-shaped energy source. In the present study, brass is taken as an example for the investigation. Experiments were conducted to capture the spectrum of the light scattered by plasma using a spectrometer. Analysis was done to estimate the material plasma. Theoretical calculation on the intensity distribution in an optical beam propagating through air was then followed for a range of parameter values taking the self-focusing effect of the medium and the impact of the plasma generated by its breakdown. Approximate values of the machining parameters for clean fabrication are deduced from the calculations, which were used to conduct a laser machining test on brass.     

Acoustic emission monitoring during laser shock cleaning of silicon wafers

A laser shock cleaning is a new dry cleaning methodology for the effective removal of submicron sized particles from solid surfaces. This technique uses a plasma shock wave produced by laser-induced air breakdown, which has applied to remove nano-scale silica particles from silicon wafer surfaces in this work. In order to characterize the laser shock cleaning process, acoustic waves generated during the shock process are measured in real time by a wide-band microphone and analyzed in the change of process parameters such as laser power density and gas species. It was found that the acoustic intensity is closely correlated with the shock wave intensity. From acoustic analysis, it is seen that acoustic intensity became stronger as incident laser power density increased. In addition, Ar gas has been found to be more effective to enhance the acoustic intensity, which allows higher cleaning performance compared with air or N₂ gas.     

Radiation breakdown in silicon wafers

Radiation breakdown in silicon slabs is observed and studied as revealed in anomalous behavior of the dose characteristics of their radiation defects when the radiative intensity is varied. A theory is constructed for reversible radiation breakdown due to the bistability which develops in a gas of radiation vacancies when the gas can be regarded as quasi-two-dimensional. In order to explain the exponential saturation of the dose characteristics as the irradiation intensity is increased, scenarios are proposed in which different forms of the constituent radiation defects develop. Some parameters of the bistable gas of primary vacancies are estimated, including diffusion coefficients, dimensions of inhomogeneity regions, and the rate of movement of the stratification line. On the whole, satisfactory agreement with experiment is obtained. Discrepancies between the diffusion coefficient for neutral vacancies obtained here and in the literature are attributed to the role of interband recombination accompanying radiation defect formation during electron bombardment. Zh. éksp. Teor. Fiz. 114, 1067–1078 (September 1998)     

New features of laser-induced gas breakdown in the ultraviolet

A study of gas breakdown induced by the ultraviolet second harmonic of ruby laser radiation has revealed an anomalously high threshold intensity for hydrogen. In addition, it has been demonstrated for the first time that the focal volume dependence of the breakdown threshold, of a number of molecular and noble gases, is influenced by the frequency of the incident radiation.     

横向表面放电光泵浦源特性研究

 介绍了一种用以泵浦XeF(C-A)激光的横向表面放电辐射源,比较详细地研究了这种泵浦源的放电击穿特性、放电电流与充电电压及不同气体介质的关系、表面放电均匀性以及不同气体成分对表面放电辐射特性的影响。得到了放电击穿时间、放电峰值电流随充电电压、不同气体介质变化的曲线;分析了提高放电均匀性的途径,在电极长50cm、间距6cm、充电电压25kV条件下获得了均匀放电。获得了各种实验条件下放电辐射的光谱曲线;通过对辐射光谱的分析,研究了有利于光解离XeF₂的最佳实验条件,当p_Ar:p_N2=1:1时,放电在远紫外波段产生的辐射最强。     

气体压强对纳秒激光诱导空气等离子体特性的影响

环境气体的压强对激光诱导等离子体特性有重要影响.基于发射光谱法开展了气体压强对纳秒激光诱导空气等离子体特性影响的研究,探讨了气体压强对空气等离子体发射光谱强度、电子温度和电子密度的影响.实验结果表明,在10-100 kPa空气压强条件下,空气等离子体发射光谱中的线状光谱和连续光谱依赖于气体压强变化,且原子谱线和离子谱线强度随气体压强的变化有明显差别.随着空气压强增大,激光击穿作用区域的空气密度增加,造成激光诱导击穿空气几率升高,从而等离子体辐射光谱强度增大.空气等离子体膨胀区域空气的约束作用,增加了等离子体内粒子间的碰撞几率以及能量交换几率,并且使离子-电子-原子的三体复合几率增加,因此造成原子谱线OⅠ777.2 nm与NⅠ821.6 nm谱线强度随着气体压强增大而增大,在80 kPa时谱线强度最高,随后谱线强度缓慢降低.而离子谱线N Ⅱ 500.5 nm谱线强度在40 kPa时达到最大值,气体压强大于40 kPa后,谱线强度随压强增加而逐渐降低.空气等离子体电子密度均随压强升高而增大,在80 kPa后增长速度变缓.等离子体电子温度在30 kPa时达到最大值,气体压强大于30 kPa后,等离子体电子温度逐渐降低.研究结果可为不同海拔高度的激光诱导空气等离子体特性的研究提供重要实验基础,为今后激光大气传输、大气组成分析提供重要的技术支持.     

Exploration of the Townsend regime by discharge light emission in a gas discharge device

The Townsend discharge mechanism has been explored in a planar microelectronic gas discharge device(MGDD) with different applied voltages U and interelectrode distance d under various pressures in air. The anode and the cathode of the MGDD are formed by a transparent SnO2 covered glass and a GaAs semiconductor, respectively. In the experiments, the discharge is found to be unstable just below the breakdown voltage Ub, whereas the discharge passes through a homogeneous stable Townsend mode beyond the breakdown voltage. The measurements are made by an electrical circuit and a CCD camera by recording the currents and light emission(LE) intensities. The intensity profiles, which are converted from the 3D light emission images along the semiconductor diameter, have been analysed for different system parameters. Different instantaneous conductivity σtregimes are found below and beyond the Townsend region. These regimes govern the current and spatio-temporal LE stabilities in the plasma system. It has been proven that the stable LE region increases up to 550 Torr as a function of pressure for small d. If the active area of the semiconductor becomes larger and the interlectrode distance d becomes smaller, the stable LE region stays nearly constant with pressure.     

Spark Breakdown - An Electrostatic Criterion

This paper examines one basic process causing the d. c. high voltage breakdown of a gas. Specifically, it contains a modified criterion for the transition of a Townsend Avalanche to a Streamer Discharge. Instead of the assumption that the electrons in the avalanche are spread laterally by multiple scattering on gas molecules (thermal diffusion), the assumption used is that the electrons are spread solely by mutual electrostatic repulsion. A criterion is found that yields quantitative results comparable to those obtained using the "multiple scattering" assumption. This modified criterion is probably most useful where "multiple scattering" is improbable - where an electron loses almost all of its energy on colliding with complex gas molecules. In addition, the classical (thermal-diffusion-dominated) breakdown criterion is re-derived in a much simpler form.     

Reversibler Gleichfeld- und Mikrowellen-Durchschlag in n-Typ-Germanium bei 4,2 °K

The microwave induced breakdown characteristic inn-type germanium at 4.2 °K has been observed and compared with the d.c. induced breakdown characteristic obtained from the same sample. The effective microwave breakdown field intensity is nearly equal to the field intensity observed in d.c. induced breakdown. However, in the breakdown region with conductivities greater than 0.1 ohm^?1 cm^?1 a relaxation effect was found and interpreted qualitatively as momentun relaxation. In the initial breakdown the relaxation time τ _m is small,ω ²τ _m ² being ?1 whereω/2π=9·10⁹s^?1 is the microwave frequency. The relaxation time is determined by predominant neutral impurity scattering with at last 5·10¹⁴ impurities per cm³. This scattering mechanism becomes ineffective when the impurities are ionized by hot carriers. Ionized impurity scattering or acoustic phonon scattering will then be predominant with increased and energy dependent values of τ _m . The increased phase shift between carriers and field causes a decreased energy transfer from the field to the carriers, an accordingly smaller ionization rate, and finally results in a nearly constant a.c. conductivity. The observed anisotropy of the breakdown field intensity is in qualitative agreement with the assumption that only carriers in “hot” valleys of the conduction band initiate the breakdown by impact ionization. The unsufficient quantitative agreement may be due to an inhomogeneity of doping which is suggested by comparing the values of the ohmic low temperature conductivity of the samples.     

Modeling of breakdown of a gas by electrons of the boundary layer during irradiation of metal targets by picosecond laser pulses

Results are presented from a numerical simulation of the breakdown of a dense inert gas by boundary-layer electrons upon irradiation of a metal target by high-power picosecond laser pulses. It is shown that taking the electric field of the boundary layer into account leads to a substantial increase in the concentration of the seed electrons near the target surface, which accelerates the ionization process. The dependence of the breakdown time on the electric field intensity of the incident wave and the concentration of the gas atoms is derived. Zh. Tekh. Fiz. 68, 48–53 (August 1998)     

The electrical breakdown of gases in the presence of crossed electric and magnetic fields

The breakdown characteristics of a gas in the presence of crossed electric and magnetic fields are discussed in terms of the Townsend ionization coefficients. The “equivalent pressure” concept is used to assess the effect of a transverse magnetic field on the first Townsend coefficient and the objections which have been raised to the application of this approach to breakdown potentials are shown to be removed by a consideration of the dependence of the second Townsend coefficient upon electric and magnetic field strengths.     

Der Einfluß metastabiler Atome auf den statischen Durchschlag in Argon

A gas discharge in argon is initiated by an ultraviolet light pulse which releases 10⁵ electrons at the cathode within 20 nsec. The time of built-up for the static breakdown is found to be in the order of msec. The oscillogram of the current near breakdown shows a fast component consisting of several successive avalanches caused by secondary electrons liberated by ions at the cathode and a slow component which is due to the electrons liberated by metastable atoms at the cathode. The second Townsend coefficient for argon ions and a nickel cathode is determined to beγ ₊= 2.5·10^?3. The amount of electrons liberated by metastables at static breakdown is 25% of the total number of secondary electrons. The mean life-time of the metastables at 1,1 Torr and an electrode separation of 10 mm is found to be 2.2 msec, which is mainly due to de-exciting collisions at the electrodes.