Temperature and density spectroscopic measurements in different laser-generated plasmas

A pulsed Nd:Yag laser, at intensities of the order of 10¹⁰ W/cm², is employed to irradiate different thick metallic targets (Ti, Fe, Ag, and Ni) placed in vacuum. The obtained non-equilibrium plasmas are investigated with various analytical techniques. An electrostatic ion energy analyzer and different ion collectors are employed to monitor in situ the ions ejected from the plasma and to determine the core plasma temperature, the ion energy distributions and the ion angular emission. An optical spectrometer is employed to analyze the plasma corona emitted light vs. wavelength and to identify the emitted characteristic lines. The optical spectroscopy permitted to evaluate the electron temperatures and densities. Results show strong temperature and density gradients occurring in the laser-generated plasma plume.     

Volume effect of laser produced plasma on X-ray emissions

An investigation of x-ray emission from Cu plasma produced by 1.054 μm Nd:glass laser pulses of 5 ns duration, at 2 × 10¹² − 2 × 10¹³ W cm⁻² is reported. The x-ray emission has been studied as a function of target position with respect to the laser beam focus position. It has been observed that x-ray emissions from ns duration plasma show a volume effect similar to subnanosecond plasmas. Due to this effect the x-ray yield increases when target is moved away relative to the best focal plane of the laser beam. This result supports the theoretical model of Tallents and has also been testified independently using suitably modified theoretical model for our experimental conditions. While above result is in good agreement with similar experimental results obtained for sub-nanosecond laser produced plasmas, it differs from result claiming filamentation rather than pure geometrical effect leading to x-ray enhancement for ns plasmas.     

吸积盘的X射线辐射对周围星际物质的离化研究

利用辐射流体力学程序对三倍频纳秒激光与靶物质相互作用进行了模拟研究，得到了可以产生黑体辐射谱分布的激光等离子体X射线辐射靶的最佳厚度；数值模拟研究了黑体谱分布的X射线辐射场对等离子体系统平均离化度分布的影响，它有助于深入理解天体物理中吸积盘对它周围星际物质的离化影响. 关键词： 辐射流体力学 激光等离子体 X射线辐射 吸积盘 离化     

Investigations on laser plasma soft X-ray sources generated with low energy laser systems

Spectra of laser-induced plasmas at low laser energies and intensities (around 100 mJ and 10¹⁰ W cm^–2 respectively) have been recorded in the spectral range of 20 to 100 nm for different target materials, laser intensities at the target and laser wavelengths. For heavy target materials, a broadband spectrum with a spectral maximum typically around 30 nm is obtained. This broadband radiation source is well suited for photoionization processes and the generation of short wavelength inner-shell photoionization lasers. For the cadmium-photoionization laser, the influence of different soft X-ray spectra on the laser energy has been investigated. The potential of laser plasma soft X-ray sources for scientific and technical applications is briefly reviewed.Dedicated to Prof. Dr. Herbert Welling on the occasion of his 60th birthday     

Energy analysis of protons emitted from Nd:YAg laser-generated plasmas

Hydrogenated targets have been irradiated in vacuum with the pulsed Nd:YAg laser at intensities of the order of 10¹⁰ W/cm². The laser-generated plasma, produced by the interaction with the solid, emits protons and other ions along the normal to the target surface. Ion collectors and ion energy analyzer were used to measure the current, the angular emission and the energy distributions of the emitted protons. Time-of-flight measurements, Coulomb–Boltzmann-distributions and the fits of experimental data were also used in order to evaluate the equivalent ion plasma temperature and the ion acceleration developed in the non-equilibrium-pulsed plasma.     

Temperature Distributions Across the Plasma Layer of Planar Low Pressure Microwave Plasmas — A Comparative Investigation by Optical Emission Spectroscopy

Nowadays low temperature non-equilibrium plasmas received considerable attention in very different fields of plasma processing. The subject of the present paper is the comparative measurement of neutral gas temperature and optical excitation temperature to analyze the temperature distributions across the plasma layer of H₂ non-equilibrium plasmas (p = 0.2 – 1.5 kPa) with small admixtures of hydrocarbons in a novel planar microwave plasma source (2.45 GHz) used for plasmachemical deposition purposes by means of optical emission spectroscopy. Typical microwave power flux densities into the plasma lie within a range of 2 W cm^?2 to 20 W cm^?2. Results of neutral gas temperature measurements derived from H_α line Doppler profiles are compared with rotational temperatures of H₂ and N₂ molecules. The neutral gas temperature (800–1700 K) corresponds to the rotational temperature of the H₂ molecules (Fulcher band, R 0–0 branch) but shows a more distinct spatial gradient. The rotational temperature of admixtured N₂ molecules (2000–3000 K) is much more higher although Boltzmann distribution was ensured. The spatially resolved measured excitation temperature (1–3 eV) determined with the help of line intensity ratios of admixtured Ar well agrees with Langmuir probe measurements. The reported measurements as a whole demonstrate the feasibility of comparative investigations of different optically determined temperatures for expressive characterization of low pressure microwave plasmas.     

Neutrals' temperature in laser-generated plasma at LNS

A Nd–YAG laser operating at 532 nm with a maximum intensity of 10¹⁰ W/cm² was used to ablate aluminium and tantalum targets placed in vacuum.

A mass quadrupole spectrometer (MQS) at high sensitivity, operating in the range of 1–300 amu, with a resolution better than 1 amu, was used to analyse the atomic emission produced by the laser ablation. The neutrals' emission produced by laser-generated plasma at INFN-LNS was investigated in terms of temperature, ablation yield, angular distribution and velocity.

The neutrals' detection through the MQS permitted to measure the mass energy distribution. Results demonstrate that the maximum temperatures of the neutral species are of the order of 100 eV. The angular emission of neutrals is peaked along the normal to the target surface, as it was observed for the ions; the ablation yield increases suddenly at low laser intensity and decreases at high laser intensity, owing to the higher ionization processes; the flow velocity follows the adiabatic expansion of the plasma in vacuum and it is of the order of 10⁴ m/s.

Measurements will be presented and discussed, according to the available models.     

Advanced targets,diagnostics and applications of laser-generated plasmas

High-intensity sub-nanosecond-pulsed lasers irradiating thin targets in vacuum permit generation of electrons and ion acceleration and high photon yield emission in non-equilibrium plasmas. At intensities higher than 10¹⁵?W/cm² thin foils can be irradiated in the target-normal sheath acceleration regime driving ion acceleration in the forward direction above 1?MeV per charge state. The distributions of emitted ions in terms of energy, charge state and angular emission are controlled by laser parameters, irradiation conditions, target geometry and composition. Advanced targets can be employed to increase the laser absorption in thin foils and to enhance the energy and the yield of the ion acceleration process. Semiconductor detectors, Thomson parabola spectrometer and streak camera can be employed as online plasma diagnostics to monitor the plasma parameters, shot by shot. Some applications in the field of the multiple ion implantation, hadrontherapy and nuclear physics are reported.     

X-Rays emission by high-intensity pulsed lasers irradiating thin foils at PALS laboratory

X-rays and forward ion emission from laser-generated plasma in the Target Normal Sheath Acceleration regime of different targets with 10-μm thickness, irradiated at Prague Asterix Laser System (PALS) laboratory at about 10¹⁶ W/cm² intensity, employing a 1,315 nm-wavelength laser with a 300-ps pulse duration, are investigated. The photon and ion emissions were mainly measured using Silicon Carbide (SiC) detectors in time-of-flight configuration and X-ray streak camera imaging. The results show that the maximum proton acceleration value and the X-ray emission yield growth are proportional to the atomic number of the irradiated targets. The X-ray emission is not isotropic, with energies increasing from 1 keV for light atomic targets to about 2.5 keV for heavy atomic targets. The laser focal position significantly influences the X-ray emission from light and heavy irradiated targets, indicating the possible induction of self-focusing effects when the laser beam is focalized in front of the light target surface and of electron density enhancement for focalization inside the target.     

Protons emission from thin doped polymeric films using pulsed laser

Protons production and acceleration via laser-generated plasma from thin Fe₂O₃ and carbon nano-tubes doped polyethylene films are investigated at relatively low laser pulse intensity, of the order of 10¹⁰ W/cm². Time-of-flight technique is employed in order to measure the proton energy and the relative yield with respect to the carbon one. Two ion collectors are used in opposite directions to detect the proton energy and yield both in backward and forward directions, normally to the irradiated target surface, as a function of the thin target doping concentration. The comparison between the results obtained with thin films doped with two nano-particle species is presented and discussed, with a special regard to the high proton emission.     

Study of the plasmas produced during the deposition of TiC/SiC thin films in a hybrid magnetron-laser system

Time- and spatially-resolved optical emission spectroscopy was performed to characterize the plasma produced in a hybrid magnetron-sputtering-laser deposition system, which is used for TiC or SiC thin films preparation. A graphite target was ablated by a KrF excimer laser (λ=248 nm,τ=20 ns) and either Ti or Si targets were used for DC magnetron sputtering in argon ambient. Spectra were measured in the range 250–850 nm. The evolution of the spectra with varying magnetron powers (0–100 W) and argon pressures (0.3–10 Pa) was studied. Spectra of the plasmas produced by a) the magnetron alone, b) the ablation laser alone, and c) the magnetron and the ablation laser together, were recorded. Spectra (a) were dominated by Ar atoms and Ar⁺ ions. Emission lines of Ti and Si were detected, when Ti target and Si target was used, respectively. Spectra (b) revealed emission of C, C⁺, C₂, Ar, Ar⁺. Spectra (c) showed presence of all previously mentioned species and further of Ti⁺ ions emission was detected. The research was supported by Grant Agency of the Czech Republic No. 202/06/02161, GA ASCR project number A1010110/01 and Institutional Research Plan AV CR No. AV0Z 10100522.     

Mechanism of generation of high-intensity terahetrz radiation under the action of high-power laser pulsed on a target

The mechanism of generation of terahertz radiation upon irradiation of a target by short (∼ 0.1 ns) high-intensity laser pulses (I ∼ 10¹⁸−10¹⁹ W cm⁻²) is investigated by numerical simulation using the relativistic electromagnetic PIC code. The interaction of such a pulse with the target, a plasma is formed on it. Electrons emitted from the plasma form a virtual cathode whose oscillations are determined not only by their self-field, but also by the field of ions of the plasma. Generation occurs in the terahertz frequency range with the efficiency thrice as high as in the absence of ions (i.e., with traditional reditron generation mechanism). The explanation for this effect is also given.     

Temperature measurements in plasmas generated by using lasers at different intensities

The temperature of laser-generated pulsed plasmas is an important property that depends on many parameters, such as the particle species and the time elapsed from the laser interaction with the matter and the surface characteristics.

Laser-generated plasmas with low intensity (<10¹⁰ W/cm²) at INFN-LNS of Catania and with high intensity (>10¹⁴ W/cm²) in PALS laboratory in Prague have been investigated in terms of temperatures relative to ions, electrons, and neutral species. Time-of-flight (ToF) measurements have been performed with an electrostatic ion energy analyzer (IEA) and with different Faraday cups, in order to measure the ion and electron average velocities. The IEA was also used to measure the ion energy, the ion charge state, and the ion energy distribution.

The Maxwell–Boltzmann function permitted to fit the experimental data and to extrapolate the ion temperature of the plasma core.

The velocity of the neutrals was measured with a special mass quadrupole spectrometer. The Nd:Yag laser operating at low intensity produced an ion temperature core of the order of 400 eV and a neutral temperature of the order of 100 eV for many ablated materials. The ToF of electrons indicates the presence of hot electron emission with an energy of ～1 keV.     

Retrieval of currents of multiply charged ions emitted from laser-produced carbon plasma

The emission of ions from laser-produced carbon plasmas is investigated by a deconvolution of ion collector signals. The deconvolution is based on the use of Kelly and Dreyfus function expressing the time-resolved ion current to recover hidden peaks in an ion collector signal. The parameters of recovered C ^q+ (1?≤?q?≤?6) currents make possible the quantification of properties of laser-produced plasmas. The drift and peak velocities of C ^q+ ions, the abundance of ions and the plasma temperature are presented in the dependence on focused laser beam energy. The carbon plasma was generated employing either single 9-ns pulses of second harmonics (532 nm) of Nd:YAG laser or pulses repeated at a stable repetition rate of 30 Hz.     

Comparative Investigation on the Excitation of Copper Emission Lines Between Argon and Helium Spark Discharge Plasmas

The emission intensities and the signal‐to‐background ratios (SBRs) of copper emission lines in the wavelength range 200–360 nm were observed from a medium‐voltage spark discharge plasma when argon or helium was employed as the surrounding gas. The observed copper spectra comprised Cu(I) lines having excitation energy of 3.8–9.3 eV, and Cu(II) lines assigned to three different transitions: 3d ⁸4p–3d ⁸4s transition (excitation energy of 8.2–9.2 eV), 3d ⁸5s–3d ⁸4p transition (13.4–13.6 eV), and the 3d ⁸4d–3d ⁸4p transition (14.2–14.8 eV). The Cu(I) lines have much smaller intensities in the helium plasma compared with the argon plasma, whereas the Cu(II) lines have similar intensities between both plasmas. The SBRs of some ionic copper lines are larger in the helium plasma compared with the argon plasma. Therefore, when an ionic line has to be measured in the analytical applications, the helium plasma should be recommended.     

神光Ⅲ激光装置直接驱动内爆靶产生的连续谱X光源

激光驱动的内爆靶通过轫致辐射过程可以产生覆盖1—100 keV能区的小尺寸、短脉冲和高亮度的光滑连续谱X光源,可用于高密度等离子体的点投影照相和吸收谱诊断等.本文对30—180 k J输出能量的神光Ⅲ激光装置直接驱动氘氚冷冻靶产生的连续谱X光源辐射特性进行了模拟研究,为优化内爆光源提供物理基础.采用了美国OMEGA激光装置和美国国家点火装置(NIF)使用的定标率来给出不同驱动能量时的靶参数和激光脉冲参数.研究发现,内爆靶丸在停滞阶段瞬时的密度和温度剧增可以产生尺寸约100μm、发光时间约150 ps的X光脉冲;X光辐射主要产生于被压缩的氘氚冰壳层内侧、而不是中心的高温气体热斑区;等离子体的自吸收可以显著降低1—3 keV的较低能区的X光发射,但对更高能区没有影响;X光辐射主要集中在30 keV的较低能区,氘氚聚变反应可以增强30 keV的硬X光辐射、但对30 keV的较软的X光辐射没有明显贡献.     

Diagnostik des Xenon-Plasmas bei hoher Leistung

Several experimental methods for temperature measurements are discussed with respect to their qualities as diagnostic tool for xenon plasmas in the pulsed mode. In this work the continuum radiation intensity corrected for infinite optical thickness is used for the determination of plasma temperature. An uncertainty δT = 1,5% follows from a detailed error calculation. Results are given for p = 6 ?20 at and i = 1 ?3,5 kA/cm². It is proposed to use a xenon pulse discharge as an calibaration standard for black body radiation at T ≈ 11000 ºK in a special pressure range where the plasma temperature is nearly independent on pressure.     

Unipolarbögen

Unipolar arcs have achieved considerable importance as a plentiful source of contaminations in tokamak plasmas. A definition of unipolar and plasma induced discharges is given, and the main properties of the space charge sheath between plasma and wall as well as the existence conditions of unipolar arcs (necessary voltage drop, current balance) are considered. From a simple model that includes the superposition of two completely different plasmas — the tokamak plasma and the cathode spot plasma of the arc — the radial dependencies of the current densities and voltages between plasma and wall and of the plasma parameters are derived. It is shown, that — in typical cases — the anode region is a ring-shaped area situated at distances from the arc cathode spot r > b with 0.3 cm ? b ? 1 cm, and that the density decrease of the arc plasma n ～ r^?μ necessitates an exponent 2 ? μ ? 3 because of the current balance condition (arc current = backflow current). Moreover some thermodynamic aspects (unipolar arcs as a kind of dissipative structures), the ignition problem, the effects of magnetic fields, the problems of diagnostics and the possibilities of simulations are shortly discussed.     

Radiation of the Plasma of a Transverse Discharge in a Helium–Krypton–Elegas Mixture

We carried out a spectroscopic investigation of the degradation of the active medium of a pulsed-periodic KrF emitter based on a He/Kr/SF₆ mixture (P = 10–150 kPa) with pumping by a transverse volumetric discharge. The plasma radiation spectra in the range 200–620 nm at different stages of degradation of the working mixture and the dynamics of the radiation of inert gases as well as of the products of decomposition of SF₆ molecules in the plasma are studied. It is shown that since the number of discharge pulses is 10⁴, rather effective formation of excited sulfur molecules is observed which decompose with emission in the spectral range 260–550 nm. This can be employed for developing a wideband lamp based on the system of KrF(B–X; D–X), S₂(B–X), and S₂(f–a) bands.     

The Visible Intensified Cameras for Plasma Imaging in the TJ‐II Stellarator

Visible cameras are widely used in fusion experiments for diagnosis and for machine safety issues. They are generally used to monitor the plasma emission, but are also sensible to surface Blackbody radiation and Bremsstrahlung. Fast or high speed cameras capable of operating in the 10⁵ frames per second speed range are today commercially available and offer the opportunity to plasma fusion researchers of two‐dimensional (2D) imaging of fast phenomena such as turbulence, ELMs, disruptions, dust, etc. The tracking of these fast phenomena requires short exposure times down to the μ s range and the light intensity can be often near the signal to noise ratio limit especially in low plasma emission regions such as the far SOL Additionally, when using interference filters to monitor, e.g. impurity line emission, the photon flux is strongly reduced and the emission cannot be imaged at high speed. Therefore, the use of image intensifiers that amplify the light intensity onto the camera sensor can be of great help. The present work describes the use of intensifiers in the visible fast cameras of TJ‐II stellarator. We have achieved spectroscopic plasma imaging of filtered impurity atomic line emission at short exposure times down to the 10 μ s range depending on atomic line and concentration. Additionally, plasma movies at velocities of 2x10⁵ frames per second near the camera operation limit can be recorded with exposure times well below 1 μ s with sufficient signal to noise ratio. Although an increasing degradation of the image quality appears when raising the light amplification, an effective gain of up to two orders of magnitude of the light intensity is feasible for many applications (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)