Extreme ultraviolet interferometry of warm dense matter in laser plasmas

We demonstrate that interferometric probing with extreme ultraviolet (EUV) laser light enables determination of the degree of ionization of the "warm dense matter" produced between the critical and ablation surfaces of laser plasmas. Interferometry has been utilized to measure both transmission and phase information for an EUV laser beam at the photon energy of 58.5 eV, probing longitudinally through laser-irradiated plastic (parylene-N) targets (thickness 350 nm) irradiated by a 300 ps duration pulse of wavelength 438 nm and peak irradiance 10(12) W cm(-2). The transmission of the EUV probe beam provides a measure of the rate of target ablation, as ablated plasma becomes close to transparent when the photon energy is less than the ionization energy of the predominant ion species. We show that refractive indices η below the solid parylene N (η(solid) = 0.946) and expected plasma values are produced in the warm dense plasma created by laser irradiation due to bound-free absorption in C(+).     

The Role of Carbonaceous Particles in Low Current Arc Duration Enhancment. II. Arcs Occurring on Approach of Electrodes

Part per million concentrations of some organic vapors in telephone relay environments are known to produce carbonaceous particles on arcing contact surfaces. Enhanced arcing then occurs and the reliability and contact lifetime of the relays is impaired. The enhancement of break arcs due to the presence of carbonaceous particles has been described in paper I. Break Arcs.6 In this present paper the enhancement of arcs occurring on approach of electrodes (make arcs) is discussed and is thought to be due to the injection of plasmas formed by the electronic and ionic bombardment of carbon particles on the anode and cathode respectively. The plasmas formed from these particles can then be related to arc-duration enhancement. The observation that under `activated' conditions no anode-type arcs exist but those that form are all of the cathode variety, can also be explained by the model. A calculation of the amount of carbon required to fully activate a 10-2 cm2 contact area yields 6.3 × 10-8 gm in agreement with values of other workers.     

New source of dense, cryogenic positron plasmas

We have developed a new method, based on the ballistic transfer of preaccumulated plasmas, to obtain large and dense positron plasmas in a cryogenic environment. The method involves transferring plasmas emanating from a region with a low magnetic field (0.14 T) and relatively high pressure (10(-9) mbar) into a 15 K Penning-Malmberg trap immersed in a 3 T magnetic field with a base pressure better than 10(-13) mbar. The achieved positron accumulation rate in the high field cryogenic trap is more than one and a half orders of magnitude higher than the previous most efficient UHV compatible scheme. Subsequent stacking resulted in a plasma containing more than 1.2 x 10(9) positrons, which is a factor 4 higher than previously reported. Using a rotating wall electric field, plasmas containing about 20 x 10(6) positrons were compressed to a density of 2.6 x 10(10) cm(-3). This is a factor of 6 improvement over earlier measurements.     

Stable generation of high-order harmonics of femtosecond laser radiation from laser produced plasma plumes at 1 kHz pulse repetition rate

We present a method for the creation of stable weakly ionized plasmas from laser ablation of solid targets using a 1 kHz pulse repetition rate laser, which can be used for stable high-order harmonic generation from plasma plumes. The plasma plumes were generated from cylindrical rotating targets. Without target rotation the intensity of harmonics in the 40-80 nm range drops by more than one order of magnitude during less than 10(3) shots, while, with rotation of the target at typically 30 revolutions per minute, stable emission of high-order harmonics from aluminum plasma plumes with variation of less than 10% was maintained for >10(6) laser shots.     

Characterization of compact bright soft X-ray source based on picosecond laser plasma

Radiation emission of silicon and aluminum plasmas produced by 40-ps laser pulses with peak intensity above 10¹⁴ W/cm² was studied. High-resolution soft X-rayspectra of H-like and He-like ions were analyzed to determine plasma parameters. We compared the line shape of resonance transitions and their intensity ratios to corresponding dielectronic satellites and the intensities of the intercombination lines of He-like ions with the results of model calculations. Such comparisons gave average values of the electron number density N_e=(1-1.9)×10²¹ cm^-3 and the electron temperature T_e=460–560 eV for Si plasmas and about 560 eV for Al plasmas produced by the first and the second laser harmonics. The plasma size is about 100 μm. According to our estimations, more than 10¹² photons were produced within the resonance line spectral width and in the solid angle 2π during the total decay period. PACS 41.50.+h; 52.25.Os; 52.50.Jm     

Two Methods of Amplification of Coherent Extreme Ultraviolet Radiation During Harmonic Generation in Plasmas

Different recently revealed approaches of harmonic enhancement in plasmas are reviewed. The 4-step analytical model of resonant enhancement of high-order harmonic generation is extended to the systems possessing resonant transitions of inner-shell electrons. Role of inelastic scattering is discussed by simulation of excited state’s population dynamics. The enhancement of harmonics in the In plasma using different pumps is analyzed to prove the concept. We also discuss the plasma emission and harmonic generation spectra in the case of resonance enhancement of single harmonic using various laser-produced plasmas. The analysis of these spectra showed that the enhancement of harmonics depended on the oscillator strengths of the nearby ionic transitions rather than the plasma emission transitions. Finally, we review some recent experimental studies of the phase-matching of high-order harmonic generation in multi-jet plasmas.     

Organic compound-contained plasmas as the media for frequency conversion of ultrashort pulses

We demonstrate the high-order harmonic generation in the plasmas produced on the surfaces of organic targets. The role of resonances and two-color pump-induced enhancement of harmonics is analyzed. We show that the harmonic spectra from organic molecules-contained plasmas demonstrate the similarities and distinctions with the one from graphite ablation.     

Resonance processes during harmonic generation in plasmas using mid-infrared radiation

A few recently introduced approaches of the high-order harmonic generation in laser-produced plasmas are reviewed. We show how the tuning of odd and even high-order harmonics of ultrashort pulses along the strong resonance of laser-produced indium plasma using optical parametric amplifier of white-light continuum radiation (1250?1400 nm) allows observation of different harmonics enhanced in the vicinity of the transition of In II ions possessing high oscillator strength. We discuss various peculiarities and discuss the theoretical model of the phenomenon of tunable harmonics enhancement in the region of 62 nm using indium plasma. With the theoretical analysis, we present the approach allowing reproduce the experimental observations and characterize the dynamics of the resonant harmonic emissions. We also discuss the resonance enhancement of harmonics using mid-infrared radiation in the tin, chromium, and antimony plasmas and show the calculations of this process.     

Near-complete absorption of intense, ultrashort laser light by sub-lambda gratings

We demonstrate near-100% light absorption and increased x-ray emission from dense plasmas created on solid surfaces with a periodic sub-lambda structure. The efficacy of the structure-induced surface plasmon resonance, responsible for enhanced absorption, is directly tested at the highest intensities to date (3 x 10{15} W cm{-2}) via systematic, correlated measurements of absorption and x-ray emission. An analytical grating model as well as 2D particle-in-cell simulations conclusively explain our observations. Our study offers a definite, quantitative way forward for optimizing and understanding the absorption process.     

Ion heating and thermonuclear neutron production from high-intensity subpicosecond laser pulses interacting with underdense plasmas

Thermonuclear fusion neutrons produced by D(d,n)3He reactions have been measured from the interaction of a high-intensity laser with underdense deuterium plasmas. For an input laser energy of 62 J, more than (1.0+/-0.2)x10(6) neutrons with a mean kinetic energy of (2.5+/-0.2) MeV were detected. These neutrons were observed to have an isotropic angular emission profile. By comparing these measurements with those using a secondary solid CD2 target it was determined that neutrons are produced from direct ion heating during this interaction.     

Anomalous enhancement of a single high-order harmonic by using a laser-ablation tin plume at 47 nm

We have successfully demonstrated intensity enhancement of a single high-order harmonic at a wavelength of 46.76 nm by using low-ionized tin ions in a laser-ablation plume as the nonlinear medium. The ablation plume was produced by irradiating a solid tin target with a 10 mJ energy picosecond laser pulse. Strong 17th-harmonic generation at a wavelength of 46.76 nm was observed with a conversion efficiency of about 10(-4). We attribute the strong enhancement of the single high-order harmonic to multiphoton resonance with a strong radiative transition of the Sn II ion.     

高剥离态离子谱学研究

利用“星光”Ⅰ激光装置,聚焦激光束于真空室内平面箔靶,产生高离化态等离子体,分别用四个不同晶格间距的平晶摄谱仪和１ｍ掠入射光栅谱仪测量了＿（２２）Ｔｉ、＿（２６）Ｆｅ、＿（２８）Ｎｉ、＿（２９）Ｃｕ、＿（３０）Ｚｎ、＿（３２）Ｇｅ等中Ｚ元素及＿（７２）Ｈｆ、＿（７３）Ｔａ、＿（７４）Ｗ、＿（７５）Ｒｅ等高Ｚ元素的高离化态离子发射谱,其波长为３．６～３００Ａ分别用组态相互作用的Ｈａｒｔｒｅｅ－Ｆｏｃｋ－Ｒｅｌａｔｉｖｉｓｔｉｃ（ＨＦＲ）方法和多组态的Ｄｉｒａｃ－Ｆｏｃｋ方法分析辨认发射谱中的线跃迁,用自旋轨道劈裂的未分辨跃迁组理论分析发射谱中的带结构,归类了六个中Ｚ元素类钠直至类氦离子的数百条跃迁线,以及四个高Ｚ元素的类钻、类镍、类铜直至类锗离子的线跃迁和跃迁带． The soft X-ray emission spectra from very highly stripped ions of the medium Z elements of_(22)T,_(26)Fe,_(28)Ni,_(29)Cu,_(30)Zn and_(32)Ge, and the high-Z elements of_(72)Hf,_(73)Ta,_(74)Wand_(75)Re have been observed in laser-produced plasmas generated by focusing a Nd-glasslaser beam onto the surface of the plane solid targets at the "Xing Guang" laser facility. Thespectra in the range of 3. 6-300A were recorded by using four flat crystal spectrographyswith different 2d spacting...     

Pushing the limits of plasma length in inertial-fusion laser-plasma interaction experiments

We demonstrate laser beam propagation and low backscatter in laser produced hohlraum plasmas of ignition plasma length. At intensities I < 5 x 10(14) W cm(-2) greater than 80% of the energy in a blue (3 omega, 351 nm) laser is transmitted through a L=5-mm long, high-temperature (Te = 2.5 keV), high-density (ne = 5 x 10(20) cm(-3)) plasma. These experiments show that the backscatter scales exponentially with plasma length which is consistent with linear theory. The backscatter calculated by a new steady state 3D laser-plasma interaction code developed for large ignition plasmas is in good agreement with the measurements.     

Electromagnetic field propagation in neutral isotropic plasmas

Interested in electromagnetic waves inside plasmas with known dielectric properties, we prove that Courant-Hilbert fields of the focus wave mode type can propagate in a steady-state, isotropic, neutral plasma which is in fact a dielectric with the refractive index where is the plasma frequency. We also prove that neutral, isotropic plasmas with memory behave for harmonic plane waves as a time reversal mirror when the memory function is a decreasing exponential.Received: 1 December 2003, Published online: 16 March 2004PACS: 42.25.Bs Wave propagation, transmission and absorption - 52.35.Hr Electromagnetic waves (e.g., electron-cyclotron, Whistler, Bernstein, upper hybrid, lower hybrid) - 52.27.Aj Single-component, electron-positive-ion plasmas     

Formation of Rydberg atoms in an expanding ultracold neutral plasma

We study the formation of Rydberg atoms in expanding plasmas at temperatures of 1-1000 K and densities from 10(5)-10(10) cm(-3). Up to 20% of the initially free charges recombine in about 100 micros, and the binding energy of the Rydberg atoms approximately equals the increase in the kinetic energy of the remaining free electrons. Three-body recombination is expected to dominate in this regime, yet most of our results are inconsistent with this mechanism.     

Hollow-Cathode Low-Pressure Arc Discharges and Their Application in Plasma Generators and Charged-Particle Sources

This paper presents the results of a study of hollow-cathode arc discharges which generate gas-discharge plasmas of densities 10¹⁰–10¹² cm^–3 in large volumes (1 m³) at low pressures (10^–2–1 Pa) and at discharge currents of up to 200 A. Consideration is given to the design and peculiarities of hot-cathode and cold-cathode discharge systems. The parameters of plasma generators and charged-particle sources where use is made of arc discharges are cited and the problems of the most efficient application of such systems in technological processes of solid surface modification are discussed.     

Comparative extreme ultraviolet emission measurements for lithium and tin laser plasmas

Detailed spectroscopic studies on extreme UV emission from laser plasmas using tin and lithium planar solid targets were completed. At 13.5 nm, the best conversion efficiency (CE) for lithium was found to be 2.2% at intensities near 7 x 10(10) W/cm(2). The highest CE measured for tin was near 5.0% at an intensity close to 1 x 10(11) W/cm(2).     

X-ray emission from short-pulse laser plasmas

Highly intense picosecond and subpicosecond laser pulses interacting with solids can create hot and dense plasmas which emit x-ray pulses in a broad spectral range from 100 eV up to MeV. The duration of these x-ray pulses depends on the transient behaviour of the relaxation and recombination mechanisms, as well as on the lifetime of energetic electrons produced via nonlinear processes in the plasma. This paper reports experiments using a 1.5-ps laser pulse with high constrast ratio (up to 10¹⁰) and intensities up to 10¹⁸ W cm^-2 irradiating solid targets. Both the line spectrum characteristics of a magnesium plasma, recorded using crystal spectrometers with high spectral resolution, and kinetic calculations have allowed the deduction of plasma parameters in the process of plasma evolution. In addition, hard x-ray pulses from a tantalum plasma were measured and their scaling was explained as bremsstrahlung emission from energetic electrons. Absolute dose values of x-ray pulses are given.     

All-electron path integral Monte Carlo simulations of warm dense matter: application to water and carbon plasmas

We develop an all-electron path integral Monte Carlo method with free-particle nodes for warm dense matter and apply it to water and carbon plasmas. We thereby extend path integral Monte Carlo studies beyond hydrogen and helium to elements with core electrons. Path integral Monte Carlo results for pressures, internal energies, and pair-correlation functions compare well with density functional theory molecular dynamics calculations at temperatures of (2.5-7.5)×10(5) K, and both methods together form a coherent equation of state over a density-temperature range of 3-12 g/cm(3) and 10(4)-10(9) K.     

Two regimes of asymmetry-induced transport in non-neutral plasmas

Confinement of charged particles in cylindrical Penning-Malmberg traps depends strongly on cross-magnetic-field transport induced by electric and/or magnetic asymmetries. New measurements in pure-electron plasmas demonstrate two separate transport regimes depending on the particle bounce-to-rotation ratio, or rigidity, R identical with&fmacr;(b)/f(E). For R<10, the transport scales as V(a)R-2, where V(a) is the strength of an applied electrostatic asymmetry. For R greater, similar10-20, this " R-2 transport" ceases abruptly, leaving " B-independent" transport which scales as V(2)(a) and does not depend directly on the rigidity.