Dependence of spatial coherence of 23.2-23.6-nm radiation on the geometry of a multielement germanium x-ray laser target

The spatial coherence of a nanosecond pulsed germanium collisionally excited x-ray laser is measured experimentally for three target configurations. The diagnostic is based on Young's slit interference fringes with a dispersing element to resolve the 23.2- and 23.6-nm spectral lines. Target configurations include a double-slab target, known as the injector, and geometries in which the injector image is image relayed to seed either an additional single-slab target or a second double-slab target. A special feature of this study is the observation of the change in the apparent source size with angle of refraction across the diverging laser beam. Source sizes derived with a Gaussian source model decrease from 44 mu m for the injector target by a variable factor of as much as 2, according to target configuration, for beams leaving the additional amplifiers after strong refraction in the plasma. (C) 1998 Optical Society of America [S0740-3224(98)00810-8].

The effects of multi-pulse irradiation on X-ray laser media

Multipulse irradiation with 100 ps pulses of stripe Germanium targets is shown to enhance by up to several orders-of-magnitude the output of Ne-like Ge lasing on the J = 0-1 line at 196 Angstrom compared to single pulse pumping. Various pre-pulse and multipulse configurations have been experimentally investigated for irradiances of approximate to 4 x 10(13) W/cm(2) with a 1.06 mu m wavelength pumping laser. The ionisation balance measured by a KeV crystal spectrometer (KAP crystal) has been found to not affect the X-ray laser output. Good agreement between the experimental results and a fluid code incorporating atomic physics, gain and X-ray beam ray tracing is obtained. The code results show that the enhanced X-ray laser output is produced by multipulse irradiation reducing the electron density gradients in the gain region and simultaneously increasing the gain region spatial size. These changes reduce the effect of refraction on the X-ray laser beam propagation.

Polar distribution of ablated atomic material during the pulsed laser deposition of Cu in vacuum: Dependence on focused laser spot size and power density

Experiments have been carried out to investigate the polar distribution of atomic material ablated during the pulsed laser deposition of Cu in vacuum. Data were obtained as functions of focused laser spot size and power density. Thin films were deposited onto flat glass substrates and thickness profiles were transformed into polar atomic flux distributions of the form f(theta)=cos(n) theta. At constant focused laser power density on target, I=4.7+/-0.3X10(8) W/cm(2), polar distributions were found to broaden with a reduction in the focused laser spot size. The polar distribution exponent n varied from 15+/-2 to 7+/-1 for focused laser spot diameter variation from 2.5 to 1.4 mm, respectively, with the laser beam exhibiting a circular aspect on target. With the focused laser spot size held constant at phi=1.8 mm, polar distributions were observed to broaden with a reduction in the focused laser power density on target, with the associated polar distribution exponent n varying from 13+/-1.5 to 8+/-1 for focused laser power density variation from 8.3+/-0.3X10(8) to 2.2+/-0.1X10(8) W/cm(2) respectively. Data were compared with an analytical model available within the literature, which correctly predicts broadening of the polar distribution with a reduction in focused laser spot size and with a reduction in focused laser power density, although the experimentally observed magnitude was greater than that predicted in both cases. (C) 1996 American Institute of Physics.

Collisional exitation soft X-ray laser at 23.6 nm in a laser-produced cylindrical target

We have tested soft X-ray lasing in neon-like germanium with cylindrical targets where wave guiding and plasma confinement may affect lasing. An intense soft X-ray laser beam of 0.05 MW peak power and a narrow beam divergence (8 mrad) was produced at 23.6 nm with a 4 cm long straight cylindrical target of 0.72 mm inner diameter. Bending the cylindrical target to form a toroidal shape increased the lasing intensity by a factor of 3 accompanied with reduction of the beam divergence from 8 to 6 mrad.

PRELIMINARY STUDIES OF RADIATION COUPLING BETWEEN REMOTE SOFT-X-RAY LASER-AMPLIFIERS

Coupling of a soft X-ray laser beam with a relaying concave mirror in a sequentially pumped amplifier geometry using the Ne-like Ge system has been studied experimentally. Preliminary observations indicate an increase in the spatial coherence of the amplified relayed beam. In addition, near-field imaging of one of the amplifier plasmas shows a double-lobed intensity pattern of the emergent beam indicating refractive guiding of the amplified beam with components both normal and tangential to the target surface.

COHERENCE AND DOUBLE-PASS AMPLIFICATION IN GE SOFT-X-RAY LASER

From measurements of spatial coherence and beam divergence of Ge soft x-ray laser at a far field, the x-ray laser beam has been characterized as a partially coherent Gaussian beam. Double-pass amplification will improve spatial and temporal coherence, spectral brightness and efficiency. Close to 100% geometrical coupling efficiency has been obtained in double pass amplification in Ge. Transient loss of feedback is attributed to mirror structure damage within the build-up time of the x-ray laser. Prospect for generation of coherent x-ray laser beam is discussed.

COLLISIONALLY EXCITED X-RAY LASER SCHEMES - PROGRESS AT RUTHERFORD APPLETON LABORATORY

Characteristics of the 3p-3s amplified spontaneous emission from Ne-like Ge plasma columns, generated by ablation from massive targets, have been studied in detail. In particular, the gain coefficients of the J = 2-1 lines at 23.2 and 23.6 nm have been measured as a function of incident intensity for a 1.05-mu-m wavelength pump laser beam. For 100-mu-m wide stripe targets and a fixed energy pump laser the maximum gain length product is achieved at an irradiance of

Efficient extreme UV harmonics generated from picosecond laser pulse interactions with solid targets

The generation of high harmonics created during the interaction of a 2.5 ps, 1053 nm laser pulse with a solid target has been recorded for intensities up to 10(19) W cm(-2). Harmonic orders up to the 68th at 15.5 nm in first order have been observed with indications up to the 75th at 14.0 nm in second-order diffraction. No differences in harmonic emission between s and p polarization of the laser beam were observed. The power of the 38th high harmonic at 27.7 nm is estimated to be 24 MW.

High power laser production of short-lived isotopes for positron emission tomography

Positron emission tomography (PET) is a powerful diagnostic/imaging technique requiring the production of the short-lived positron emitting isotopes C-11, N-13, O-15 and F-18 by proton irradiation of natural/enriched targets using cyclotrons. The development of PET has been hampered due to the size and shielding requirements of nuclear installations. Recent results show that when an intense laser beam interacts with solid targets, megaelectronvolt (MeV) protons capable of producing PET isotopes are generated. This report describes how to generate intense PET sources of C-11 and F-18 using a petawatt laser beam. The work describing the laser production of F-18 through a (p,n) O-18 reaction, and the subsequent synthesis of 2-[F-18] is reported for the first time. The potential for developing compact laser technology for this purpose is discussed.

Imaging of high harmonic radiation emitted during the interaction of a 20 TW laser with a solid target

We present images of the source of extreme ultraviolet (XUV) harmonic emission at a wavelength of 220 Angstrom from the interaction of a 20 TW, 1.053 mu m Nd:glass laser beam focused to intensities up to 4x10(18) W cm(-2) onto a solid target. From these measurements we determine an upper limit to the source size and brightness of the harmonic emission to show its efficacy as a novel source of short-pulse, coherent XUV radiation. We also demonstrate the empirical scaling of the harmonic generation efficiency with irradiance up to 10(19) W mu m(2) cm(-2), and extrapolate to estimate the possible source brightness at higher irradiances. These source brightnesses are compared to those available from an x-ray laser system. (C) 1997 American Institute of Physics.

2ND-HARMONIC LIGHT GENERATION IN THE INTERACTION OF LASER-BEAMS WITH UNDERCRITICAL PLASMAS

The process of second harmonic generation (SHG) in undercritical plasmas is studied. It is shown that filamentation and self-focusing of the laser beam in the plasma can break the plasma density symmetry and lead to SHG by free electrons. In turn, second harmonic emission may be used to investigate the plasma parameters and to diagnose the process of laser beam filamentation itself.

Dynamics of intense laser propagation in underdense plasma: Polarization dependence

We present a comprehensive numerical study of the dynamics of an intense laser pulse as it propagates through an underdense plasma in two and three dimensions. By varying the background plasma density and the polarization of the laser beam, significant differences are found in terms of energy transport and dissipation, in agreement with recently reported experimental results. Below the threshold for relativistic self-focusing, the plasma and laser dynamics are observed to be substantially insensitive to the initial laser polarization, since laser transport is dominated by ponderomotive effects. Above this threshold, relativistic effects become important, and laser energy is dissipated either by plasma heating (p-polarization) or by trapping of electromagnetic energy into plasma cavities (s-polarization) or by a combination of both (circular polarization). Besides the fundamental interest of this study, the results presented are relevant to applications such as plasma-based accelerators, x-ray lasers, and fast-ignition inertial confinement fusion. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4737151]

A theoretical assessment of surface defect machining and hot machining of nanocrystalline silicon carbide

In this paper, a newly proposed machining method named “surface defect machining” (SDM) [Wear, 302, 2013 (1124-1135)] was explored for machining of nanocrystalline beta silicon carbide (3C-SiC) at 300K using MD simulation. The results were compared with isothermal high temperature machining at 1200K under the same machining parameters, emulating ductile mode micro laser assisted machining (µ-LAM) and with conventional cutting at 300 K. In the MD simulation, surface defects were generated on the top of the (010) surface of the 3C-SiC work piece prior to cutting, and the workpiece was then cut along the <100> direction using a single point diamond tool at a cutting speed of 10 m/sec. Cutting forces, sub-surface deformation layer depth, temperature in the shear zone, shear plane angle and friction coefficient were used to characterize the response of the workpiece. Simulation results showed that SDM provides a unique advantage of decreased shear plane angle which eases the shearing action. This in turn causes an increased value of average coefficient of friction in contrast to the isothermal cutting (carried at 1200 K) and normal cutting (carried at 300K). The increase of friction coefficient however was found to aid the cutting action of the tool due to an intermittent dropping in the cutting forces, lowering stresses on the cutting tool and reducing operational temperature. Analysis shows that the introduction of surface defects prior to conventional machining can be a viable choice for machining a wide range of ceramics, hard steels and composites compared to hot machining.

Photo-ionized neon plasmas induced by radiation pulses of a laser-plasma EUV source and a free electron laser FLASH

In this work, a laser-produced plasma extreme ultraviolet source and a free electron laser were used to create Ne photo-ionized plasmas. In both cases, a radiation beam was focused onto a gas stream injected into a vacuum chamber synchronously with the radiation pulse. Extreme ultraviolet radiation from the plasma spanned a wide spectral range with pronounced maximum centered at lambda = 11 +/- 1 nm while the free electron laser pulses were emitted at a wavelength of 32 nm. The power density of the focused plasma radiation was approximately 2 x 10(7) W/cm(2) and was seven orders of magnitude lower compared with the focused free electron laser beam. Radiation fluences in both experimental conditions were comparable. Despite quite different spectral characteristics and extremely different power densities, emission spectra of both photo-ionized plasmas consist of the same spectral lines within a wavelength range of 20 to 50 nm, however, with different relative intensities of the corresponding lines. The dominating spectral lines originated from singly charged ions (Ne II); however, Ne III lines were also detected. Additionally, computer simulations of the emission spectra, obtained for photo-ionized plasmas, driven by the plasma extreme ultraviolet source, were performed. The corresponding measured and calculated spectra are presented. An electron temperature and ionic composition were estimated. Differences between the experimental spectra, obtained for both irradiation conditions, were analyzed. The differences were attributed mainly to different energies of driving photons.

Molecular dynamics simulation (MDS) to study nanoscale machining processes

Molecular Dynamics Simulations (MDS) are constantly being used to make important contributions to our fundamental understanding of material behaviour, at the atomic scale, for a variety of thermodynamic processes. This chapter shows that molecular dynamics simulation is a robust numerical analysis tool in addressing a range of complex nanofinishing (machining) problems that are otherwise difficult or impossible to understand using other methods. For example the mechanism of nanometric cutting of silicon carbide is influenced by a number of variables such as machine tool performance, machining conditions, material properties, and cutting tool performance (material microstructure and physical geometry of the contact) and all these variables cannot be monitored online through experimental examination. However, these could suitably be studied using an advanced simulation based approach such as MDS. This chapter details how MD simulation can be used as a research and commercial tool to understand key issues of ultra precision manufacturing research problems and a specific case was addressed by studying diamond machining of silicon carbide. While this is appreciable, there are a lot of challenges and opportunities in this fertile area. For example, the world of MD simulations is dependent on present day computers and the accuracy and reliability of potential energy functions [109]. This presents a limitation: Real-world scale simulation models are yet to be developed. The simulated length and timescales are far shorter than the experimental ones which couples further with the fact that contact loading simulations are typically done in the speed range of a few hundreds of m/sec against the experimental speed of typically about 1 m/sec [17]. Consequently, MD simulations suffer from the spurious effects of high cutting speeds and the accuracy of the simulation results has yet to be fully explored. The development of user-friendly software could help facilitate molecular dynamics as an integral part of computer-aided design and manufacturing to tackle a range of machining problems from all perspectives, including materials science (phase of the material formed due to the sub-surface deformation layer), electronics and optics (properties of the finished machined surface due to the metallurgical transformation in comparison to the bulk material), and mechanical engineering (extent of residual stresses in the machined component) [110]. Overall, this chapter provided key information concerning diamond machining of SiC which is classed as hard, brittle material. From the analysis presented in the earlier sections, MD simulation has helped in understanding the effects of crystal anisotropy in nanometric cutting of 3C-SiC by revealing the atomic-level deformation mechanisms for different crystal orientations and cutting directions. In addition to this, the MD simulation revealed that the material removal mechanism on the (111) surface of 3C-SiC (akin to diamond) is dominated by cleavage. These understandings led to the development of a new approach named the “surface defect machining” method which has the potential to be more effective to implement than ductile mode micro laser assisted machining or conventional nanometric cutting.