Production of radioactive nuclides by energetic protons generated from intense laser-plasma interactions

Nuclear activation has been observed in materials exposed to the ablated plasma generated from high intensity laser-solid interactions (at focused intensities up to 2x10(19) W/cm(2)) and is produced by protons having energies up to 30 MeV. The energy spectrum of the protons is determined from these activation measurements and is found to be consistent with other ion diagnostics. The possible development of this technique for

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.

Guiding of a 10-TW picosecond laser pulse through hollow capillary tubes

Efficient guiding of 1-ps infrared laser pulses with power exceeding 10 TW has been demonstrated through hollow capillary tubes with 40- and 100-mu m internal diameters and lengths up to 10 mm, with transmission greater than 80% of the incident energy coupled into the capillary. The beam is guided via multiple reflections off a plasma formed on the walls of the guide by the pulse's rising edge, as inferred from optical probe measurements.

Relativistic channeling of a picosecond laser pulse in a near-critical preformed plasma

Relativistic self-channeling of a picosecond laser pulse in a preformed plasma near critical density has been observed both experimentally and in 3D particle-in-cell simulations. Optical probing measurements indicate the formation of a single pulsating propagation channel, typically of about 5 mu m in diameter. The computational results reveal the importance in the channel formation of relativistic electrons traveling with the light pulse and of the corresponding self-generated magnetic field.

SPECTRALLY MODULATED 2ND-HARMONIC EMISSION FROM LASER-PLASMA FILAMENTS

Spectra of forward emitted second harmonic light from laser interaction with filamentary plasmas have been experimentally studied. Rather regular modulations in the frequency domain have been observed into overall red-shifted spectra. The observed spectral features are consistent with self-phase-modulation of the intense laser light in growing filaments. A model accounts for this effect.

Role of low-energy electrons in Ar emission from low-pressure radio frequency discharge plasma

Optical emission spectra from a low-pressure Ar plasma were studied with high spatial resolution. It has been shown that the intensity ratios of Ar lines excited through metastable levels to those excited directly from the ground state are sensitive to the shape of electron energy distribution function. From these measurements, important information on the spatial variation of plasma parameters can be obtained. (C) 1999 American Institute of Physics. [S0003-6951(99)01629-0].

Structure observed in measured electron energy distribution functions in capacitively coupled radio frequency hydrogen plasmas

Electron energy probability functions measured with a passively compensated Langmuir probe in asymmetric capacitively coupled hydrogen and deuterium plasmas exhibit structure. The otherwise relatively continuous distribution appears to have an abrupt peak in electron density near 5 eV. This structure occurs at a higher energy in deuterium than hydrogen and there is a correlation between floating potential and the voltage at which the structure is observed in the second derivative of the I(V) characteristic. While the cause of the structure has yet to be clarified, spectroscopic observations and computer-based hydrogen models indicate that the high energy tail of the distribution is strongly modulated during the radio frequency cycle. The effect of this modulation on plasma properties and probe measurements has yet to be explored. (C) 1999 American Institute of Physics. [S0003-6951(99)00819-0].

Measurements of negative ion densities by absorption spectroscopy

A novel diagnostic technique for the measurement of negative ions is presented. The absorption of a cw-laser beam with a wavelength close to the maximum of the photodetachment cross section is measured. The laser beam undergoes multiple reflections through the discharge volume using a multipass cell there a Herriott arrangement). Proof-of-principle measurements of negative hydrogen ions in a hybrid volume source are presented. The measured H- density is in agreement with results of conventional, Langmuir probe-based photodetachment measurements. (C) 1998 American Institute of Physics. [S0003-6951(98)02119-6].

Comparison of plasma transport theory with measurements in a H- discharge

This paper and its companion paper describe the comparison between a one-dimensional theoretical model of a hydrogen discharge in a magnetic multipole plasma source and experimental measurements of the plasma parameters. The discharge chamber, described here, has been designed to produce significant densities of H- ions by incorporating a weak transverse field through the discharge to obtain electron cooling so as to maximize H- production. Langmuir probes are used to monitor the plasma, determining the ion density, the electron density and temperature and the plasma potential. The negative density is measured by photo-detachment of the extra electron using an intense laser beam. The model, described in the companion paper, uses the presented source geometry to calculate these plasma quantities as a function of the major are parameters; namely the are current and voltage and gas pressure. Good agreement is obtained between theory and experiment as a function of position and arc parameters.

THE ROLE OF GAS-PHASE OXIDATION AND COMBINATION DURING LASER DEPOSITION OF YBA2CU3O7-X IN AMBIENT OXYGEN

Spectroscopic absorption and emission measurements have been used to study laser deposition of YBCO films. They show that >95% of the monatomic Y and Ba initially ablated from the target undergo gas-phase chemical combination before film deposition. In contrast, considerable monatomic Cu persists into the deposition region. in this region, equilibrated gas temperatures are of the order of 2700 K. It is suggested that this high temperature facilitates film crystallization and epitaxial growth. The survival of monatomic Cu in the plume to the site of deposition is a manifestation of its endothermic reaction with O-2.

PROPERTIES OF ZNS THIN-FILMS PREPARED BY 248-NM PULSED-LASER DEPOSITION

Pulsed laser deposition (PLD) from a hot pressed manganese doped ZnS target using a KrF laser, has produced a high rate deposition method for growing luminescent thin films. Good stoichiometric quality and typical luminescent crystal structures have been observed with a predominant hexagonal phase and little evidence of the cubic phase. The luminescent characteristics were determined by cathodoluminescence and photoluminescence excitation and stable electroluminescence was observed under pulsed dc conditions with a minimum brightness of 150 cd/m2. PLD film characteristics are compared with those observed in radio-frequency sputtered samples.

TIME-RESOLVED OPTICAL-EMISSION AND ELECTRON-ENERGY DISTRIBUTION FUNCTION MEASUREMENTS IN RF PLASMAS

Comparisons between experimentally measured time-dependent electron energy distribution functions and optical emission intensities are reported for low-frequency (100 and 400 kHz) radio-frequency driven discharges in argon. The electron energy distribution functions were measured with a time-resolved Langmuir probe system. Time-resolved optical emissions of argon resonance lines at 687.1 and 750.4 nm were determined by photon-counting methods. Known ground-state and metastable-state excitation cross sections were used along with the measured electron energy distribution functions to calculate the time dependence of the optical emission intensity. It was found that a calculation using only the ground-state cross sections gave the best agreement with the time dependence of the measured optical emission. Time-dependent electron density, electron temperature, and plasma potential measurements are also reported.

SUPERCONDUCTING YBA2CU3O7 THIN-FILMS ON MGO BY KRF LASER ABLATION - OPTIMIZATION OF DEPOSITION PARAMETERS

The optimization of interrelated deposition parameters during deposition of in situ YBa2Cu3O7 thin films on MgO substrates by KrF laser ablation was systematically studied in a single experimental chamber. The optimum condition was found to be a substrate temperature of 720-degrees-C and a target-substrate distance of 5 cm in an oxygen partial pressure of 100 mTorr. These conditions produced films with T(c) = 87 K. The presence of YO in the plasma plume was found to be important in producing good quality films. The films were characterized by resistance-temperature measurements, energy dispersive x-ray analyses, scanning electron microscopy, and x-ray-diffraction measurements, and the physical reasons underlying film quality degradation at parameter values away from optimal are discussed.

An effective method for comparing the turbulence intensity from LDA measurements and CFD predictions within a ship propeller jet

The mean velocity and turbulence intensity are the two main inputs to investigate the ship propeller induced seabed scouring resulting from a vessel is manoeuvring within a port where the underkeel clearances are low. More accurate data including the turbulence intensity is now available by using the laser doppler anemometry (LDA) measurement system and computational fluid dynamics (CFD) approach. Turbulence intensity has a loose definition, which is the velocity fluctuation as the root mean square (RMS) referenced to a mean flow velocity. However, the velocity fluctuation and mean velocity can be the overall value includingx, y and z directions or the value of a single component. LDA and CFD results were obtained from two different acquisition systems (Dantec LDA system and Fluent CFD package) and therefore the outputs cannot be compared directly. An effective method is proposed for comparing the turbulence intensity between the experimental measurements and the computational predictions within a ship propeller jet. The flow patterns of turbulence intensity within a ship propeller jet are presented by using the LDA measurements and CFD results from turbulence models of standard k-e, RNG k-e, realizable k–e, standard k–?, SST k–?and Reynolds stresses.