Influence of the photon orbital angular momentum on electric dipole transitions: negative experimental evidence.

We describe an experiment of atomic spectroscopy devoted to ascertaining whether the orbital angular momentum (OAM) of photons has the same property of interacting with atoms or molecules as occurs for the spin angular momentum (SAM). In our experiment, rubidium vapors are excited by means of laser radiation with different combinations of OAM and SAM, particularly selected to inhibit or enhance the fluorescence according to the selection rules for the electric dipole transitions between the fundamental state and the first excited doublet. Our results clearly show that an electric-dipole-type transition is insensitive to the OAM value, and provide an original validation of a problem long debated in theoretical works.

Jet outflow and open field line measurements on the C-2U advanced beam-driven field-reversed configuration plasma experiment.

Knowledge and control of the axial outflow of plasma particles and energy along open-magnetic-field lines are of crucial importance to the stability and longevity of the advanced beam-driven field-reversed configuration plasma. An overview of the diagnostic methods used to perform measurements on the open field line plasma on C-2U is presented, including passive Doppler impurity spectroscopy, microwave interferometry, and triple Langmuir probe measurements. Results of these measurements provide the jet ion temperature and axial velocity, electron density, and high frequency density fluctuations.

Note: Real-time monitoring via second-harmonic interferometry of a flow gas cell for laser wakefield acceleration.

The use of a gas cell as a target for laser wakefield acceleration (LWFA) offers the possibility to obtain stable and manageable laser-plasma interaction process, a mandatory condition for practical applications of this emerging technique, especially in multi-stage accelerators. In order to obtain full control of the gas particle number density in the interaction region, thus allowing for a long term stable and manageable LWFA, real-time monitoring is necessary. In fact, the ideal gas law cannot be used to estimate the particle density inside the flow cell based on the preset backing pressure and the room temperature because the gas flow depends on several factors like tubing, regulators, and valves in the gas supply system, as well as vacuum chamber volume and vacuum pump speed/throughput. Here, second-harmonic interferometry is applied to measure the particle number density inside a flow gas cell designed for LWFA. The results demonstrate that real-time monitoring is achieved and that using low backing pressure gas (<1 bar) and different cell orifice diameters (<2 mm) it is possible to finely tune the number density up to the 10(19) cm(-3) range well suited for LWFA.

Preparation of small size palladium nanoparticles by picosecond laser ablation and control of metal concentration in the colloid.

We assessed a method for the preparation of small, highly stable and unprotected Pd nanoparticles by picosecond laser ablation in 2-propanol. The nanoparticles can be extracted from 2-propanol by centrifugation and redispersed in water, where a strongly negative ζ-potential assures long term stability. The proposed procedure permits reduction of particle size down to 1.6nm and optimization of the Pd(0):Pd(II) ratio which, in the best cases, was of the order of 6:1. The increase of this ratio with ablation times has been correlated to the high temperature conversion of PdO to metallic Pd by a simple theoretical model. A study of the relationship between colloid absorption at 400nm and Pd concentration permitted the role of PdO in the determination of the UV-vis spectra to be clarified and the limits of the Mie theory for the evaluation of colloid concentration to be established. The absorption at 400nm can be used as a fast method to estimate the Pd content in the colloids, provided that a calibration of the ablation process is preliminarily performed.

Long-path second-harmonic interferometer with nanosecond time resolution: reliable diagnostic tool for electron density measurement in pulsed plasma devices.

We describe the performance of a second-harmonic interferometer (SHI) to measure, on an optical path exceeding 12 m, the electron plasma density of two plasmoids formed in separate theta-pinch chambers and then merged in a central compression chamber after undergoing acceleration and compression. The excellent mechanical stability and a time resolution better than 50 ns suggest the application of SHI, especially in pulsed plasma devices with limited optical accesses.

Sensitive and robust second-harmonic interferometer for measuring the dispersion of the electro-optic coefficients in bulk materials.

Second-harmonic interferometry (SHI) is proposed for measuring the electro-optic (EO) coefficients of massive media. It combines the advantages of interferometric techniques with the mechanical stability of single-beam methods, simultaneously skimming the wavelength dispersion of the EO response. For demonstrating the effectiveness of the SHI technique, the EO coefficients r(33)(T) and r(13)(T) of the EO crystal lithium niobate are measured simultaneously at 1064 and 532 nm.

Temporal and spatial characterization of a pulsed gas jet by a compact high-speed high-sensitivity second-harmonic interferometer.

A compact modular high-speed high-sensitivity second-harmonic interferometer is used to characterize a pulsed gas jet. The temporal evolution of the line-integrated gas density is measured with a resolution of 1 µs revealing detailed information on its dynamics. The actual radial gas density distribution in the jet is obtained applying the Abel's inversion method. The sensitivity of the interferometer is 1 mrad, and its robustness, compactness and modularity make the instrument suitable for practical application. Possible use of the instrument in monitoring cluster formation, and phase-dispersion microscopy is discussed.

Doppler spectroscopy and D-alpha emission diagnostics for the C-2 FRC plasma.

Two Doppler spectroscopy diagnostics with complementary capabilities are developed to measure the ion temperatures and velocities of FRC plasmas in the C-2 device. First, the multichord ion doppler diagnostic can simultaneously measure 15 chords of the plasma using an image intensified camera. Second, a single-chord fast-response ion Doppler diagnostic provides much higher faster time response by using a 16-channel photo-multiplier tube array. To study the neutral density of deuterium under different wall and plasma conditions, a highly sensitive eight-channel D-alpha diagnostic has been developed and calibrated for absolute radiance measurements. These spectroscopic diagnostics capabilities, combined with other plasma diagnostics, are helping to understand and improve the field reversed configuration plasmas in the C-2 device.

Dynamic formation of a hot field reversed configuration with improved confinement by supersonic merging of two colliding high-ß compact toroids.

A hot stable field-reversed configuration (FRC) has been produced in the C-2 experiment by colliding and merging two high-ß plasmoids preformed by the dynamic version of field-reversed θ-pinch technology. The merging process exhibits the highest poloidal flux amplification obtained in a magnetic confinement system (over tenfold increase). Most of the kinetic energy is converted into thermal energy with total temperature (T{i}+T{e}) exceeding 0.5 keV. The final FRC state exhibits a record FRC lifetime with flux confinement approaching classical values. These findings should have significant implications for fusion research and the physics of magnetic reconnection.

Electron density measurements of a field-reversed configuration plasma using a novel compact ultrastable second-harmonic interferometer.

A compact high-sensitivity second-harmonic interferometer for line-integrated electron density measurements on a large plasma machine is presented. The device is based on a fiber coupled near-infrared continuous-wave Nd:YAG laser and is remotely controlled. The performances of the instrument are tested on the Irvine field-reversed configuration machine, and a sensitivity of few 10(14) cm(-2) in measuring line integrated electron density is demonstrated with a time resolution of a few microseconds. The interferometer is self calibrated, has an impressive stability, and it does not require any further alignment after proper installation. These features make this device a real turn-key system suitable for electron density measurement in large plasma machines.

Spectral redshift in harmonic generation from plasma dynamics in the laser focus.

High-precision spectral measurements on the 9th harmonic generated in xenon gas are compared with calculations of the plasma dynamics resulting from multiphoton ionization in the laser focus. For the regime of 300 ps pulses and above-saturation intensities a novel mechanism producing redshifts in the harmonics is uncovered and explained. Ions play a double role: the nonlinear susceptibility of the ions is decisive for the harmonic intensity, while their mutual repulsion and the associated increase of the index of refraction is identified as the cause of the redshift.

Wavelength-dependent collective effects in the multiphoton ionization of atomic deuterium.

This paper presents the results of an experimental investigation into collective effects in the transient plasma formed by multiphoton ionization of atomic deuterium with a pulsed laser. The laser wavelength is varied in a narrow range around 243 nm, so that the photoionization is resonant with the metastable 2S(1/2) state. The ion yield, the ion time-of-flight spectra, and the yield of Lyman-alpha photons have been measured as a function of laser intensity (from 1 to 340 MW/cm(2)) and laser detuning around the 1S(1/2)-2S(1/2) two-photon resonance. During and shortly after the laser pulse, collective effects resulting from the mutual interaction of the photoelectrons and the ions affect the spatial and temporal distribution of the ions. Because of the near-degeneracy of the 2S(1/2), 2P(1/2), and 2P(3/2) states, the resonant multiphoton ionization is affected by the Stark mixing of these states in the collective field. As a result, the time-dependent yields of ions and of Lyman-alpha photons are modulated by the interplay of the multiphoton ionization of the atoms and the collective effects in the plasma. From the measurements it is deduced that collective effects are important above a critical charge density of 3x10(8) ions/cm(3). An asymmetry is observed in the line profile of the total ion yield as a function of laser detuning. This asymmetry is interpreted to be due to the effect of the collective field upon the intermediate resonant 2S(1/2) state of the photoionization process.