The excitation of nonlinear electrostatic waves, such as shock and solitons, by ultraintense laser interaction with overdense plasmas and related ion acceleration are investigated by numerical simulations. Stability of solitons and formation of shock waves is strongly dependent on the velocity distribution of ions. Monoenergetic components in ion spectra are produced by "pulsed" reflection from solitary waves. Possible relevance to recent experiments on "shock acceleration" is discussed.In the interaction of superintense (I = 10 18 − 10 21 W cm −2 ) laser pulses with overdense plasmas (i.e. having electron density n e > n c , the cut-off density), the light pressure ranges from Gigabar to Terabar values and, like a piston, may sweep out and compress the laserproduced plasma pushing its surface at nearly relativistic speeds. Such combination of strong compression and acceleration is often described as the generation of strong shock waves (briefly, "shocks") propagating towards the bulk of the plasma. These light-pressure driven shocks may cause heating of solid targets, as experimentally inferred [1] and be of interest for the production of warm dense matter. In addition, shocks may lead to acceleration of ions if the latter are reflected by the shock front as a moving wall. In moderately overdense and hot plasmas, where the shock waves are of collisionless nature, shock acceleration may lead to higher ion energies than the widely studied target normal sheath acceleration (TNSA) mechanism [2], as suggested on the basis of numerical simulations [3]. Recently, two experiments employing hydrogen gas jet targets and CO 2 lasers reported on monoenergetic proton beams produced via shock acceleration [4,5]. The study of laser-driven shocks in the laboratory may also help the understanding of similar processes in astrophysical environments [6, and references therein]The above mentioned simple picture of ion acceleration indicates that, as far as the shock velocity v s is constant, the reflected ions should have velocity 2v s and produce a monoenergetic peak in the spectrum. Silva et al.[3] report that such peak would evolve into a spectral plateau due to further acceleration in the sheath field at the rear side of the target, so that suppression of such field (for instance by producing a smooth density gradient at the rear side) could preserve monoenergeticity. However, in the present Letter we show that obtaining monoenergetic spectra is not straightforward, even neglecting the side effect of the sheath field. We observe monoenergetic peaks in the simulations only when shortduration ion bunches are accelerated by solitary waves generated by the laser-plasma interaction. Moreover, the formation and the evolution of both soliton-or shock-like waves is highly dependent on the velocity distribution of background ions. A crucial point is that, for a ion distribution without a velocity spread, either all the ions or none of them may be reflected from a moving electrostatic field front, depending on the ratio Φ max /v 2 s wher...

Plasma diagnostics of atmospheric plasmas is a key tool in helping to understand processing performance issues. This paper presents an electrical, optical and thermographic imaging study of the PlasmaStream atmospheric plasma jet system. The system was found to exhibit three operating modes; one constricted/localized plasma and two extended volume plasmas. At low power and helium flows the plasma is localized at the electrodes and has the electrical properties of a corona/filamentary discharge with electrical chaotic temporal structure. With increasing discharge power and helium flow the plasma expands into the volume of the tube, becoming regular and homogeneous in appearance. Emission spectra show evidence of atomic oxygen, nitric oxide and the hydroxyl radical production. Plasma activated gas temperature deduced from the rotational temperature of nitrogen molecules was found to be of order of 400 K: whereas thermographic imaging of the quartz tube yielded surface temperatures between 319 and 347 K.

The effect of varying process parameters on atmospheric plasma characteristics and properties of nanometre thick siloxane coatings is investigated in a reel-to-reel deposition process. Varying plasma operation modes were observed with increasing applied power for helium and helium/oxygen plasmas. The electrical and optical behaviour of the dielectric barrier discharge were determined from current/voltage, emission spectroscopy and time resolved light emission measurements. As applied power increased, multiple discharge events occurred, producing a uniform multi-peak pseudoglow discharge, resulting in an increase in the discharge gas temperature. The effects of different operating modes on coating oxidation and growth rates were examined by injecting hexamethyldisiloxane liquid precursor into the chamber under varying operating conditions. A quenching effect on the plasma was observed, causing a decrease in plasma input power and emission intensity. Siloxane coatings deposited in helium plasmas had a higher organic component and higher growth rates than those deposited in helium/oxygen plasmas.

Simulation results are reported for two ion acceleration mechanisms driven by radiation pressure. Threedimensional (3D) simulations of the acceleration of thin foils by circularly polarized pulses ("light sail" regime) at ultra-relativistic intensities (a 0 > 100) show an ion energy that is higher than observed in 1D and 2D simulations, presumably due to density rarefaction and self-wrapping of the laser pulse as the foil is deformed. Simulations of the interaction of linearly polarized pulses with long-scalelength, moderately overdense plasmas at mildly relativistic intensities (a 0 = 1÷ 10) show radiation-pressure driven formation of both solitary and shock waves leading to ion acceleration in the target bulk. In 1D simulations, the spectrum of the accelerated ions is monoenergetic within some range of the initial ion temperature. In 2D simulations, the onset of rippling at the shock surface apparently leads to broadening of the energy spectrum.

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