We have demonstrated the guiding of laser pulses with peak intensities up to 2:2 10 17 W=cm 2 in a 5.5 cm long plasma column containing highly charged Ar ions generated by a fast capillary discharge. A rapid discharge-driven hydrodynamic compression guides progressively lower order modes through a plasma with increasing density and degree of ionization, until the guide collapses on axis. The lowest order mode (FWHM 50 m) is guided with 75% transmission efficiency shortly before the plasma reaches the conditions for lasing in Ne-like Ar. The subsequent rapid plasma expansion forms a significantly leakier and more absorbent guide. The guiding of intense laser beams in plasmas has attracted significant attention [1][2][3][4][5][6][7][8][9]. This is motivated by the need for extending the interaction length between intense laser pulses and plasmas beyond the limitations set by diffractive defocusing and ionization-induced refraction for important applications such as the generation of intense coherent soft x-ray radiation [9][10][11][12] and wakefield accelerators [13,14]. Preformed index waveguides, plasma channels with an electron density minima on axis, were first demonstrated using the hydrodynamic expansion of a cylindrical plasma following a laser-produced spark [3,4,9]. More recently, several approaches based on electrical discharges have been studied, including plasmas created by discharge ablation of the walls of a microcapillary [5], a discharge through a hydrogen filled microcapillary [6], and the plasma implosion in Z-pinch discharges in helium and methane [7,8].An application of considerable interest for plasma waveguides is the longitudinal excitation of soft x-ray lasers that can potentially result in saturated amplifiers with reduced laser pump energy and increased efficiency [9][10][11][12]. Lasing at 60.8 nm by collisional electron excitation of S VIII [10], collisional recombination in Li III at 13.5 nm [11], and optical-field ionization driven collisional excitation in Pd-like Xe at 41.8 nm [12] has been reported in laser-driven plasma channels created in wall ablated [10,11] and gas filled [12] microcapillaries excited by relatively slow discharge current pulses. Particularly promising is the development of transient collisional lasers using Ne-like or Ni-like ions [10,15,16].Herein we report the characterization of multiply ionized plasma waveguides created by a fast Ar capillary discharge of the type used to develop discharge-pumped collisional soft x-ray lasers [17][18][19] and the demonstration of the guiding of laser pulses with peak intensity up to 2:2 10 17 W cm ÿ2 . In contrast to slow capillary discharges these discharges can reach the Ne-like or Ni-like stage of ionization for several atoms of interest [18]. A rapid compression of the plasma column results from the strong current-induced J B force [17]. A shock wave that originates in the vicinity of the capillary wall propagates towards the axis forming a plasma waveguide of continuously decreasing diameter, and increasing density and...
Abstract-Highly ionized Ar plasma channels were created by a fast capillary discharge and used to guide laser pulses with peak intensities up to 2 2 10 17 W/cm 2 over a 5.5-cm distance.These plasmas are of interest for the generation of efficient soft X-ray lasers by longitudinal laser excitation. The guides were characterized using plasma interferometry, modeling, and near field imaging. T HE guiding of intense laser beams in plasmas has attracted significant attention [1]-[7]. This is motivated by the need of extending the interaction length between intense laser pulses and plasmas beyond the limitations set by diffractive defocusing andionization-inducedrefraction.Anapplicationof considerable interest for plasma waveguides is the longitudinal excitation of soft X-ray lasers that can potentially result in saturated amplifiers with reduced laser pump energy and increased efficiency.Herein, we report images related to the characterization of multiply ionized plasma waveguides created by a fast Ar capillary discharge of the type used to develop collisional soft X-ray lasers [8]. The experiments used ps, 800-nm Ti:Sa laser pulses, except for the demonstration of high-intensity beam guiding that was done using fs pulses. The discharge utilized 3.2-mm diameter, 5.5-11-cm-long Al O capillaries filled with 180 to 150 mtorr of Ar. The discharge current pulses had a peak amplitude of 15-21 kA,and a half-period of ns. Interferometry of the plasma shows the evolution of an annular plasma shell with density minimum at the center that constitutes a waveguide of continuously decreasing diameter and increasing density until it collapses on axis (Fig. 1). The plasma compression is accompanied by an increase in the degree of ionization, reaching shortly before the collapse, as corroborated during these experiments by the observation of lasing at 46.9 nm in Ne-like Ar.The guiding properties of a 5.5-cm-long capillaries were investigatedbyimaging theoutputofthecapillary[ Fig.2(right) modelingthebeampropagationusingtheKirchhoff-Fresnel integral inputing the radially dependent phase delay determined from the interferometrically measured density profile [ Fig. 2(left)]. For times less than 30 ns, with respect to beginning of the current pulse, the plasma density is low and the peak of the electron density is far from the axis, resulting in no guiding of the beam. When theplasmacolumncompressesto m indiameter,thewings of the laser pulse begin to experience a phase shift that results in output modes with concentric circles as shown in Fig. 2(a). As the column continues to compress a variety of lower order modes, such as that illustrated in Fig. 2(b), are seen. Fig. 2(c) shows an approximately gaussian output mode with a full-width half-maximum (FWHM) of m. Subsequently, the guide is lost as the plasma pinches and the electron density profile becomes convex, resulting in the beam being strongly refracted, as shown in Fig. 2(d). The development of a second guiding phase was observed to take place during the first few nanoseconds of the...
The output energy dependence of high repetition rate grazing incidence pumped Ni-like Mo, Ni-like Ag, and Ne-like Ti transient collisional soft x-ray lasers on the duration of the pump pulse was studied combining experiments and model simulations. Lasing is observed to occur for a wide range of pump pulse widths ͑e.g., 2 to 18 ps for Ni-like Ag͒, with maximum output occurring for pump pulses of 4-6 ps FWHM, corresponding to pump intensities of 1.4-2.0ϫ 10 14 W/cm 2. Moderately short pump pulses are observed to be optimum for lasers that make use of preplasmas in which the mean degree of ionization approaches the charge of the lasing ion, while long pump pulses produce over-ionization and weaker lasing. However, long pump pulses are capable of producing gain and lasing in low Z ions even when the preplasma has a very low degree of ionization after expanding for 5 ns. As the duration of the pump pulse shortens the optimum delay respect to the prepulse is observed to decrease. The physics that determines these and other measured trends is discussed.
We report the generation of dense plasma waveguides containing a large concentration of silver ions by means of a fast ͑ϳ55 ns first half-cycle͒ microcapillary discharge. Concave plasma density profiles with axial electron density Ͼ1 ϫ 10 19 cm −3 were measured from discharge ablation of 330 or 440 m diameter Ag 2 S capillaries with 3 -5 kA peak amplitude current pulses. The dynamic of this plasma waveguide was studied with interferometry, absorption measurements, and hydrodynamic model simulations. The results are relevant to the development of efficient longitudinally pumped metal vapor soft x-ray lasers, in particular those employing transient excitation of Ni-like ions. An approach to the design of a gain saturated waveguided 13.9 nm laser in Ni-like Ag is discussed.
Axial injection in plasma gun through the cathode has clear benefit of longer particle residence time and optimum particle trajectory in the plume; however, accelerated wear of the cathode seem to be the major issue in this approach. This study investigates the arc instability phenomena in an axially injecting single cathode plasma torch design. Gun voltage measurements were used to evaluate the arc behavior. For comparison purpose, arc fluctuations with a standard solid cathode torch design under identical operating parameters have also been studied. A comparison of different internal hardware configurations is also done to understand and establish the important factors in the design of the axial injection and solid cathode systems. Further, this study presents the influence of plume elongation and accelerated gas velocities on the arc behavior in different configurations under low pressure environment.
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