Soft X-ray laser diagnostics of exploding aluminum wire plasmas

Abstract

The evolution of current-driven thin single Al wire plasmas was studied with soft X-ray shadowgraphy and interferometry using a very compact capillary discharge-driven λ = 46.9 nm laser probe. Wires of 25 µm diameter excited by current pulses with a 78 A/ns increase rate were observed to expand uniformly at a rate of 3.5 µm/ns. In contrast, an increase in the rate of energy deposited per unit mass was observed to give rise to significant early plasma instabilities. The results illustrate the use of table-top soft X-ray lasers as a new tool for the diagnostics of dense pulse power driven plasmas.PACS 52.70.Kz; 52.59.Qy; 52.25.Vy; 52.50.Nr IntroductionInterferometry with optical lasers has been used for decades to obtain multi-dimensional maps of the electron density in a large variety of plasmas [1]. However, free-free absorption and refraction of the probe beam limit the maximum electron density, plasma size, and plasma density gradient that can be probed [2]. More recently, laboratory-size soft X-ray lasers have been used to extend the limits of plasma interferometry to larger plasma-scale lengths and higher plasma densities. A 15.5 nm Ne-like Y laser pumped by the Nova laser has successfully probed dense laser-created plasmas produced at Lawrence Livermore National Laboratory [2]. Due to limitations in the repetition rate of the X-ray laser, the experiments were limited to a few shots per day [2,3]. Recently, the use of compact saturated soft X-ray lasers [4][5][6] and the development of more robust interferometers [7] have advanced soft X-ray laser diagnostics as a practical tool for the study of high density plasmas [4][5][6][7]. Results of the study of the dynamics of several different laser-created plasmas have been recently reported [5][6][7]. Extremely compact capillary discharge soft-X-ray lasers [8,9], which are portable and have spectral brightness similar to or higher than that of their laboratory-size predecessors, produce laser pulses of about 1 ns duration that are particularly well suited for the diagnostics of dense pulse-power driven plasmas. Herein we report the results of the study of the evolution of current-induced u Fax: +1-970/491-8671, E-mail: rocca@engr.colostate.edu explosions of thin wires using a capillary discharge 46.9 nm laser in combination with a Mach-Zehnder interferometer in which diffraction gratings are used as beam splitters.The dynamics of thin exploding metal wire plasmas is of significant interest for the efficient generation of incoherent X-ray pulses of extremely high energy [10][11][12]. Fast Z-pinch implosions of cylindrical wire arrays have been shown to generate high energy density plasmas that emit very intense bursts of incoherent X-ray radiation capable of driving inertial confinement fusion targets [11]. In those experiments the main implosion phase, driven by multi-mega ampere currents, is preceded by a phase in which the heating, vaporization, and ionization of the wires in the array causes an expansion and merging of the individual plasmas into a co...