Alexey Astanovitskiy

Effect of current prepulse on wire array initiation on the 1-MA ZEBRA accelerator.

Experiments on the 1-MA ZEBRA accelerator with reduced current prepulse duration, using a flashover switch, demonstrate a significant increase of initial energy deposition into the tungsten wire array before breakdown, and of total radiation energy from the Z pinch. Shorter current prepulse raises the current rate through each individual wire in the array and results in an increase of the energy deposition into wire cores before breakdown. In our experiments, the inferred tungsten wire temperature increases from ∼800K (with 250ns prepulse) up to ∼3700K (with 60ns prepulse). Total radiation energy increases from 12 to 16kJ. Our experimental results relate wire-array initiation to heating of the individual array wires up to the time of breakdown.

Development of UV Laser Probing Diagnostics for 1-MA Z-Pinches

Laser probing diagnostics at the wavelength of 266 nm were developed for investigation of the 1-MA z-pinch plasmas. The absorption and refraction in plasma are significantly smaller at 266 nm than at the regular wavelength of 532 nm. These features allow observation of fine details in the z-pinch plasma at the ablation, implosion, and stagnation phases. Two-color shadowgraphy at 532/266 nm presents a structure of ablating wires and implosion bubbles in wire arrays. Plasma distribution and dynamics in compact cylindrical, star, and planar wire arrays can be studied at the wavelength of 266 nm. An electron density Ne > 5 · 1019 cm-3 was reconstructed with interferometry at 266 nm in the stagnated z-pinch. Further development of laser probing diagnostics of the z-pinch plasmas is discussed.

High-Resolution UV Laser Diagnostics on the 1-MA Zebra Generator

Laser diagnostics at the wavelength of 266 nm with a spatial resolution of 5-8 μm were developed for the investigation of dense Z-pinches at the 1-MA generator. The absorption and refraction in Z-pinch plasma are significantly smaller at the ultraviolet (UV) wavelength of 266 nm compared to the optical range. This allows for observation of the fine internal structure of Z-pinches at the stagnation phase. A UV laser beam penetrates through the trailing plasma around Z-pinches and shows strong instabilities of the dense pinch inside this plasma column. Kink instability, necks, and areas of disruption are seen in Z-pinches at the peak of the X-ray pulse and later in time. UV two-frame side-on and end-on shadowgraphy show plasma dynamics in the pinch at stagnation.

Development of z-pinch laser diagnostics at the Zebra generator

The 50 TW Leopard laser coupled with the 1-MA Zebra generator was used for the development of new diagnostics for z-pinch plasmas. Two plasma diagnostics are presented: an x-ray broadband backlighting for z-pinch absorption spectroscopy and parametric two-plasmon decay of the laser beam in dense z-pinch plasma. The implementation of the new diagnostics on the Zebra generator and the first results are discussed. The absorption spectroscopy is based on backlighting the z-pinch plasma with broadband x-ray radiation from the Sm laser plasma. The absorption spectroscopy can deliver data about the electron temperature and density of z-pinch plasma at the non-radiative stage. The parametric two-plasmon decay of intensive laser radiation generates 3/2ω and 1/2ω harmonics. These harmonics can be used to derive the temperature of the z-pinch plasma with the electron density near the quarter of critical plasma density [1]. Ultraviolet (UV) laser probing diagnostics at the wavelength of 266 nm have been developed for the investigation of the 1-MA Z-pinch plasma. The smaller absorption and refraction in dense plasma at the wavelength of 266 nm allows for a deeper penetration into the dense Z-pinch plasma. These features allow for the observation of fine details in the Z-pinch plasma at the implosion and stagnation phases. A shadowgraphy channel with a spatial resolution of 4 µm was tested. An electron density Ne>5·1019 cm-3 was measured directly in the stagnated Z-pinch with interferometry at 266 nm. Further development of laser probing diagnostics includes a UV Faraday rotation diagnostic and two-frame UV shadowgraphy. UV laser probing diagnostics give the opportunity to investigate the micro structures and current distribution in the stagnated Z-pinch.