UV Laser-Probing Diagnostics for the Dense Z Pinch

Abstract

Laser diagnostics at 266 nm were developed for the investigation of dense Z-pinch plasma at the 1 MA Zebra generator. A three-channel diagnostic can be configured as shadowgraphy and interferometry with two temporal frames or as a Faraday rotation polarimeter. Absorption and refraction of ultraviolet (UV) radiation in dense plasma is significantly smaller compared with regular diagnostics at the wavelength of 532 nm. Therefore, UV diagnostics allow direct investigation of the fine structure of the dense Z-pinch, development of instabilities, and a distribution of magnetic fields in Z-pinch plasma. Micropinches and instabilities with characteristic scales of 15-200 μm were observed in 1 MA wire-array Z pinches. Development of instabilities in wire-array Z pinches is in agreement with the magnetohydrodynamic simulations. Interferometry at the wavelength of 266 nm allows measurement of plasma density in the range (1-2)×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">20</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> in the ablating wires, imploding plasma, stagnating pinch, and trailing material. A fast plasma motion was observed at the stagnation stage with two-frame shadowgraphy. Plasma motion at stagnation and prolonged implosion of trailing mass can provide the additional kinetic energy in the Z pinch and can be a source of enhanced X-ray radiation. A Faraday rotation diagnostic reveals a distribution of magnetic fields in the pinch and trailing material. The magnetic field strength and current were reconstructed from the rotation angles and phase shifts in plasma using the Abel transform. Current in the pinch can switch from the high-inductance neck and redistribute to the trailing material when resistance of peripheral plasma drops owing to heating by X-ray radiation. Further development of UV diagnostics to short wavelengths can help to apply well-established optical methods to Z-pinch plasma in multiMA pulsed power facilities.