D. Papp

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.

Study of micro-pinches in wire-array Z pinches

Bright and hot areas with a high plasma density and temperature are observed in all kinds of Z pinches. We studied bright radiating spots produced by micro-pinches in cylindrical and planar wire-arrays at the 1 MA Zebra pulsed power generator using an x-ray streak camera synchronized with laser diagnostics, x-ray time-gated pinhole camera, and spectroscopy. Hot spots with extremely dense and relatively hot plasma arise during the collapse of the micro-pinches. These hot spots radiate a continuum spectrum with energy >2.5 keV. Typical micro-pinches in Al wire arrays generate x-ray bursts with durations of 0.4–1 ns in the soft x-ray range and 0.1–0.4 ns in the keV range. UV two-frame shadowgraphy shows spatial correlation of hot spots with the collapse and explosion of micro-pinches. Micro-pinches typically occur at the necks of the Z pinch, but can demonstrate a variety of parameters and different dynamics. An analysis of x-ray streak images shows that micro-pinches can generate >20% of the x-ray energy in some types of wire-array Z pinches.

Study of transparent and nontransparent regimes of implosion in star wire arrays

Star wire arrays were used to control the imploding plasma flows and study plasma interpenetration. These arrays consisted of linear “rays” aligned azimuthally and extending from the vertical axis. Star arrays with two close located wires (“gates”) instead of a single wire on the inner cylinder were studied for transparent and nontransparent regimes of propagation of imploding plasma through the gates. Nontransparent mode of collision is typical for regular star wire arrays and it was also observed in Al stars with gate wires of regular length and with the gate width of 0.3–2 mm. The cascade process of implosion in stars and trapping of imploding plasma in 1–2 mm gates were modeled with the three-dimensional resistive magnetohydrodynamics code. The intermediate semitransparent mode of collision was observed in Al stars with long Al “gate” wires. A transparent mode was observed in Al stars with long stainless steel or W gate wires. Applications of wire arrays with controlled plasma flows are discussed.

UV Laser-Probing Diagnostics for the Dense Z Pinch

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)×1020 cm-3 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.

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.

A Proposed 100-kHz fs Laser Plasma Hard X-Ray Source at the ELI-ALPS Facility

Femtosecond lasers are a proven source of hard X-rays for ultrafast probing applications; however, such X-ray sources do not offer significant advantages over accelerator-based photon sources. In this paper, the use of the proposed 100-kHz two-cycle ALPS-HR laser at the ELI-ALPS facility to drive a femtosecond hard X-ray source is investigated and target-related issues for high repetition rates are also discussed. A particle-in-cell simulation is used to determine the optimal plasma scale length for the investigated laser parameters to generate the necessary hot electron spectrum. Electron transport and photon creation are simulated using a Monte Carlo method in liquid droplet targets. The generated photon characteristics are postprocessed to build the synthetic X-ray spectrum and temporal pulse shape. The X-ray source characteristics are compared with other similar laser plasma X-ray sources. The parameters for the preliminary experiments are also discussed.

Study of ablation and implosion stages in wire arrays using coupled ultraviolet and X-ray probing diagnostics

Star and cylindrical wire arrays were studied using laser probing and X-ray radiography at the 1-MA Zebra pulse power generator at the University of Nevada, Reno. The Leopard laser provided backlighting, producing a laser plasma from a Si target which emitted an X-ray probing pulse at the wavelength of 6.65 A. A spherically bent quartz crystal imaged the backlit wires onto X-ray film. Laser probing diagnostics at the wavelength of 266 nm included a 3-channel polarimeter for Faraday rotation diagnostic and two-frame laser interferometry with two shearing interferometers to study the evolution of the plasma electron density at the ablation and implosion stages. Dynamics of the plasma density profile in Al wire arrays at the ablation stage were directly studied with interferometry, and expansion of wire cores was measured with X-ray radiography. The magnetic field in the imploding plasma was measured with the Faraday rotation diagnostic, and current was reconstructed.

Study of the precursor and non-precursor implosion regimes in wire array Z-pinches

Star-like and closely spaced nested wire array configurations were investigated in precursor and non-precursor implosions. Closely spaced nested cylindrical arrays have inner and outer arrays with equal wire numbers, and inner and outer wires aligned to each other. The gap between the outer and inner wires is not more than 1 mm. Calculation of magnetic fields shows that the small gap results in a reversed, outward j × B force on the inner wires. Closely spaced arrays of 6–16 wires with outer diameter of 16 mm and with gaps of ΔR = 0.25–1 mm were tested. 6–8-wire arrays with a gap of ΔR = 0.4–1 mm imploded without precursor, but precursor was present in loads with 12–16 wires and ΔR = 0.25–1 mm. Implosion dynamics of closely spaced arrays was similar to that of star-like arrays. Implosion time was found to decrease with decreased wire numbers. Star array configurations were designed with a numerical scheme to implode with or without precursor. The lack of precursor resulted in a marginal improvement in tot...

Study of magnetic fields and current in the Z pinch at stagnation

The structure of magnetic fields in wire-array Z pinches at stagnation was studied using a Faraday rotation diagnostic at the wavelength of 266 nm. The electron plasma density and the Faraday rotation angle in plasma were calculated from images of the three-channel polarimeter. The magnetic field was reconstructed with Abel transform, and the current was estimated using a simple model. Several shots with wire-array Z pinches at 0.5–1.5 MA were analyzed. The strength of the magnetic field measured in plasma of the stagnated pinch was in the range of 1–2 MG. The magnetic field and current profile in plasma near the neck on the pinch were reconstructed, and the size of the current-carrying plasma was estimated. It was found that current flowed in the large-size trailing plasma near the dense neck. Measurements of the magnetic field near the bulge on the pinch also showed current in trailing plasma. A distribution of current in the large-size trailing plasma can prevent the formation of multi-MG fields in the Z pinch.

X-ray emission from a liquid curtain jet when irradiated by femtosecond laser pulses

Laser-based sources of ionizing radiation have attracted considerable attention in the last years for their broad potential applications. However, the stability and robustness of such sources are still issues that need to be addressed. Aiming to solve such problems, we propose a source that uses a liquid jet—rather than a solid—as a target for the production of X-rays. Liquid jets offer always a clean surface for every laser shot which represent a clear advantage over solids. In this work, we present an experimental characterization of the X-ray emission of such targets, and study the efficiency of the process when two temporally delayed pulses are used. According to the obtained results, the X-ray yield is comparable with commonly used targets.