A. L. Astanovitskiy

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.

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.

Observation of impact of eddy current on laser targets in a strong fast rising magnetic field

Pulsed power generators are capable of producing strong magnetic fields. Foil laser targets for high intensity interactions with matter were characterized in a strong magnetic field produced by different types of loads. Longitudinal magnetic fields generated by coil loads were measured by Faraday rotation of the polarization plane of laser beams in a small glass sample at two wavelengths. Magnetic fields of 0.8–1.5 MG were measured in different coil loads at 1 MA current. For the first time, the impact of eddy current generated by the fast rising magnetic field with dB/dt = 109 T/s was observed in Al laser targets using shadowgraphy and interferometry. Material of the Al 10 μm foil target expanded from both sides at 0.1–0.2 mm. This effect was not observed in dielectric targets. The impact of eddy current on targets is important for designing experiments for laser plasma interactions in strong magnetic fields.

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.

Experimental platform for investigations of high-intensity laser plasma interactions in the magnetic field of a pulsed power generator

An experimental platform for the studying of high-intensity laser plasma interactions in strong magnetic fields has been developed based on the 1 MA Zebra pulsed power generator coupled with the 50-TW Leopard laser. The Zebra generator produces 100-300 T longitudinal and transverse magnetic fields with different types of loads. The Leopard laser creates plasma at an intensity of 1019 W/cm2 in the magnetic field of coil loads. Focusing and targeting systems are integrated in the vacuum chamber of the pulsed power generator and protected from the plasma debris and strong mechanical shock. The first experiments with plasma at laser intensity >2 × 1018 W/cm2 demonstrated collimation of the laser produced plasma in the axial magnetic field strength >100 T.

Study of 1 MA wire array Z pinches using x-ray radiography and UV laser diagnostics

Summary form only given. X-ray laser-based imaging backlighting was developed at the 1-MA Zebra generator at UNR. Al and Ti star and cylindrical wire arrays were studied using backlighting at the wavelength of 6.65 Å and UV laser diagnostics at 266 nm. X-ray imaging allows for the viewing of the dense core of plasma column during the ablation stages with a resolution of 20 um. X-ray backlighting was provided by hitting a Si target with the 50 TW Leopard laser, which provides the advantage of flexible timing during the ablation and implosion stages.UV laser diagnostics were fielded at the same stage. UV shadowgraphy provides a laser backlit images of wire arrays Z pinches at all stages. UV interferometry allows for the measurement of electron density of plasma of ablating wires. UV and x-ray probing diagnostics allow study of the plasma corona and dense core during ablation stage of the 1 MA wire arrays.

UV laser diagnostics for the dense Z-pinch

Summary form only given. Ultraviolet (UV) laser diagnostics are powerful tools for investigation of high-energy-density plasma. UV diagnostics at 266 nm were developed for investigation of dense Z-pinch plasma at the 1 MA Zebra generator. A three-channel diagnostic can be configured as two-frame shadowgraphy and interferometry or a Faraday rotation diagnostic. Spatial resolution of diagnostics is 5-15 μm depending on the beampath configuration. Absorption and refraction of UV radiation in dense plasma is significantly smaller compare to 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 and currents in Z-pinch plasma with an unprecedented level of detail [1]. Mictopinches with diameters of 60-100 μm 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 3D MHD Gorgon simulations. Interferometry at the wavelength of 266 nm allow measurement of plasma density in the range of (13)x1020 cm-3 in the ablating wires, imploding plasma, stagnating pinch, and trailing material. Fast plasma motion with a speed > 100 km/s was observed at the stagnation stage with two-frame shadowgraphy. Plasma motion at stagnation and prolonged implosion of trailing mass provide additional kinetic energy in the stagnated 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. Magnetic fields >0.5 MG were measured in the pinch. Current in the pinch can switch from the highinductance neck and redistribute to the trailing material when resistance of peripheral plasma drop due to heating by x-ray radiation. The formation of hot spots in the Z-pinch was analyzed with UV diagnostics and x-ray streak camera. Further development of UV diagnostics to 211nm VUV range can help to apply well established optical methods to Z-pinch plasma in multi-MA pulsed power facilities.

Study of dynamics of hot spots in wire-array Z-pinches

Summary form only given. Hot spots with a high plasma density and temperature spontaneously arise in all kinds of Z-pinches with currents from 0.1 to 4 MA. Instabilities generate bright and hot spots with the enhanced electron temperature and density. The electron temperature of 9 keV was measured in the hot spot of the X-pinch with current of 5 MA. The formation of hot spots can be interpreted as a result of the collapse of the Z-pinch due to the radiative loss of energy and plasma flowing out of the spot. Hot spots carry current and can generate strong MG magnetic fields in the Z pinch. Hot spots contribute to the energy balance and neutron yield in Z pinches. In our experiments bright radiating spots were observed in small diameter cylindrical and planar wire-arrays. Bubble-like implosion in wire arrays initiates the formation of necks on the pinch which produce hot spots. We investigated hot spots at the 1 MA Zebra generator using synchronized x-ray streak camera, laser diagnostics at 266 and 532 nm, and x-ray time-gated pinhole camera. Hot spots in Al wire arrays generate x-ray bursts with durations of 0.4-1 ns in the soft range and 150-400 ps in the keV range. Continuum radiation with photon energy > 2.5 keV was observed in hot spots generated in Al Z pinches. UV two-frame shadowgraphy shows spatial correlation of hot spots with necks on the pinch. An analysis of x-ray streak images shows that a collapse of a single hot spot can generate >10% of radiated x-ray power of the 1 MA Z pinch.

Study of implosion dynamics, the x-ray yield and plasma interpenetration in star wire arrays with gates in the inner cylinder

Star wire arrays with two closely located wires (“gates”) on the inner cylinder of star wire arrays were studied. The gate wires were used to study plasma interpenetration and reproduce transparent and non-transparent regimes of propagation of the imploding plasma through the gates. The non-transparent mode of collision is typical for regular star wire arrays and it was also observed in Al stars with gate wires of regular length. Gated star arrays demonstrate similar x-ray yield but slightly different delay of x-ray generation compared to regular stars. Double length wires were applied as gate wires to increase their inductance and resistance and to increase transparency for the imploding plasma. The wires of the gates were made of Al or high atomic number elements, while the rest of the arrays were regular length Al wires. An intermediate semi-transparent mode of collision was observed in Al stars with long Al gate wires. Arrays with long heavy-element gate wires demonstrated transparency to plasma passing through. Shadowgraphy at the wavelength of 266 nm showed that plasma moved through the gate wires. Double implosions, generating a double-peak keV X-ray pulse, were observed in star arrays when the gates were made of high atomic number elements. A new laser diagnostic beampath for vertical probing of the Z-pinch was built to test how wires could be used to redirect plasma flow. This setup was designed to test gated arrays and further configurations to create a rotating pinch. Results on plasma flow control obtained are discussed, and compared to numerical calculations.