A. A. Anderson

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.

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.

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.

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.

PPPS-2013: Study of ablation and implosion stages of 1 MA wire array z-pinches using UV laser diagnostics

The ablation and implosion stages of wire array z-pinches was studied using UV diagnostics at the wavelength of 266 nm. Experiments were conducted using Al cylindrical, star, and planar wire arrays at the 1 MA Zebra pulsed-power generator at UNR. UV interferometry allows direct study of electron plasma density > 1020 cm-3. We measured the density structure and dynamics in the ablating wires with 2-frame UV interferometry. Faraday rotation diagnostics at the wavelength of 266 nm were developed and applied to the ablation and implosion stages in wire arrays. Comparison of Faraday images with shadowgrams and interferograms allows for the measurement of magnetic fields to reconstruct the current in the imploding plasma and non-imploded material in 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.