S. D. Altemara

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.

Observation of Cascade Implosions in “Star”-Like Wire-Array

The dynamics of implosions in ldquostarrdquo-like wire arrays were investigated in the 1-MA Zebra generator. The hydrodynamic mode of implosion was confirmed by several plasma diagnostics, including laser probing, an optical streak camera, and a time-gated charge-coupled-device camera. Implosion begins on the edge wires and cascades from wire to wire accelerating toward the center.

Z

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.

-Pinches on the Zebra Generator

Summary form only given. Copper and aluminum mm-diameter rods have been driven by a mega-ampere current pulse at UNRs Nevada Terawatt Facility. The facilitys z-pinch delivers 1 M A in ~100 ns producing megagauss surface magnetic fields that diffuse into the skin layer, ohmically heating the load and causing plasma formation. The load radius is designed such that it is in the “thick wire” regime; the radius is much thicker than the skin depth. With the novel “barbell” design of our loads, plasma formation due to arcing or electron avalanche is avoided, allowing for the study of ohmically heated loads. Work presented here will show first evidence of a magnetic field threshold for plasma formation in copper and compare with previous work done with aluminum1. Similarities and differences between these metals will be presented, giving motivation for continued work with different material loads. During the current rise, the metal is heated to temperatures that cause multiple phase changes. When the surface magnetic field reaches a threshold, the metal ionizes and the plasma becomes pinched against the underlying cold liquid metal. Diagnostics fielded include visible light radiometry, two-frame shadowgraphy in both 266 and 532 nm wavelengths, 266 nm interferometry, time gated EUV spectroscopy, 12-frame/5 ns gated imaging, and single frame/2 ns gated imaging with an ICCD camera. Surface temperature, expansion speeds, instability growth, time of plasma formation and plasma uniformity are determined from the data. The interplay between an ohmically heated conductor and a magnetic field is important for the field of Magnetized Target Fusion (MTF). MTF compresses a magnetized fuel by imploding a flux conserving metal liner. During compression, fields reach several megagauss, with a fraction of the flux diffusing into the metal liner. The magnetic field induces eddy currents in the metal, leading to ionization and potential mixing of metal contaminant into the fusion fuel.

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

Plasma dynamics in cylindrical closely coupled and star wire arrays at ablation and implosion stages were studied. Formation of the Z-pinch and its radiative properties were studied in the non-precursor regime and compared with regular ablation regime with a precursor.

Plasma formation and evolution on a copper surface driven by megaampere current pulse

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.

The study of ablation and implosion dynamics in closely coupled nested cylindrical and star wire array Z pinches

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.

Development of z-pinch laser diagnostics at the Zebra generator

Star-like wire arrays [1] with two closely located wires (“gates”) on the inner cylinder were studied on the 1-MA Zebra generator. The hydrodynamic mode of collision is typical for regular star-like wire arrays [2] and it was observed with shadowgraphy and optical streak cameras. In this mode, star-like wire arrays generate a short 8-12 ns x-ray pulse [1]. The star-like wire arrays were modified to reach a transparent mode typical for multiwire nested arrays [3–4].

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

The 532 nm laser diagnostic set at the Zebra generator shows the details of the ablation and stagnation phases in cylindrical, planar, and star‐like wire arrays but it cannot show the structure of the stagnated z‐pinch and the implosion in small diameter loads, 1–3 mm in diameter. The absorption increment and the refraction angle of the 532 nm laser, when passing through the plasma, are too great to obtain quality images. An ultraviolet probing beam at the wavelength of 266 nm was developed to study small‐diameter loads and to investigate the structure of the 1‐MA z‐pinch. The UV radiation has a much smaller absorption increment and refraction angles in plasmas than the 532 nm light and allows for better imaging of the z‐pinch plasmas. Estimates showed that UV probing would be able to probe the high‐density z‐pinch plasma in experiments on the Zebra generator, and the early results of UV probing on the Zebra generator have shown promise.

Study of implosions in star-like wire arrays with “gates” on the inner cylinder

Summary form only given. The effect of current prepulse on wire array core temperature on the 1-MA z-pinch installation ZEBRA was investigated using current, light emission, 4-channel laser shadowgraphy, visible ICCD camera and PCD/XRD diagnostics. A vacuum flashover switch reduced the original ~250ns prepulse on ZEBRA to as short as ~50ns. Shortened prepulse increases the initial current rate through the individual wires and this in turn leads to more energy deposited into the wire cores (higher temperature) at breakdown. Shorter current prepulse also improved the axial homogeneity for deposited energy. Better axial homogeneity of deposited Joule energy improves the axial symmetry of precursor plasma and reduces its typically observed m=l instability. Reduction of the precursor instability results in the generation of a single X-ray pulse rather than the multiple pulses observed for unstable precursor. Multi-frame shadowgraphy shows that discrepancy in the initial wire-electrode contacts results in differences in energy deposition and ablation from wire to wire. Keeping similar contacts for all wires in the array is critical for azimuth ablation uniformitv. Shortening the current prepulse using a flashover switch results in an increase of light emission peak and wire core temperature. Core temperature rises from ~800K to ~1100K and finally to ~3100K. at moment of breakdown. Moment t=0 corresponding to the beginning of main current pulse.