D. A. Chalenski

Diagnostics on the COBRA pulsed power generator

The COBRA pulsed power generator has a variable current pulse wave form and amplitude (95–180ns rise time, up to 1MA peak current). It was designed to study wire array Z pinches and X pinches, including plasma formation, pinch implosion dynamics, and pinch plasma parameters as a function of current rise time. These loads have been studied using an extensive set of diagnostics with spatial and/or temporal resolution. The set of electrical diagnostics on the COBRA generator includes Rogowski coils to monitor the total load current and the current through individual return current posts, and there is also an inductive voltage monitor. A set of extreme ultraviolet and x-ray detectors is used to study the load radiation. Wire array and X pinch plasma formation and dynamics are studied using two-frame, point projection X-pinch x-ray imaging as well as with multiframe laser probing. Flat potassium acid phtalate crystal (KAP), convex, extreme luminosity imaging conical spectrograph, and focusing spectrograph with...

The Role of Flux Advection in the Development of the Ablation Streams and Precursors of Wire Array Z‐pinches

B‐dot probes for the first time have successfully measured the field through implosion of wire arrays on COBRA. The probe data confirm an advective magnetic evolution of closed field lines during the onset of ablation that was first seen in 2D GORGON simulations.

Implosion dynamics and radiation characteristics of wire-array Z pinches on the Cornell Beam Research Accelerator

Experimental results are presented that characterize the implosion dynamics and radiation output of wire-array Z pinches on the 1-MA, 100-ns rise-time Cornell Beam Research Accelerator (COBRA) [J. B. Greenly et al., Rev. Sci. Instrum. 79, 073501 (2008)]. The load geometries investigated include 20-mm-tall cylindrical arrays ranging from 4to16mm in diameter, and consisting of 8, 16, or 32 wires of either tungsten, aluminum, or Invar (64% iron, 36% nickel). Diagnostics fielded include an optical streak camera, a time-gated extreme-ultraviolet framing camera, a laser shadowgraph system, time-integrated pinhole cameras, an x-ray wide-band focusing spectrograph with spatial resolution, an x-ray streak camera, a load voltage monitor, a Faraday cup, a bolometer, silicon diodes, and diamond photoconducting detectors. The data produced by the entire suite of diagnostics are analyzed and presented to provide a detailed picture of the overall implosion process and resulting radiation output on COBRA. The highest x-r...

Ablation dominated implosion dynamics of aluminum and stainless steel nested cylindrical wire arrays

The results of experiments with combined aluminum (Al) and stainless steel (SS) alloy 304, nested wire arrays from the 1 MA COBRA generator at Cornell University are presented. The loads studied consisted of a 6 mm diameter inner array and a 13 mm diameter outer array with a different material in each array: SS or aluminum. Al implodes before SS in all loads studied, even when Al was on the inner array. The new wire ablation dynamic model and spectroscopic modeling are used to interpret these data. The observed implosion dynamics are likely a result of the higher ablation rate of Al. These initial results suggest that combining wire materials with different ablation rates in wire array loads could be developed into a useful technique for x-ray pulse shaping and radiation yield optimization.

Ablation dynamics in coiled wire-array Z-pinches

Experiments to study the ablation dynamics of coiled wire arrays were performed on the MAGPIE generator (1 MA, 240 ns) at Imperial College, and on the COBRA generator at Cornell Universitys Laboratory of Plasma Studies (1 MA, 100 ns). The MAGPIE generator was used to drive coiled wires in an inverse array configuration to study the distribution of ablated plasma. Using interferometry to study the plasma distribution during the ablation phase, absolute quantitative measurements of electron line density demonstrated very high density contrasts between coiled ablation streams and inter-stream regions many millimetres from the wire. The measured density contrasts for a coiled array were many times greater than that observed for a conventional array with straight wires, indicating that a much greater axial modulation of the ablated plasma may be responsible for the unique implosion dynamics of coiled arrays. Experiments on the COBRA generator were used to study the complex redirection of plasma around a coiled wire that gives rise to the ablation structure exhibited by coiled arrays. Observations of this complex 3D plasma structure were used to validate the current model of coiled array ablation dynamics [Hall et al., Phys. Rev. Lett. 100, 065003 (2008)], demonstrating irrefutably that plasma flow from the wires behaves as predicted. Coiled wires were observed to ablate and implode in the same manner on both machines, indicating that current rise time should not be an issue for the scaling of coiled arrays to larger machines with fast current rise times.Experiments to study the ablation dynamics of coiled wire arrays were performed on the MAGPIE generator (1 MA, 240 ns) at Imperial College, and on the COBRA generator at Cornell Universitys Laboratory of Plasma Studies (1 MA, 100 ns). The MAGPIE generator was used to drive coiled wires in an inverse array configuration to study the distribution of ablated plasma. Using interferometry to study the plasma distribution during the ablation phase, absolute quantitative measurements of electron line density demonstrated very high density contrasts between coiled ablation streams and inter-stream regions many millimetres from the wire. The measured density contrasts for a coiled array were many times greater than that observed for a conventional array with straight wires, indicating that a much greater axial modulation of the ablated plasma may be responsible for the unique implosion dynamics of coiled arrays. Experiments on the COBRA generator were used to study the complex redirection of plasma around a coile...

Soldered contact and current risetime effects on negative polarity wire array Z pinches

The experimental results described in this paper were motivated by earlier, low current, single wire experiments. In these experiments, single 10–25 μm diameter wires were driven by 1–5 kA current pulses with variable dI/dt from 5 to 60 A/ns. The amount of energy deposited in the wires, the expansion rate, and expansion uniformity that occurred before a plasma induced voltage collapse were found to depend on the polarity, dI/dt, and the quality of the contacts between the wires and the electrodes. This paper reports the results of experiments with cylindrical wire arrays driven by Cornell Beam Research Accelerator (COBRA) [J. B. Greenly, J. D. Douglas, D. A. Hammer et al., Rev. Sci. Instrum. 79, 073501 (2008)] current pulses that reached 1 MA. The pulse lengths were varied from 100 to 200 ns. These larger current pulses drove the wires of the array through the initiation phase studied in the single wire experiments and through ablation and Z-pinch implosion to stagnation on the cylindrical axis of the arr...

Azimuthally correlated ablation between z-pinch wire cores

Azimuthally correlated wire core ablation was compared for closely spaced versus widely spaced wires in a 1 MA Z-pinch. X-ray point-projection diagnostics revealed that 240 μm spaced wires exhibited a correlation coefficient approaching unity in both real space and in k-space. This correlated ablation between wires at a fixed axial location is believed to occur due to an enhanced, localized Joule heating. Wires separated by 2.47 mm or greater were uncorrelated in real space, but correlated in k-space, indicating the ablation structure between wires was shifted in phase.

Modifying Wire Array Z-pinch Ablation Structure and Implosion Dynamics Using Coiled Arrays

Coiled arrays, a cylindrical array in which each wire is formed into a helix, suppress the modulation of ablation at the fundamental wavelength. Outside the vicinity of the wire cores, ablation flow from coiled arrays is modulated at the coil wavelength and has a 2‐stream structure in the r,θ plane. Within the vicinity of the helical wires, ablation is concentrated at positions with the greatest azimuthal displacement and plasma is axially transported from these positions such that the streams become aligned with sections of the coil furthest from the array axis. The GORGON MHD code accurately reproduces this observed ablation structure, which can be understood in terms of J×B forces that result from the interaction of the global magnetic field with a helical current path as well as additional current paths suggested by the simulations. With this ability to control where ablation streamers occur, large wavelength coils were constructed such that the breaks that form in the wires had sufficient axial separ...

Development of the Axial Instability in Low Wire Number Wire Array Z‐Pinches

We are investigating the development of the axial instability, a modulation of the size of the coronal plasma that develops around each wire in wire‐array Z‐pinches. The modulation is a result of nonuniform ablation of material from the relatively cold wire core. It has long been known that the wavelength of this modulation is constant late in time and, since it is unique to different materials, it has come to be known as the fundamental mode. In these experiments we imaged individual wires with laser shadowgraphy early in time primarily in low wire number, large wire diameter aluminum arrays for ease of viewing. We Observe the development of this modulation from the time of initiation of coronal plasma, obtaining its dominant wavelength and amplitude growth as a function of time. We also studied the instability on coiled wires, which modify the wire ablation mechanism and completely suppress the fundamental mode[Hall2008]. time is discussed.

Soldered Contact and Current Risetime Effects on Negative Polarity Wire Array Z-pinches

The Cornell University COBRA pulser is a nominal 1 MA machine, capable of driving up to 32 wire cylindrical Z‐pinch arrays. COBRA can operate with variable current risetimes ranging from 100 ns to 200 ns (short and long pulse, respectively). Wires are typically strung with a “press” contact to the electrode hardware, where the wire is loosely pulled against the hardware and held there to establish electrical contact. The machine is normally negative, but a bolt‐on convolute can be used to modify the current path and effectively produce positive polarity operation at the load.Previous research with single wires on a 1–5 kA pulser [1] has shown that soldering the wire, thereby improving the wire/electrode contact, and operating in positive polarity can improve the energy deposition into the wire and enhance wire core expansion. Negative polarity showed no difference. Previous experiments on the negative polarity, 20 MA, 100 ns Z accelerator [2] have shown that improving the contact improved the x‐ray yield....