Project Category: Experimental Research

An X-pinch is formed by two or more wires that cross at a single point and is driven by a large current pulse (>80 kA) from a pulsed power device.  The currents through each of the wires of an n-wire X-pinch combine at the cross-point and the magnetic pressure at that location increases as n² relative to a single wire.  The increased magnetic pressure creates a localized a pinch column (mini z-pinch) approximately 100-400 μm long.  The column undergoes instabilities that cause an implosion of the plasma into small regions of high temperature and density, often referred to as micropinches.  From these regions, one or more micron-scale, sub-nanosecond soft x-ray bursts (1-10 keV) are emitted.

The emission from the center of the X-pinch acts as a near instantaneous (sub-nanosecond) point source (micron-scale).  Such a source can be used for point-projection imaging of objects both static (housefly) or moving (another X-pinch as seen above, or a z-pinch shown below).

In order to image a z-pinch, the X-pinch must be placed in the return path as illustrated in the diagram below.

A new area research involves placing an X-pinch below a z-pinch in order to image the axis of an array as illustrated below.

Wire Arrays

A majority of the laboratory research effort is focused on wire array z-pinch physics. During the last years, we have shown that wire-array Z-pinch experiments, including on Z at 20 MA, proceed via four distinct phases: plasma initiation by current driven wire explosion, ablation from stationary dense wire cores and redistribution of the array mass by global-field-driven flow of the ablation plasma, snowplow-like implosion of the redistributed mass, and stagnation of the imploding plasma on axis, at which time an intense x-ray pulse is generated. The optical streak camera image shown below illustrates the four phases of wire-array z-pinch dynamics identified above, except that the initiation phase, which takes place close to the beginning of the streak, does not emit enough light to be visible in the streak.

In addition to standard cylindrical wire array loads, several novel configurations have been investigated experimentally, including nested, radial and conical arrays and X-pinches, because they enable detailed measurements of particular aspects of z-pinch physics and extend the available range of plasma parameters and applications. We begin with a discussion of the four phases of cylindrical wire-arrays.

Initiation

At early times, the flow of current through the wires increases their temperature and resistance, melts them and then partially vaporizes and ionizes them, resulting in the formation of coronal plasmas around the wires. The voltage across the wire then abruptly collapses as the current transfers to the coronal plasma and continues to rise. By measuring the load current and voltage, the energy deposited in the wires prior to plasma formation has been measured in both single wire experiments and wire-arrays. For most wires, the wire cores are only partially vaporized, which is consistent with radiographic images showing a foam-like mixture of liquid and vapor states. The energy deposited in the wire cores and their subsequent size has been shown to be sensitive to the wire – electrode contacts and to the polarity and geometry of the electric field. When the current transfers to the coronal plasmas around the wires, the ablation phase is initiated.

These radiographs are being used to evaluate quantitatively the temporal and spatial evolution of the ablation plasma, including the axial modulations, in individual pulses. High-resolution 3D MHD simulations are now showing axial modulations that are similar to the observed modulations, suggesting that an MHD instability mode is responsible. Joint experiments with Sandia scientists using chemically etched wires showed that variations in initial wire diameter do not remove the “natural” wavelength of this modulation, but allow introduction of controlled perturbations. The results of these experiments were used to benchmark Sandia’s ALEGRA and the laboratory’s GORGON MHD codes. The only configuration tested so far that eliminates this modulation is the helical wire array, in which the introduction of the axial magnetic field associated with the helical current path appears to lock the modulation wavelength to the period of the helices.

Implosion

This phase of wire-array dynamics begins when ~50% of the initial wire material has been ablated. As plasma is swept up by the implosion “snowplow,” the variation of the ablation rate along the wires provides a large-amplitude seed perturbation for the R-T instability and also results in a fraction of the initial array mass being left behind (as “trailing mass”) by the implosion. The growth of the instability in the implosion surface and evolution of the trailing mass can be seen in the series of XUV images from COBRA shown below.

Our investigations showing the importance of the trailing mass are now accepted by the Z-pinch community. he plasma filling the interior of the array is accreted by the implosion surface giving rise to “snowplow” radiation which can be used to tailor the foot of the radiation pulse. laboratory experiments have shown how further control over radiation pulse shaping can be achieved using nested wire arrays in the current transfer mode. Methods to increase the duration of the radiation pulse using zippered implosions in conical wire arrays have also been explored.

Stagnation

The assembly of the imploding plasma on axis leads directly to the first high intensity x-ray pulse, the part of the x-ray output that is important for ICF. Detailed analysis of gated XUV images has shown that the temporal spread in the stagnation of different parts of the plasma structure onto the axis can be directly correlated with the x-ray rise-time. The helical instability visible in the post-stagnation frames like the last one in the above figure may be responsible for the late-time breakup of the z-pinch.
Very careful spectroscopic measurements of gas-puff z-pinches by our Weizmann Institute partner show that the implosion kinetic energy was adequate to explain the radiation pulse. However, in wire-array z-pinches, this may not be so. For example, in experiments with wire arrays in which the current through the stagnated plasma column was held constant but the implosion kinetic energy was changed by a factor of two, the X-ray power was observed to be unchanged. There remains uncertainty over what fraction of the x-ray pulse is generated by the kinetic energy of the implosion versus other mechanisms that might convert magnetic energy into electron thermal energy and radiation, such as turbulence, Ohmic heating or MHD instabilities.