Implosion characteristics and applications of combined tungsten–aluminum Z-pinch planar arrays

High Energy Density Physics 9 (2013) 653e660

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Implosion characteristics and applications of combined tungstenealuminum Z-pinch planar arrays G.C. Osborne a, b, *, V.L. Kantsyrev a, b, A.A. Esaulov a, b, A.S. Safronova a, b, M.E. Weller a, b, I. Shrestha a, b, K.M. Williamson a, b,1, V.V. Shlyaptseva a, b a b

University of Nevada, Reno, 1664 N. Virginia St., Reno, NV 89557, USA Sandia National Laboratories, 1515 Eubank S.E., Albuquerque, NM 87123, USA

a r t i c l e i n f o

a b s t r a c t

Article history: Received 16 January 2013 Received in revised form 17 June 2013 Accepted 17 June 2013 Available online 4 July 2013

An exploration of the implosion properties and X-ray radiation pulses from tungsten-based planar wire array Z-pinch experiments is presented, with an emphasis on loads mixed with aluminum. These experiments were carried out on Zebra, the 1.0 mA pulse power generator at the Nevada Terawatt Facility. A suite of diagnostics was used to study these plasmas, including X-ray and EUV Si diodes, optical imaging, laser shadowgraphy, and time-gated and time-integrated X-ray pinhole imagers and spectrometers. Specifically, loads with relatively large inter-wire gaps where tungsten is placed in the center of a planar configuration composed primarily of aluminum showed unusual characteristics. These loads are shown to generate a “bubbling” effect in which plasma from the ablation of outer aluminum wires is temporarily hindered from converging at the center of the array where the tungsten wire is located. Reproduction of these experiments with variations to load geometry, materials, and mass distribution are also presented and discussed in an attempt to better understand the phenomenon. In addition, a theoretical model has also been applied to better understand the dynamics of the implosions of these loads. Applications of this effect to radiation pulse shaping, particularly with multi-planar arrays, are also discussed. Ó 2013 Published by Elsevier B.V.

Keywords: Z-pinch Tungsten Plasma applications Pulsed power Planar arrays

1. Introduction Planar wire arrays have been shown to be very efficient radiators [1] and are currently under consideration for driving hohlraum-based inertial confinement fusion experiments at Sandia National Laboratory [2]. A new phenomenon has been discovered in Z-pinch loads where tungsten is used in the center of a planar wire array (PWA), arranged so that the wires form a single straight row, composed primarily of aluminum with similar linear mass and wide inter-wire spacing, and those results are shown here. Additionally, tungsten has been the topic of much interest in recent years and has also proven to be an efficient radiator. As pre-pulses are important for driving hohlraum configurations [3], it has become a point of interest to study the dependencies of variations

* Corresponding author. University of Nevada, Reno, 1664 N. Virginia St., Reno, NV 89557, USA. Tel.: þ1 775 303 3936. E-mail addresses: [email protected] (G.C. Osborne), [email protected] (V.L. Kantsyrev), [email protected] (A.A. Esaulov), [email protected] (A.S. Safronova), [email protected] (M.E. Weller), [email protected] (I. Shrestha), [email protected] (K.M. Williamson), [email protected] (V.V. Shlyaptseva). 1 K.M. Williamson is now located at Sandia National Laboratories, 1515 Eubank S. E., Albuquerque, NM 87123, USA. 1574-1818/$ e see front matter Ó 2013 Published by Elsevier B.V. http://dx.doi.org/10.1016/j.hedp.2013.06.004

in pre-pulse shapes, particularly with regard to load configuration, so the possibility of taking advantage of this phenomenon for such purposes shows promise for future applications. Experiments were performed on the Zebra generator [4] at the University of Nevada, Reno. Zebra is a pulse power Z-pinch machine with a current rise time z 100 ns, maximum current amplitude z 1.0 mA and impedance z 1.9 U. A laser probing system was utilized to provide shadowgraphy data with a pulse length of 150 ps and wavelength 532 nm. Four CCD cameras recorded images spaced apart w3 ns along two channels angled 22.5
from each other. Additionally, an intensified CCD (ICCD) camera with a gate time of 3 ns captured optical images of the radiating plasma at a 90
angle to the shadowgraphy laser probing path. This was used in conjunction with a full diagnostics suite [5] to analyze plasma radiation, but the devices most relevant to studying these loads are a time-integrated pinhole imager (TIPH) with resolution of 220 mm and cutoff wavelength l1/10 < 0.31 nm (E1/10 > 4 keV) using layered Bio-max MS Kodak X-ray film, a time-integrated KAP convex crystal (2 inch radius, 2d ¼ 2.663 nm) spectrometer (TISP) that is axially spatially resolved with spatial resolution 1.5 mm, a 0.2 mm Al filtered (15 eV < hv < 70 eV, hv > 240 eV) silicon diode (EUV), 5 mm Kimfoil filtered (180 eV < hv < 280 eV, hv > 700 eV) X-ray diode (XRD), and an 8 mm Be filtered (hv > 750 eV) photo-conducting

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detector (PCD). A time-gated pinhole imager (TGPH) was also fielded with a spatial resolution of 230 mm, filtered to a cutoff energy of E1/10 > 1.24 keV (l1/10 < 10  A), and the microchannel plate (MCP) was gated for 3 ns. Spectroscopic analysis provides an informative means of determining plasma conditions, so non-LTE K-shell aluminum and magnesium models [6] developed at UNR were used to provide values for aluminum temperatures and densities. This is utilized in conjunction with an M-shell tungsten model [7] for 3de5l transitions based on atomic data from HULLAC to determine the charge state balance of the tungsten plasma. Spectrogram tracings were taken from X-ray film recordings and integrated along small axial portions of featurerich bright spots. Line intensities were adjusted to account for background noise, film sensitivity, and filter transmission. The phenomenon shown is also expanded to include multiplanar wire arrays, particularly double and triple planar wire arrays (DPWA and TPWA, respectively). The former is a configuration in which two planar rows of wires are placed in parallel to each other. The latter is similar, but with three wire arrays. These load configurations show particularly interesting results in regards to their pre-pulse shapes, and an analysis and comparison of the data from those shots are also presented. All wires shown throughout this paper, including the single planar wire arrays (SPWA) are 20 mm from anode to cathode. 2. The “bubbling” phenomenon Observed in this research is an effect in which incoming ablated plasma from outer wires of an array is hindered from converging on

axis, as would normally occur due to Lorentz forces, due to interactions with plasma expanding out from a central wire of different material. The term “bubbling” arose from the appearance of bubble-shaped instabilities on shadowgraphy at the interaction points of these two plasmas. In this and following sections, possible explanations for this instability will be discussed in detail. This phenomenon was first witnessed in SPWA loads where a 10 mm W wire was placed central to an array of widely spaced (2 mm) Al(5056) wires with thickness 28 mm. The wire diameters were chosen such that they would have similar linear mass, which combined made the array 82 mg/cm. Shadowgraphy and TIPH images for this shot are shown in Fig. 1(a), and the timings for all shadowgraphs are given in Fig. 2. The bubbling that occurs around the central W wire is shown clearly in contrast to the comparatively much more stable and uniform Al plasma flows on either side. An ICCD camera image is also shown in Fig. 1(a) and gives evidence of the aluminum wires ablating much earlier than the tungsten. The inverse case where the Al wire is placed on the center and is surrounded by tungsten wires is show in Fig. 1(b) for comparison; it shows “standard” plasma column formation on its central axis where the Al wire resides. Comparing the two TIPH images reveals that the amount of X-ray output with energy >4 keV is substantially less in the case of the shot where bubbling occurred than when it did not. Additionally, the comparison of XRD signals shown in Fig. 2 (with significant values tabulated in Table 1) indicate that while both the original bubbling load and its inverse case implode at nearly the same time, each with gradually increasing X-ray output of nearly the same duration (approximately 20 ns), only the bubbling load forms a pre-pulse. It should also be noted that the

Fig. 1. Shadowgraphy (shown at the left-most position of each block of images, viewed parallel to the laser probe with the exception of (a), which is at a 12.5
angle) and pinhole imaging (resolution of 220 mm and cutoff wavelength l1/10 < 3.1  A, viewed at a 45
angle, shown to the right of the shadowgraphy image) for shots a) 1955 e SPWA with wires spaced 2 mm apart, taken 39 ns before stagnation, b) 1765 e SPWA with wires spaced 2 mm apart, taken 24 ns before implosion, c) 2162 e SPWA of Au and Al (rather than W and Al, as in all other shots) with wires spaced 2 mm apart, taken 28 ns before stagnation, and d) 2456 e SPWA with Al wires spaced 1.4 mm apart and 2.8 mm from the central W wire, taken 66 ns before stagnation. Intense CCD (ICCD) image, viewed 12.5
from parallel to the camera and taken approximately 65 ns before the highest XRD peak, provided for shot 1955 as well, shown in the right-most position of (a).

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Fig. 2. Signals for shots a) 1955 e Al/W/Al SPWA, 2 mm inter-wire gap, b) 1765 e W/Al/W SPWA, 2 mm inter-wire gap, c) 2456 e Al/W/Al SPWA, 1.4/2.8/1.4 mm inter-wire gap, d) 2301 e Al/W/Al TPWA, 0.7 mm inter-wire gap, 3 mm inter-planar gap, e) 2162 e Al/Au/Al SPWA, 2 mm inter-wire gap, f) 1961 e W TPWA, 0.7 mm inter-wire gap, 3 mm inter-planar gap, center array was 12.5 mm thick wires and outer arrays were 5 mm, g) 1037 e W/Al DPWA, 0.7 mm inter-wire gap, 3 mm inter-planar gap, and h) 1828 e Al TPWA, 0.7 mm interwire gap, 3 mm inter-wire gap. Arrows indicate timing of shadowgraphy images shown in Fig. 1 and vertical gray lines indicate the approximate transition times between the prepulse phase and the implosion phase.

bubbles observed are relatively stable for a period of time, as shown in Fig. 3, where Fig. 3(b) takes place approximately 6 ns after Fig. 3(a), and of the three identical shots shown in Table 1 (#1955, #1931, and #2161), bubbling was observed in time periods ranging between 15 and 39 ns prior to stagnation. One of the earlier hypotheses about the cause of the bubbling instabilities was that they were due to the method in which the tungsten wires were manufactured, a process which leaves a thin coating of hydrocarbons on the wire that could be blown off during the current rise and cause destabilization of the ablating plasma.

Tungsten wires also tend to be less uniform than thin wires of other materials, so it was postulated that the wire shapes could possibly seed instabilities, despite recent publications to the contrary [8]. To test both of these postulates, a load was developed that replaced the central tungsten wire with gold, which is more uniform and lacks any hydrocarbon coating. Shadowgraphy and time-integrated pinhole images of this experiment are shown in Fig. 1(c). While the shape of the instabilities shown in this figure do not correspond precisely with the cases where W is present instead of Au, the evidence of a higher-density, non-uniform interface between the

Table 1 Single planar configurations are denoted using dots that represent single wires and multi planar arrays are shown using bars that indicate wire rows. Black indicates Al wires, gray indicates W, and light gray indicates Au. Implosion time is measured from current start to PCD X-ray peak. The section of time between the start of the of X-ray output on the XRD signal and the implosion peak is given as Zone 1 and the section between the pre-pulse peak and the implosion peak is given as Zone 2. Pre-pulse percentage is calculated as the integral of the XRD signal from time t ¼ 0 to the minimum point between the pre-pulse peak and the implosion peak divided by the integral of the entire signal. All temperature and density measurements have a model fitting error under 10%; please see Section 3 for further explanation and examples. Shot

1931 1955

Configuration

K-shell Al/Mg Te (eV) K-shell Al/Mg Ne (cm3) Zone 1 time (ns) Zone 2 time (ns) Current rise to X-ray peak (ns) Pre-pulse percentage

300

1.5  1020

47

12

87

25%

375

6  10

19

20

7

93

25%

1.5  10

20

22

e

77

2  1020

29

11

116

25%

e

1765

420

2456

370

1037

e

e

26

15

87

17%

2301

e

e

44

26

63

30%

20

23

e

2161

390

1  10

78

e

2162

355

1.5  1020

53

7

87

14%

1961

e

e

22

16

105

31%

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than in other experiments, resulting in the W wire carrying higher current at an earlier time. This is the primary reason the central wire visually appears less stable than in shadowgraphy images for other shots shown in Fig. 1(aec) and is also possibly a large contributing factor to why bubbles were witnessed at such early times (66 ns before stagnation). Comparison of the signal data shown in Fig. 2(c) displays a much earlier (by approximately 9 ns) and more pronounced pre-pulse peak than the original bubbling load, as well. This ability to adjust the time of the pre-pulse with minor changes to load geometry is one of the reasons these shots could be of particular interest in the development of Z-pinch driven inertial confinement fusion experiments that utilize hohlraum configurations [3]. 3. Spectroscopic analysis

Fig. 3. Shadowgraphy images of bubbling shot showing time evolution of instabilities over an approximately 6 ns time frame, cropped for emphasis on the center of the array.

Au and Al wires is clear. This shows that neither of tungsten’s manufacturing imperfections are the cause, nor it is more likely due to differences in radiative properties of high-Z and low-Z elements. Slight variations in load geometry were also investigated in an attempt to better understand the mechanics behind the bubbling phenomenon. First, it was postulated that if the effect was caused by a pinching of the magnetic field around the tungsten wire, it could be significantly mitigated by reducing the inter-wire gap of the entire load, and indeed, the results of these experiments showed no noticeable evidence of bubbling. Furthermore, investigation of the radiation output indicates a lack of pre-pulse X-ray radiation output commonly associated with bubbling, however this hypothesis was later observed to be inconsistent with current modeling (see Section 4) and also does not explain why bubbling was not seen in the inverse case. Second, the aluminum wires were moved a greater distance from the central tungsten wire, such that the gap between Al and W was 2.8 mm and the distance between the Al wires was 1.4 mm. This was done to investigate whether or not the instabilities would be localized around the tungsten wire, however the shadowgraph displayed in Fig. 1(d) shows clearly that this is not the case, and the bubbling spans the entire 2.8 mm gap between the W and Al wires. It is also worth noting that the change in geometry for this experiment also altered the load impedance in such a way that current distribution favored the central wire more

Analysis of time-integrated, axially resolved spectral data was performed in an attempt to make comparisons between plasma parameters of bubbling shots of various configurations and nonbubbling shots. Spectral tracings were taken at axial positions where the plasma column was radiating most brightly, as these areas are not only the most feature-rich, but also because these bright spots are formed early in the plasma’s lifecycle, during the ablation phase, and so plasma parameters extracted at those locations from the time-integrated spectra tend to correlate closely with data taken at pre-stagnation times [9]. All aluminum used in this research was the alloy Al(5056), which contains 95% Al and 5% Mg, so spectroscopic modeling was done for both K-shell Al and Mg, as well as M-shell W, to estimate the density and temperature of the Al plasma and the charge balance of the W ions, respectively. The K-shell Al and Mg data was generated using a model developed at UNR [6]. In most cases, the Al lines could be modeled and parameters extracted from them directly, however in optically thick cases, film saturation of the most intense Al lines often occur. In the converse case (more optically thin plasma), K-shell Mg lines are barely observed at all, which makes it a simple matter to determine which model should be applied. A more detailed discussion of Al:Mg Z-pinch plasmas and this method are available in Refs. [7,10]. The Hebrew University Lawrence Livermore Atomic Code (HULLAC) was used to calculate charge balancing and average atomic number of the M-shell W lines [11]. All spectra were calibrated against prominent and well known Al He and H-like (particularly He-a, He-b, He-g, Ly-a) and W Ni-like lines. Modeling for the K-shell Al or K-shell Mg lines for each shot (see Table 1) do not show any obvious discrepancy from shot to shot in density, but do indicate significantly lower temperatures for K-shell plasma in the shots with bubbling compared to without. Examples of this modeling are provided in Fig. 4, where the optically thin Kshell Mg lines for shot #1955 were modeled in Fig. 4(a) with an estimated 8% fitting error and optically thin K-shell Al lines of shot #1765 were modeled in Fig. 4(b) with a 2% fitting error. This error is calculated by taking the difference between the modeled and experimental peak intensities of diagnostically important spectral lines. Of particular note are the calculations from shot #2456, where the Al wires were spaced further from the central W wire than in other experiments. While it may be expected that the wider spacing would cause a decrease in density in the Al plasma, it is actually high in comparison to other bubbling shots, which is in agreement with the shadowgraphy image shown in Fig. 1(d) where it appears that the ablated Al material is accumulating near the inner-most Al wire instead of passing directly into the broad gap, as would happen in a non-bubbling configuration. Due to the nature of bubbling and dramatic differences in Al and W plasmas in general, M-shell W is modeled and treated separately from K-shell. In general, for Z-pinch experiments on Zebra, X-ray

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Fig. 4. Modeling of W 3de5l transitions (a), with model indicated in gray and experiment in black. Time-integrated spectra of shots (b) 1955 (Al/W/Al bubbling load with 2 mm inter-wire gap) including Mg modeling in gray, time-integrated pinhole image (d), and time-gated pinhole image (e) taken 35 ns before stagnation, and (f) 1765 (W/Al/W nonbubbling load with 2 mm inter-wire gap) including Al modeling in gray, time-integrated pinhole image (h) and time-gated pinhole image (i) taken 2 ns before stagnation, with prominent K-shell Al/Mg and M-shell W lines marked. Locations where line tracings were obtained are indicated by arrows on the left of each spectrum. W lines follow the identification labels used in Ref. [10]. All time-gated pinhole images shown are at a 90
angle from the time-integrated pinhole images.

M-shell W emission is generated by a much hotter plasma (Te > 1 keV) than K-shell Al (Te w 300e500 eV) and therefore is probably originated by a hotter part of the same axially resolved bright spot. Al. In particular, charge state balancing (a property that is highly sensitive to temperature) of the W 3de5l lines (which are much less optically thick than the most intense 3de4l lines) show no discernible trend toward higher or lower ionization states, suggesting that the W plasma undergoes no significant temperature change in shots where the instabilities occur. An example of the W modeling is shown in Fig. 4(c). The calculated for all of the SPWAs considered in Table 1 is 44.4  0.3, which indicates the dominance of Ni- and Cu-like W ions. The benefit of modeling these lines instead of the more intense 3de4l transitions is discussed in further detail in Refs. [10], and lines in Fig. 4 are labeled in accordance with that paper. Future spectroscopic work will focus on having X-ray spectral data with radial spatial resolution as well as Al and W spectra in the EUV region with possible correlations made to the bubbling phenomenon. 4. Comparison with modeling In an attempt to better understand the mechanisms behind bubbling instabilities, the Wire Array Ablation Dynamics Model (WADM) [12] and a radiation magnetohydrodynamic (RMHD) model [12,13] were employed, where all calculations were done to simulate shot #1955. Previous studies of numerous planar wire array configurations (see, for example, Ref. [12]) do not indicate any cause for magnetic field strengthening around the central wire regardless of its material, in addition to the fact that we have proven that the instability does not occur in the case where the Al and W wire positions are swapped (Fig. 1(b)), so focus was given

instead to understanding the differences in parameters between the tungsten and aluminum plasmas. Because bubbling is shown to be stable over significant periods of time, it was postulated that the structure is being supported by material on either side. Since we also do not see bubbling in shots with pure Al, we can hypothesize that the instability arises at the interaction point between the Al and W plasmas. Results from the WADM for wire ablation at times corresponding to the shadowgraphy image shown in Fig. 1 for shot #1955 are presented in Fig. 5. Although the WADM works with discrete parameters, a special data postprocessing procedure can be applied to estimate a quasicontinuous spatial distribution of mass density of the ablated plasma flow shown in Fig. 5. Detailed description of this WADM amplification can be found in Ref. [14]. The approximate boundaries of the bubble region around the central tungsten wire in Fig. 5 are averaged vertically (along the z-axis). In order for the ablated aluminum plasma flow to be stopped from converging on the central tungsten wire due to Lorentz and hydrodynamic forces, the following condition should be fulfilled at the boundary between the two materials:

rAl v2Al 2

þ pAl

g g1

¼

rW v2W 2

þ pW

g : g1

(1)

Eq. (1) is derived from the Bernoulli Equation for compressible flow, where the indices Al and W correspond to the parameters of the aluminum and tungsten plasmas, respectively. Velocity of ablated plasma is denoted as v, p corresponds to pressure, r to flow density, and g to the adiabatic index. Since the spatial gradients of the magnetic field at the center of single planar arrays has been shown to be negligibly small [12], the magnetic pressure terms on both sides of the equation will be nearly equal and can be neglected.

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Fig. 5. Quasi-continuous two dimensional (overhead view) distribution of the (a) plasma mass density r(x,y) and (b) current density through the plasma jz(x,y) calculated by the WADM for Zebra shot #1955 (Al/W/Al bubbling load with 2 mm inter-wire gap) at time t ¼ 65 ns, where the shadowgraphy image shown in Fig. 1(a) is taken. Dotted lines show approximate boundaries of the bubble region.

It can also be assumed that the velocity of incoming aluminum plasma at the interaction region in the center of the array is much larger than the radial expansion velocity of the tungsten plasma, such that vAl >> vW (in fact, modeling predicts vAl z 150 mm/ns and vW z 15 mm/ns), which means that the first term from the righthand side of the equation can also be neglected. WADM calculations predict the flow density of aluminum plasma at the boundary to be rAl ¼ 3  104 g/cm3. The pressure of the aluminum plasma can be estimated assuming a temperature of Te ¼ 20 eV (see Ref. [15]) as pAl ¼ 1.3  108 Pa. The adiabatic index at this condition can be taken as g ¼ 1.3 [15], and as a result, the second term on the left side of Eq. (1) comes out to approximately 15% of the value of the right-hand term, so its contributions are insignificant as well. The reduced formula, then, is

pw z

g  1 rAl v2Al ¼ 8  108 Pa: g 2

(2)

Unfortunately, this equation is still unable to determine parameters of the tungsten plasma, since pressure itself is a function of plasma mass density r (or ion number density ni) and temperature. To calculate the pressure of the tungsten plasma using the ion number density, if we assume that Ti z Te, it can be written that


pw ¼ ni 1 þ Z Te ;

(3)

where Z is the mean ion charge. Fig. 6 shows the calculated volume power density for the radiation emission Prad and Ohmic heating PU in both tungsten (a) and aluminum (b) plasmas for shot #1955. The parameters Prad and PU have been calculated using the ionization and emissivity tables of the radiation RMHD code [13,16]. The ionization balance component [17] exploits the local thermodynamic equilibrium approximation, the resistivity component is based on the Spitzer approximation (see, for example, Ref. [18]), and the radiation emissivity component [17] accounts for the radiation due to freee free and freeebound electron transitions. The current through the bubbling region is calculated by the WADM to be Ib ¼ 60 kA (9% of the total 670 kA being delivered to the load at time t ¼ 65 ns, which corresponds to shadowgraphy imaging shown previously). Calculations from the WADM indicate that the ablated mass of the central tungsten wire at t ¼ 65 ns is approximately 10% of its total starting mass, or M/Mb z 10, where

Fig. 6. Volume densities of the radiation power losses Prad and Ohmic heating PU in (a) tungsten and (b) aluminum plasmas at constant pressure 8  108 Pa, as a function of electron temperature Te or the ratio of total wire mass M to the mass of the ablated plasma Mb.

M is the total wire mass and Mb is the ablated mass. This corresponds to a parameter window where ohmic heating is fully compensating the plasma radiation and therefore fulfilling power conservation, as shown in Fig. 6(a), leading to the conclusion that the bubbles observed in shadowgraphy images are likely to be formed by higher temperature (Te w 200 eV) tungsten plasma. A higher temperature (and consequently lower density) would explain the transparency of the inner bubble regions to laser optical probing. The appearance of non-transparent instabilities between these two plasmas can then be explained by the large differences in density and temperature, where the more optically thick interface regions correspond to a build-up of plasma material where these parameters shift suddenly. The modeling remains consistent for the gold comparison as well, since its plasma properties and radiation features are quite similar to that of tungsten. The reason bubbling is not observed in the inverse case, where Al is central and W surrounds it, is apparent in Fig. 6(b) where we can see that the only parameter window for Ohmic heating compensation occurs at high ablation mass (M/ Mb z 2, or 50% of the initial wire mass). This means that the Al won’t be able to ablate a sufficient amount of material fast enough to create the pressures required to stop incoming plasma from the outer wires at an interface region beyond the central axis, as witnessed in loads where bubbling instabilities are observed. By the time the central Al wire had ablated enough material to do so, the W plasma would have already reached the center of the load. 5. Signal shapes and multi-planar configurations The strong pre-pulse formations in the SPWA shots discussed in previous sections have shown a strong potential to be utilized in pulse shaping studies. The initial bubbling configuration (as in shot #1931) in particular was attempted numerous times, only three shots of which have been included in this paper, and has been shown to have a consistent signal shape. Many other load configurations have also been surveyed in an attempt to collect information on the magnitudes and timings of the X-ray radiation pulses from bubbling and non-bubbling experiments, including SPWA

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Fig. 7. Shadowgraphy images for shots (a) 1961 e W TPWA with 0.7 mm inter-wire gap, 3 mm inter-planar gap where the center array was composed of 12.5 mm thick wires and the outer two arrays of 5 mm wires, taken 47 ns before stagnation parallel to the wire rows, (b) 2301 e Al/W/Al TPWA, 0.7 mm inter-wire gap, 3 mm inter-planar gap taken 37 ns before stagnation parallel to the wire rows, and (c) 1037 e W/Al DPWA, 0.7 mm inter-wire gap, 3 mm inter-planar gap, with positions of arrays marked, taken 44 ns before stagnation at a 45
angle to the wire rows.

loads with even spacing, SPWA loads with uneven spacing, nonuniform triple planar wire array (TPWA) loads, and non-uniform DPWA loads. Each of these load types utilizes various arrangements of W and/or Al wires. Radiation output signals from a SPWA bubbling load is shown in Fig. 2(a), which clearly shows the formation of a strong pre-pulse that occurs 7 ns before the primary X-ray peak on both the X-ray diode and the photo-conducting detector. Comparing to the other shots surveyed (shown in Table 1), we can see that this configuration had the fastest total implosion time, due primarily to its relatively small size and low number of wires. The ratio of pre-pulse output to total X-ray output shown in Table 1 is calculated by integrating the XRD signal. First, the pre-pulse is calculated by integrating from the signal start, past the first peak, and ending at the minimum point between the pre-pulse peak and the primary peak. Then an integration of the entire signal is performed and is used to calculate the ratio. Two more SPWA configurations were also tested. The first was the same as the original bubbling load, except that the wires were each spaced 1 mm apart instead of 2 mm. This load was attempted numerous times, and in each attempt, no bubbling instabilities were witnessed on shadowgraphy and pre-pulse formation was nonexistent. The second configuration was the one discussed in Section 2 where the Al wires were spaced even further away from the central W wire. While the original uniform 2 mm gap configuration often shows pre-pulse formation very near the implosion peak (sometimes not at all; see, for example, shot #2161 in Table 1), it was postulated that this load with wider spacing would have a much more pronounced X-ray pre-pulse. Indeed, both XRD and PCD prepulses are apparent, and though they are smaller in magnitude than the shots with uniform 2 mm spacing, the pre-pulse signal on the XRD is still approximately 25% of the total integrated output. Triple planar wire arrays have previously been shown to be excellent candidates for pulse shaping experiments [1], so another load was tested in an attempt to create bubbling effects in a triple planar configuration, and a pure W load of varying wire mass (shot #1961), chosen for its particularly prominent pre-pulse, was used for comparison and is shown in Fig. 7(a). In this shot, 12.5 mm W was used in the center and 5 mm W was used in the outer arrays, such that the center array was of significantly heavier mass. In the

bubbling shot, #2301, 12.7 mm Al and 16 mm W was used, also making the central W array heavier in total linear mass than the two outer Al arrays. The shadowgraphy results in Fig. 7(b) clearly show bubble-like instabilities along the center array that are not apparent in the pure W load. Both shots have three pronounced XRD peaks, including a pre-pulse. Of particular note is that neither of the pre-pulses in these two shots registered on the PCD, suggesting they radiated at a much lower energy than the SPWA loads. The bubbling shot also took significantly longer (44 ns) to implode than its SPWA counterparts and had an earlier pre-pulse (26 ns prior to stagnation, shown in Fig. 2), yet the total integrated ratio of pre-pulse to total X-ray output was still 0.30. Lastly, an asymmetric double planar wire array (DPWA) was tested in which 6 mm W made up one array and 15 mm Al composed the other, putting the two planes at similar total linear mass. While shadowgraphy, displayed in Fig. 7(c) as shot #1037, shows some formation of instabilities where the two arrays meet, the signals in Fig. 2(d) lack the prominent pre-pulse witnessed in other bubbling shots. A pre-pulse still exists at the foot of the primary implosion peak on the XRD signal, however, and like the TPWAs, is not apparent on the PCD. Notably, the pre-pulse to total output ratio is much lower than in other bubbling shots (0.17). 6. Conclusions Bubble-shaped instabilities have been witnessed in planar arrays where a high-Z element, specifically W or Au, is surrounded by Al wires. A full analysis of these shots, including shadowgraphy, X-ray signals, and spectroscopy has been presented and discussed. The possibility that these effects are caused by drastic differences in the parameters and radiation features of low-Z and high-Z plasmas has been shown via modeling, while numerous other hypotheses have also been presented and proven false. The modeling has shown to be consistent with experimental results in all cases, including the lack of bubble-like instabilities witnessed in inverted (Al in the center, W on the outside) configurations. The hypothesis derived through modeling also explains the cases where bubbling was not witnessed in loads with smaller inter-wire gaps, as the change to a smaller geometry affects the load impedance such that the inner wire receives current at an even later time than in arrays with wider

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inter-wire spacing. This means that the central wire would not have received current and started ablating early enough to generate bubbles. Additionally, a method has been developed for identifying parameter windows where bubbling can and cannot occur, allowing future load designs to either take advantage of these instabilities or avoid them, depending on their intended application. An extensive study of the signal shapes of bubbling loads of various configurations has also been presented with the expectation that they may be used in pulse shaping experiments in the future. The SPWAs appear to implode at nearly the same times whether bubbling instabilities are formed or not, however all bubbling shots appear to have lower hard x-ray radiation yield than other similar loads. It was shown that the timing and shape of these pre-pulses can be altered by varying the load geometry. However, with the exception of the DPWA and Au shots, all loads consistently had a prepulse to total radiation output ratio of approximately 25e30%. Acknowledgments We would like to thank the NTF team at UNR for their effort in Zebra operations during the experiments and with their help in data collection. This research was supported by DOE/NNSA under Cooperative Agreements DEFC52-06NA27586, DE-FC52-06NA27588, DENA0001984, and in part by DE-FC52-06NA27616. References [1] V.L. Kantsyrev, A.S. Safronova, A.A. Esaulov, K.M. Williamson, I. Shrestha, G.C. Osborne, et al., J. Phys. Conf. Ser. 244 (2010) 032030.

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