Experimental Demonstration of the Stabilizing Effect of Dielectric Coatings on Magnetically Accelerated Imploding Metallic Liners

Enhanced implosion stability has been experimentally demonstrated for magnetically accelerated liners that are coated with $70\mu \mathrm{m}$ of dielectric. The dielectric tamps liner-mass redistribution from electrothermal instabilities and also buffers coupling of the drive magnetic field to the magneto-Rayleigh-Taylor instability. A dielectric-coated and axially premagnetized beryllium liner was radiographed at a convergence ratio [$\mathrm{CR}=R_{\text{in},0}/R_{\text{in}}(z,t)$] of 20, which is the highest CR ever directly observed for a strengthless magnetically driven liner. The inner-wall radius $R_{\text{in}}(z,t)$ displayed unprecedented uniformity, varying from 95 to $130\mu \mathrm{m}$ over the 4.0 mm axial height captured by the radiograph.

Figure 1

(a) Load current waveforms $I(t)$ and liner inner-wall trajectories $R_{\text{in}}(t)$. Experimental $I(t)$ data are from Bdot probes located at $R\sim 6\mathrm{cm}$. All data are time shifted so that $I(t=3000\mathrm{ns})=5\mathrm{MA}$. Squares indicate axially averaged $R_{\text{in}}(t)$ data from experimental radiographs. Simulated (1D) $I(t)$ and $R_{\text{in}}(t)$ curves pertain to Z2390 ($R_{\text{in},0}=2.89\mathrm{mm}$) and Z2772 ($R_{\text{in},0}=2.325\mathrm{mm}$, Z2772 will be discussed in detail later in the Letter). For Z2772 (and for all experiments which include axial premagnetization, which require a higher-inductance extended final current feed), Bdot measurements are unphysically low, and load currents are empirically derived. (b) Cross section of typical liner hardware used in Z experiments. (c) Initial geometry of mass-matched Al and Be Epon-coated liners (above horizontal axis) and of a premagnetized ($B_{z,0}=7\mathrm{T}$, indicated by blue shading) and coated Be liner (below axis). Epon (shown in green) was cast onto the target surface, and then single-point diamond turned on a lathe to $70\mu \mathrm{m}$ thick. Red bars indicate platinum coating locations (thickness not to scale).

Figure 2

Radiographs of $\mathrm{AR}=8.94$ Al liners ($R_{\text{in},0}=3.082$, and $R_{\text{out},0}=3.47\mathrm{mm}$). (a) Dielectric coated. (b) Uncoated. (c), and (d) Transmission contours of the right-hand side surfaces of the radiographs in (a) and (b), respectively. Transmission contours are (percentage,color): (10%,blue), (15%,green), (20%,magenta), (40%,black), (60%,cyan), and (80%,yellow). Figure (c) extends to the initial radius of the Epon (${\mathrm{R}}_{\text{Epon},\text{out},0}=3.54\mathrm{mm}$). Transmission is nearly 100% for radii from 2.5 to 3.5 mm, which demonstrates that the Epon coating has imploded with the liner. The gray scale applies to (a)–(d).

Figure 3

Radiographs of imploding Be liners with $\mathrm{AR}=6$, $R_{\text{in},0}=2.89$, and $R_{\text{out},0}=3.47\mathrm{mm}$. Axes in [mm]. Transmission scales in [%]. (a)–(b) Dielectric coated. (c)–(d) Uncoated liner experiments previously reported in Ref. [20]. Rods are placed on axis in select experiments [black vertical strips in the center of radiographs (a), (c), and (d)] to limit the time-integrated self-emission that is sometimes generated at stagnation, and can obscure radiographs. To accentuate the high-opacity coatings on the liner’s inner wall, the gray scale ranges were limited to 0%–30% transmission in (a), (c), and (d) ($0\%=\text{black}$, $30\%=\text{white}$), and 0%–20% transmission in (b). Therefore, much of the Be and Epon mass in the limb regions of the radiographs has been omitted in this representation of the data.

Figure 4

(a) and (b) Radiographs of a dielectric coated and premagnetized AR-6 Be liner. In (b), data from a single radiograph are displayed using two transmission scales. For $R<300\mu \mathrm{m}$, the transmission range is limited to 0%–10% to accentuate the liner’s inner wall. For $R>300\mu \mathrm{m}$, the transmission range is 0%–100% allowing observation of the full liner mass. The liner in (c) (data previously reported in Ref. [32]), was similar to the liner in (a) and (b), including the 7 T axial premagnetization field, but did not include a dielectric coating. Remarkably, even at $\mathrm{CR}=6.4$, the liner in (c) has larger amplitude inner-wall instabilities than the liner in (b), which is at $\mathrm{CR}\sim 20$. The liner in (a)–(b) contained a 60 psi deuterium gas fill, whereas the liner in (c) contained s 120 psi deuterium fill; simulations suggest that inclusion of cold gas does not play a meaningful role in the dynamics of these liners, as the back pressure (even at high convergence) is insignificant. (d) Transmission profile of the central region of the radiograph in (b), axially averaged from $z=2.0$ to $z=2.67\mathrm{mm}$. (a)–(c) Axes in [mm]. Transmission scales in [%].