Nested Multi-Wire Array Implosions For KeV X-Ray Generation

Introduction: The brightest sources of keV X-ray radiation used for nuclear weapons effects testing, weapons physics, and a variety of other applications are pulsed-power driven Z-pinch plasma radiation sources (PRS). Hundreds of kJ of 2-4 keV radiation have been produced on the 20 MA Z facility at Sandia National Laboratories. The production of harder X rays is increasingly difficult (Fig. 5), largely because heating of high-atomic-number plasmas is a serious problem due to the energy losses through subkilovolt line radiation that rapidly increase with the atomic number.¹ Even the "optimistic" scaling of the experimental results to the photon energy range of >10 keV predicts the yields not exceeding ~1 kJ (Fig. 5). To do better than this, we need improved PRS load design and maybe other than K-shell line radiation emission mechanisms. Combining the innovative approaches recently suggested at NRL—nested wire array load design and the use of continuum recombination radiation of low-Z elements^2,3—seems to be the most promising way to increase the PRS X-ray yields in the high-energy range.

FIGURE 5
K-shell yields produced in recent experiments on Z and the corresponding K-shell energies vs atomic number. Dashed lines show interpolation of these results to higher photon energies.

Nested Wire Arrays: Magnetic field lines in a conventional cylindrical wire array are shown in Fig. 6(a). The pressure of global azimuthal magnetic field accelerates the wires to the axis, where their kinetic energy is eventually converted into radiation. In a nested load, an inner concentric cylindrical wire array is placed inside the outer one, as shown in Fig. 6(b). When the imploded outer array hits the inner, the two arrays can interact either as continuous gas shells (colliding shell mode), or the outer array plasma can penetrate through the gaps between the inner wires, all or some of its current being switched to the inner array. In either case, this interaction tends to stabilize the implosion, which results in a higher radiation power. The nested wire array loads are now used at Sandia for generating both softX rays (nesting increased the record radiation power on Z by 40%) and hard X rays. The efficiency of the nested load design for the latter purpose is illustrated by Figs. 5 and 7. The Al single-wire array shot Z70 in 1997 is compared to the Al nested wire array shot Z811 (October 2001). The improved load design contributed to increasing the K-shell yield and power by factors of 3 and 4, respectively.

Recombination Continuum: When higher atomic-number elements are difficult to heat to multi-keV temperatures, we suggest using the continuum radiation of lower atomic-number elements at energies well in excess of their K-shell excitation potentials. An advantage of lower atomic-number plasmas is that one need not pay the large energetic price of stripping many electrons to reach a K-shell radiating hard X-ray lines. Our theoretical and simulation results indicate that the recombination continuum may have an advantage for generating 7-10 keV quanta on pulse power facilities operating in the range of 15-20 MA. The largest recombination yield is expected from a highest atomic-number element that could efficiently produce K-shell yield on a given pulse power machine, at conditions corresponding to higher plasma temperatures, tighter pinches, and lower load masses than in most conventional single arrays loads. It could be achieved using either aluminum or titanium low-mass nested wire array loads. Figure 7(a) shows the K-shell and high-energy yields predicted for Al nested wire arrays with outer/inner mass and radius ratio 2:1, as in shot Z811, imploded on Z in a 1D radiative-MHD simulation for the best-case "colliding shells" mode of the interaction between the inner and the outer arrays. The optimum conditions for the K-shell emission are close to those of shot Z811, total mass 1.5 mg/cm, and the predicted K-yield is even higher than the observed one because of energy coupling, which seems to be unrealistically high in the colliding shell mode. Optimum masses for generating higher-energy quanta are predicted to be slightly lower. Figure 7(b) shows similar predictions for nested Ni-clad Ti wire arrays, with inner/outer arrays interacting at collision in a more realistic 50-50 current splitting mode. Although the reduction in the energy coupling efficiency and 30% of the higher atomic-number Ni ions decrease the temperature of the pinch, and hence, the keV X-ray yields, the predicted radiative performance, if confirmed in experiment, will be of high practical significance.

(a) Al in a "colliding shells" mode (experimental K-shell yields from shots Z70 and Z811 are also shown).	(b) Ti-Ni in a 50-50 current-splitting mode.
FIGURE 7 Simulated mass scans of K-shell and recombination yield for 2:1 nested wire array loads imploded on Z.

Summary: We describe an approach to load design that can significantly improve the radiative performance of PRS in the keV energy range. It has already proved to be very successful in increasing both the yield and power of Al K-shell emission produced on the Z. Experiments underway at Sandia National Laboratories are testing our predictions of considerable yields in higher-energy photons that could be produced with low-mass nested wire array loads.

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