Polar drive on OMEGA

Abstract

Abstract. High-convergence polar-drive experiments are being conducted on OMEGA [T. R. Boehly et al., Opt. Commum. 133, 495 (1997)] using triple-picket laser pulses. The goal of OMEGA experiments is to validate modeling of oblique laser deposition, heat conduction in the presence of nonradial thermal gradients in the corona, and implosion energetics in the presence of laser-plasma interactions such as crossed-beam energy transfer. Simulated shock velocities near the equator, where the beams are obliquely incident, are within 5% of experimentally inferred values in warm plastic shells, well within the required accuracy for ignition. High, near-one-dimensional areal density is obtained in warm-plastic-shell implosions. Simulated backlit images of the compressing core are in good agreement with measured images. Outstanding questions that will be addressed in the future relate to the role of cross-beam transfer in polar drive irradiation and increasing the energy coupled into the target by decreasing beam obliquity.Polar drive (PD) [1, 2] enables one to conduct direct-drive experiments while the National Ignition Facility (NIF [3]) is in the x-ray-drive configuration. To achieve nearly symmetric illumination in the absence of beam ports at the equator, higher-latitude beams are repointed toward the equator, compensating for reduced drive. This repointing results in oblique irradiation at the equator. Since laser energy from oblique beams is absorbed at lower densities in the corona, repointing results in reduced hydrodynamic efficiency. The polar-drive NIF ignition design [4] also uses other means to increase laser drive at the equator and achieve adequate symmetry including higher energy for the beams pointed to the equator and specialized laser-spot shapes.The goal of polar-drive experiments on OMEGA [5] is to validate models used in simulating ignition designs. These include models of oblique laser deposition, any plasma-induced effects such as the energy transfer between laser beams [6], heat conduction to the ablation surface [7], and nonuniformity seeding and growth [8]. Previous PD OMEGA implosion studies [9] used low-adiabat continuous pulse shapes. A relatively low convergence ratio (defined as the ratio of the initial inner shell radius to the inner shell radius at stagnation) of ∼12 was obtained. Recent experiments on OMEGA have used triple-picket laser pulse shapes followed by a square main laser pulse to achieve higher convergence ratios of ∼19, closer to the value of 23 for the ignition design. The triple-picket laser pulse shape is more relevant for ignition since successful compression with a high near-one-dimensional (1-D) areal density of ∼300 mg/cm 2 was obtained in OMEGA triple-picket, deuterium-tritium (DT)-implosion experiments [10]. a

Polar-drive implosions on OMEGA and the National Ignition Facility

“…This result is consistent with independent PD shock-timing measurements using cone-in-shell geometries. 14 Backlit images indicate that simulations reproduce the gross features of the converging shell. Figure 9 shows the images obtained by backlighting the converging shell with 26 which transports x rays through the DRACO profiles accounting for absorption using opacities generated by the PRISM group.…”

“…A flat foot to a continuous rise 13 [ Fig. 5(b)] and three pickets preceding a main pulse are used [12][13][14]]. The PD ignition design uses the latter pulse shape since nearly 1D high areal densities have been demonstrated with this pulse shape in symmetric drive.…”