Temperature dependence of the nuclear symmetry energy

We have studied the properties of A=54 and A=64 isobars at temperatures T \leq 2 MeV via Monte Carlo shell model calculations with two different residual interactions. In accord with empirical indications, we find that the symmetry energy coefficient, b_{sym}, is independent of temperature to within 0.6 MeV for T \leq 1 MeV. This is in contrast to a recent suggestion of a 2.5 MeV increase of b_{sym} for this temperature, which would have significantly altered the supernova explosion scenario.Comment: 7 pages, including 2 figures, Caltech preprint MAP-17

Shell model Monte Carlo calculations for Dy-170

We present the first auxiliary field Monte Carlo calculations for a rare earth nucleus, Dy-170. A pairing plus quadrupole Hamiltonian is used to demonstrate the physical properties that can be studied in this region. We calculate various static observables for both uncranked and cranked systems and show how the shape distribution evolves with temperature. We also introduce a discretization of the path integral that allows a more efficient Monte Carlo sampling.Comment: 11 pages, figures available upon request, Caltech Preprint No. MAP-16

Pairing correlations in N~Z pf-shell nuclei

We perform Shell Model Monte Carlo calculations to study pair correlations in the ground states of $N=Z$ nuclei with masses A=48-60. We find that $T=1$, $J^{\pi}=0^+$ proton-neutron correlations play an important, and even dominant role, in the ground states of odd-odd $N=Z$ nuclei, in agreement with experiment. By studying pairing in the ground states of $^{52-58}$Fe, we observe that the isovector proton-neutron correlations decrease rapidly with increasing neutron excess. In contrast, both the proton, and trivially the neutron correlations increase as neutrons are added. We also study the thermal properties and the temperature dependence of pair correlations for $^{50}$Mn and $^{52}$Fe as exemplars of odd-odd and even-even $N=Z$ nuclei. While for $^{52}$Fe results are similar to those obtained for other even-even nuclei in this mass range, the properties of $^{50}$Mn at low temperatures are strongly influenced by isovector neutron-proton pairing. In coexistence with these isovector pair correlations, our calculations also indicate an excess of isoscalar proton-neutron pairing over the mean-field values. The isovector neutron-proton correlations rapidly decrease with temperatures and vanish for temperatures above $T=700$ keV, while the isovector correlations among like nucleons persist to higher temperatures. Related to the quenching of the isovector proton-neutron correlations, the average isospin decreases from 1, appropriate for the ground state, to 0 as the temperature increases

The beta-beta two neutrino decay in 48Ca

A schematic study of the $\beta \beta 2\nu$-decay of $^{48}Ca$ is made in a shell-model approach. The emphasis is especially put on the role of the spin-orbit potential in relation with the contribution of other terms in the strong interaction. This is discussed with a particular attention to the behavior of these ones under the SU(4) symmetry. Different methods in calculating the transition amplitude are also looked at with the aim to determine their reliability and, eventually, why they don't work. Further aspects relative to the failure of the Operator Expansion Method to reproduce the results of more elaborate calculations are examined.Comment: 24 pages, 5 figure

Polar-direct-drive experiments on OMEGA

Polar direct drive (PDD), a promising ignition path for the NIF while the beams are in the indirect-drive configuration [S. Skupsky et al., Phys. Plasmas 11, 2763 (2004)], is currently being investigated on the OMEGA laser system by using 40 beams in six rings repointed to more uniformly illuminate the target [R. S. Craxton et al., Phys. Plasmas 12, 056304 (2005).]. The OMEGA experiments are being performed with standard, “warm” targets (865-$\mu$m-diam, 20-$\mu$m-thick, CH shells filled with 15-atm D$_{2})$ with and without the use of an equatorial “Saturn-like” toroidally shaped CH ring [R. S. Craxton and D. W. Jacobs-Perkins, Phys. Rev. Lett. 94, 095002 (2005)] (nominal dimensions: 2.2-mm diam measured to ring center, 0.3-mm thick). For the Saturn case, the plasma formed around the ring refracts light toward the target equator as the ring plasma expands. The nominal laser drive is a 1-ns flat pulse, ${\sim}$400 J per beam, employing 1-THz, 2-D SSD with polarization smoothing. Target implosion symmetry is diagnosed with framed x-ray backlighting using additional OMEGA beams and by time-integrated x-ray imaging of the stagnating core. The best results have been obtained with Saturn targets by varying the beam pointing and ring diameter, achieving ${\sim}$75% of the fusion yield from symmetrically illuminated targets with the same total energy (60 beams, 15.3 kJ)

Polar direct drive – Ignition at 1 MJ

Target designs to achieve direct-drive ignition on the NIF using the x-ray-drive beam configuration are examined. This approach, known as polar direct drive (PDD), achieves the required irradiation uniformity by repointing some of the beams toward the target equator, and by increasing the laser intensity at the equator to compensate for the reduced laser coupling from oblique irradiation. Techniques to increase the equatorial intensity can include using phase plates that produce elliptical spot shapes, increasing the power in beams directed toward the equator, and using a ring offset from the equator to redirect rays toward the target normal. The requirements for beam pointing, power balance, single-beam smoothing, and inner-ice-surface roughness are examined. Designs with an incident laser energy of 1.0 MJ are presented. The simulations use the 2-D hydrocode DRACO with 3-D ray trace to model the laser irradiation and Monte Carlo alpha particle transport to model the thermonuclear burn

Polar drive on OMEGA

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