10,675,309 research outputs found
Fast ignition of fusion targets by laser-driven electrons
We present hybrid PIC simulations of fast electron transport and energy
deposition in pre-compressed fusion targets, taking full account of collective
magnetic effects and the hydrodynamic response of the background plasma.
Results on actual ignition of an imploded fast ignition configuration are shown
accounting for the increased beam divergence found in recent experiments [J.S.
Green et al., Phys. Rev. Lett. 100, 015003 (2008)] and the reduction of the
electron kinetic energy due to profile steepening predicted by advanced PIC
simulations [B. Chrisman et al. Phys. Plasmas 15, 056309 (2008)]. Target
ignition is studied as a function of injected electron energy, distance of
cone-tip to dense core, initial divergence and kinetic energy of the
relativistic electron beam. We found that beam collimation reduces
substantially the ignition energies of the cone-guided fuel configuration
assumed here.Comment: 15 pages, 9 figures. accepted for publication in Plasma Physics and
Controlled Fusio
Observers and their notion of spacetime beyond special relativity
It is plausible that quantum gravity effects may lead us to a description of
Nature beyond the framework of special relativity. In this case, either the
relativity principle is broken or it is maintained. These two scenarios (a
violation or a deformation of special relativity) are very different, both
conceptually and phenomenologically. We discuss some of their implications on
the description of events for different observers and the notion of spacetime.Comment: 10 page
The frustrated spin-1/2 J1-J2 Heisenberg ferromagnet on the square lattice: Exact diagonalization and Coupled-Cluster study
We investigate the ground-state magnetic order of the spin-1/2 J1-J2
Heisenberg model on the square lattice with ferromagnetic nearest-neighbor
exchange J1<0 and frustrating antiferromagnetic next-nearest neighbor exchange
J2>0. We use the coupled-cluster method to high orders of approximation and
Lanczos exact diagonalization of finite lattices of up to N=40 sites in order
to calculate the ground-state energy, the spin-spin correlation functions, and
the magnetic order parameter. We find that the transition point at which the
ferromagnetic ground state disappears is given by J2^{c1}=0.393|J1| (exact
diagonalization) and J2^{c1}=0.394|J1| (coupled-cluster method). We compare our
results for ferromagnetic J1 with established results for the spin-1/2 J1-J2
Heisenberg model with antiferromagnetic J1. We find that both models (i.e.,
ferro- and antiferromagnetic J1) behave similarly for large J2, although
significant differences between them are observed for J2/|J1| \lesssim 0.6.
Although the semiclassical collinear magnetic long-range order breaks down at
J2^{c2} \approx 0.6J1 for antiferromagnetic J1, we do not find a similar
breakdown of this kind of long-range order until J2 \sim 0.4|J1| for the model
with ferromagnetic J1. Unlike the case for antiferromagnetic J1, if an
intermediate disordered phase does occur between the phases exhibiting
semiclassical collinear stripe order and ferromagnetic order for ferromagnetic
J1 then it is likely to be over a very small range below J2 \sim 0.4|J1|.Comment: 15 pages, 7 figures, 2 table
Equilibrium spin-glass transition of magnetic dipoles with random anisotropy axes on a site diluted lattice
We study partially occupied lattice systems of classical magnetic dipoles
which point along randomly oriented axes. Only dipolar interactions are taken
into account. The aim of the model is to mimic collective effects in disordered
assemblies of magnetic nanoparticles. From tempered Monte Carlo simulations, we
obtain the following equilibrium results. The zero temperature entropy
approximately vanishes. Below a temperature T_c, given by k_B T_c= (0.95 +-
0.1)x e_d, where e_d is a nearest neighbor dipole-dipole interaction energy and
x is the site occupancy rate, we find a spin glass phase. In it, (1) the mean
value , where q is the spin overlap, decreases algebraically with system
size N as N increases, and (2) D|q| = 0.5 (T/x)^1/2, independently of N,
where D|q| is the root mean square deviation of |q|.Comment: 7 LaTeX pages, 7 eps figures. Submitted to PRB on 30 December 200
Mechanical properties of plastics predetermined by empirical method
To predetermine the mechanical properties of rigid plastics as a function of plasticizer content and composition, a set of equations has been empirically derived. These relate strain rate, yield stress, temperature, and weight fraction of the plasticizer
A New Method for Multi-Bit and Qudit Transfer Based on Commensurate Waveguide Arrays
The faithful state transfer is an important requirement in the construction
of classical and quantum computers. While the high-speed transfer is realized
by optical-fibre interconnects, its implementation in integrated optical
circuits is affected by cross-talk. The cross-talk between densely packed
optical waveguides limits the transfer fidelity and distorts the signal in each
channel, thus severely impeding the parallel transfer of states such as
classical registers, multiple qubits and qudits. Here, we leverage on the
suitably engineered cross-talk between waveguides to achieve the parallel
transfer on optical chip. Waveguide coupling coefficients are designed to yield
commensurate eigenvalues of the array and hence, periodic revivals of the input
state. While, in general, polynomially complex, the inverse eigenvalue problem
permits analytic solutions for small number of waveguides. We present exact
solutions for arrays of up to nine waveguides and use them to design realistic
buses for multi-(qu)bit and qudit transfer. Advantages and limitations of the
proposed solution are discussed in the context of available fabrication
techniques
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