Search Results (21 total)

A general procedure is discussed to formulate a coupling function capable of targeting desired responses such as synchronization, antisynchronization, and amplitude death in identical as well as mismatched chaotic oscillators. The coupling function is derived for unidirectional, mutual, and matrix type coupling. The matrix coupling, particularly, is able to induce synchronization, antisynchronization, and amplitude death simultaneously in different state variables of a response system. The applicability of the coupling is demonstrated in spiking-bursting Hindmarsh-Rose neuron model, Rössler oscillator, Lorenz system, Sprott system, and a double scroll system. We also report a scaling law that defines a process of transition to synchronization.

We propose a design of coupling for stable synchronization and antisynchronization in chaotic systems under parameter mismatch. The antisynchronization is independent of the specific symmetry (reflection symmetry, axial symmetry, or other) of a dynamical system. In the synchronization regimes, we achieve amplification (attenuation) of a chaotic driver in a response oscillator. Numerical examples of a Lorenz system, Rössler oscillator, and Sprott system are presented. Experimental evidence is shown using an electronic version of the Sprott system.

Longitudinal bunching factors in excess of 70 of a 300-keV, 27-mA K⁺ ion beam have been demonstrated in the neutralized drift compression experiment [P. K. Roy , Phys. Rev. Lett. 95, 234801 (2005)] in rough agreement with particle-in-cell source-to-target simulations. A key aspect of these experiments is that a preformed plasma provides charge neutralization of the ion beam in the last one meter drift region where the beam perveance becomes large. The simulations utilize the measured ion source temperature, diode voltage, and induction-bunching-module voltage waveforms in order to determine the initial beam longitudinal phase space which is critical to accurate modeling of the longitudinal compression. To enable simultaneous longitudinal and transverse compression, numerical simulations were used in the design of the solenoidal focusing system that compensated for the impact of the applied velocity tilt on the transverse phase space of the beam. Complete source-to-target simulations, that include detailed modeling of the diode, magnetic transport, induction bunching module, and plasma neutralized transport, were critical to understanding the interplay between the various accelerator components in the experiment. Here, we compare simulation results with the experiment and discuss the contributions to longitudinal and transverse emittance that limit the final compression.

An experiment to inject and match a 10  μs, singly charged K⁺ ion bunch at an ion energy of 0.3 MeV, current of 45 mA, and dimensionless perveance of 10^-3 into a solenoid lattice has been carried out at LBNL. The principal objective of this experiment is to match and transport the space-charge dominated ion beam and compare predicted and measured emittance. Initial investigation also presented the opportunity to study electron cloud effects and the effects of misalignments. A qualitative comparison of experimental and calculated results are presented, which include time resolved current density, transverse distributions, and phase space of the beam at different diagnostic planes.

In a first beam dynamics validation experiment for a new Pulse Line Ion Acceleration (PLIA) concept, the predicted energy amplification and beam bunching were experimentally observed. Beam energy modulation of -80 to +150  keV was measured using a PLIA input voltage waveform of -21 to +12  kV. Ion pulses accelerated by 150 keV, and bunching by a factor of 4 were simultaneously achieved. The measured longitudinal phase space and current waveform of the accelerated beam are in good agreement with 3D particle-in-cell simulations.

Heavy ion drivers for heavy ion fusion and high energy density physics applications use space-charge-dominated ion beams which must undergo longitudinal bunch compression in order to meet the requisite beam intensities desired at the target. The Neutralized Drift Compression Experiment-1A (NDCX-1A) at Lawrence Berkeley National Laboratory is used to determine the effective limits of neutralized drift compression, which occurs due to an imposed longitudinal velocity tilt on the drifting beam and subsequent neutralization of the beam’s space charge with background plasma. The accurate and temporally resolved measurement of the ion beam’s current and pulse length, which has been longitudinally compressed to a few nanoseconds duration at its focal plane, is a critical diagnostic. This paper describes the design and experimental results for a fast and accurate ion beam probe, which reliably measures the absolute beam current in the presence of high density plasma at the focal plane as a function of time. A particle-in-cell code has been used to model the propagation of the intense ion beam and to design the diagnostic probe.

Longitudinal compression of a velocity-tailored, intense neutralized K⁺ beam at 300 keV, 25 mA has been demonstrated. The compression takes place in a 1–2 m drift section filled with plasma to provide space-charge neutralization. An induction cell produces a head-to-tail velocity ramp that longitudinally compresses the neutralized beam, enhancing the beam peak current by a factor of 50 and producing a pulse duration of about 3 ns. This measurement has been confirmed independently with two different diagnostic systems.

The neutralized transport experiment (NTX) at the Heavy Ion Fusion Virtual National Laboratory is exploring the performance of neutralized final-focus systems for high perveance heavy ion beams. The final-focus scenario in a heavy ion fusion driver consists of several large aperture quadrupole magnets followed by a drift section in which the beam space charge is neutralized by a plasma. This beam is required to hit a millimeter-sized target spot at the end of the drift section. The objective of the NTX experiments and associated theory and simulations is to study the various physical mechanisms that determine the final spot size (radius r_s) at a given distance (f) from the end of the last quadrupole. In a fusion driver, f is the standoff distance required to keep the chamber wall and superconducting magnets properly protected. The NTX final quadrupole focusing system produces a converging beam at the entrance to the neutralized drift section where it focuses to a small spot. The final spot is determined by the conditions of the beam entering the quadrupole section, the beam dynamics in the magnetic lattice, and the plasma neutralization dynamics in the drift section. The main issues are the control of emittance growth due to high order fields from magnetic multipoles and image fields. In this paper, we will describe the theoretical and experimental aspects of the beam dynamics in the quadrupole lattice, and how these physical effects influence the final beam size. In particular, we present theoretical and experimental results on the dependence of final spot size on geometric aberrations and perveance.

In heavy-ion inertial-confinement fusion systems, intense beams of ions must be transported from the exit of the final-focus magnet system through the fusion chamber to hit spots on the target with radii of about 2 mm. For the heavy-ion-fusion power-plant scenarios presently favored in the U.S., a substantial fraction of the ion-beam space charge must be neutralized during this final transport. The most effective neutralization technique found in numerical simulations is to pass each beam through a low-density plasma after the final focusing. To provide quantitative comparisons of these theoretical predictions with experiment, the Virtual National Laboratory for Heavy Ion Fusion has completed the construction and has begun experimentation with the neutralized-transport experiment. The experiment consists of three main sections, each with its own physics issues. The injector is designed to generate a very high-brightness, space-charge-dominated potassium beam, while still allowing variable perveance by a beam aperturing technique. The magnetic-focusing section, consisting of four pulsed quadrupoles, permits the study of magnet tuning, as well as the effects of phase-space dilution due to higher-order nonlinear fields. In the final section, the converging ion beam exiting the magnetic section is transported through a drift region with plasma sources for beam neutralization, and the final spot size is measured under various conditions of neutralization. In this paper, we discuss the design and characterization of the three sections in detail and present initial results from the experiment.

We report first band-offset measurements obtained by multiphoton internal-photoemission induced second-harmonic generation. Our two-color contactless laser technique involves (1) optically pumping electrons into the oxide and (2) probing the resulting interface electric field using time-dependent second-harmonic generation. One- and two-photon internal-photoemission thresholds for the Si/SiO₂ interface were measured to be 4.5 and 2.25 eV, respectively. This method promises to become a valuable experimental tool in determining band offsets in wide variety of semiconductor interfaces.

The point stressed by Michel and Reidemeister is that the Michel potential in our work is not final and an adjustment of the parameter α gives an elastic distribution and a potential similar to that obtained for the molecular one. This is usual in the absence of the elastic data at large angles as commented on by Tariq et al. [Phys. Rev. C 59, 2558 (1999)]. However, the inadequacy of the Michel potential with the parameter α=7.4 in the description of the (α,t) reaction data leads to a finding of a better value for the parameter by Michel and Reidemeister. This justifies the use of the molecular and Michel potentials in the analysis of the reaction and conforms to the suggestion by Satchler (Ref. [40] in Tariq et al.) that the performance of a potential in nonelastic processes is a stronger criterion in its selection.

Full finite-range distorted-wave Born approximation calculations have been performed using molecular, Michel, and normal optical potentials to analyze the angular distributions of cross sections for the 53 transitions populating the bound and unbound states of ²⁸Si via the (α,t) reaction. The parameters of these three potentials have been determined from analyses of the elastic scattering data in the entrance channel at the incident energy involved. The molecular and optical potentials are found to produce satisfactory fits to the data, but the Michel potential seems to be inadequate. For all three potentials in the entrance channel, the deduced l transfers for the transitions to the 15.02, 15.85, and 16.11 MeV states differ from the assignments previously reported. The extracted spectroscopic factors are compared with shell-model predictions.

We report a new and surprising enhancement of the electric field at the Si/SiO ₂ interface following the cessation of intense pulsed near-infrared radiation. The phenomenon, measured by optical second-harmonic generation, occurs only for photon energies and oxide film thickness that exceed respective thresholds. We attribute the new effect to multiphoton hole injection into the oxide and to an asymmetry in electron and hole dynamics, in particular to distinctly different trapping and detrapping processes.

The production of low-mass dileptons and soft photons from thermalized quark-gluon plasma (QGP) and hadronic matter in relativistic heavy ion collisions is evaluated. A boost invariant longitudinal and cylindrically symmetric transverse expansion of the systems created in central collision of lead nuclei at CERN SPS, BNL RHIC, and CERN LHC, and undergoing a first-order phase transition to hadronic matter is considered. A large production of low-mass (M< 0.3 GeV) dileptons, and soft photons (p_T<0.4 GeV) is seen to emanate from the bremsstrahlung of quarks and pions. We find an increase by a factor of 2 to 4 in the low-mass dilepton and soft-photon yield as we move from SPS to RHIC energies, and an increase by an order of magnitude as we move from SPS to LHC energies. Most of the soft radiations are found to originate from pion driven processes at SPS and RHIC energies, while at the LHC energies the quark and the pion driven processes contribute by a similar amount. The study of the transverse mass distribution is seen to provide interesting details of the evolution. We also find a unique universal behavior for the ratio of M² weighted transverse mass distribution for M= 0.1 GeV to that for M= 0.2 and 0.3 GeV, as a function of M_T, for SPS, RHIC, and LHC energies, in the absence of transverse expansion of the system. A deviation from this universal behavior is seen as a clear indication of the flow.

Production of soft photons in relativistic heavy ion collisions due to bremsstrahlung processes in quark matter and hadronic matter is studied. The contribution of pion-driven processes is found to dominate the yield. © 1996 The American Physical Society.

Radiation of photons off quarks escaping from a quark-gluon plasma and thereby accelerated by the confining color field is studied. The rate of emission of these photons is found to increase with their rapidity for a given energy. The p_T spectrum of the photons thus radiated is compared to those due to the Compton and annihilation processes in the quark-gluon plasma (QGP) after accounting for the space-time evolution of the plasma. The largest contributions are seen if the current mass for the quarks is used and then the photons produced due to the color-confining mechanism operating on the quarks are found to give a contribution of about 100% at very low p_T and about 10–20% at the transverse momenta of 2–3 GeV as compared to the photons due to the Compton and annihilation processes in the plasma. However, if a larger value is associated with the mass of the quark, this radiation is very strongly suppressed. This uncertainty raises doubts about the usefulness of such radiations as a signature of a QGP, which is rather unfortunate as these photons are known to be polarized.

It is shown that the eigenvalue problem of the L operator for the sine-Gordon equation can be put in a supersymmetric form. We comment on the connection between the conserved quantities of the Korteweg–de Vries and sine-Gordon systems.

A ladder-operator technique for generating isospectral Hamiltonians is presented. The most attractive feature of the scheme is that the results are obtained without any specific realization of the ladder operators. The method is applied to the N-dimensional Coulomb and isotropic oscillator problems. Few new results are presented in addition to those obtained using supersymmetric quantum mechanics.

The rearrangement collision problem of positronium formation into arbitrary n,l,m states by positrons from hydrogen atoms in the ground state has been investigated employing the first Born approximation and the first-order exchange approximation. It is shown that for fixed l,m, n³ times the asymptotic cross section, if multiplied by the factor Π_α=0^l(1-α²n^-2), gives a good estimate of the n³ cross section of the corresponding n,l,m state when n is not too low. For s state the first-order-exchange-approximation results are greater than those of the first Born approximation. The momentum-transfer cross sections in the first Born approximation overestimate the first-order-exchange-approximation results throughout the energy range considered.

The Coulomb-Born (CB) and a modified Coulomb-Born approximations where distortion has been taken into account in the initial channel have been applied to calculate the electron-capture cross sections by protons passing through some heliumlike ions. Results for the charge-transfer cross sections in CB approximation for different isoelectronic ions have been computed, retaining both the post and prior forms of the interaction potential. The results for the process H⁺+Li⁺→H+Li²⁺ have been compared with the available theoretical and experimental findings.