Search Results (31 total)

Effects of dimensionality on magnetic and electric properties of one- and two-dimensional GeMn systems and the role of defects in magnetic ordering are investigated by means of electron spin resonance (ESR) and superconducting quantum interference device magnetometry techniques. Arrays of Ge_1−xMn_x nanowires and thin Ge:Mn films with similar concentrations of the magnetic impurity (x=1%–8%) have been fabricated by chemical synthesis and ion implantation, respectively. In magnetically homogeneous Ge_1−xMn_x nanowires, all observed electron spin resonances are related to absorption on individual magnetic centers (Mn³⁺ and Mn²⁺ ions and polarized charge carriers) in a broad temperature range, T=5–300 K. On the other hand, in strongly inhomogeneous 2D GeMn films, a collective spin excitation, the spin-wave resonance, is observed at low temperatures, T=5–60 K. This signifies the presence of long-range spin states and a cooperative magnetic response originating from crystalline Mn₅Ge₃ precipitates and Mn-rich amorphous nanoclusters as well as diluted Mn ions. Additionally, a strong negative background was observed and attributed to the microwave magnetoresistance of the Ge:Mn thin films. The absence of the magnetoresistance in Ge_1−xMn_x nanowires indicates that the scattering of charge carriers is determined by dimensions of the structure. Overall, our analysis of magnetic-resonance phenomena reveals a significant difference between one-dimensional and two-dimensional magnetic semiconductors. It emphasizes the important role of dimensionality as well as the type and distribution of magnetic defects in spin-dependent scattering and dynamic magnetic properties of GeMn semiconductors.

Using the ORBIT code we study the sensitivity of electron cloud properties with respect to different proton beam profiles, the secondary electron yield (SEY) parameter, and the proton loss rate. Our model uses a cold proton bunch to generate primary electrons and electromagnetic field for electron cloud dynamics. We study the dependence of the prompt and swept electron signals vs the bunch charge and the recovery of electron clouds after sweeping on the beam loss rate and the SEY. The simulation results are compared with the experimental data measured at the proton storage ring at the Los Alamos National Laboratory. Our simulations indicate that the fractional proton loss rate in the field-free straight section may be an exponential function of proton beam charge and may also be lower than the averaged fractional proton loss rate over the whole ring.

Thin carbon foils are used as strippers for charge exchange injection into high intensity proton rings. However, the stripping foils become radioactive and produce uncontrolled beam loss, which is one of the main factors limiting beam power in high intensity proton rings. Recently, we presented a scheme for laser stripping an H^- beam for the Spallation Neutron Source (SNS) ring. First, H^- atoms are converted to H⁰ by a magnetic field, then H⁰ atoms are excited from the ground state to the upper levels by a laser, and the excited states are converted to protons by a magnetic field. In this paper we report on the proof-of-principle demonstration of this scheme to give high efficiency (around 90%) conversion of H^- beam into protons at SNS in Oak Ridge. The experimental setup is described, and comparison of the experimental data with simulations is presented.

We report the synthesis and magnetic characterization of metal/metal oxide one-dimensional heterostructures with a radial geometry—coaxial nanocables. High-density arrays of cobalt and magnetite- (Fe₃O₄) based nanocables with the diameter of ∼100 nm and length of ∼60 μm have been synthesized within the pores of anodized aluminum oxide membranes using a supercritical fluid inclusion-phase technique. All investigated heterostructures demonstrate ferrimagnetic/ferromagnetic properties at room temperature. We show that the magnetic properties of cobalt-magnetite nanocables can be tuned by varying the thickness of the sheath and core layers. The exchange bias effect, due to formation of CoO thin layers at the nanocable interfaces, was observed at low temperatures. The strength and manifestation of this effect depends on the structural characteristics and oxidation state of the Co layer. We show that the vast increase of the coercive field (up to 450 times) at low temperatures in samples containing the large fraction of magnetite is caused by a governing influence of the temperature-dependent magnetoelastic term of the anisotropy.

We report an observation of room temperature ferromagnetism in Ge nanowires doped with Mn. High-density arrays of Ge_1−xMn_x (x=1%–5%) nanowires with the smallest diameter of 35 nm have been synthesized within the pores of anodized aluminium oxide membranes using a supercritical fluid inclusion-phase technique. Structural analysis of the nanowires proved the existence of a highly crystalline germanium host lattice containing discrete manganese atoms. All of the nanowires studied displayed ferromagnetic properties at room temperature. Ferromagnetic ordering reaches a maximum at intermediate Mn concentrations. The magnetic properties of the nanowires can be understood by considering the influence of co-dopant nonmagnetic impurities and nanowire/membrane interfaces.

We present experimental data from the Los Alamos Proton Storage Ring (PSR) showing long-lived linac microbunch structure during beam storage with no rf bunching. Analysis of the experimental data and particle-in-cell simulations of the experiments indicate that space charge, coupled with energy spread effects, is responsible for the sustained microbunch structure. The simulated longitudinal phase space of the beam reveals a well-defined separatrix in the phase space between linac microbunches, with particles executing unbounded motion outside of the separatrix. We show that the longitudinal phase space of the beam was near steady state during the PSR experiments, such that the separatrix persisted for long periods of time. Our simulations indicate that the steady state is very sensitive to the experimental conditions. Finally, we solve the steady-state problem in an analytic, self-consistent fashion for a set of periodic longitudinal space-charge potentials.

Finding self-consistent distributions of beam particles interacting with each other via the space charge force is one of the challenges of accelerator physics. Exactly solvable models are used for simulation benchmarks, instability threshold calculations, etc. Since such distributions have been found only in one and two dimensions (Kapchinsky-Vladimirsky distribution), it is not possible to apply them to a general three dimensional motion. This paper shows how to construct new sets of self-consistent distributions, extending even to the three dimensional case.

We present studies of space-charge-induced beam profile broadening at high intensities in the Proton Storage Ring (PSR) at Los Alamos National Laboratory. We investigate the profile broadening through detailed particle-in-cell simulations of several experiments and obtain results in good agreement with the measurements. We interpret these results within the framework of coherent resonance theory. With increasing intensity, our simulations show strong evidence for the presence of a quadrupole-mode resonance of the beam envelope with the lattice in the vertical plane. Specifically, we observe incoherent tunes crossing integer values, and large amplitude, nearly periodic envelope oscillations. At the highest operating intensities, we observe a continuing relaxation of the beam through space charge forces leading to emittance growth. The increase of emittance commences when the beam parameters encounter an envelope stop band. Once the stop band is reached, the emittance growth balances the intensity increase to maintain the beam near the stop band edge. Additionally, we investigate the potential benefit of a stop band correction to the high intensity PSR beam.

We present a particle core model study of the space charge effect on high intensity synchrotron beams, with specific emphasis on the Proton Storage Ring (PSR) at Los Alamos National Laboratory. Our particle core model formulation includes realistic lattice focusing and dispersion. We transport both matched and mismatched beams through real lattice structure and compare the results with those of an equivalent uniform-focusing approximation. The effects of lattice structure and finite momentum spread on the resonance behavior are specifically targeted. Stroboscopic maps of the mismatched envelope are constructed and show high-order resonances and stochastic effects that dominate at high mismatch or high intensity. We observe the evolution of the envelope phase-space structure during a high intensity PSR beam accumulation. Finally, we examine the envelope-particle parametric resonance condition and discuss the possibility for halo growth in synchrotron beams due to this mechanism.

Because of a long-range resistive wake, the closed orbit may experience an unstable drift. Unlike the conventional betatron instabilities, this closed orbit instability is not sensitive to the spread of the betatron frequencies. For bunched beams, feedback appears to be the only way to stabilize the closed orbit above threshold. This new instability can be significant for both existing and designed high-intensity rings.

Space charge presents a fundamental limitation to high intensity circular accelerators. Its effects are especially important in the latest designs for high-intensity proton rings, which require beam losses much smaller than presently achieved in existing facilities. It is therefore necessary to understand the major space-charge effects which could lead to emittance growth and associated beam loss. In this paper, we explore the excitation of high-order collective beam modes and associated instabilities driven by space-charge coupling resonances. Such studies help us to understand energy exchange and emittance growth driven by space-charge coupling. They also have direct application to the choice of a good working point in a high-intensity machine. The studies are performed using an earlier version of the Spallation Neutron Source lattice, which was used as a generic example of a circular machine. In this way, we explore the nature of the observed space-charge coupling effect and its applicability to high-intensity rings in general.

Transverse beam profiles are observed to broaden with increasing intensity in the Proton Storage Ring at the Los Alamos Neutron Scattering Center. Measured profiles are simulated with an H^- injection model that includes a 2D particle-in-cell space charge calculation. Inclusion of space charge effects in the simulation improves the agreement between the experimentally observed profiles and the calculated profiles. The comparisons are made for a range of injected intensities.

Numerical calculations for the Spallation Neutron Source accumulator ring indicate that lattice resonances excited by the space-charge potential can increase a mismatch significantly by deforming the beam distribution in phase space. Hence increased mismatch leads to enhanced envelope oscillations that are driving the 2:1 parametric resonance leading to halo formation, even for initially matched beams. We have observed this behavior for the 2ν_x-2ν_y=0 resonance and for the 4ν_y=23 resonance. This mechanism for halo formation peculiar to rings through resonance driven mismatch is very sensitive to the tunes, which emphasizes the importance of a careful choice of operating point in tune space.

Uncontrolled beam losses due to space-charge-induced halo generation are a concern in high intensity rings, which are characterized by high beam intensities and low uncontrolled beam loss requirements. It is therefore important to investigate the dynamics of space charge in high intensity rings. We report here the results of extensive calculations using a particle-tracking approach with a self-consistent particle-in-cell model and alternatively with a particle core model. We find that the inclusion of space charge forces provides agreement between calculated and experimentally observed beam profile shapes in the high intensity proton storage ring. We also confirm computationally the extension to rings of the accepted dynamics of halo generation with rms beam mismatch exciting the parametric resonance. In addition, we propose a new two-stage mechanism for halo production in rings in which space-charge-driven lattice resonances generate beam mismatch that excites the parametric resonance. Because of its dependence on lattice resonances, this mechanism is peculiar to rings and is capable of generating halo even from initially matched beams. It is also very sensitive to the operating point in tune space, as we show in the results of a vertical tune scan simulating injection into the Spallation Neutron Source accumulator ring. Our results extend and enhance the understanding of fundamental space charge physics, which has been developed for linear accelerators, to rings.

We report experimental measurements of electron spectra resulting from the scattering of 9.5-eV electrons by helium atoms through an angle of 9° in the presence of a high-intensity (∼10⁸ W cm^-2) CO₂ laser. The intensities of the additional peaks which occur separated from the elastic scattering peak by multiples of the photon energy in the presence of the laser are much greater than expected. These data suggest that calculations based on the Kroll-Watson approximation, usually applied to this type of experiment, are inappropriate for these scattering conditions.

We are reporting electron spectra resulting from the inelastic scattering of 45-eV electrons from helium atoms through a scattering angle of 12° in the presence of an intense carbon dioxide laser. The spectra presented are in the region of the 2 ¹S and 2 ¹P states of helium. These data show additional structure when the laser is present at scattered electron energies which correspond to an increase or decrease in electron kinetic energy of the equivalent of one laser quantum. These measurements demonstrate the simultaneous electron-photon excitation of an atom at high incident-electron energies.

The transport effects induced by resistive ballooning modes are estimated from a theory, and are found to be mainly thermal electron conduction losses. An expression for electron thermal diffusivity χ_e is derived. The theoretical predictions agree well with experimental values of χ_e obtained from power balance for the ISX-B plasmas at high poloidal beta.

This paper describes observations of magnetohydrodynamic instability with neutralbeam heating in the ISX-B tokamak and the theory specifically developed to support these experiments. The observed magnetohydrodynamic activity is explained by the resistive model presented but is not responsible for the observed degradation of confinement. Increasingly important n>1 pressure-driven modes are predicted by the theory for the higher experimental β_p values, but there is no experimental verification of their presence.

Multiphoton free-free transitions are detected in the scattering of electrons on argon atoms and on hydrogen molecules in the presence of an intense pulsed CO₂ laser field. The authors present measurements for several values of the incoming electron energy, the electron scattering angle, and the angle between the laser polarization vector and the electron momentum transfer. All observations are in qualitative agreement with a recent theoretical model based on a low-frequency approximation.

We propose a mechanism for the major disruption in tokamaks that involves the nonlinear destabilization of tearing modes by the (m=2) / (n=1) tearing mode, where m and n denote the poloidal and toroidal mode numbers, respectively. The magnetic islands generated can extend across the plasma cross section. For resistivities of the order of magnitude of these in TOSCA and LT-3, the time scale for their appearance is consistent with the time for the major disruption.

Multiphoton processes are detected in the scattering of electrons on argon atoms in the presence of a strong CO₂-laser field. The observations are in accordance with a recently developed semiclassical model.

In this paper we present the nonlinear effects we obtained so far through ferromagnetic-transmission-resonance experiments in iron and nickel. The experiments were performed in the geometry in which both the applied and microwave fields are mutually parallel and also parallel to the sample surface at the input cavity. In this geometry, we have found two interesting transmitted waves through ferromagnetic samples several microns thick. One is polarized parallel to and has the same frequency as the applied microwave field. The other, polarized perpendicular to the applied field, has a frequency half that of the applied field and is generated in the ferromagnetic sample. The results are compared to the nonlinear theory presented in the previous paper. The agreement between the experimental and theoretical results is reasonable. The theory shows that the nonlinear phenomena are very sensitive to the exchange stiffness constant. It is thus hoped that these phenomena will be useful in determining the exchange stiffness constant, and ultimately the exchange integral, as a function of temperature. Further, they might provide new information on spin-relaxation processes.

Transition probabilities for the 3p-3s array of Ne i have been measured in emission from a glow discharge containing a He-Ne-Ar mixture. The neon spontaneous emission at 632.8 nm (3s₂-2p₄) was taken as the reference line and the emission intensities of the 2p_i-1s_j lines were compared with it to obtain the relative transition probabilities A(2p_i-1s_j) / A_632.8. The discharge condition was carefully controlled so that the ratio of the populations of the 3s₂ and a given 2p_i level was precisely equal to the ratio of the statistical weights of the energy levels: N(3s₂) / N(2p_i)=g(3s₂) / g(2p_i), eliminating two population unknowns encountered in an ordinary gas discharge. The relative A values were then converted to the absolute values by means of the accurate value of A_632.8, which was obtained in our recent measurements. The results were subjected to extensive comparisons with other experimental data of Bridges and Wiese, of Bennett and Kindlmann, and the intermediate coupling calculations by Mehlhorn and by Feneuille et al. Excellent agreement was observed throughout between these data sets and our results. The following has been found: Our results are more consistent with the J-file-sum rule than any other experimental data; our relative line strengths are in excellent agreement with the theoretical calculations of Feneuille et al.; our radial transition integral σ² exhibits least variations over different 2p levels—its weighted average has been found to be 6.66±0.02 (a.u.) on an absolute basis, which is very close to the value obtained from the Coulomb approximation; and our transition-probability sums are in excellent agreement with those evaluated from the intermediate coupling calculations combined with the Coulomb approximation. Based on these findings and the error analysis, we estimate the uncertainty of our A values, except for a few weak lines, to be 2% on the relative scale and 6% on the absolute scale. Also, a discussion is given concerning the extent of configuration interaction on the 2p⁵3p states.