Search Results (21 total)

In a particle accelerator with a periodic structure beam space charge force may excite resonant beam emittance growth if the particle’s transverse phase advance approaches 90°. A recent simulation study with the PARMILA code [D. Jeon , Phys. Rev. ST Accel. Beams 12, 054204 (2009)] has shown the feasibility of measuring the stop band of this fourth order resonance in the GSI Universal Linear Accelerator UNILAC and proposed its experimental verification, which is reported here. Measurements of transverse phase space distributions behind a periodically focusing structure reveal a fourfold symmetry characteristic of fourth order resonances as well as a resonance stop band above σ₀=90° per focusing cell. These experimental findings agree with results from three different beam dynamics simulation codes, i.e., DYNAMION, PARMILA, and TRACEWIN.

It is discovered that, for a high intensity beam, the 4σ=360° (or 4ν=1) resonance of a linear accelerator is manifested through the octupolar term of space charge potential when the depressed phase advance per cell σ is close to and below 90° but no resonance effect is observed when σ is just above 90°. To verify that this is a resonance, a frequency analysis is performed and a study of resonance crossing from above and from below the resonance is conducted. It is observed that this fourth order resonance is dominating over the better known envelope instability and practically replacing it. The simulation study shows a clear emittance growth by this resonance and its stop band. A proposal to GSI was made to perform an experiment to measure the stop band of this resonance using the UNILAC. The experiment confirmed this resonance and will be published in a separate paper.

Transverse emittance growth along the Alvarez drift tube linac (DTL) section is a major concern with respect to the preservation of beam quality of high current beams at the GSI UNILAC. In order to define measures to reduce this growth, appropriate tools to simulate the beam dynamics are indispensable. This paper is about the benchmarking of three beam dynamics simulation codes, i.e. DYNAMION, PARMILA, and PARTRAN against systematic measurements of beam emittances for different transverse phase advances along the DTL. Special emphasis is put on the modeling of the initial distribution for the simulations. The concept of rms equivalence is expanded from full intensity to fractions of less than 100% of the beam. The experimental setup, data reduction, preparation of the simulations, and the evaluation of the simulations are described. In the experiments and in the simulations, a minimum of the rms-emittance growth was observed at zero current phase advances of about 60°. In general, good agreement was found between simulations and experiment for the mean values of horizontal and vertical emittances at the DTL exit.

By adopting classical models of molecular chirality, contributions of the coupled-oscillator and helix natures to the chiral surface second-order susceptibilities are identified through introduction of a molecular orientational distribution. Experimentally, surface orientational distribution functions at interfaces of an isotropic chiral chitosan polymer film are determined from second harmonic generation measurement. The largest chiral component of surface nonlinear optical susceptibility is from the electric-magnetic coupling with dominant contribution from the helix nature of chitosan.

A halo generation mechanism in the nonperiodic lattices such as the SNS (Spallation Neutron Source) linac MEBT [medium-energy beam transport between radio-frequency quadrupole and DTL (drift tube linac)] is reported. We find that the nonlinear space charge force resulting from large transverse beam eccentricity ∼2:1 in the ∼1.6 m-long MEBT chopper section is responsible for halo formation. As a result, the beam distribution, based on the front end emittance measurements and multiparticle simulation studies, develops halo that leads to beam loss and radioactivation of the SNS linac. Designing lattices with transverse beam eccentricity close to 1:1 suppresses this kind of halo generation. Modifying the MEBT optics and introducing adjustable collimators in the MEBT significantly reduced beam losses in the coupled cavity linac, which is a preferred scheme for mitigating halo. It turns out that the DTL collimation does not effectively remove halo and presents a risk of overheating drift tubes.

We study halo emittance growth in anisotropic beams and show that the rms emittance growth resulting from mismatch is highly anisotropic, depending on the tune ratio. We find that the free-energy limit calculated by Reiser [J. Appl. Phys. 70, 1919 (1991)] for an axisymmetric 1D halo can be extended to 2D if understood as an upper bound to the rms emittance growth averaged per degree of freedom. The thus-obtained “free-energy limit” of an ideal transport system is compared with the halo rms emittance growth in simulations of the Spallation Neutron Source linac.

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.

A new type of spin depolarization resonance has been observed at the Brookhaven Alternating Gradient Synchrotron (AGS). This spin resonance is identified as a strong closed-orbit sideband around the dominant intrinsic spin resonance. The strength of the resonance was proportional to the 9th harmonic component of the horizontal closed orbit and proportional to the vertical betatron oscillation amplitude. This “hybrid” spin resonance cannot be overcome by the partial snake at the AGS, but it can be corrected by the harmonic orbit correctors.

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.

The evolution of the beam distribution in a double-rf system with a phase modulation on either the primary or secondary rf cavity was measured. We find that the particle diffusion process obeys the Einstein relation if the phase space becomes globally chaotic. When dominant parametric resonances still exist in the phase space, particles stream along the separatrices of the dominant resonance, and the beam width exhibits characteristic oscillatory structure. The particle-tracking simulations for the double-rf system are employed to reveal the essential diffusion mechanism. Coherent octupolar motion has been observed in the bunch beam excitation. The evolution of the longitudinal phase space in the octupole mode is displayed.

Experimental observation of particle diffusion mechanism in the presence of overlapping parametric resonances generated by a time dependent rf phase modulation is analyzed. We find that the regime of fast emittance growth is associated with the rapid particle motion along the separatrix of a dominant parametric resonance, the slow growth regime is related to particle diffusion in the chaotic sea, and the emittance saturation occurs when beam particles fill the chaotic region bounded by an invariant torus. Experimental data observed at the Indiana University Cyclotron Facility (IUCF) Cooler Ring are shown to agree well with the theoretical analysis.

We investigate effects of quantum fluctuation, potential well distortion, quantum lifetime, and Touschek lifetime of the quasi-isochronous (QI) dynamical system. The Fokker-Planck equation is employed to study the equilibrium bunch distribution. The quantum lifetime in the moderate damping regime is compared with analytical formulae. The effects of harmonic radio-frequency phase modulation on equilibrium distribution function, quantum lifetime reduction, and the occurrence of stochastic resonance are studied. The formula for the Touschek lifetime for the QI dynamical system is derived and studied.

The quasi-isochronous (QI) dynamical system, in the presence of synchrotron radiation damping and rf phase modulation, exhibits a sequence of period-2 bifurcations en route towards global chaos (instability) in a region of modulation tune. The critical modulation amplitude for the onset of the global chaos shows a cusp as a function of the modulation tune. This cusp is shown to arise from the transition from the 2:1 to the 1:1 parametric resonances. We have also studied the effect of the rf voltage modulation on the QI dynamical system and found that the tolerance of the rf voltage modulation is much larger than that of the rf phase modulation. © 1996 The American Physical Society.

The synchrotron equation of motion in quasi-isochronous (QI) storage rings is transformed to a universal Weierstrass equation, where the solution is given by Jacobian elliptic functions. Scaling properties of the QI Hamiltonian are derived. The effects of phase space damping and the sensitivity of particle motion to external harmonic modulation are studied. We find that the rf phase modulation is particularly enhanced in QI storage rings. Exact formula and sum rules for resonance strength coefficients are derived. When the QI dynamical system is subject to harmonic modulation, it exhibits a sequence of period-2 bifurcations leading to global chaos in a region of modulation tune. This means that the operators of QI storage rings should pay special attention to rf phase noise. © 1996 The American Physical Society.

The Mn-induced Cu(100)-c(2×2) structure has been investigated using scanning tunneling microscopy (STM). Mn atoms adsorb as isolated atoms or small clusters at low coverages. Adsorbed Mn atoms replace the substrate Cu atoms to form an ordered surface alloy with a c(2×2) structure at ∼0.5 ML. Large buckling of Mn atoms in the c(2×2) structure, which was proposed to explain a surface ferromagnetism, is not observed by STM.

We report a study of the isolated zinc vacancy (V_Zn) in ZnSe using optical detection of magnetic resonance (ODMR) via magnetic circular dichroism (MCD) in absorption. Three MCD bands associated with V_Zn^- are observed at 1.0, 1.4, and 2.65 eV. The 1.4- and 2.65-eV bands have previously been observed by stress-induced linear dichroism studies. Detection of the band at 1.0 eV provides important confirmation of a previously proposed molecular orbital model for the defect. The MCD-ODMR signals are absorptionlike and disappear for the applied magnetic field along the defect axis. This unusual behavior is explained by calculating the dipole transition probabilities of the MCD transitions using this model, but expanded to include spin-orbit interaction.

Na-induced 3×1 reconstruction on the Si(111) surface was studied using a scanning tunneling microscope and other tools. Atomic images showed that this surface consisted of Na zigzag chains separated by a missing row, which covered the bulk-terminated Si(111) 1×1 substrate at the coverage of 2/3. Upon further deposition, a Na multilayer was grown following the Stranski-Krastanov mode. Tunneling I-V curves showed that the first layer of Na was insulating but the insulator-metal transition occurred in the second layer. We propose, based on the structural analysis, that this transition represents a two-dimensional Mott-Hubbard system.

The origin of a triplet (S=1) optically detected magnetic-resonance (ODMR) spectrum observed in the luminescence of GaP:O has been the subject of recent controversy. Originally identified with the negative-charge state of a substitutional oxygen impurity, more recent suggestions have interpreted a resolved I=3/2 hyperfine interaction in the ODMR to indicate an interstitial-gallium-related defect. We report here optical detection of electron-nuclear double resonance (ODENDOR) of the central I=3/2 nucleus and confirm unambiguously that it is gallium. Its magnetic hyperfine and nuclear quadrupole interactions are determined and an improved analysis of the electron spin Hamiltonian is presented. A possible model for the defect is suggested: a 〈100〉-oriented Ga_iV_Ga pair stabilized by the presence of one or two nearby substitutional oxygen donors. The ODENDOR spectrum arises from the M=0 state, causing unusual magnetic-field and orientation effects. The origin of these effects is described and the procedure of analysis to extract the hyperfine parameters is outlined.

Optical detection of electron-nuclear double resonance (ODENDOR) has been used in a study of an optically detected magnetic resonance spectrum previously assigned to the shallow effective-mass-like 1S(E) state of the substitutional oxygen donor in GaP. The ODENDOR spectra were observed via photoluminescence transitions which are known to be oxygen related. Hyperfine interactions with several shells of both P and Ga neighbors were observed showing that the defect center is on the P sublattice and has full T_d (tetrahedral) symmetry. A substantial hyperfine interaction was observed on the Ga sublattice inconsistent with an effective-mass-like state made up from the lowest X₁ conduction-band valleys. Further, the full tetrahedral symmetry of the defect as deduced from the ODENDOR spectra reveals an A₁ state not consistent with the 1S(E) state believed to be involved in the electron-capture luminescence transition. Analysis of the defect wave-function distribution gives a binding energy of about 0.22 eV.

Optically detected electron-nuclear double resonance (ODENDOR) is reported for the P_In antisite in p-type InP. In both electron-irradiated and as-grown samples, ENDOR signals from the four nearest P neighbors and the next In shell were detected by monitoring radio-frequency-induced changes in magnetic circular dichroism associated with absorption near the band edge of InP while maintaining microwave resonance of the P_In antisite. The electronic wave function of the P_In⁺ state is highly localized, essentially all of it accounted for within these first two neighbor shells. The optical-absorption band associated with the antisite is centered above the InP band gap, and only its tail is observed.