Search Results (15 total)

In this paper, we present a new lattice design for the International Linear Collider (ILC) positron damping ring. The same lattice could be used also for the electron damping ring. This lattice is based on modified FODO arc cells, and has a freely tunable momentum compaction factor from 2×10^-4 to 6×10^-4. In comparison with the baseline design which uses the theoretical minimum emittance arc cells, the advantages of this design are: freely tunable momentum compaction factor; smaller number of quadrupole and sextupole magnets; larger dynamic aperture; simplified layout and lower cost. The design principles and analysis techniques used in this paper to change the momentum compaction factor by a large amount while keeping a very low emittance and large dynamic aperture are very important for the ILC damping ring; at the same time it could be very useful for any high bunch density storage ring.

A new ion acceleration method, namely, phase-stable acceleration, usingcircularly-polarized laser pulses is proposed. When the initial target density n₀ and thickness D satisfy a_L∼(n₀/n_c)D/λ_L and D>l_s with a_L, λ_L, l_s, and n_c the normalized laser amplitude, the laser wavelength invacuum, the plasma skin depth, and the critical density of the incident laserpulse, respectively, a quasiequilibrium for the electrons is established bythe light pressure and the space charge electrostatic field at the interactingfront of the laser pulse. The ions within the skin depth of the laser pulseare synchronously accelerated and bunched by the electrostatic field, andthereby a high-intensity monoenergetic proton beam can be generated. The protondynamics is investigated analytically and the results are verified by one-and two-dimensional particle-in-cell simulations.

The effects of interfacial coupling at the boundary of ferromagnetic and antiferromagnetic components in a nanoscale columnar-structured thin film of Ni₈₀Fe₂₀∕CoO have been examined. Field-cooling the film results in very different temperature dependences of the enhanced coercivity and exchange-bias shift of the hysteresis loop. The exchange-bias temperature dependence is well described by thermal fluctuations of the interfacial spins while the coercivity temperature dependence indicates that single-domain-like columns are being coherently rotated by the thermal fluctuations of the interface spins. Furthermore, only a portion of the spins in the antiferromagnetic layer seem to be associated with the spin coupling that results in exchange bias. X-ray magnetic resonant scattering measurements show clearly the presence of canted Co interfacial moments that provide a local field which enables exchange bias at a significantly higher temperature than the onset of an enhanced coercivity.

A Ni₈₀Fe₂₀/(Ni,Fe)O thin film exhibits a positive exchange bias when cooled in a zero field and a negative exchange bias when field cooled. With transmission electron microscopy and electron energy loss spectrometry, the composition and magnetic structure has been ascertained and a distribution of magnetization easy axes about the interface extrapolated. The results indicate that the positive exchange bias is from antiferromagnetic interface moments perpendicular to their ferromagnetic counterparts. With field cooling the alignment is put into a parallel configuration resulting in a negative exchange bias.

Molecular dynamics simulations have been carried out to investigate the fluid wetting and flow in nanochannels whose surfaces are structured by an array of nanoscale triangular modules. We find that the surface nanostructures have a dual effect on the boundary slip and friction of the liquid nanoflow. On the one hand, the nanostructures can enhance the surface hydrophilicity for a hydrophilic liquid-solid interaction, and can increase the hydrophobicity for a hydrophobic interaction due to a nanoscale lotus effect. In particular, the nanostructured surface may show superhydrophobicity and lead to the large velocity slip of the liquid flow. On the other hand, simultaneously, the nanostructures distort the nanoscale streamlines of the liquid flow near the channel surfaces and block the nanoflow directly, which decreases the apparent slip length equivalently. The dual effect of the nanostructures on the surface wettability and the hydrodynamic disturbance results in a nonmonotonic dependence of the slip length on the nanostructure size. The simulations imply that the surface nanostructures can be applied to control the friction of liquid micro- and nanoflows.

The simultaneous acceleration of O⁺ and O^- beams in a radio frequency quadruple accelerator has been numerically simulated and the results are presented in this paper. The micropulses of O⁺ and O^- beams have been measured in dual beam acceleration experiments, which verified the numerical simulations and feasibilities of dual beam acceleration.

A 201.5 MHz, 50 mA, 2.0 MeV deuteron radio frequency quadrupole accelerator is proposed as the neutron generator for the neutron experiment facility project at Peking University, China. Based on better understanding of beam losses, some new optimization procedures concerning both longitudinal and transverse dynamics are adopted. Accordingly, the beam transmission efficiency is improved from 91.2% to 98.3% and the electrode length is shortened from 2.91 to 2.71 m. The fundamental physical analyses are performed to look inside the new design recipe and explain why it works.

The electron cloud instability (ECI) in the Beijing Electron Positron Collider (BEPC) was studied systematically with experiments and simulations in recent years. Several electron cloud detectors have been installed into the BEPC storage ring to measure the electron yield. The effects of solenoid, the clearing electrode, the chromaticity, and the octupole on ECI have been investigated in BEPC experimentally. To evaluate the ECI in BEPCII, the upgrade project of BEPC, a computer code has been developed to simulate the electron density in the antechamber under different conditions, as well as the progress of the single bunch and coupled-bunch instabilities caused by the electron cloud. The experimental and simulation results, as well as the activities, are reported in this paper.

The reaction cross sections of ^27,28P and the corresponding isotones on Si target were measured at intermediate energies. The measured reaction cross sections of the N=12 and 13 isotones show an abrupt increase at Z=15. The experimental results for the isotones with Z≤14 as well as ²⁸P can be well described by the modified Glauber theory of the optical limit approach. The enhancement of the reaction cross section for ²⁸P could be explained in the modified Glauber theory with an enlarged core. Theoretical analysis with the modified Glauber theory of the optical limit and few-body approaches underpredicted the experimental data of ²⁷P. Our theoretical analysis shows that an enlarged core together with proton halo are probably the mechanism responsible for the enhancement of the cross sections for the reaction of ²⁷P+²⁸Si.

A vertical coupled-bunch instability was observed for a positron beam at the Beijing Electron Positron Collider (BEPC). The experimental results show that the instability has similar characteristics as that observed in the Photon Factory of KEK several years ago. The instability at BEPC can be explained by the effect of an electron cloud which is produced in the beam chamber by synchrotron light hitting the wall.

Reaction cross section σ_R of proton-rich isotones (N=10) near ²³Al and Al isotopes (^23-28Al) on C target have been measured at intermediate energies around 30 MeV/nucleon. An abnormal increase of the experimental σ_R is observed for ²³Al and it suggests that there is an anomalously large matter rms radius in ²³Al. Together with the very weakly binding of the last proton (S_p=0.125 MeV), it indicates that there is a proton halo in ²³Al. This conclusion is also supported by the difference factor d which is deduced from the measured and theoretical σ_R in the Glauber or Boltzmann-Uehling-Uhlenbeck model and is used for the manifest of halo phenomena. The theoretical calculation based on the relativistic density dependent Hartree model shows that there is a proton halo when the last proton is in the 2s_1/2 orbit in ²³Al. The significance of the proton halo in ²³Al is discussed.

Measurements of the total reaction cross section for ^12-16C, ^14-17N, and ^16-18O on carbon target at intermediate energies were performed on the Radioactive Ion Beam Line of the Heavy Ion Research Facility in Lanzhou. A larger enhancement of σ_R for ¹⁵C was observed than for its neighbors. Evidence for possible anomalous nuclear structure in ¹⁵C was revealed in the analysis of the total reaction cross section in terms of the difference factor d.

The fragments produced in the reaction of ¹⁸O+⁹Be at 60 MeV/nucleon were measured experimentally. The isotopic distribution of the fragmentation reaction products was well reproduced by using a modified statistical abrasion-ablation model. This model predicts that the fragment isotopic distribution at a fixed atomic number Z shifts towards the neutron rich side for the neutron-rich projectile. This isospin effect will decrease with decreasing the fragment atomic number Z and disappear when (Z_proj-Z)/Z_proj becomes larger than 0.5. It was shown that the disappearance of the isospin effect of fragmentation reaction is induced by the geometry effect in abrasion stage and the evaporation effect later.

The isotropic distributions of projectile fragmentation have been measured for the reactions of 44 MeV/nucleon ¹²⁹Xe on ⁹⁰Zr and ¹⁹⁷Au targets. The systematics of the production cross sections are discussed via some macroscopic models.

Photoemission spectroscopy now reaches a lateral resolution in the submicrometer range. It is possible, therefore, to perform chemical analysis in the microscopic domain required for life-science experiments. We present a successful test of this approach, including photoemission spectra taken on microscopic areas of a fixed neuron culture, and photoelectron two-dimensional microdistribution maps of specific elements. The test demonstrates that scanning photoemision spectromicroscopy can be successfully applied to biological specimens, although one cannot automatically connect results on fixed cultures and the in vivo properties.