Search Results (7 total)

Theoretical and numerical studies of the transport in vacuum of multi-nC, multi-MeV electron beams are performed using several methods, including envelope models, a novel semianalytic approach using ellipsoidal shell decomposition, a modified electrostatic particle-in-cell method, and a point-to-point interaction model. The effects of space-charge forces on the longitudinal and transverse bunch properties are evaluated for various bunch lengths, energies, energy spreads, and charges. An evaluation of the various methods for studying space-charge effects in large energy spread, high charge beams is summarized. Examples are given for beam distributions typical of those generated by plasma-based accelerators. It is found that, for the highly correlated beams produced in the self-modulated regime, the high energy portion of the beam can gain significant energy while propagating in vacuum due to space-charge effects.

This paper outlines a formalism for the description of the linear transverse dynamics of charged particles in an axisymmetric ionization cooling channel. The particle trajectories in the absence of Coulomb scattering are described in terms of lattice functions à la Courant and Snyder, which depend only on the electric and magnetic fields in the channel. The process of multiple Coulomb scattering, which introduces stochastic terms into the particle equations of motion, is treated (in Gaussian approximation) by obtaining the distribution function in phase space as a solution of a Fokker-Planck equation. The distribution function is then used to obtain moment equations for the transverse variables and for combinations of variables such as the emittance and angular momentum. The distribution function is also used to obtain an expression for the peak four-dimensional phase space density and for the fraction of the beam that is within a certain area in phase space. The special case of a periodic channel is then considered and expressions for the asymptotic rms emittance and peak phase space density are obtained. Finally, the application of the general formalism to a numerical example, based on the reported design of a cooling channel for a neutrino source, is considered, and comparisons are made with numerical simulations of that channel.

This paper will explore the possibility of the formation of localized modes in the coupled transverse oscillations of bunches in an accelerator. Such modes are characterized by a large amplitude excitation of one of the bunches, with little or no excitation of the other bunches. They are very similar to intrinsic localized modes predicted to appear in lattice excitations of anharmonic condensed matter systems. The existence of the phenomenon in accelerators requires long-range coupling between the bunches, and quartic terms in the effective potential experienced by the bunches. The discussion in this paper considers the specific cases of a long-range coupling due to the resistive wall impedance, and nonlinearities in the transverse force caused by octupoles. The general equations for the localized mode frequencies and amplitudes are derived. The equations are applied to the specific cases of 9 and 21 equally spaced, equally populated bunches in the Cornell electron storage ring. For both cases, with the currently available octupole strength, and with currents of 8 mA per bunch, localized modes with amplitudes of about 2 mm, and with frequency shifts of 50–100s^-1 relative to the linear coupled-bunch mode frequencies, may be possible.

The pionic x-ray energies of the 4f-3d transition in π-P and the 5f-4d transition in π-Ti were measured with a bent-crystal spectrometer at the Nevis synchrocyclotron; and a new value of the pionic mass is deduced to be 139 567.5±0.9 keV, leading to an improved value for the μ-neutrino mass of m_νμ²=0.102±0.119 MeV²; m_νμ<0.52 MeV, at 90% confidence level.

A newly designed, large-aperture and high-resolution bent-crystal spectrometer has been used to observe high-intensity sources of pionic x rays. The pionic x-ray source was a target of natural titanium which was placed adjacent to a copper pion-production target in the external beam of the Nevis synchrocyclotron. The energy difference between the 5g→4f and 5f→4d transitions in pionic titanium was measured to be 87.6 ± 1.8 eV. Comparison with the prediction of the Klein-Gordon equation is made.

The small-angle (13°-40°) part of the elastic differential cross section for the scattering of low-energy (T_π∼100 MeV) positive and negative pions from natural lead was measured. The same quantity was also measured for the scattering of low-energy muons from lead. The muon-lead data were used to determine the rms charge radius of the lead nucleus. Assuming a Fermi shape for the charge density with a skin thickness of 2.21 fm, the result was 〈r²〉_ch^{1 / 2}=5.50±0.06 fm. The π^±-Pb data, taken in the region of large Coulomb nuclear interference, were compared with predictions of the pion-nucleus optical model according to Watson and Kisslinger. Excellent fits to the data were obtained by treating the model phenomenologically; however, the best-fit optical-model parameters were not in agreement with the values computed from experimental pion-nucleon phase shifts. Since the π^±-Pb elastic cross sections are sensitive to the neutron density distribution, a determination of the rms neutron-matter radius could be made. Assuming a Fermi shape for the neutron-matter density, with a skin thickness of 2.21 fm, the result was 〈r²〉_n.m.^{1 / 2}=5.45_-0.25^+0.13 fm.

Elastic scattering of 190-MeV / c muons has been measured to ∼1% in the angular range 140 mrad≤θ≤590 mrad corresponding to invariant four-momentum transfers 0.016 fm^-2≤q²≤0.33 fm^-2. Magnetostrictive wire spark chambers were used to measure the incident momentum and scattering angle. A comparison is made with electron scattering experiments which extend down in q² to 0.02 fm^-2. A change of slope of the carbon form factor in the range below 0.05 fm^-2 would indicate unexpected structure in the outer regions of the nuclear charge distribution. Disagreement with electron scattering where both processes are measured would indicate a breakdown of muon-electron universality. No such effects are observed.