Search Results (16 total)

We recently achieved the first experimental observation of laser-driven particle acceleration of relativistic electrons from a single Gaussian near-infrared laser beam in a semi-infinite vacuum. This article presents an in-depth account of key aspects of the experiment. An analysis of the transverse and longitudinal forces acting on the electron beam is included. A comparison of the observed data to the acceleration viewed as an inverse transition radiation process is presented. This is followed by a detailed description of the components of the experiment and a discussion of future measurements.

We present the first direct observation of a higher-order inverse-free-electron-laser (IFEL) interaction. Interaction at the fourth, fifth, and sixth harmonics is observed from an IFEL operating at 800 nm. The harmonic spacing, relative harmonic strength, and transverse beam overlap of the interaction are all in good agreement with tracking simulations.

We demonstrate a new particle acceleration mechanism using 800 nm laser radiation to accelerate relativistic electrons in a semi-infinite vacuum. The experimental demonstration is the first of its kind and is a proof of principle for the concept of laser-driven particle acceleration in a structure loaded vacuum. We observed up to 30 keV energy modulation over a distance of 1000λ, corresponding to a 40  MeV/m peak gradient. The energy modulation was observed to scale linearly with the laser electric field and showed the expected laser-polarization dependence. Furthermore, as expected, laser acceleration occurred only in the presence of a boundary that limited the laser-electron interaction to a finite distance.

We report on measurements of quantum electrodynamic processes in an intense electromagnetic wave, where nonlinear effects (both multiphoton and vacuum polarization) are prominent. Nonlinear Compton scattering and electron-positron pair production have been observed in collisions of 46.6 GeV and 49.1 GeV electrons of the Final Focus Test Beam at SLAC with terawatt pulses of 1053 nm and 527 nm wavelengths from a Nd:glass laser. Peak laser intensities of ≈0.5×10¹⁸ W/cm² have been achieved, corresponding to a value of ≈0.4 for the parameter η=eE_rms/mω₀c and to a value of ≈0.25 for the parameter Υ_e=E_rms^⋆/E_crit=eE_rms^⋆ħ/m²c³, where E_rms^⋆ is the rms electric field strength of the laser in the electron rest frame. We present data on the scattered electron spectra arising from nonlinear Compton scattering with up to four photons absorbed from the field. A convolved spectrum of the forward high energy photons is also given. The observed positron production rate depends on the fifth power of the laser intensity, as expected for a process where five photons are absorbed from the field. The positrons are interpreted as arising from the collision of a high-energy Compton scattered photon with the laser beam. The results are found to be in agreement with theoretical predictions.

A signal of 106±14 positrons above background has been observed in collisions of a low-emittance 46.6 GeV electron beam with terawatt pulses from a Nd:glass laser at 527 nm wavelength in an experiment at the Final Focus Test Beam at SLAC. The positrons are interpreted as arising from a two-step process in which laser photons are backscattered to GeV energies by the electron beam followed by a collision between the high-energy photon and several laser photons to produce an electron-positron pair. These results are the first laboratory evidence for inelastic light-by-light scattering involving only real photons.

Nonlinear Compton scattering has been observed in the collision of a low-emittance 46.6-GeV electron beam with terawatt pulses from a Nd:glass laser at 1054 and 527 nm wavelengths in an experiment at the Final Focus Test Beam at SLAC. Peak laser intensities of 10¹⁸  W/cm² have been achieved, corresponding to a value of 0.6 for the parameter η  =  eE_rms/mω₀c. Results are presented for multiphoton Compton scattering in which up to four laser photons interact with an electron, in agreement with theoretical calculations.

Elastic charge exchange cross sections are calculated using the Kisslinger and Michigan State University potentials. Each is consistent with elastic sccattering data, and predicts observable charge exchange cross sections. However, the charge exchange cross sections obtained by the two models have substantial differences.

NUCLEAR REACTIONS ²⁰⁸Pb(π⁺, π^-)²⁰⁸Po (double analog); cross sections predicted for 50 MeV π⁺.

Angular distributions for the elastic and inelastic scattering of 0.8 GeV protons from ¹²C, ¹³C, and ²⁰⁸Pb have been measured. Reported are data for the ground, 4.4 MeV 2+, 7.6 MeV 0+, 9.6 MeV 3^-, and 14.1 MeV 4+ states of ¹²C; the ground, 3.1 MeV 1 / 2⁺, 3.7 MeV 3 / 2^-, 6.9 MeV 5 / 2⁺, 7.6 MeV 5 / 2^-, and 11.9 MeV (?) states of ¹³C; and the ground, 2.6 MeV 3^-, and 3.2 MeV 5^- states of ²⁰⁸Pb. Analyses of the elastic angular distributions are made using the partial wave formalism and the Kerman-McManus-Thaler approach to the nucleon-nucleus optical potential; a realistic spin-orbit term is included. The inelastic transitions are analyzed within the framework of the distorted-wave Born-approximation, using transition strengths consistent with previous low-energy studies of inelastic scattering. A simple single-particle-plus-excited-core model is used for ¹³C. In terms of this analysis, an assignment of J^π=(5 / 2, 7 / 2)⁺ is made for the 11.9 MeV state in ¹³C.

NUCLEAR REACTIONS ^{12,  13}C, ²⁰⁸Pb(p, p^′), E=0.8 GeV; measured σ(θ); enriched targets; resolution ≥ 80 keV, θ_c.m.=2-40°, Δθ=0.2°. Optical potential analysis, DWBA, inelastic deformation lengths, β_lR.

The first elastic-scattering analyzing-power (A_y) data at 0.8 GeV are presented and discussed. Angular distributions of A_y over the range 2° to 20° (30° for ¹²C) have been obtained for ¹²C, ⁵⁸Ni, ⁹⁰Zr, and ²⁰⁸Pb using the high-resolution spectrometer at the Clinton P. Anderson Meson Physics Facility. The analyzing powers oscillate sharply over positive values in qualitiative agreement with expectations.

The zero-degree differential cross section for pion double-charge exchange on ¹⁸O(π⁺,π^+-)¹⁸Ne was measured at three incident pion energies and found to be 2.00±0.34, 2.19±0.44, and 1.67±0.38 μb/sr at 139, 126, and 95 MeV, respectively. A similar measurement for ¹⁶O at an energy of 145 MeV resulted in a value of 0.87 ± 0.21 μb/sr. The ratio of the ground-state transitions near 140 MeV is σ(¹⁸O) / σ(¹⁶O)=2.3±0.7. The mass excess determined for ¹⁶Ne is 24.4 ± 0.5 MeV. The experimental setup and data analysis are discussed in detail. Results are compared with various calculations and discussed from the standpoint of their implications for learning about reaction dynamics and nuclear structure. The reaction appears to be quite sensitive to ground-state correlations of the target.

NUCLEAR REACTIONS ¹⁸O(π⁺,π^-), E=95-139 MeV; measured σ(E,θ=0°). ¹⁶O(π⁺,π^-), E=145 MeV; measured σ(θ=0°), deduced ΔM(¹⁶Ne).

Differential cross sections for elastic scattering of 0.8-GeV protons from ¹²C, ⁵⁸Ni, and ²⁰⁸Pb have been measured. Preliminary analysis of the data in terms of the Kerman-McManus-Thaler formalism with spin-dependent nucleon-nucleon amplitudes shows sensitivity to details of proton and neutron matter distributions.

The pion double-charge-exchange reaction ¹⁸O(π⁺, π^-)¹⁸Ne has been observed. A value of (dσ / dΩ)(0°)=1.78±0.30 μb/sr was obtained at a pion energy of 139 MeV for the ΔT_z=2 isobaric analog transition from the ground state of oxygen-18 to the ground state of neon-18.

Isobaric analog resonances in proton scattering experiments are analyzed using a set of coupled optical equations which include both elastic and inelastic channels as well as other channels which can couple to these via the charge-exchange operator (τ→·T→). The method has been applied to the particular case of Sr⁸⁸+p, in which only the p₀(0₁⁺) and p₁(2₁⁺) proton channels are dominant and mixing of the (j×0₁⁺) and (j^′×2₁⁺) configurations describes the primary fragmentation of the 2d_{5 / 2}, 3s_{1 / 2}, 2d_{3 / 2}, and j^′×2₁⁺ neutron states in the parent nucleus Sr⁸⁹. Our purpose in this investigation was to see whether or not one could obtain a consistent match to both the level structure of the bound parent analog system and the analog resonance data in the various open channels with a general expression for the effective nucleon-nucleus interaction. Restricting as many parameters and potential terms as possible using independent sources of data, we obtained a detailed fit to the level structure of Sr⁸⁹ and the resonant Sr⁸⁸(p,p₀) and (p,p₁^′) excitation functions, the resonant Sr⁸⁸(p,n) total cross-section data, and the elastic polarization functions for an incident proton energy from E_p=4.8 to 8.2 MeV. The predicted on-resonance p^′(2₁⁺) angular distributions follow the main features of the data but not the finer details. It was found that the interference between the resonance and the direct inelastic background depends mostly on the nuclear rather than the Coulomb core excitation, and that direct inelastic charge-exchange does not affect the results. Our ability to fit all of the data in a consistent manner is interpreted as a confirmation of the main features of the generalized potential model assumed.

The isobaric analogs of levels in Ti⁵¹ from E_x=2.0-5.5 MeV are observed here as resonances in (p,p^′) scattering on Ti⁵⁰. Excitation functions for the inelastic p₁^′(2⁺) and p₂^′(4⁺) transitions have been determined at θ_{la b}=90^∘ and 160° from an incident energy of 3.4-7.1 MeV. Twenty-two analog resonances have been identified within this range, and aspects of the accompanying intermediate and fine structure are discussed. Several on-resonance angular distributions have been measured. In some cases, they display marked asymmetries about 90°, indicating the presence of direct-reaction interference effects. A Breit-Wigner analysis of the elastic and inelastic data was carried out, and the resulting partial and total widths are given. The associated spectroscopic factors and excitation energies are compared with the values found from Ti⁵⁰(d,p)Ti⁵¹. Several possible reaction processes are considered in the description of the resonant inelastic proton transitions to the first 0⁺, 3⁺, and 4⁺ states in Ti⁵⁰, and information on the levels of Ti⁵¹ is discussed in terms of the core-excited model.

The fragmentation of single-particle analog resonances in proton elastic scattering is accomplished here by means of an effective nonlocal optical potential added to the Lane equations. This potential is inferred from the analogous potential in the parent isobaric nucleus, where it arises from the coupling of the single-neutron states to other degrees of freedom in that system. The new coupled equations are solved numerically by an iterative procedure for the case of Sr⁸⁸+p using an average set of optical-model parameters and the known (d, p) spectroscopic factors. The elastic excitation functions calculated in this way compare closely to experiment over a range of energies of E_p=4.8 to 8.1 MeV, which includes six 2d_{5 / 2}, 2d_{3 / 2}, 3s_{1 / 2}, and 1g_{7 / 2} resonances. The sensitivity of the calculations to the various input parameters indicates that the method may be useful for providing independent determinations of both optical-model parameters and spectroscopic factors. Some shortcomings of the model are discussed and future improvements indicated.