Search Results (34 total)

Electron injectors delivering relativistic electron beams with very high brightness are essential for a number of current and proposed electron accelerator applications. These high brightness beams are generally produced from photoemission cathodes. We formulate a limit on the electron beam brightness from such cathodes set by the transverse thermal energy of the electrons leaving the photocathode and the accelerating field at the cathode. Two specific examples—direct measurement of the transverse phase space of a space charge dominated beam from a high-voltage photoemission electron gun and a numerical optimization of the same at a higher gun voltage—illustrate the importance of this limit.

We present a comparison between space charge calculations and direct measurements of the transverse phase space of space charge dominated electron bunches from a high voltage dc photoemission gun followed by an emittance compensation solenoid magnet. The measurements were performed using a double-slit emittance measurement system over a range of bunch charge and solenoid current values. The data are compared with detailed simulations using the 3D space charge codes GPT and Parmela3D. The initial particle distributions were generated from measured transverse and temporal laser beam profiles at the photocathode. The beam brightness as a function of beam fraction is calculated for the measured phase space maps and found to approach within a factor of 2 the theoretical maximum set by the thermal energy and the accelerating field at the photocathode.

To achieve the lowest emittance electron bunches from photoemission electron guns, it is essential to limit the uncorrelated emittance growth due to space charge forces acting on the bunch in the vicinity of the photocathode through appropriate temporal shaping of the optical pulses illuminating the photocathode. We present measurements of the temporal profile of electron bunches from a bulk crystal GaAs photocathode illuminated with 520 nm wavelength pulses from a frequency-doubled Yb-fiber laser. A transverse deflecting rf cavity was used to make these measurements. The measured laser pulse temporal profile and the corresponding electron beam temporal profile have about 30 ps FWHM duration, with rise and fall times of a few ps. GaAs illuminated by 520 nm optical pulses is a prompt emitter within our measurement uncertainty of ∼1  ps rms. Combined with the low thermal emittance of negative electron affinity photocathodes, GaAs is a very suitable photocathode for high-brightness photoinjectors. We also report measurements of the photoemission response time for GaAsP, which show a strong dependence on the quantum efficiency of the photocathode.

Substantially more than half of the electromagnetic nuclear physics experiments conducted at the Continuous Electron Beam Accelerator Facility of the Thomas Jefferson National Accelerator Facility (Jefferson Laboratory) require highly polarized electron beams, often at high average current. Spin-polarized electrons are produced by photoemission from various GaAs-based semiconductor photocathodes, using circularly polarized laser light with photon energy slightly larger than the semiconductor band gap. The photocathodes are prepared by activation of the clean semiconductor surface to negative electron affinity using cesium and oxidation. Historically, in many laboratories worldwide, these photocathodes have had short operational lifetimes at high average current, and have often deteriorated fairly quickly in ultrahigh vacuum even without electron beam delivery. At Jefferson Lab, we have developed a polarized electron source in which the photocathodes degrade exceptionally slowly without electron emission, and in which ion back bombardment is the predominant mechanism limiting the operational lifetime of the cathodes during electron emission. We have reproducibly obtained cathode 1/e dark lifetimes over two years, and 1/e charge density and charge lifetimes during electron beam delivery of over 2×10⁵   C/cm² and 200 C, respectively. This source is able to support uninterrupted high average current polarized beam delivery to three experimental halls simultaneously for many months at a time. Many of the techniques we report here are directly applicable to the development of GaAs photoemission electron guns to deliver high average current, high brightness unpolarized beams.

We have conducted a multiobjective computational optimization of a high brightness, high average current photoinjector under development at Cornell University. This injector employs a dc photoemission electron gun. Using evolutionary algorithms combined with parallel computing resources, the multivariate parameter space of the photoinjector was explored for optimal values. This powerful computational tool allows an extensive study of complex and nonlinear systems such as the space-charge dominated regions of an accelerator, and has broad areas of potential application to accelerator physics and engineering problems. In the present case, the optimized injector is simulated to deliver beam of very high quality (e.g., a rms normalized emittance of 0.1 mm mrad for 0.1 nC, and 0.7 mm mrad for 1 nC bunches). The field strengths of the active elements of the injector are moderate and technically practical. The relevance of these results to various novel linac-based accelerator proposals is pointed out.

Precision measurements of the relative analyzing powers of five electron beam polarimeters, based on Compton, Møller, and Mott scattering, have been performed using the CEBAF accelerator at the Thomas Jefferson National Accelerator Facility (Jefferson Laboratory). A Wien filter in the 100 keV beam line of the injector was used to vary the electron spin orientation exiting the injector. High statistical precision measurements of the scattering asymmetry as a function of the spin orientation were made with each polarimeter. Since each polarimeter receives beam with the same magnitude of polarization, these asymmetry measurements permit a high statistical precision comparison of the relative analyzing powers of the five polarimeters. This is the first time a precise comparison of the analyzing powers of Compton, Møller, and Mott scattering polarimeters has been made. Statistically significant disagreements among the values of the beam polarization calculated from the asymmetry measurements made with each polarimeter reveal either errors in the values of the analyzing power or failure to correctly include all systematic effects. The measurements reported here represent a first step toward understanding the systematic effects of these electron polarimeters. Such studies are necessary to realize high absolute accuracy (ca. 1%) electron polarization measurements, as required for some parity violation measurements planned at Jefferson Laboratory. Finally, a comparison of the value of the spin orientation exiting the injector that provides maximum longitudinal polarization in each experimental hall leads to an independent and very precise (better than 10^-4) absolute measurement of the final electron beam energy.

Bunch lengths as short as 84 fs (rms) have been measured at Jefferson Laboratory using a rf zero-phasing technique. To the best of our knowledge, this is the first accurate measurement of the longitudinal distribution function in this regime. In this paper, an analytical approach for computing the longitudinal distribution function and bunch length is described for arbitrary longitudinal and transverse distributions for the case where the intrinsic energy spread of the bunch is small compared to the correlated energy spread imparted by zero-phasing cavities. The measurement results are presented, which are in excellent agreement with numerical simulations.

Forty-seven charm events have been observed in an exposure of the SLAC Hybrid Facility bubble chamber to a 20-GeV backward-scattered laser beam. Thirty-seven events survive all the necessary cuts imposed. Based on this number the total charm cross section is calculated to be 63_-28⁺³³ nb.

The asymmetries in forward π-N, π-Δ, and K⁺-(Λ+Σ) photoproduction have been measured with a 16-GeV linearly polarized beam. The experimental method and the procedures for extracting cross sections and asymmetries from the data are discussed in detail. Information on the energy and momentum-transfer dependence of cross sections for natural- and unnatural-parity exchange, interference between exchanges of opposite G parity, and vector-meson dominance is obtained and discussed.

We report a verification of the theory of special relativity at a value of γ≈2.5×10⁴ based upon a comparison of electron g-2 measurements at meV and GeV kinetic energies. Specially we obtain a measure of the equivalence between the quantities γ≡(1-β²)^{-1 / 2} and γ̃≡(p / m₀)dp / dE.

J / ψ photoproduction has been measured from beryllium and tantalum targets by observing the yield of single muons at a transverse momentum of 1.65 GeV/c with a bremsstrahlung beam of E₀=20 GeV. The results have been interpreted in terms of a nuclearoptics model to yield the ψ-nucleon total cross section. The result is σ_ψN=3.5±0.8 mb.

We report measurements of the asymmetry in deep inelastic scattering of longitudinally polarized electrons by longitudinally polarized protons. The antiparallel-parallel asymmetries are positive and large in agreement with predictions of quark-parton models of the proton. A limit is obtained on parity nonconservation in the scattering of longitudinally polarized electrons by unpolarized nucleons.

We report on a new type of high-energy electron-proton scattering experiment in which longitudinally polarized electrons are scattered from longitudinally polarized protons. The asymmetry in elastic scattering at Q²=0.765 (GeV/c)² was measured; our result agrees with the theoretical asymmetry and determines the sign of G_E / G_M to be positive.

We report measurements of the invariant cross section in the forward hemisphere for inclusive photoproduction of π^±, K^±, p, and p̅ from hydrogen and deuterium with an incident photon energy of 18 GeV. A small amount of data was also taken at incident energies of 9 and 13 GeV. The measurements were made using the SLAC 20-GeV/c spectrometer, and a bremsstrahlung-subtraction technique was used to obtain the cross sections at the specified incident energy. The data are compared with those from lower-energy experiments and interpreted within the context of the Mueller-Regge model and the constituent-interchange model.

We have extended the search for new resonances and thresholds in the hadronic final state X⁺ in e p→e^′ X for missing masses from 3 GeV to 5.5 GeV. At a level of roughly 5% of the cross sections we found no unexpected structure.

The s and t dependence of incoherent ψ(3100) photoproduction from deuterium has been measured at the Stanford Linear Accelerator Center. ψ(3700) photoproduction and ψ(3100) photoproduction from hydrogen have also been measured.

The longitudinal polarization of the new Yale University-Stanford Linear Accelerator Center polarized-electron beam has been determined at laboratory energies between 6.47 and 19.40 GeV. Spin-dependent elastic electron-electron scattering (Møller scattering) has been found to be a practical technique for polarization measurements at high energies. The results are consistent with the energy and angular dependence predicted by quantum electrodynamics and with an energy-independent beam polarization of 0.76±0.03.

Using a 16-GeV linearly polarized photon beam, we have measured asymmetries in the process γN→K⁺(Λ+Σ) from hydrogen and deuterium, for square of four-momentum transfer, t, between -0.01 and -0.8 (GeV/c)². The data show that for -t≳0.1 (GeV/c)², the cross sections for γp→K⁺Λ, γp→K⁺Σ⁰, and γn→K⁺Σ^- are strongly dominated by natural-parity exchange, as is the case in single-pion photoproduction.

We report γp total, topological, and channel cross sections at 9.3 GeV from a bubblechamber experiment using a nearly monoenergetic photon beam.

Asymmetries in charged-pion photoproduction from hydrogen and deuterium have been measured with 16-GeV linearly polarized photons. Considerable energy dependence is seen in the natural-parity contribution to the π^- / π⁺ ratio from deuterium, and in the unnatural-parity part of the cross section for γn→π^-p. The energy dependence of this latter cross section is consistent with the expected from a conventional pion Regge trajectory.

We study the inclusive spectra of π^- mesons from the events obtained in three exposures of the SLAC 82-in. hydrogen bubble chamber to a nearly monochromatic polarized photon beam of mean energies 2.8, 4.7, and 9.3 GeV. The data are presented in terms of transverse momentum p_⊥ and three suggested choices for the other independent variable, i.e., the longitudinal momentum p_∥ in the laboratory system, the rapidity variable y=1 / 2ln[(E+p_∥) / (E-p_∥)], and the variable suggested by Feynman x=p_∥^* / p_max^* in the c.m. system. The 4π geometry of the bubble chamber allows us to cover the entire kinematically allowed range of these variables. We show that exact limiting fragmentation does not occur at our energies, but the data are compatible with an approach to a limiting distribution as A+Bs^{-1 / 2}. The qualitative features of the structure function f(x,p_⊥²) in terms of Feynman's x variable are similar at the three energies. Quantitatively, we find 5-10% differences between the 4.7- and 9.3-GeV data near x=0. We find f(x,p_⊥²) is not factorizable into independent functions of x and p_⊥². For our data the mean π^- multiplicity is described well by 〈n^-〉=c^-lns+d^-, where c^-=0.44±0.04 and d^-=0.07±0.08. Following the procedure suggested by Bali et al., we calculate c^- from our experimentally observed 9.3-GeV structure function at x=0 and find c^-=0.44±0.02 in agreement with the value obtained directly. We find a correlation between the azimuth of the π^- and the photon polarization plane only for x>0.3 when elastic ρ⁰ photo-production is excluded. Lastly, we note that the distribution of π^- longitudinal momentum is not symmetric in the "quark frame" where p_target=1.5p_beam.

Cross sections for the reaction ep→epX⁰, measured by detecting both the scattered electron and the proton, are given for missing masses m_x between 0 and 1 GeV / c². Enhancements are observed at m_x=0 and in the region of the ρ meson at q²=0.1 and 0.5 (GeV / c)². The electroproduction of all states with m_x between 0.7 and 0.83 GeV / c² varies like e^-7.5t for q²=0.1 (GeV / c)², similar to missing-mass photoproduction, but for q²=0.5 (GeV / c)² it varies like e^-3.5t. In this mass region the resolution tail of the large bremsstrahlung peak at m_x=0 is small.

Photoproduction is studied at 2.8 and 4.7 GeV using a linearly polarized monoenergetic photon beam in a hydrogen bubble chamber. We discuss the experimental procedure, the determination of channel cross sections, and the analysis of the channel γp→pπ⁺π^-. A model-independent analysis of the ρ⁰-decay angular distribution allows us to measure nine independent density-matrix elements. From these we find that the reaction γp→pρ⁰ proceeds almost completely through natural parity exchange for squared momentum transfers |t|<1 GeV² and that the ρ production mechanism is consistent with s-channel c.m. helicity conservation for |t|<0.4 GeV². A cross section for the production of π⁺π^- pairs in the s-channel c.m. helicity-conserving p-wave state is determined. The ρ mass shape is studied as a function of momentum transfer and is found to be inconsistent with a t-independent Ross-Stodolsky factor. Using a t-dependent parametrization of the ρ⁰ mass shape we derive a phenomenological ρ⁰ cross section. We compare our phenomenological ρ⁰ cross section with other experiments and find good agreement for 0.05<|t|<1 GeV². We discuss the discrepancies in the various determinations of the forward differential cross section. We study models for ρ⁰ photoproduction and find that the Söding model best describes the data. Using the Söding model we determine a ρ⁰ cross section. We determine cross sections and nine density-matrix elements for γp→Δ⁺⁺π^-. The parity asymmetry for Δ⁺⁺ production is incompatible with simple one-pion exchange. We compare Δ⁺⁺ production with models.