Tech-X, Boulder, Colorado, USA
Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA
BNL, Upton, Long Island, New York, USA
High acceptor doping of GaAs and (Cs, O) or (Cs, F) surface coating leads to downward band bending terminating with effective negative electron affinity surface. The periodicity breaking at the surface together with the formed potential leads to one or more localized states in the band bending region together with effective Fermi level pinning. We report results on how to calculate the band bending potential, the Fermi level pinning, and localized states as functions of GaAs p-doping density, surface density of states, and temperature. We also consider how these surface properties affect electron emission.
Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA
We have developed a cost-effective, fast rotating wire scanner for use in accelerators where high beam currents would otherwise melt even carbon wires. This new design uses a simple planetary gear setup to rotate a carbon wire, fixed at one end, through the beam at speeds in excess of 20 m/s. We will present results from bench tests, as well as transverse beam profile measurements taken at Cornell's high-brightness ERL photoinjector, for a beam energy of 4 MeV and currents up to 35 mA.
Cornell University, Ithaca, New York, USA
Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA
Beam brightness from modern day photoinjectors is limited by the photocathode. A multifaceted photocathode development program has been undertaken at Cornell University with a goal to develop the ultimate photocathode which has high quantum efficiency, low mean transverse energy, quick response time and a long lifetime. Positive affinity cathodes like CsK2Sb and NaK2Sbhave been grown using different kinds of alkali metal sources (alkali-azide and pure metal) , characterized and tested in the Cornell-ERL photoinjector. Novel layered structures of various III-V semiconductors like GaAs and AlGaAs grown using Molecular Beam Epitaxy and activated to negative electron affinity using Cs and NF3 are also being investigated. Surface and photoemission diagnostics like Auger spectroscopy, LEED, RHEED and the 2D-electron energy analyzers have been connected in vacuum to the photocathode growth and preparation chambers to fully characterize the surface and emission properties of the materials grown. A Monte Carlo based simulation has also been developed to predict photoemission from layered semiconductor structures and help design novel structures to optimize the photoemission properties.
Tech-X, Boulder, Colorado, USA
Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA
The need for a bright electron beam is increasing in the fields of x-ray science, electron diffraction and electron microscopy which are required for colliders. GaAs-based photocathodes have the potential to produce high-brightness, unpolarized and polarized, electron beams with performance that meets modern collider requirements. Even after decades of investigation, however, the exact mechanism of electron emission from GaAs is not well understood. Therefore, we investigate photoemission from a GaAs photocathode using detailed Monte Carlo electron transport simulations. Instead of a simple stepwise potential, we consider a triangular barrier including the effect of the image charge to take into account the effect of the applied field on the emission probability. The simulation results are compared with the experimental results for quantum efficiency, angular and energy distributions of emitted electrons without the assumption of any ad hoc parameters.
Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA
We present a detailed study of the emittances produced in the Cornell Energy Recovery Linac Photoinjector. Both the horizontal and vertical transverse phase spaces, as well as the time-resolved (sliced) horizontal phase space, were simulated and directly measured at the end of the injector for 19 pC and 77 pC bunches at roughly 8 MeV. The resulting 90% normalized transverse emittances for 19 (77) pC/bunch were 0.23 ± 0.02 (0.51 ± 0.04) μm in the horizontal plane, and 0.14 ± 0.01 (0.29 ± 0.02) μm in the vertical plane, respectively. These emittances were measured with a corresponding bunch length of 2.1±0.1 (3.0±0.2) ps, respectively. For both bunch charges, the rms momentum spread was determined to be on the order of 10-3. Excellent overall agreement between measurement and simulation has been demonstrated. The beam brightness measured in this work is significantly better than the best of modern storage rings, and represents a milestone for the field of high-brightness, high-current photoinjectors.