We have measured the beam-normal single-spin asymmetry in elastic scattering of transversely polarized 3 GeV electrons from unpolarized protons at Q 2 0:15, 0:25 GeV=c 2 . The results are inconsistent with calculations solely using the elastic nucleon intermediate state and generally agree with calculations with significant inelastic hadronic intermediate state contributions. A n provides a direct probe of the imaginary component of the 2 exchange amplitude, the complete description of which is important in the interpretation of data from precision electron-scattering experiments. DOI: 10.1103/PhysRevLett.99.092301 PACS numbers: 25.30.Bf, 13.40.ÿf, 14.20.Dh, 24.70.+s Elastic scattering of electrons from nucleons is usually treated in the single-photon exchange (Born) approximation. Higher order processes, such as two-photon exchange, are generally treated as small radiative corrections. However, interest in two-photon exchange was recently renewed when it was argued that contributions from the real part of this amplitude play a role in the discrepancy between the Rosenbluth separation and polarization transfer measurements of the ratio of the elastic form factors G p E =G p M [1][2][3]. In addition, although the twophoton exchange contribution is small, it is comparable to the parity-violating elastic electron-nucleon scattering asymmetry [4], and recent parity-violation measurements have had to consider possible systematic corrections due to this effect. A good understanding of two-photon exchange contributions can be extended to calculations of diagrams PRL 99,

Operation of the JLab IR Upgrade FEL at CW powers in excess of 10 kW requires sustained production of high electron beam powers by the driver ERL. This in turn demands attention to numerous issues and effects, including: cathode lifetime; control of beamline and RF system vacuum during high current operation; longitudinal space charge; longitudinal and transverse matching of irregular/large volume phase space distributions; halo management; management of remnant dispersive effects; resistive wall, wake-field, and RF heating of beam vacuum chambers; the beam break up instability; the impact of coherent synchrotron radiation (both on beam quality and the performance of laser optics); magnetic component stability and reproducibility; and RF stability and reproducibility. We discuss our experience with these issues and describe the modus vivendi that has evolved during prolonged high current, high power beam and laser operation.

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We report on the progress of the conceptual development of the energy recovering linac (ERL)-based electron-light ion collider (ELIC) at CEBAF that is envisioned to reach luminosity level of 103-_1035 /cm2s with both beams polarized to perform a new class of experiments in fundamental nuclear physics. Four interaction points with all light ion species longitudinally or transversally polarized and fast flipping of the spin for all beams are planned. The unusually high luminosity concept is based on the use of the electron cooling and crab crossing colliding beams. Our recent studies focused on the design of low beta interaction points, exploration on raising the polarized electron injector current to the level of 3-30 mA with the use of electron circulatorcollider ring, forming a concept of stacking and cooling of the ion beams, and specifications of the electron cooling facility.

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