Brr2 is a DExD/H-box helicase responsible for U4/U6 unwinding during spliceosomal activation. Brr2 contains two helicase-like domains, each of which is followed by a Sec63 domain with unknown function. We determined the crystal structure of the second Sec63 domain, which unexpectedly resembles domains 4 and 5 of DNA helicase Hel308. This, together with sequence similarities between Brr2’s helicase-like domains and domains 1–3 of Hel308, led us to hypothesize that Brr2 contains two consecutive Hel308-like modules (Hel308-I and II). Our structural model and mutagenesis data suggest that Brr2 shares a similar helicase mechanism with Hel308. We demonstrate that Hel308-II interacts with Prp8 and Snu114 in vitro and in vivo. We further find that the C-terminal region of Prp8 (Prp8-CTR) facilitates the binding of the Brr2/Prp8-CTR complex to U4/U6. Our results have important implications for the mechanism and regulation of Brr2’s activity.

In 2008 the Cornell Electron/Positron Storage Ring (CESR) was reconfigured from an electron/positron collider to serve as a test bed for the International Linear Collider damping rings. One of the primary goals of the CESR Test Accelerator (CesrTA) project is to develop a fast low-emittance tuning method which scales well to large rings such as the ILC damping rings, and routinely achieves a vertical emittance of order 10 pm at 2.085 GeV. This paper discusses the tuning methods developed at CesrTA to achieve lowemittance conditions. One iteration of beam-based measurement and correction requires about 10 min. A minimum vertical emittance of 10.3ðþ3.2= − 3.4Þ sys ðAE0.2Þ stat pm has been achieved at 2.085 GeV. In various configurations and beam energies the correction technique routinely achieves vertical emittance around 10 pm after correction, with rms coupling < 0.5%. The measured vertical dispersion is dominated by beam position monitor systematics. The propagation of uncertainties in the emittance measurement is described in detail. Simulations modeling the effects of magnet misalignments, beam position monitor errors, and the emittance correction algorithm suggest the residual vertical emittance measured at the conclusion of the tuning procedure is dominated by sources other than optics errors and misalignments.

We describe the construction and operation of an x-ray beam size monitor (xBSM), a device measuring e + and e − beam sizes in the CESR-TA storage ring using synchrotron radiation. The device can measure vertical beam sizes of 10 − 100 µm on a turn-by-turn, bunch-by-bunch basis at e ± beam energies of ∼ 2 GeV. At such beam energies the xBSM images x-rays of ≈1-10 keV (λ ≈ 0.1 − 1 nm) that emerge from a hard-bend magnet through a single-or multiple-slit (coded aperture) optical element onto an array of 32 InGaAs photodiodes with 50 µm pitch. Beamlines and detectors are entirely in-vacuum, enabling single-shot beam size measurement down to below 0.1 mA (2.5 × 10 9 particles) per bunch and inter-bunch spacing of as little as 4 ns. At E b = 2.1 GeV, systematic precision of ∼ 1 µm is achieved for a beam size of ∼ 12 µm; this is expected to scale as ∝ 1/σ b and ∝ 1/E b . Achieving this precision requires comprehensive alignment and calibration of the detector, optical elements, and x-ray beam. Data from the xBSM have been used to extract characteristics of beam oscillations on long and short timescales, and to make detailed studies of low-emittance tuning, intra-beam scattering, electron cloud effects, and multi-bunch instabilities.

We report on the observation of incoherent Cherenkov radiation emitted by a 5.3 GeV positron beam circulating in the Cornell electron-positron storage ring as the beam passes in the close vicinity of the surface of a fused silica radiator (i.e., at a distance larger than 0.8 mm). The shape of the radiator was designed in order to send the Cherenkov photons towards the detector, consisting of a compact optical system equipped with an intensified camera. The optical system allows both the measurements of 2D images and angular distribution including polarization study. The corresponding light intensity has been measured as a function of the distance between the beam and the surface of the radiator and has shown a good agreement with theoretical predictions. For highly relativistic particles, a large amount of incoherent radiation is produced in a wide spectral range. A light yield of 0.8×10^{-3} photon per particle per turn has been measured at a wavelength of 600±10  nm in a 2 cm long radiator and for an impact parameter of 1 mm. This will find applications in accelerators as noninvasive beam diagnostics for both leptons and hadrons.

Intrabeam scattering (IBS) limits the emittance and single-bunch current that can be achieved in electron or positron storage ring colliders, damping rings, and light sources. Much theoretical work on IBS exists, and while the theories have been validated in hadron and ion machines, the presence of strong damping makes IBS in lepton machines a different phenomenon. We present the results of measurements at CesrTA of IBS-dominated beams, and compare the data with theory. The beams we study have parameters typical of those specified for the next generation of wiggler-dominated storage rings: low emittance, small bunch length, and an energy of a few GeV. Our measurements are in good agreement with IBS theory, provided a tail-cut procedure is applied.