M. Sullivan

The Gravity Probe-B star-tracking telescope

Abstract We describe the design, development, and testing of a cryogenic star-tracking telescope that provides the inertial pointing reference for the Gravity Probe-B gyroscopes, as established by a distant guide star. The telescope has a modified Cassegrainian configuration with an aperture of 14.4 cm and an effective focal length of 3.8 m, and operates at 2.5 K. It consists of high-grade fused-quartz components silicated together to maximize strength dimensional stability. The star image is split at the focal plane to provide quadrant pointing information for subtraction from the gyro readout signal and for spacecraft attitude control. We also briefly describe the status of the telescope readout electronics and the guide star selection.

Development of the Gravity Probe B flight mission

Abstract Gravity Probe B is an experiment to measure the geodetic and frame-dragging precessions, relative to the “fixed” “stars”, of a gyroscope placed in a 650 km altitude polar orbit about the earth. For Einsteins general relativity, the precessions are calculated to be 6.6 arcsec/yr for the geodetic precession and 0.042 arcsec/yr for the frame-dragging precession. The goal of the experiment is to measure these precessions to better than 0.01% and 1%, respectively. This paper gives an overview of the experiment and a discussion of the flight hardware development and its status. This paper also includes an estimate of the geodetic and frame-dragging errors expected for the experiment.

Status and future plans of the PEP-II B-factory

The PEP-II e/sup +/e/sup -/ collider has been operating for two years with the BaBar detector at the energy of the Upsilon 4S resonance. The peak luminosity has reached 3.3 /spl times/ 10/sup 33//cm/sup 2//s with 693 bunches with a positron current of 1.5 A and an electron current of 0.8 A. PEP-II has delivered in excess of 38 fb/sup -1/ of data to BaBar. The beam-beam tune shift limits are approaching 0.05-0.07 horizontally and 0.03-0.05 vertically. The electron cloud instability enlarges the positron beam size at high currents but is reduced by a solenoidal field on the vacuum chambers. The beam currents in PEP-II are being raised to increase the number of bunches and the luminosity. Over the next few years the luminosity goal for PEP-II is 10/sup 34//cm/sup 2//s.

Beam‐Beam Performance Of The SLAC B‐Factory

The beam‐beam performance of PEP‐II has been monitored by parasitic measurements recorded during routine physics running, and by a few dedicated accelerator physics experiments. These measurements indicate that in some cases, the beam‐beam interaction and the electron‐cloud‐induced blowup of the low‐energy positron beam are somehow coupled and enhance each other. Tailoring the bunch pattern to carefully balance these effects has proven very effective in maximizing the integrated luminosity. The comparison of recent simulation results with experimental data is encouraging.

Beam-beam collisions at the PEP-II B factory

We describe first beam collisions at the PEP-II B Factory, a collaboration of SLAC, LBNL, and LLNL. The beams are brought close to each other in the transverse (x,y) and longitudinal (timing) directions through the use of two shared beam position monitors located 0.72 m from either side of the interaction point (IP). Transverse beam-beam deflection scans and the use of a zero-angle luminosity detector allow us to center the collisions. Beam collisions were also seen by exciting one beam at its tune frequency and observing a response in the other beam at the same frequency. Shifts in betatron tunes have also been measured. To date, the peak measured luminosity attained is 5.2/spl plusmn/1/spl times/10/sup 32/ cm/sup -2/ s/sup -1/ with 786 bunches in each beam and with beam currents of 354 mA for the high-energy beam (HEB) and 680 mA for the low-energy beam (LEB).

A new Q2-bellows absorber for the PEP-II SLAC B-factory

A new Q2-bellows absorber will damp only transverse wake fields and will not produce additional beam losses due to Cherenkov radiation. The design is based on the results of HOM analysis. Geometry of the slots and absorbing tiles was optimized to get maximum absorbing effect.

The PEP-N interaction region

The PEP-N project consists of a small, very low-energy e/sup -/ storage ring (VLER) located in one of the interaction-straight regions of PEP-II. The small ring is brought into collision with the low-energy (3.1 GeV) e/sup +/ beam (LER). The center-of-mass energies from this collision are between the /spl Phi/ and J//spl psi/ resonances. We achieve a head-on collision through the use! of a central magnetic dipole field that generates a large horizontal bending field. This field is also the central field of the detector. The large energy range of the VLER, in order to maximize the center-of-mass energy range, complicates the collision point geometry. In order to maintain the beam orbits near the collision point two techniques are used. The first is to scale the central dipole field up and down with the energy of the VLER and the second is to use passive shielding to decrease the integral B/spl middot/dl of the dipole field seen by the VLER. Changes in the orbit of the LER are corrected with local bending magnets. Further details of the interaction region geometry as well as design issues that include synchrotron radiation from the high-current positron beam are discussed.

FUTURE VERY HIGH LUMINOSITY OPTIONS FOR PEP-II *

A Super B Factory, an asymmetric e + e - collider with a luminosity of 10 36 cm -2 s -1 , can provide a sensitive probe of new physics in the flavor sector of the Standard Model. The success of PEP-II and KEKB in producing unprecedented luminosity with unprecedented short commissioning time has taught us about the accelerator physics of asymmetric e + e - colliders in a new parameter regime. It appears to be possible to build on this success to advance the state of the accelerator art by building a collider at a luminosity approaching 10 36 cm -2 s -1 . Such a collider would produce an integrated luminosity up to 10,000 fb -1 (10 ab -1 ) in a running year. Design studies are underway to arrive at a complete parameter set based on a collider in the PEP-II tunnel but with an upgraded RF system (perhaps a higher frequency) and an upgraded interaction region [1-6].

Interaction region design for a Super-B factroy

We present a preliminary design of an interaction region for a Super-B Factory with luminosity of 1times1036 cm-2 sec-1. The collision has a plusmn17 mrad crossing angle and the first magnetic element starts 0.3 m from the collision point. We show that synchrotron radiation backgrounds are controlled and are at least as good as the backgrounds calculated for the PEP-II accelerator. How the beams get into and out of a shared beam pipe is illustrated along with the control of relatively high synchrotron radiation power from the outgoing beams. The high luminosity makes radiative bhabha backgrounds significantly higher than that of the present B-Factories and this must be addressed as the design is further improved.

The center-of-mass energy of PEP-II

The PEP-II B-factory is designed to operate at a center-of-mass energy (E/sub cm/) of 10.58 GeV, the mass value of the upsilon 4S resonance. It is important to set and maintain the E/sub cm/ to the peak of this resonance in order to maximize the production of B mesons that enable the BaBar detector to measure CP violation. There are several elements in the determination of the beam energies. Aside from the strength of the main bending magnets, there is a contribution to the beam energy from horizontal correctors. In addition, the frequency of the RF system also influences the ring energies by controlling the closed orbit circumference. The low-energy ring (LER) in PEP-II has a wiggler magnet for emittance control that also contributes to the beam energy of the LER. We discuss these aspects of beam energy determination and the algorithms used to monitor the beam energies.