S. Ecklund

SLC positron source pulsed flux concentrator

SLC (SLAC Linear Collider) positron beams produced by a very high energy electron beam, impinging on a high Z target, have initially small transverse size but large divergence, a situation ill matched to the following S-band accelerator. The flux concentrator is an adiabatic matching device placed between the target and this accelerator, which trades divergence for size. It produces a magnetic field which rises sharply over less than 5 mm to its peak value, and then falls off adiabatically over 10 cm. It is a 12-turn, 10-cm-long copper coil with a cylindrical outside radius of 4 cm and a conical inside radius growing from 3.5 mm to 2.6 cm. The 0.2-mm gaps between the individual windings were manufactured by electric discharge machining out of one copper block. Excitation current and water cooling are provided by a hollow rectangular conductor brazed to the outside of the coil (also 12 turns).<<ETX>>

SIMULATION AND MEASUREMENT OF THE FRINGE FIELD OF THE 1.5 T BABAR SOLENOID

In the context of the SLAC PEP-II asymmetric e{sup +}e{sup {minus}} collider and the BABAR detector with its 1.5 Tesla solenoid, we have calculated and measured the fringe field at the nearby beam elements and at the position of the photomultipliers external to the return iron but within a specially designed iron shield. The comparisons of these measurements with the simulations based on finite element analysis are remarkably good, within about 5 Gauss at the most critical beam element. The field at the photomultipliers is less than 1 Gauss, in agreement with the simulation. With a simple method of demagnetization of the shield, a maximum field of 0.6 Gauss is obtained.

Results from a prototype permanent magnet dipole-quadrupole hybrid for the PEP-II B-factory

We describe the construction of a prototype hybrid permanent magnet dipole and quadrupole. The magnet consists of two concentric rings of Sm/sub 2/Co/sub 17/ magnetic material 5 cm in length. The outer ring is made of 16 uniformly magnetized blocks assembled as a Halbach dipole and the inner ring has 32 blocks oriented in a similar fashion so as to generate a quadrupole field. The resultant superimposed field is an offset quadrupole field which allows us to center the field on the high-energy beam in the interaction region of the PEP-II B-factory. The dipole blocks are glued to the inside surface of an outer support collar and the quadrupole blocks are held in a fixture that allows radial adjustment of the blocks prior to potting the entire assembly with epoxy. An extensive computer model of the magnet has been made and from this model we developed a tuning algorithm that allowed us to greatly reduce the n=3-17 harmonics of the magnet.

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.

A fast luminosity monitor system for PEP II

The PEP II fast luminosity system provides a measurement of luminosity to the control system with a time constant of 0.3 seconds and fluctuations less than 0.1% for this interval, adequate for use in feedback systems. Continuous visual updates of luminosity are provided. The alignment of the positron beam at the collision point can also be monitored, and there is a visual display of the luminosity associated with each bunch-pair in the machine, sampled approximately every two seconds.

Performance of the 1994/95 SLC final focus system

A major upgrade to the SLC final focus was installed in 1994 to eliminate the dominant third-order aberration of the system, and thereby to reduce the vertical beam size at the IP by a factor of two. At low current, the optimal beam size of about 400 nm is now routinely established, and its sensitivity to orbit variations, to changes of emittance and energy spread, and to other beam parameters has been studied. For intensities above 3/spl times/10/sup 10/ particles per bunch, tuning is more difficult due to increased fluctuations of energy, orbit, and emittances. Nonetheless, the expected beam size of about 600 nm has been observed. New procedures and diagnostics allow easier tuning and optimization of the final focus, and also a first measurement of the emittance increase in the arcs.

High order mode heating observations in the PEP-II interaction region

High order mode (HOM) heating is observed in the PEP-II interaction region vacuum system by monitoring chamber temperatures in several locations. The region where the electron and positron beams enter a common chamber results in a RF cavity where the beams generate HOM heating. By determining the temperature dependence on the beam current, beam bunch length, and the phase of the two beams, information about the modes is extracted. The time response of the temperature and the geometry also aid in determining the source of the heat flow.

Positrons for linear colliders

The pair production process of obtaining positrons has the advantage of being essentially instantaneous, allowing production of beams with a time structure or bunching required for the linear collider. Photons which are used for the pair production are produced via electromagnetic cascades from electron interactions in matter. We describe Monte Carlo simulations of this method of positron production. (AIP)

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.

Recent developments in the design of the NLC positron source

Recent developments in the design of the Next Linear Collider (NLC) positron source based on updated beam parameters are described. The unpolarized NLC positron source consists of a dedicated 6.2 GeV S-band electron accelerator, a high-Z positron production target, a capture system and an L-band positron linac. The 1998 failure of the SLC target which is currently under investigation may lead to a variation of the target design. Progress towards a polarized positron source is also presented. A moderately polarized positron beam colliding with a highly polarized electron beam results in an effective polarization large enough to explore new physics at NLC. One of the schemes towards a polarized positron source incorporates a polarized electron source, a 50 MeV electron accelerator, a thin target for positron production and a new capture system optimized for high-energy small angular divergence positrons.