M. Billing

Beam position monitors for storage rings

Abstract Accurate monitoring of the position of particle beams and synchrotron light beams is becoming more important for the types of experiments that are currently being performed and for those experiments being planned for the future. This paper reviews the present state of beam position monitors for synchrotron light sources and storage rings. It includes descriptions of position monitors for both the stored particle beam and the synchrotron light beam. Typical interfaces for these monitors and their system characteristics (resolution, stability, bandwidth and problems or limitations) are discussed. General considerations for the design of beam position monitoring systems and results from operational position monitoring systems are presented. The paper also reviews several types of diagnostic measurements using beam position monitors which are useful in improving accelerator operations.

Beam Diagnostic Instrumentation at CESR

We discribe various beam diagnostic devices in use at CESR, an 8 GeV electron-positron storage ring operating primarily in the 4.7 to 5.5 GeV beam energy range. Getting the last 20% of performance depends to some extent on empirical tuning and appropriate presentation of various parameters is very important. Several devices are most useful in machine studies and we describe their operation. The individually regulated quadrupoles in CESR provide unique opportunities for lattice measurements and calibration of beam position monitors.

Status of A Plan for an ERL Extension to CESR

We describe the status of plans to build an Energy-Recovery Linac (ERL) X-ray facility at Cornell University. This 5 GeV ERL is an upgrade of the CESR ring that currently powers the Cornell High Energy Synchrotron Source (CHESS) [1]. Due to its very small electron-beam emittances, it would dramatically improve the capabilities of the light source and result in X-ray beams orders of magnitude better than any existing storage-ring light source. The emittances are based upon simulations for currents that are competitive with ring-based sources [2, 4]. The ERL design that is presented has to allow for non-destructive trans port of these small emittances. The design includes a series of X-ray beamlines for specific areas of research. As an upgrade of the existing storage ring, special attention is given to reuse of many of the existing ring components. Bunch compression, tolerances for emittance growth, simulations of the beam-breakup instability and methods of increasing its threshold current are mentioned. This planned upgrade illustrates how other existing storage rings could be upgraded as ERL light sources with vastly improved beam qualities.

CESR status and performance

Machine performance for the running period is reviewed with an emphasis on phenomena associated with the large number of parasitic crossings peculiar to a single ring collider with multi-bunch beams.

Beam test of a superconducting cavity for the CESR luminosity upgrade

The prototype superconducting cavity system for CESR-Phase III was tested in CESR in August 1994. The performance of the system was very gratifying. The cavity operated gradients of 4.5-6 MV/m and accelerated beam currents up to 220 mA. This current is a factor of 3 above the world record 67 mA for SRF[1]. The high circulating beam current did not increase the heat load or present any danger to the cavity. No instability attributable to the SRF cavity was encountered. A maximum of 155 kW of rf power was transferred to a 120 mA beam. The window was subjected to 125 kW reflected power and processed easily. In the travelling wave mode, vacuum bursts and are trips prevented us from going above 165 kW. The maximum HOM power extracted was 2 kW. Beam stability studies were conducted for a variety of bunch configurations. In other tests a 120 mA beam was bumped horizontally and vertically by /spl plusmn/10 mm. While supporting a 100 mA beam, the cavity was axially deformed with the tuner by 0.4 mm to sweep the HOM frequencies across dangerous revolution harmonics. In all such tests, no resonant excitation of HOMs or beam instabilities were observed, which confirms that the potentially dangerous modes were damped strongly enough to be rendered harmless.

Effect on luminosity from bunch-to-bunch orbit displacements at CESR

In routine operation the Cornell electron/positron storage ring, CESR, collides 9 trains of 4 or 5 bunches of electrons and positrons. The luminosity of individual bunch pairs can suffer if the transverse positions of those bunches are displaced differentially. This paper will present the results of studies of the effect of bunch-by-bunch or train-by-train variations in the luminosity by measuring (1) the deflection from the beam-beam interaction, (2) the relative positions of the bunches, and (3) the bunch dependent luminosity from the CLEO detector. Comparisons between these three methods are discussed.

Observation of a longitudinal coupled bunch instability with trains of bunches in CESR

A longitudinal coupled bunch instability has been observed in the operation of CESR with multiple trains of bunches. The instability threshold is dependent on the number of bunches per train and the spacing between those bunches. The threshold is also sensitive to parameters in the RF system leading to the conclusion that the RF cavities play a significant role in the dynamics. A summary of these observations is presented along with the design and status of a feedback system to stabilize the beams.

Operation of a fast digital transverse feedback system in CESR

We have developed a time domain transverse feedback system with the high bandwidth needed to control transverse instabilities when the CESR e/sup +/e/sup -/ collider is filled with trains of closely spaced bunches. This system is based on parallel digital processors and a stripline driver. It is capable of acting on arbitrary patterns of bunches having a minimum spacing of 14 ns. Several simplifying features have been introduced. A single shorted stripline kicker driven by one power amplifier is used to control both counter-rotating beams. The desired feedback phase is achieved by sampling the bunch position at a single location on two independently selectable beam revolutions. The system adapts to changes in the betatron tune, bunch pattern, or desired damping rate through the loading of new parameters into the digital processors via the CESR control system. The feedback system also functions as a fast gated bunch current monitor. Both vertical and horizontal loops are now used in CESR operation. The measured betatron damping rates with the transverse feedback system in operation are in agreement with the analytical prediction and a computer simulation developed in connection with this work.

Beam-beam interaction with a horizontal crossing angle

Abstract We report measurements of the dependence of luminosity and beam-beam tune-shift parameter on horizontal crossing angle at a single interaction point in the Cornell Electron Storage Ring. The report is based on data collected between September 1991 and January 1992 at CESR. For head-on collisions (zero crossing angle) the achieved tune-shift parameter is ξ v = 0.03 ± 0.002 at 11 mA/bunch. For a crossing half-angle of θ c = ± 2.4 mrad, we achieve ξ v = 0.024 ± 0.002 at similar bunch currents. The data suggest some degradation of performance if the trajectory through the interaction region is distorted magnetically even while head-on collisions are preserved. Therefore at least some of the observed dependence of tune-shift parameter on crossing angle may be due to the associated large displacement of the beam trajectories in the interaction region optics. Furthermore, with the introduction of the crossing angle, the algorithm for optimizing luminosity is significantly complicated due to linear optical errors and the solenoid compensation. We interpret the measured tune-shift parameter at θ c = 2.4 mrad as a lower limit to what can ultimately be achieved.

Simulation Measurement of Bunch Excited Fields and Energy Loss in Vacuum Chamber Components and Cavities

Following the method of Sands and Rees, we have developed a two arm comparison apparatus of high sensitivity and good temporal resolution for the measurement of bunch engendered fields. With a relative timing uncertainty of .5 picoseconds the apparatus has demonstrated a sensitivity of .05¿ for the resistive impedance in the presence of a reactive impedance of 4.2¿ for a gaussian shaped bunch of standard deviation 2.1 centimeters. The apparatus is described and results for a typical component are presented. Both energy loss and bunch generated fields are discussed.