N. Phinney

Report on the SLC Control System

The SLC control system is based on a VAX 11/780 Host computer with approximately 50 microprocessor clusters which provide distributed intelligence and control of all CAMAC interface modules. This paper will present an overview of the system including current status and a description of the software architecture and communication protocols.

Correlation plot facility in the SLC control system

The correlation plot facility is a powerful interactive tool for data acquisition and analysis throughout the SLC (Stanford Linear Collider). A generalized interface allows the user to perform a wide variety of machine physics experiments. The user may step one or two independent parameters such as magnet or feedback setpoints while measuring or calculating up to 160 others. Measured variables include all analog signals available to the control system as well as a variety of derived parameters such as beam size or emittance. Various fitting algorithms and display options are provided for data analysis. A software-callable interface is also provided. Applications based on this facility are used to phase klystrons, measure emittance and dispersion, minimize beam size at the interaction point and maintain beam collisions.<<ETX>>

Beam-based monitoring of the SLC linac optics with a diagnostic pulse

The beam optics in a linear accelerator may be changed significantly by variations in the energy and energy spread profile along the linac. In particular, diurnal temperature swings in the SLC klystron gallery perturb the phase and amplitude of the accelerating RF fields. If such changes are not correctly characterized, the resulting errors will cause phase advance differences in the beam optics. In addition RF phase errors also affect the amplitude growth of betatron oscillations. We present an automated, simple procedure to monitor the beam optics in the SLC linac routinely and non-invasively. The measured phase advance and oscillation amplitude is shown as a function of time and is compared to the nominal optics.

Use of simulation programs for the modelling of the Next Linear Collider

The Next Linear Collider is an electron-positron accelerator unprecedented in its size, energy, and tight tolerances. We describe the suite of simulation tools which are widely used in designing and modelling the performance of the NLC. In order to achieve a uniform beamline description and permit simulation of all facets of the collider, an extended version of the Standard Input Format (xSIF) has been developed and implemented in MAD and DIMAD. We discuss several enhancements to the MAD and DIMAD calculation engines necessary to properly simulate the most challenging regions of the facility. We also describe enhancements to LIAR which allow it to be used as the tracking engine for a tuning/feedback simulation written in MATLAB. Finally, we discuss the additional software needed to model the beam stabilization and tuning processes.

Timing System Control Software in the SLC

A new timing system1 that allows precision (~1-2 ns) control of the trigger times of klystrons, beam position monitors, and other devices on a pulse-to-pulse basis at up to 360 Hz is in operation in the first third of the SLAC linear accelerator. The control software is divided between a central host VAX and local Intel 8086-based microprocessor clusters. Facilities exist to set up and adjust the timing of devices or groups of devices independently for beam pulses having different destinations and purposes, which are run in an interlaced fashion during normal machine operation. Upgrading of the system is currently underway, using a new version of the Programmable Delay Unit CAMAC module to allow pipelining of timing information for three machine pulses. An overview of the current state of the system is presented in this paper, with an emphasis on software control.

Beam-based analysis of day-night performance variations at the SLC linac

Diurnal temperature variations in the linac gallery of the Stanford Linear Collider (SLC) can affect the amplitude and phase of the rf used to accelerate the beam. The SLC employs many techniques for stabilization and compensation of these effects, but residual uncorrected changes still affect the quality of the delivered beam. This paper presents methods developed to monitor and investigate these errors through the beam response. Variations resulting from errors in the rf amplitude or phase can be distinguished by studying six different beam observables: betatron phase advance, oscillation amplitude growth, rms jitter along the linac, measurements of the beam phase with respect to the rf, changes in the required injection phase, and the global energy correction factor. By quantifying the beam response, an uncorrected variation of 14/spl deg/ (S-band) during 28/spl deg/F temperature swings was found in the main rf drive line system between the front and end of the linac.

SLC Final Performance and Lessons

The Stanford Linear Collider (SLC) was the first prototype of a new type of accelerator, the electron-positron linear collider. Many years of dedicated effort were required to understand the physics of this new technology and to develop the techniques for maximizing performance. Key issues were emittance dilution, stability, final beam optimization and background control. Precision, non-invasive diagnostics were required to measure and monitor the beams throughout the machine. Beam-based feedback systems were needed to stabilize energy, trajectory, intensity and the final beam size at the interaction point. variety of new tuning techniques were developed to correct for residual optical or alignment errors. The final focus system underwent a series of refinements in order to deliver sub-micron size beams. It also took many iterations to understand the sources of backgrounds and develop the methods to control them. The benefit from this accumulated experience was seen in the performance of the SLC during its final run in 1997-98. The luminosity increased by a factor of three to 3*10 30 and the 350,000 Z data sample delivered was nearly double that from all previous runs combined.

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.

Pulse to pulse stability issues in the SLC

Although the SLC achieved record luminosity in 1994, a major hindrance to further increases is pulse-to-pulse stability of the machine, often referred to as jitter. Raising the intensity of the SLC beams has gained luminosity but the intensity-normalized luminosity has decreased due to additional emittance dilution and to increasing jitter at higher intensities. Precision tuning of the final focus using beam-beam deflection scans is hampered by the pulse-to-pulse variations in both beam position and beam size. These were traced to position, intensity and energy jitter in various subsystems of the collider. Contributions to both the origin and amplification of the jitter have been identified as coming from wakefields in the linac, instabilities in the damping rings, acceptance limitations and feedback performance. The intensity fluctuations from the source can easily be amplified as a result of the SLC configuration of accelerating the two electron and positron bunches in the same linac.

Error Message Recording and Reporting in the SLC Control System

Error or information messages that are signaled by control software either in the VAX host computer or the local microprocessor clusters are handled by a dedicated VAX process (PARANOIA). Messages are recorded on disk for further analysis and displayed at the appropriate console. Another VAX process (ERRLOG) can be used to sort, list and histogram various categories of messages. The functions performed by these processes and the algorithms used are discussed.