Josef Frisch

A high stability, low noise RF distribution system

Next generation linear colliders require high stability, low noise distribution of RF phase and timing signals. We describe a fiber-optics system that transmits phase at 357 MHz, at a 1500 nm wavelength, over a distance of 15 kilometers. Phase length errors in the transmission fiber are measured using the phase of the signal reflected from the fiber end. Corrections are performed by controlling the temperature of a 6-kilometer fiber spool placed in series with the main transmission fiber. This system has demonstrated a phase stability better than 10 femtoseconds per degree C, per kilometer, an improvement of a factor of >2000 relative to unstabilized fiber. This system uses standard low cost telecom fiber and components.

Very High Resolution Optical Transition Radiation Beam Profile Monitor

We have constructed and tested a 2 um resolution beam profile monitor based on optical transition radiation (OTR). Theoretical studies of OTR [1] show that extremely high resolution, of the order of the wavelength of the light detected, is possible. Such high‐resolution single pulse profile monitors will be very useful for future free electron laser and linear collider projects. Using the very low emittance 1.3 GeV electron beam at the KEK Accelerator Test Facility (ATF) [2] (1.4nm ex × 15pm ey), we have imaged transition radiation from 5 micron σ beam spots. Our test device consisted of a finely polished target, a thin fused silica window, a 35 mm working distance microscope objective (5x and 10x) and a triggered CCD camera. A wire scanner located near the target is used to verify the profile monitor performance. In this paper we report results of beam tests.

RF cavity BPM'S as beam angle and beam correlation monitors

It has been shown that high performance cavity BPMs are capable of accurate beam trajectory angle and beam tilt, (x-z or y-z correlation) measurements. Such a device will be very useful for the optimization of a variety of beamlines, such as high current linacs, bunch rotators and storage rings. The signal from a nonaxial trajectory or a tilted beam is in quadrature to that observed from a simple displacement of a very short bunch. Using in-phase/quadrature-phase (I/Q) demodulation of the cavity BPM signal, it is possible to separate position and angle/tilt. In this paper, we present results of beam angle and tilt monitor tests carried out in the KEK Accelerator Test Facility (ATF) extraction line.

Advanced collimator engineering for the nlc

The high beam intensities in the Next Linear Collider can damage the collimation system jaws if a failure of the electronic machine protection system allows the acceleration of a mis-launched pulse. We are developing a prototype collimator that will allow a new jaw surface to be moved into place after beam damage. We are also developing a prototype collimator based on continuously reforming the collimator jaws out of liquid metal. This second system would allow the use of smaller beam sizes, and thereby shorter beam line lengths in the collimation system. The status of both prototypes is presented.

The Next Linear Collider machine protection system

The Next Linear Collider (NLC) electron and positron beams are capable of damaging the linac accelerating structure and beamline vacuum chambers during an individual aberrant accelerator pulse. Machine protection system (MPS) considerations, outlined in this paper for the 1 TeV NLC design, have an impact on the engineering and design of most machine components downstream of the damping ring injector complex. The MPS consists of two functional levels. The first level provides a benign, single bunch, low intensity, high emittance pilot beam that will be used for commissioning and also whenever the integrity or the settings of the downstream components are in doubt. This level also provides for the smooth transition back and forth between high power operation and the benign diagnostic pilot bunch operation. The pilot bunch parameters in the main linac are estimated on the basis of the expected stress in the accelerator structure copper. Beam tests have been done at the SLAC linac to examine the behavior of the copper at the damage stress threshold. Typical pilot beam parameters (compared with nominal) are: 10 times reduced intensity, 10 times increased horizontal emittance and 1000 times increased vertical emittance, resulting in a reduction in charge density of 10/sup 5/. The second level is the primary protection against a single aberrant pulse. Its goal is to reduce the possibility that a substantial transverse field changes the trajectory of the high power beam from one pulse to the next. All devices that could produce such a field are (1) monitored by a fast response network and where possible have (2) deliberately slowed response times. A maximum allowable interpulse difference is evaluated for each such device as well as the beam trajectory monitors in each interpulse period.

High precision SC cavity alignment measurements with higher order modes

Experiments at the FLASH linac at DESY have demonstrated that the higher order modes (HOMs) induced in superconducting cavities can be used to provide a variety of beam and cavity diagnostics. The centers of the cavities can be determined from the beam orbit which produces minimum power in the dipole HOM modes. The phase and amplitude of the dipole modes can be used as a high resolution beam position monitor. For most superconducting accelerators, the existing HOM couplers provide the necessary signals, and the downmix and digitizing electronics are straightforward, similar to those for a conventional BPM.

SLAC's polarized electron source laser system for the E-158 parity violation experiment

SLAC E158 is an experiment to make the first measurement of parity violation in Moller scattering. The left-right cross-section asymmetry in the elastic scattering of a 45-GeV polarized electron beam off unpolarized electrons in a liquid hydrogen target will be measured to an accuracy of better than 10-8, with the expected Standard Model asymmetry being approximately 10-7. An intense circularly polarized laser beam for the polarized electron source is required with the ability to quickly switch between left and right polarization states with minimal left-right asymmetries in the parameters of the electron beam. This laser beam is produced by a unique SLAC-designed, flash-lamp pumped, Ti:Sapphire laser. We present this laser system design and initial results from recent commissioning runs.

The NLC injector system

The Next Linear Collider (NLC) injector system is designed to produce low emittance, 10 GeV electron and positron beams at 120 hertz for injection into the NLC main linacs. Each beam consists of a train of 95 bunches spaced by 2.8 ns; each bunch has a population of 1.15/spl times/10/sup 10/ particles. At injection into the main linacs, the horizontal and vertical emittances are specified to be /spl gamma//spl isin//sub x/=3/spl times/10/sup 16/ m-rad and /spl gamma//spl isin//sub y/=3/spl times/10/sup -8/ m-rad and the bunch length is 100 /spl mu/m. Electron polarization of greater than 80% is required. Electron and positron beams are generated in separate accelerator complexes each of which contain the source, damping ring systems, L-band, S-band, and X-band linacs, bunch length compressors, and collimation regions. The need for low technical risk, reliable injector subsystems is a major consideration in the design effort. This paper presents an overview of the NLC injector systems.

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.

Electronics and Algorithms for HOM Based Beam Diagnostics

The signals from the Higher Order Mode (HOM) ports on superconducting cavities can be used as beam position monitors and to do survey structure alignment. A HOM-based diagnostic system has been installed to instrument both couplers on each of the 40 cryogenic accelerating structures in the DESY TTF2 Linac. The electronics uses a single stage down conversion form the 1.7 GHz HOM spectral line to a 20MHz IF which has been digitized. The electronics is based on low cost surface mount components suitable for large scale production. The analysis of the HOM data is based on Singular Value Decomposition. The response of the OM modes is calibrated using conventional BPMs.