N. S. Sereno

Radiation effects studies at the Advanced Photon Source

At the Advanced Photon Source (APS) concern for radiation-induced demagnetization of the insertion devices (IDs) in the storage ring and in the free-electron laser has initiated systematic radiation effects studies towards the development of efficient techniques for ID protection. The studies include radiation dose monitoring, parametric study of the radiation-induced demagnetization, as well as, potentially, a dedicated radiation effects testbed at the APS providing GeV electron beams. Such studies could also be directly applicable to future generation facilities, such as the Linac Coherent Light Source (LCLS). Results and discussion of the radiation damage studies at APS are presented.

Planned use of pulsed crab cavities for short X-ray pulsed generation at the Advanced Photon Source

Recently, we have explored application to the Advanced Photon Source (APS) of Zholents[1] crab cavity scheme for production of short x-ray pulses. We assumed use of superconducting (SC) cavities in order to have a continuous stream of crabbed bunches and flexibility of operating modes. The challenges of the SC approach are related to the size, cost, and development time of the cavities and associated systems. A good case can be made [2] for a pulsed system using room-temperature cavities. APS has elected to pursue such a system in the near term, with the SC-based system planned for a later date. This paper describes the motivation for the pulsed system and gives an overview of the planned implementation and issues. Among these are overall configuration options and constraints, cavity design options, frequency choice, cavity design challenges, tolerances, instabilities, and diagnostics plans.

Performance of the Advanced Photon Source (APS) linear accelerator

A 2856-MHz S-band, electron-positron linear accelerator (linac) is the injector and source of particles for the APS. The linac is operated 24 hours per day, with 405-MeV electrons to support commissioning of the other APS accelerators, and with positrons or electrons to support linac studies. It produces electrons with energies up to 655 MeV or positrons with energies up to the design energy of 450 MeV.

Utilization of CTR to measure the evolution of electron-beam microbunching in a self-amplified spontaneous emission (SASE) free-electron laser (FEL)

We report on the first measurements of the z-dependent evolution of electron-beam microbunching as revealed through coherent transition radiation (CTR) measurements in a visible self-amplified spontaneous emission free-electron laser experiment. The increase in microbunching was detected by tracking the growth of the visible CTR signals as generated from insertable metal mirrors/foils after each of the last three undulators. The same optical imaging diagnostics that were used to track the z-dependent intensity of the undulator radiation (UR) were also used to track the electron beam/CTR information. Angular distribution, beam size, and intensity data were obtained after each of the last three undulators in the five-undulator series, and spectral information was obtained after the last undulator. The exponential growth rate of the CTR was found to be very similar to that of the UR and consistent with simulations using the code GENESIS.

Design and performance of the LCLS cavity BPM system

In this paper we present the design of the beam position monitor (BPM) system for the LCLS undulator, which features a high-resolution X-band cavity BPM. Each BPM has a TM010 monopole reference cavity and a TM110 dipole cavity designed to operate at a center frequency of 11.384 GHz. The signal processing electronics features a low- noise single-stage three-channel heterodyne receiver that has selectable gain and a phase locking local oscillator. We will discuss the system specifications, design, and prototype test results.

A potpourri of impedance measurements at the Advanced Photon Source storage ring

Machine coupling impedances were determined in the APS storage ring from measurements of the bunch length, synchronous phase, and synchrotron and betatron tunes vs. single-bunch current. The transverse measurements were performed for various numbers of small gap insertion device (ID) chambers installed in the ring. The transverse impedance is determined from measurements of the transverse tunes and bunch length as a function of single-bunch current. The shift in the synchrotron tune was measured as a function of bunch current from which the total cavity impedance was extracted. The loss factor was determined by measuring the relative synchronous phase as a function of bunch current. The longitudinal resistive impedance is calculated using the loss factor dependence on the bunch length. From these results, we can estimate what the impedance would be for a full set of ID chambers.

Bunch length measurements at the Advanced Photon Source (APS) linear accelerator

Measurements of the APS linac micro-bunch length are performed by backphasing a single 2856-MHz, S-band linac waveguide and using a downstream spectrometer to observe the beam. By measuring the beam width in the dispersive plane as a function of RF power into the linac waveguide, the bunch length can be determined absolutely provided the beam energy and dispersion at the spectrometer are known. The bunch length determined in this fashion is used to calibrate a fifth-harmonic bunch length cavity which is used for real-time bunch length monitoring.

Initial Imaging of 7-GeV Electron Beams with OTR/ODR Techniques at the APS

The development of nonintercepting (NI) diagnostics for beam size continues to be of interest in the accelerator community. In the three rings of the Advanced Photon Source (APS) facility, we use optical and x-ray synchrotron radiation generated as the electron beam transits the dipole magnetic fields as an NI mechanism to image the beam during top-up operations. However, in the straight transport lines an alternate beam-size measurement method is needed. Optical diffraction radiation (ODR) is under investigation to monitor 7-GeV beam size and trajectory in the booster synchrotron-to-storage ring (BTS) beamline during top-up operations. We have performed our initial measurements with an Aluminum blade/mirror that served as an optical transition radiation (OTR) monitor when fully inserted into the beam and as an ODR monitor when the beam passed near the edge.

Investigation of APS PAR Vertical Beam Instability

A vertical beam instability has been observed for some time in the particle accumulator ring (PAR) of APS. It appears at low beam current when a single linac bunch is selected for PAR injection and is highly reproducible. An investigation was conducted to characterize and understand this instability. We obtained some interesting data and believe this was due to ion trapping. A more stable lattice was established as result of the investigation. This report summarizes the experimental results and gives some preliminary analysis.

Far-field OTR and ODR images produced by 7-GeV electron beams at APS

We have investigated the angular distribution patterns (far-field focus) of optical transition radiation (OTR) and optical diffraction radiation (ODR) generated by 7-GeV electron beams passing through and near an Al metal plane, respectively. The 70-murad opening angles of the OTR patterns provide calibration factors for the system. Effects of the upstream quadrupole focusing strength on the patterns as well as polarization effects were observed. The OTR data are compared to an existing OTR single-foil model, while ODR profile results are compared to expressions for single-edge diffraction. ODR was studied with impact parameters of about 1.25 mm, close to the gamma lambda-bar value of 1.4 mm for 628-nm radiation. We expect angle-pointing information along the x axis parallel to the mirror edge is available from the single-lobe ODR data as well as divergence information at the sub-100-murad level. Experimental and model results will be presented.