Optical and electrical characterisation of the LNLS UVX storage ring electron beam

Abstract

The Laboratório Nacional de Luz Síncrotron (LNLS) is a research institute affiliated to the CNPq (Brazilian National Research Council) currently in its final implantation stages in Campinas, SP. LNLS will provide the scientific community of the State of São Paulo and of Brazil, in August 1996, with a high intensity broad-spectrum radiation source based upon the 1.15 GeV electron storage ring UVX. The forthcoming commissioning of the LNLS accelerators opens up a wide range of research possibilities in the fields of experimental beam physics and accelerator technology. Over the last few years, the LNLS Accelerator Physics Group has been mainly involved in the conceptual and technical design of the linear and circular accelerators, having analyzed various aspects of the electron beam dynamics that influence the performance of the storage ring (as measured in terms of the stored beam current, emittance, lifetime and beam stability). From those studies, two topics have emerged as particularly relevant to the Brazilian machine: beam stability issues related to the low-energy injection (particularly those associated with beam neutralization due to ion trapping) and the possibility of producing very short electron bunches (7 ps FWHM) in a quasi-isochronous operation mode. During the design of the machine, much effort was devoted to minimizing the potentially harmful effects of beam instabilities at low energy, e.g., by introducing clearing electrodes to reduce the density of neutralizing ions and building a vacuum chamber with as smooth cross-section variations as possible. The start of machine operation would allow us to confirm the effectiveness of those measures experimentally. The possibility of producing short light pulses from the UVX storage ring (a useful machine characteristic for synchrotron radiation users working with time resolved techniques) has been studied and the stability properties of this quasi-isochronous operation mode have been analyzed. The experimental implementation of such a mode will grant the Brazilian machine accesses to the restricted subset of the third generation radiation sources in the world which are capable of short pulse generation. In the experimental investigation of both topics mentioned above, the syncrotron radiation emitted by the beam and the high frequency electromagnetic signals induced by the beam in antennas (striplines) built into the ring vacuum chamber are essential characterization tools. In the longitudinal phase plane, the observation of the time structure of the beam allows the measurement of the bunch length, whereas observation of the transverse distribution of the radiation gives information on the transverse electron beam distribution, from which one can infer the beam emittance, a quantity directly affected by beam stability conditions. Still in the transverse plane, the simultaneous observation of variations of beam profile (with the synchrotron radiation) and beam coherent oscillation amplitude (with the pickup antennas) as a function of time after the application of a resonant beam excitation allows the measurement of the characteristic time of damping processes (either radiation or Landau damping). Finally, the spectrum of beam oscillations as detected by Fourier analyzing the stripline signals gives information on the betatron tunes or transverse electron oscillation frequencies, which are affected by various collective effects, in particular by ion trapping, and also point to the existence of unstable oscillation modes of the beam. In this RESEARCH PROJECT, we describe an optical characterization bench and a beam oscillation observation and excitation system for the UVX stored electron beam designed to study collective effects (particularly at low energy) and to perform short bunch length measurements. The optical bench includes mirrors and lenses to convey the synchrotron light to the sensitive elements and two radiation detectors: a fast photo-diode for longitudinal (time-structure) observations and a fast (400 frames per second) CCD camera coupled to a fast digitizing PC board that provides beam profile capture at a rate fast enough to allow observation of phenomena on the time scale of synchrotron radiation damping. The beam oscillation observation system includes two sets of four striplines (one for excitation of the beam and the other for collecting signals from the beam) and RF electronics to perform additions and subtractions with those signals in order to deliver signals proportional to the horizontal and vertical beam positions to a swept tuned spectrum analyzer. All of the hardware needed for the construction of the beam oscillation observation system and about half that needed for the optical characterization bench has already been purchased by LNLS. (AU)