Electron beam profile measurements and emittance manipulation at the MAX-laboratory

Abstract: The emittances of the electron beams at the MAX-laboratory accelerator system have been studied. Apart from the build-up of the diagnostic tools for precise determination of the beam spatial profiles, the objectives have been: a) to verify the accelerator design emittances at low currents and to try to determine the emittances also at high currents; b) to investigate possibilities to manipulate the emittances. The method for emittance determination was optical measurements of the electron beam profiles. Then, by different means the accelerator lattice functions were estimated at the observation point. From this knowledge and from measurements of the synchrotron frequency, the emittances in all three dimensions were determined. As a consequence of the desire to determine very low transverse emittances, a theoretical model for the emission and focusing of synchrotron radiation had to be employed. A model based on classical electrodynamics and wave-optics was chosen. The accuracy of this model was partly verified in the visible region, by measurements at low circulating currents in the MAX-II storage ring at injection energy. Simultaneously, the measurements gave indications of a possibility to measure transverse emittances even below 20 pm rad. Both the MAX-I and MAX-II storage rings were found to reach the expected natural horizontal emittances. The vertical emittance at low currents, which elucidates to which accuracy the ring magnetic elements are aligned, was in MAX-I / MAX-II measured to be 300 pm rad / 50 pm rad, respectively. At high currents, an increased energy spread was the main source of beam quality dilution in both rings if no counter-measures were taken. The introduction of a short period, narrow gap undulator into MAX-I did not bring about any beam quality deterioration. The MAX-I lattice was tuned towards small momentum compaction values, and by controlled beam energy changes the emittances were manipulated such that an rms bunch length of less than 1 mm was reached at low currents. Second order theory accounted well for the observed horizontal and longitudinal beam profiles. The lattice of MAX-II was chosen to give a finite dispersion in the long straight sections, which allowed the natural horizontal emittance to be decreased about a factor of two compared to the dispersion-free case.