Dynamic Aperture Measurements at the Advanced Light Source

A large dynamic aperture for a storage ring is of importance for long lifetimes and a high injection efficiency. Measurements of the dynamic aperture of the third generation synchrotron light source Advanced Light Source (ALS) using beam excitation with kicker magnets are presented. The experiments were done for various accelerator conditions, allowing us to investigate the influence of different working points, chromaticities, insertion devices, etc.. The results are compared both with tracking calculations and a simple model for the dynamic aperture yielding good agreements. This gives us confidence in the predictability of the nonlinear accelerator model. This is especially important for future ALS upgrades as well as new storage ring designs. 1 DEFINITION OF THE APERTURE The aperture of an electron storage ring is the maximum transverse and/or longitudinal deviation from the design orbit an electron can experience without being lost. The size of the aperture can effect both injection and lifetime. At the ALS where one injects horizontally offset from the stored beam the on-momentum aperture has to be large enough to accept the injected electron beam. From the point of view of lifetime it is important to have both sufficiently large on-momentum and off-momentum apertures. In particular the lifetime of a low energy, low emittance electron storage ring like the ALS1 is usually given by the scattering of electrons within a bunch (Touschek effect) and/or by elastic and inelastic scattering of electrons with the residual gas. When electrons scatter within a bunch, they may transfer enough momentum to be outside the momentum aperture of the storage ring. Depending on the dispersion function at the scattering position, scattered electrons start a betatron oscillation in addition to the momentum offset. An electron with a large betatron amplitude usually reaches the off-momentum aperture at smaller momentum deviations. Elastic scattering of electrons with the residual gas excites betatron oscillations. The elastic scattering lifetime is thus proportional to the on-momentum aperture. For more details about aperture measurements using lifetime investigations see [1], [2]. The aperture can be limited by several effects. The linear transverse motion of electrons is limited by the vacuum chamber aperture xvc:. In the presence of disperThis work was supported by the Director, Office of Energy Research, Office of Basic Energy Sciences, Materials Sciences Division, of the U.S. Department of Energy, under Contract No. DE-AC03-76SF00098. y Now at DESY, Hamburg, Germany. 1The ALS is operated with an energy of 1:5 1:9GeV and an emittance of 3:5 5:6 10 9 radm. sion this aperture is reduced by the off-momentum orbit. The invariant physical horizontal aperture is the minimum around the ring ofAphys;x( ) = (xvc:(s) (s) )= x(s) with = dP P0 the relative momentum deviation. Dispersion is usually only present in the horizontal plane leaving the vertical physical aperture Aphys;y momentum independent. The longitudinal motion of electrons is limited by the height of the rf-bucket provided by the accelerating voltage in the cavity. The electron motion is also confined by dynamic effects, leading to resonant or chaotic amplitude growth. The border of this motion is called the dynamic aperture Adyn;x( ) and depends on the relative momentum deviation. The dynamic aperture is often estimated through tracking calculations. One can also estimate the dynamic aperture using a simple model in the following way: Electrons are lost when their tune satisfies a resonance condition. From knowing the tune shift terms with amplitude, @ y @Ax , @ y @Ay , and momentum deviation, @ y @ , @ 2 y @ 2 , one can compute the tune shift due to momentum and transverse deviations: y = @ y @Ax Adyn;x+ @ y @Ay Adyn;x+ @ y