Orbit Measurements in the BESSY II Booster in Preparation for Quasi-Low-Alpha Operation

Diagnostic refurbishments are ongoing at the booster synchrotron in preparation for the near future Variable pulse Storage Ring (VSR) project at BESSY II. Essential orbit measurements have been re-installed after almost two decades of latency. This diagnostic will help assess the effectiveness of possible upgrade scenarios such as quasi-low alpha operation and extraction optimization. The contribution presents the preliminary Beam Position Monitor measurements from the continual global upgrade of the injector systems. INJECTION REGIME BESSY VSR A comprehensive description of the injection system at BESSY II was recently given in [1]. In a familiar fashion that characterized 3 generation light sources across the world, injection into the Storage Ring is from a low energy LINAC followed by a Booster synchrotron. Although the present injection scheme is highly reliable, a global upgrade is foreseen for the BESSY VSR project [2]. The most prominent aspect with respect to the injector is the evidence that the bunch length on injection into the Storage Ring needs to be reduced from its present value, by at least a factor two in order to keep the high injection efficiencies. The problem arises from the large difference in the bunch lengths on injection and the reduced longitudinal acceptance due to the proposed VSR technique as pictorially shown in Fig. 1. Figure 1: Longitudinal phase space comparison of BESSY II with BESSY VSR. QUASI-LOW ALPHA IN THE BOOSTER To complement an upgrade of the existing power capabilities of the accelerating structure in the Booster in order to produce a shorter bunch, a more flexible optic is envisaged. The present optic in the Booster is based on a simple FODO ∗ terry.atkinson@helmholtz-berlin.de lattice with a 16-fold symmetry. The momentum compaction and hence optic to produce shorter bunch lengths is heavily restricted. Installing independent quadrupole power supplies and breaking the symmetry allows more tunable optic towards low alpha [1]. Such a large scale refurbishment of the existing power supplies requires considerable investment. Minor alterations to the present lattice to test the effectiveness of breaking the symmetry has led to the proposal of a quasi-low alpha lattice. Such a lattice in the BESSY II Booster is achieved by reversing the polarity of each 3 quadrupole. This simple alteration transforms the original FODO lattice into a quasi-Double Bend Achromat with the quadrupole settings k1, k2, (-k1) and k2. Figure 2: Momentum compaction as a function of transverse beam degradation for a quasi-low alpha optic in the Booster. Each blue point shown in Fig. 2 represents a randomly generated stable lattice and the chromatic invariant H value of the horizontal axis portrays transverse beam degradation (emittance equilibrium) relative to the nominal optic. The transition from the present nominal setting depicted as a red cross to a quasi-low optic lattice is troublesome since no stable path therein exists. This notion was emphasized during the first attempts to find a stable optic after the polarity of each 3 quadrupole was reversed during a 24 hour test. Every single stage such as the injection, the first turn and the compensation of over-steering magnetic elements were flawed by lack of allocated time and limited diagnostics. Driving the optic upgrade onwards is the refurbishment of the latter and is described in the following sections.