Since 1996, the storage ring had been routinely operated with an intensity of 200 mA in the multibunch mode, i.e. twice the design current. Tests to increase the intensity to 300 mA have been recently initiated with a view to identifying possible experimental limitations and to define the necessary R&D program with a long-term goal of delivering this current to Users. Since higher order magnitude (HOM) driven coupled bunch instabilities would very likely prevent 300 mA from being reached in the uniform filling mode, tests were performed when filling only one third of the ring. With this partial filling (330 bunches) of the circumference, the periodic beam loading of the cavities induces a spread in synchrotron frequencies in the bunch train that prevents the constructive built-up of the instability. To guard against resistive wall instabilities, the chromaticity was increased above the routine multibunch values (x = 6.7, z = 9.5). With these precautions, the first attempt to ramp the intensity above 200 mA was rather easy (Figure 194). No abnormal pressure rise or temperature increase on critical components (crotch absorbers, RF windows) was observed. A slight retuning of cavity temperature could easily cure fugitive HOMs. However, at 250 mA, the radiation induced outside the shielding exceeded the authorised level and the tests were temporarily interrupted. The lifetime was rather moderate: 17 hours as compared to 75 h at 200 mA and uniform filling mode. This comes both from the 1/3 filling mode which is unfavourable in terms of lifetime due to the larger intra beam scattering and from the higher pressure in the ring chamber induced by the extra synchrotron radiation power incident on all absorbers.

Fig. 194: Synopsis showing the ramping of the current to 250 mA.