Figure 175 presents the distribution of the various filling modes used in 2003. The larger share of the 16 bunch filling mode as compared to 2002 is not significant since, for reasons of convenience, some of the 16 bunch shifts slipped from 2002 to 2003. The multi-bunch modes of operation (uniform and 1x1/3 filling patterns) provide the highest current and the longest lifetime of the stored beam and correspond to 59% of the total beamtime. The other modes of operation aim at providing a particular filling pattern to users making use of the time structure of the electron beam.


Fig. 175: Distribution of the 2003 user time in the various filling modes.


A new hybrid mode has been developed consisting in 24 groups of 8 bunches + 1 bunch of 4 mA. This mode is delivered with a 200 mA current and a reasonable lifetime (~ 30 hours). It is targeted to simultaneously satisfy the maximum number of users including the majority who are looking for maximum current as well as those who need such a time structure with many empty buckets in between the filled bunches. The usual RF knockout technique of cleaning the residual parasitic bunches resulted in a contrast ratio as large as 109 between the filled and the unfilled buckets. Such a filling pattern was developed for the first time in 2002. Two weeks of this beam were successfully delivered in 2003 with two refills per day. It is intended to make this the standard hybrid mode in 2004.

Some difficulties were experienced in 2002-3003 with single bunch delivery. Whereas in previous years it was possible to reach an intensity of 20 mA, this is no longer true and in spite of a great deal of effort to find the optimum parameters, the maximum single bunch current offering sufficient stability and low emittance dropped to 14 mA in 2003. Although the reason for this limitation has not been precisely determined, it is strongly suspected that it comes from an increase in both the horizontal and vertical coupling impedance induced by the discontinuities and resistivity of the recently installed small aperture vacuum vessels. In order to continue satisfying the User community using single bunch and those interested in both the time structure and a sufficient beam current, a new mode has been developed and was successfully delivered for 23 shifts during the last run of the year. It consists in 4 bunches of 10 mA equally distributed around the circumference. With a lifetime of 5 hours and a total maximum intensity of 40 mA, this mode will most likely replace the single bunch one in the future.