Increase of positrons by a high-intensity two-bunch acceleration scheme at the KEKB linac

As the accumulation current of positrons increases in the KEKB ring, the injection time is becoming longer. It will thus be one of the most important issues affecting the accumulation of the integrated luminosity. As one of the steps, we introduced a high-intensity two-bunch acceleration scheme at the KEKB linac to intensify positrons by means of doubling the primary electrons. We recently obtained test results of 0.54 nC for the first bunch and 0.49 nC for the second bunch at the linac end. This scheme increased the positron intensity by nearly 65%. Since the linac frequency is not a harmonic number of the LER frequency, the best time interval between two bunches is 96.29 ns, corresponding to 49 LER-buckets. Even with this limitation, it is undoubtedly a very useful scheme for increasing the positron injection rate. The beam test results are described.