Past 16 years of the KEK electron/positron injector linac

It is 16 years since the upgrade of the injector for KEKB, 2856 MHz electron linac, was started in 1993. The KEKB was in competition with PEP-II, and it is said that the KEKB was inferior to PEP-II because the energy of the injector at KEK was only 2.5 GeV while that at SLAC was as high as 50 GeV. The framework of the KEKB project was determined to collide 8 GeV electrons with 3.5 GeV positrons with a luminosity higher than 1034 cm2s-1. To attain the electron energy of 8 GeV, following scheme was employed: (1) The klystron output is increased from 20 to 40 MW, and the pulse width is compressed to reinforce the acceleration gain from 8 to 20 MV/m. (2) The number of accelerator units is increased from 40 to 60. These plans were successfully realized, and both of the peak luminosity and the integrated luminosity of the KEKB exceeded those of PEP II. The project of fast beam switching to three rings, KEKB electron ring (8 GeV), KEKB positron ring (3.5 GeV), and Photon Factory electron ring (2.5 GeV), is underway now. (K.Y.)