On 5 April 2016, the BEPCII, upgrade of the Beijing Electron Positron Collider (BEPC), broke the luminosity record again in the charm energy region by 10.0´1032cm-2s-1, and reached its design luminosity at the optimised beam energy of 1.89 GeV as well.
BEPC was constructed in IHEP, Beijing, from 1984 to 1988, working in the charm-tau region with fruitful physics results until July 2005, the shutdown time when the BEPCII storage rings construction started. BEPCII was designed as a double-ring factory-like collider, using the existing tunnel of BEPC and aiming at a design luminosity 100 times higher than BEPC at the beam energy of 1.89 GeV. It can also be operated as a synchrotron radiation facility with the beam energy of 2.5 GeV by combining two outer half-rings of each collision ring at the interaction point (IP) and the opposite crossing point (CP). Some key technologies, such as superconducting RF cavity and magnets, slotted pipe kicker magnet, chopper power supply, large vacuum chamber with TiN coating inside, transverse and longitudinal bunch-by-bunch feedback systems, new positron target on the linac, etc., are applied in BEPCII. These technologies realised a high beam current for collisions, a small vertical beta function at the IP, and shorter bunch lengthening, ensuring the design luminosity of 1 × 1033 cm-2s-1 at the beam energy of 1.89GeV. The main design parameters of the collision mode and their optimised values are listed in Fig. 1.
From 2009, the year when BEPCII passed the national check and test, and started running for its users, a machine study on luminosity enhancement has been carried out towards the design value, although only one year was dedicated to the physics data taken at the design energy. Every year, BEPCII ran for high energy physics from E c.m. of 2.0 GeV to 4.6 GeV, for about 6 months, and 3 months for dedicated synchrotron radiation user. Only one month a year is spent on luminosity commissioning. With the efforts on shifting horizontal tunes of two ring closer to half integer, squeezing the beta function at the IP about 10%, moving CP by 1/4 wave length of the betatron oscillation to mitigate beam-beam effect, optimising beam emittance and momentum compaction, and trying different bunch patterns, etc., we finally obtained the design luminosity at its optimised beam energy, as shown in Fig. 2.
In the meantime, a lot of physics results were obtained during the operation of BEPCII. Among them, the discovery of Zc(3900), a four-quark state particle, was the most significant result in recent physics studies. Moreover, more than 500 synchrotron radiation users completed their experiments at BEPCII every year, with abundant results in different scientific areas.