Study of initial dynamic pressure rise behaviour in Indus-2

UHV system for synchrotron radiation source Indus-2 was developed and commissioned. The specified ultimate vacuum in 10−10 mbar range was achieved. This vacuum deteriorates with the injection of electron beam into the storage ring mainly due to photon induced desorption (PID). Initially, the PID yield may be as high as 10−1 molecules/photon and the resultant dynamic pressure rise may be very high. However, the PID yield decreases with beam cleaning, provided by beam dose accumulation. For Indus-2, initially 550 MeV electron beam was injected and by now more than 5.8 A.Hr beam dose has been accumulated. The maximum beam current stored was 72.81 mA at 550 MeV. Once the beam energy was ramped up to 2.4 GeV also, but regular ramping was restricted to 2 GeV. The maximum current was 51 mA at 2 GeV. Since the Indus-2 has eight unit cells, any one-unit cell is a representative of entire storage ring. This paper shows the dynamic pressure rise behaviour of one of them. The pressure rise per mA of beam current has been measured and compared at different stages of beam dose accumulation. Variation in the values of PID yield with beam dose has also been studied. The specific dynamic pressure rise calculations were done for both the GDC cleaned as well as non-GDC cleaned dipole chambers. For non-GDC cleaned chambers these values were larger by more than one order of magnitude. This paper discusses the details of dynamic pressure rise behaviour of Indus-2 in its initial stages of commissioning.