ENABLING TECHNOLOGIES

174 ESRF

and induced radioactivity are reduced. Most of the X-rays generated along the beam path do not reach the beamlines. This excess of radiation is stopped by dedicated surfaces exposed to vacuum, generating heat load and photon- stimulated desorption (PSD), which is the main gas load source for the vacuum system.

The EBS design has two main types of X-ray stopper: lumped copper absorbers and NEG- coated chambers. Copper absorbers are designed to minimise reflection and scattering of photons and are mainly distributed along the arcs, accommodated in vacuum chambers made of stainless steel or aluminium. The use of aluminium chambers obtained by machining from bulk material is an innovation used at the ESRF. This solution eliminates the complex bending and welding usually necessary for stainless steel, and is more lightweight and cost- effective. However, in the vacuum community, there are strong concerns regarding its usage: its PSD is much higher than stainless steel, hence reflected photons could generate considerable gas load. Real pressure measurements confirm the vacuum design calculation that this aluminium effect is negligible. Figure 148 shows the pressure of all chamber 8s (CH8) distributed along the ring. No significant deviation is visible, even if four of the 32 installed chambers are made of aluminium, while the positions of all absorbers, pumps and gauges are identical. Most of the NEG-coated chambers used in many straight sections have been recovered from the previous machine, where they were pre- conditioned. Nevertheless, they are the drivers of the average pressure along the ring and hence, of the vacuum conditioning.

No vacuum system is designed to allow full current from the beginning, due to the high costs and space needed for the required pumping speed. Moreover, most of this speed will not be necessary over time once the system is conditioned. Hence, a gradual current increase is considered. For EBS, the vacuum design estimated a total dose of 50 A.h to reach full current, with an average pressure of 8×10-8 mbar. The real average pressure measured at this dose, was 8×10-9 mbar, i.e., one order of magnitude better than foreseen. The vacuum system performance exceeded expectations and it was possible to reach full current in three months, indeed, much faster compared to other synchrotrons. Figure 149 shows the storage ring normalised average pressure and beam lifetime versus cumulated dose. Improvements in both lifetime and pressure, due to vacuum interventions performed on ID02 and ID12, are clearly visible.

It is not by chance that the biggest improvements in vacuum performances are linked to insertion

Fig. 148: CH8 pressure with beam. Chambers in cells 6, 8, 15 and 27 are made of aluminium, the rest are made of stainless steel.

Fig. 149: Average pressure and beam

lifetime versus cumulated dose.

Fig. 150: RGA conditioning

curves.