Beam Loss Detectors

General Description :

Beam Loss Detectors (BLD) are devices intended to monitor the radiation losses associated with the circulation of a beam in the storage ring.

BLD can be divided into two categories: photomultiplier base Beam Loss Detectors (SBLD) (photo 1 and 2)and Ionisation chamber base detectors (Unidos or IG5 Centronic).

BLDs are installed all arround the storage ring. Each of the 32 cells of the ESRF storage ring is equipped with two SBLDs, one Unidos, and one IG5 Centronic detectors. They convert light into an electrical signal that will be carried out of the tunnel. A special electronic installed in the technical gallery will drive these detectors and convert their data into usable electric levels (acquired by a low-cost GADCM board, unipolar, 0 to 5V 12 bit input or commercial electrometers).

Uses of BLD:

Vacuum diagnostics
The lifetime may be suddenly reduced by a local reduction in physical aperture due for example to the deformation of an RF finger. This will become immediately apparent on the slow detectors in the following cells or on the fast detectors when trying to re-inject. Following interventions on vacuum on the storage ring, the vacuum improves from a relatively poor value, due to conditioning, over several weeks. This is evident from the lifetime and beam losses on straight section vacuum vessels (figure 1). A growing leak in a cell may be detected by increases in the losses on the insertion device vessel in the following cell (figure 2).

Scraper optimisation
The losses on the small vacuum vessels are a problem due to increased Bremstrahlung on the beam line and due to activation of the vacuum chamber. The losses can be dramatically reduced and concentrated at the position of the scraper by optimising its position (figure 3).

In practice the slow beam loss detectors have also been useful in detecting injection losses by reducing dramatically the injected current from the linac so as not to saturate the detectors.

Synchrotron Radiation Background
It was stated that the radiation level inside the tunnel due to synchrotron radiation is in the range of Sv/h and practically everywhere due to the scattering capability of the synchrotron radiation. The radiation level due to wide angle bremsstrahlung resulting from the electron losses at stable stored beam will be in the range of a few uSv/h only. It is therefore essential to shield the detector with at least 1cm of lead to suppress the synchrotron radiation background to less than 1uSv/h. Nevertheless, for the case of the fast detectors, synchrotron radiation is not a problem as they are only sensitive to large bursts from beam losses. For the slow detector the shielding should not be thicker than a cm since then it will start to decrease the signal. Therefore it was agreed to use 1 cm of shielding which will also help to build up the shower close to the detector. It was demonstrated that the existing beam loss detector set-up (detector used from before December 1998) although not properly shielded is showing signals at the top end of the intensity proportionality.