Long, small-aperture Insertion Device (ID) vacuum chambers are difficult to pump due to the low conductance. As a result the collision of the electrons with the residual gas may produce a large amount of Bremsstrahlung gamma rays. When hitting a material, this Bremsstrahlung produces a cascade of lower energy particles, some of them energetic enough to go through a few centimetres of lead. As a consequence a significant dose rate can be recorded outside the hutch. To reduce this dose rate to an acceptable level, one must control the residual gas pressure in the insertion device vacuum chamber. To do so, the ESRF has developed, in collaboration with CERN, a process of coating the inner surface of the ID vessels with a thin, Non Evaporable Getter (NEG) material. Such material provides distributed surface pumping along the whole length of the chamber and reduces the photon-induced desorption. By the end of 2003 14 NEG coated ID chambers of different lengths, apertures and material will have been installed on the ring totalling a cumulated length of 60 m. The first chamber was coated at CERN, but in 2002 the ESRF developed its own NEG coating facility and 4 of the most delicate ID chambers were coated in-house in 2003. These chambers, made of aluminium, are 5 m long and have an 8 x 57 mm internal aperture. They were installed on the ID6 straight section and the Bremsstrahlung produced on-axis was monitored using a detector located in the ID6 optics hutch. Out of the 4 chambers, two have been considered successful and put into operation on the ID26 and ID20 straight sections. Figure 177 shows the Bremsstrahlung dose rate recorded vs. integrated ring current for the last three chambers installed. One observes a large dispersion of the results. The two lower curves correspond to chambers for which the coating is considered as successful. These chambers are now in operation on ID26 and ID20. A large number of bursts are visible on the upper curve. These bursts were correlated with small electron beam losses and are likely to come from the collision of the beam with a small piece of NEG material falling from the aluminium surface.


Fig. 177: The Bremsstrahlung measured on axis of three different NEG coated, 8 mm aperture ID chambers as a function of the integrated electron current. Each curve corresponds to a different chamber.


A careful inspection of the defective chambers with an endoscope has confirmed the expectation of a lack of adhesion of the coating material in a few places. Experience in the past has shown that aluminium is more prone to such adhesion problems than stainless steel and that such problems are probably linked to insufficient cleaning/conditioning of the surface prior to the coating by magnetron sputtering. As a result of this, the whole coating process has since then been reviewed in depth in order to eliminate such problems in the future. Further development is going on. A so-called quadrupole vacuum chamber has been NEG coated. Such chambers have a large transverse dimension due to the pumping port and cannot be inserted into a solenoid of reasonable size. Consequently, an array of permanent magnets rather than a solenoid was used to create the field necessary for the magnetron sputtering.