On the button

Published 26th November 2015

The third contract in the procurement phase of the EBS project has been signed, for 1500 ultra-high vacuum Beam Position Monitor (BPM) buttons. These buttons will equip a total of 320 BPMs in the storage ring and constitute the most essential diagnostics system: they measure, at 320 locations around the ring, the position of the electron beam in both the horizontal and vertical planes, with respect to the centre of the vacuum chamber. In other words, they determine the ‘orbit’ of the electron beam. They then send the information to an orbit correction system, which keeps the orbit on track and highly stable. This assures that the x-ray beam produced for user experiments will also benefit from this high positional stability.

A ‘button’, is in fact a pick-up sensor welded into a stainless steel block, which is itself welded to the vacuum chamber. The button receives a small RF signal from the wall currents in the vacuum pipe. The amplitude of the 352MHz RF signal is inversely proportional to the distance between the button and the beam. By having four buttons precisely distributed on a stainless steel block it is possible to calculate the horizontal and vertical coordinates of the beam position, with respect to the centre of the BPM block. 

The button itself is in the UHV chamber, while the signal is transmitted on the ‘air’ side via coaxial cables to the signal acquisition electronics. The vacuum tightness and its reliability over a very long time-scale (more than 10 years) are absolutely crucial and are ensured by a ceramic disc in the tube.

The design and the choice of the materials of these new buttons have been very much inspired by the existing buttons in today’s storage ring. However, a thorough new design was needed, since the dimensions had to be reduced by roughly a factor two.

The newly designed BPM button prototype (right) is a feat of precision engineering and is roughly half the size of its predecessor to be able to fit into the tighter constraints of the new machine. Diagnostics draftsman Nicolas Benoist helped with the design of the buttons and holds the current (left) and new (right) prototype buttons. Behind him are the models of the two buttons. The top part shows the standard coaxial SMA connector (blue) that is in air, while the button at the lower end (dark red) is in the UHV vacuum chamber. The ceramic disc (yellow) is only 2.5mm thick and provides the vacuum tightness that is an essential requirement.

Prototypes of the new design of button made with different materials were delivered in spring 2014 by two companies and tested on the accelerator (on ID25 of the storage ring) for compliance with tight mechanical specifications, and also included measurements and assessments of reliability and stability under different beam and environmental conditions. These tests showed the buttons gave stable readings of the beam position and satisfactory behaviour in general.

A call for tender was launched in June 2015 and the contract for 900 buttons of 8mm diameter and 600 buttons of 6mm diameterwas signed in October. Delivery is expected in three batches from March to April 2016, followed immediately by site acceptance tests. Eventually the buttons will be installed on the accelerator and will help to keep the new Extremely Brilliant beam exactly where it should be!

In-vacuum: The buttons as seen from inside the vacuum aperture on the ID25 test chamber