LHC gets the ball rolling

A technique involving a small ball with a transmitter embedded inside it has been successfully tested in Sector 7-8. The ball is sent through the LHC beam pipes to check the LHC interconnections.

A game of ping-pong at the LHC? On 13 September a rather unusual test was carried out in Sector 7-8 of the accelerator. A ball just a bit smaller than a ping-pong ball was carefully introduced into one of the accelerator’s two vacuum pipes, where it travelled 800 metres in the space of a few minutes, propelled by a pumping system at the other end.

This is not just any old ball but one that contains a transmitter developed by the Beam Instrumentation group of the AB Department (AB-BI). The 40 MHz signals emitted by the transmitter are received by Beam Position Monitors (BPMs) located every 50 metres along the beam pipes, then recorded on a computer screen to allow the ball’s progress through the pipe to be monitored.

The purpose of this original experiment was to demonstrate that it is possible to monitor the interconnections of a given sector without having to open it. During the warming-up of Sector 7-8, the first to have been cooled to 1.9 Kelvin (see Bulletin No. 16-17/2007), the teams had detected a fault with one of the interconnections. One of the "plug-in" modules (PIMs) responsible for the continuity of the electrical circuit of the vacuum chamber had been damaged during the warm-up operation (see box).

The cryostats of the sector had been opened during the summer to check all the interconnections, and X-rays had revealed four other faulty modules. However, the opening and closing procedure is difficult and time-consuming: "It takes three weeks to open a sector and five weeks to close it up again", explains Sylvain Weisz of the TS-IC group. It was then that the scientists in charge of LHC installation came up with a simpler idea: "We realised that it would be quicker to insert a transmitter into the magnets and switch on the beam position monitors", continues Sylvain Weisz.

The team had the idea of blowing a ball containing a transmitter into the beam pipes. They chose a diameter of 34 mm, just a little smaller than the 36 mm beam screen, so that the ball would be stopped in its tracks by any obstacles. Its progress is monitored by the signals from the BPM every 50 metres. The absence of a signal indicates that the ball has come to a halt, at which point the team can concentrate on the small number of interconnections between the two beam position monitors concerned.

The solution seemed simple but a bit outlandish to say the least! An object can’t be inserted just like that into a vacuum pipe that has been meticulously prepared to guarantee the strict conditions of cleanliness required for an ultra-high vacuum. However, the method was so straightforward and efficient that any doubts soon evaporated. The various groups involved - AB-BI for the BPMs, AT-VAC for the vacuum, AT-MCS for the interconnections, TS-MME for the materials and TS-IC for the installation - pooled their expertise to implement it as quickly as possible.

On 13 September the teams carried out a first test, which proved conclusive, and the ball detected a sixth faulty module. They have since performed further tests to adjust the pumping and the speed of the ball.

Sector 4-5, which is currently being cooled down and will subsequently be warmed up again, will be the next to use this method to pinpoint any faulty modules. A further advantage of this clever system is the early opportunity it affords to test the beam position monitors.

The plug-in modules (PIMs) are installed at the interconnections of the vacuum chambers of the accelerator magnet assemblies. Their job is to ensure the electrical continuity along the vacuum chambers so that the "mirror" currents produced by the beam on the walls of the beam pipe can circulate freely. Any resistance (or impedance, to be more precise) would create hot points, thus reducing the intensity of the beam. The modules consist of copper "fingers" which are able to slide along a cylinder. The system allows the contraction and dilation of the machine components during cooling and warming to be accommodated. Each module expands or shrinks by about 40 millimetres but continues to ensure electrical continuity as the fingers remain in contact with the cylinder in which they are sliding. Six of the 366 modules of the arc were damaged when Sector 7-8 was warmed up, with the fingers buckling into the space reserved for the beam. The fingers had failed to slide properly when the vacuum pipes returned to their initial length during the warm-up back to ambient temperature.