Delving into the heart of materials

In the middle of September, the HiRadMat platform was equipped with a new test bench designed by CERN teams, which should provide a greater insight into how matter behaves when impacted by high-energy, high-intensity particle beams.


Section of the new machine where images of the sample fragments being projected upon beam impact are recorded.

The HiRadMat – High Irradiation to Materials – facility is designed to test the resistance to high-intensity beams of materials or components destined for particle accelerators. As part of the EU programme EUCARD, which finances a number of R&D projects including one to develop new materials for protecting the LHC against accidental beam losses, CERN's Mechanical and Materials Engineering (MME) Group, supported by teams from the EN, BE, TE and PH Departments, has designed a machine capable of testing six different materials in a single experiment. In total, twelve rows of up to ten 40-mm-diameter samples can be subjected to a series of high-intensity proton pulses.

"With the power of accelerators increasing, research into the behaviour of materials under extreme conditions of temperature and pressure is becoming more and more urgent," says the experiment coordinator Alessandro Bertarelli, who is also Head of the PE Section in the Engineering Department's Mechanical Design Office. "Our facility is the first to capture - simultaneously and in real time - the shock wave and the surface movement brought about by the impact of the beam and, at the same time, to record images of the sample fragments being projected upon impact." These tests will help validate the simulation models and understand the mechanisms which regulate behaviours of matter at these extreme conditions.

Part of the team working on the new HiRadMat facility.

No fewer than 244 strain gauges, 36 temperature gauges and a laser doppler vibrometer are connected to a 4 MHz fast acquisition system – 4 million measurement points per second – which are triggered at the passage of the beam. In addition, a high-speed camera operating at an image every 50 µs (20,000 frames per second) will be built into the system. It took a whole year of development to prepare such a high-performance acquisition system.

Before this complex system was inserted into the HiRadMat tunnel in mid-September, each component was individually tested and checked. Since no previous system had ever reached these acquisition rates and strain levels in such an environment, numerous tests had to be performed in order to validate the measurements of the strain gauges.

Testing will start in the first week of October. Once the test bench has been positioned in the tunnel, the samples will be bombarded by beams from the SPS with an energy of 440 GeV and an intensity varying from 1010 to 1013 particles. The data recorded in real time will give information to a precision of a few percent (less than 10%). Corrections will be made in the processing of the data and the reconstructed signal will provide even better results. Complementary tests will then be performed on the samples to measure their mechanical properties after irradiation. But it will take at least one month for the radiation levels in the experimental area to fall sufficiently for it to be safe for the samples to be handled. What secrets will the materials reveal? We await the answer with bated breath…

Old-fashioned flash bulbs get the job done!

Numerous technical problems had to be overcome by the MME team designing the test bench, notably the requirement for all the components in the experimental facility to be radiation-hard. Since the high-speed camera and the laser vibrometer are sensitive to radiation, they had to be located in a bunker 40 m from the experiment. The images were conveyed to the camera's lens by a system of mirrors and flashes. For the high-speed camera, high-luminosity flash-bulbs had to be installed above the impact zone, but the closer the impact, the closer the radiation! Modern-day flash-bulbs contain semi-conductors which cannot survive in such an environment, so the Group's engineers had to get hold of photographic flash-bulbs from the 1970s and 80s and adapt them in order to solve this problem. So you can see that CERN is definitely keeping up with fashion - vintage is "in"!


by Caroline Duc