EuCARD >> News >> Newsletters >> Issue 10 >> Article 3


To Infinity and beyond

Tasked with verifying the fabrication of CLIC components, "Infinity" is a new precision measuring machine that is now part of the CERN Metrology Service. The EuCARD editor got an exclusive glimpse at this remarkable instrument.

With plastic slippers over our shoes, we were led into a specially installed room of the CERN Metrology Service, temperature controlled, double glazed and gleaming white.

There stood Infinity, a sleek machine of granite base, steel gantry and ceramic measuring axis. With a click of the computer mouse, it whizzed into action selecting a probe head from a bank of possible choices. We heard a faint "click" as the machine readjusted to take the weight of the probe head, before advancing smoothly towards the component to be measured.

Co-financed by CERN and the CLIC (Compact LInear Collider) collaboration, and made by Leitz, Infinity is an ultra high accuracy "coordinate measuring machine" (CMM). It is one of only seven machines of its type in the world. It weighs in at 7.5 tonnes and is mounted on special absorbers to filter vibrations (it would lose precision if placed directly on the ground).

Four sensors positioned around the machine measure ambient temperature, which needs to be kept at a constant 20 °C during measurement taking. Temperature variations do not exceed 0.2 °C per hour, 0.4 °C per day and 0.1 °C per metre. When the machine is in action, the "whirr" of the ventillators stops and the technician exits to look on through a window as Infinity gets to work.

CLIC precision



Infinity is the name of the new, ultra-precise, measuring machine now operating at the CERN Metrology Service to measure CLIC components. Image and thumbnail image on main page courtesy of Maximilien Brice, CERN.

The CLIC study is for a future electron-positron compact linear collider that explores energy regions beyond the capabilities of today's particle accelerators. Through a world-wide linear collider collaboration, the CLIC study aims at a nominal center-of-mass energy of 3 TeV for electron-positron collisions.

This results in extremely demanding requirements for the accelerating structures in terms of the accelerating gradient (100 MV/m or higher), high-power (of the order of 100 MW), strong higher-order mode damping (complex geometries) and tight mechanical tolerance (the shape accuracy of the CLIC accelerating structure iris is 5 μm).  

The tight mechanical tolerances that CLIC requires were not measurable with the existing measuring equipment, which had much larger tolerances and heavier force measuring probes (see images below), hence the need for Infinity.


                 
Whenever a component is measured, the equipment leaves behind a tiny, circular mark on the component's surface. The above images show two components, magnified using the same objective lens (20 x). The left-hand image shows the mark left behind by older measuring equipment. The mark has a depth of 4.7 μm (about 1/20 the width of a human hair*). For CLIC, this surface damage is unacceptable. The right-hand image shows the mark left behind by Infinity, with a depth of 80 nm (around 600 times smaller), which is acceptable for CLIC.
Images courtesy of Didier Glaude, CERN.

How Infinity works

Infinity has a measuring range of 1.2 m x 1 m x 0.7 m and can measure components as heavy as 1 tonne. It has a precision of 0.3 micrometres (μm), around 300 times smaller than the width of a human hair*, and a Maximum Permissible Error (MPEE) of 0.3 μm + L/1000 μm, where L is the length in mm.

The images below show how Infinity uses probes to measure a component. The probe exerts a tiny force on the component (from 0.02 N to 0.16 N) and takes a series of measurements to then enable a 3D plot. As CLIC is at prototype stage, each component is different and set-up can take around 3 days before measuring, which can take about an hour.

The team are currently collaborating with the manufacturer on a new measurement head to lower the measuring force to from 0.005 N to 0.10 N. Tests of this new head will take place this autumn.


 
Left: Infinity uses a series of probes to measure a component. In this image a right-angle probe is used, while another vertical probe rests on the stand to the right of the image. The machine can be programmed to switch between probes during the measuring process.
Centre: Close-up of the probe tip as it approaches the component.
Right: Once Infinity has measured a component, a computer simulation can be generated to show miniscule deviations from theoretical measurements. The rainbow scale is from dark blue (a deviation of +0.0016 mm) through green (no deviation) to red (-0.0016 mm). Images courtesy of Maximilien Brice, CERN.

Infinity and EuCARD

Within the EuCARD project, task 9.2 concerns the Normal Conducting High Gradient Radio Frequency (RF) Cavities of the CLIC Test Facility 3 (CTF3). CTF3 has been constructed to demonstrate feasibility of a multi-TeV linear collider based on CLIC technology.

EuCARD research complements ongoing efforts and concentrates primarily on integration i.e. satisfying simultaneously the requirements of highest possible gradient, power handling, tight mechanical tolerances and heavy High Order Mode (HOM) damping.

Within EuCARD are explorations into how alignment errors influence wakefields and HOM damping, and how assembly of CLIC accelerating and power extraction structures can satisfy the 10 micrometre precision requirement of positioning both radial and longitudinal, taking into account dynamical effects present during accelerator operation. EuCARD researchers, like other members of the CLIC collaboration, are using the measurements from Infinity to improve their investigations.

For now the machine is measuring CLIC components, but who knows for the future. You could say the possibilities are infinite! 

- Kate Kahle, CERN, EuCARD-DCO (WP2).

[*Average width of human hair is 99 µm, range is from 18 µm to 180 µm].

Please contact the EuCARD editor with news, events, achievements, images and ideas that you would like added to the public website, newsletter or Intranet.