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A Study on Particle Accelerator Components Metrology and Alignment to the Nanometre scale

Final Report Summary - PACMAN (A Study on Particle Accelerator Components Metrology and Alignment to the Nanometre scale)

Project name: PACMAN Project No: 606839
Start date: 01/09/2013 Duration: 48 months

PACMAN is a study on Particle Accelerator Components’ Metrology and Alignment to the Nanometre scale. It is an Innovative Doctoral Program, hosted by CERN, providing training to 10 Early Stage Researchers (ESRs) working towards a PhD thesis. 17 partners: 9 companies and 8 universities are part of the PACMAN network.

Context and motivation
The recent discovery of the Higgs particle has put in evidence the enormous discovery potential of accelerators and in particular colliders. To keep up with the expectations of the high energy physics community, the next generation of particle colliders ought to produce a very large number of collisions at very high energy. In practice this means that two beams of transversal dimensions in the micron range have to meet each other over a shooting length of tens of meters. The size of the beam will also affect the number of final collisions and in order to prevent the beam from blowing-up, it has to follow a very well defined path in the machine. This translates into very tight tolerances concerning the position of the components focusing, accelerating or detecting the beam all along the accelerator. These objects are large and heavy, weighting sometimes more than 100 kg and measuring 1 m in length or more. They shall be aligned to within a few microns over a distance of several hundreds of metres. Similar kinds of problems are faced when aligning the parts of a large telescope which needs to produce a sharp image from far away stars where the accuracy required in positioning the optical elements is less than 1/1000 mm over a hundred metres.
The most challenging collider project up to now is the so-called compact linear collider (CLIC) in which collaborate more than 30 international institutions in close relationship with CERN. The baseline alignment strategy for CLIC reflects the current state of the art and consists of a succession of independent steps on which each element central axis is referenced to external objects called fiducials in the environment of a coordinate measuring machine (CMM). All elements are then mounted and aligned in a common support using those fiducials and, at last, the support itself is positioned inside the accelerator. These operations are costly, lengthy and generate systematic and random errors at each step.
Technical objectives
The EU-funded network PACMAN aims to improve the alignment accuracy of the CLIC components by developing new alignment methods and tools that can be applied simultaneously to all different accelerator components. By sharing a common reference system materialized by a stretched wire, the different components like magnets, beam position monitors, accelerating structures and even the fiducials can be positioned around a common axis in one single step and always in a CMM environment which guarantees sub-micron accuracy. To this end, ten early stage researchers are concentrating on the different aspects required namely:
• finding the axis of the element with micron precision by using electromagnetic methods in magnets, BPM and accelerating structures (ESR2.1 ESR2.2 ESR4.1 ESR4.2);
• measuring relative positions of the axes represented by the wire and the alignment targets by non-contact methods and respecting the accuracy using a CMM, Frequency Scanning Interferometry (FSI) or micro-triangulation (ESR1.1 ESR1.2 and ESR1.2);
• moving and detecting a movement in the nanometre scale (ESR3.3 ESR4.1)
• characterizing the environment during the measurements using a seismic sensor compatible with magnetic fields (ESR 3.2)
• keeping the total error budget under control and evaluating the final uncertainty of the final measurement (ESR 3.1)
The methods and tools developed will be integrated in a single automatic bench in order to prove their compatibility and feasibility.

Figure 1 (please see attachment: Figure 1: PACMAN subjects in pictures)

Training objectives
PACMAN is an ITN network which main motivation is the training of young researchers. As such, it offers the ESRs a multi-disciplinary project based on topics like metrology, micro-wave technologies, electromagnetism, nano-technologies, vibrational motion, or precision mechanics. The training program is undertaken at CERN as well as in the university partners. The essential on-the-job training is also reinforced by secondments in the industrial partners of the network. Researchers have the opportunity to learn from visiting scientists, attend workshops and conferences and exchange with a large community of professionals. Training is also provided on transferable skills like project management, writing and presenting or entrepreneurship. A special emphasis is also given to intellectual property and knowledge transfer especially from academia to business.
Work performed since the beginning of the project and main results achieved so far
The PACMAN project started in September 2013. During the first six months of the project, the management and outreach work packages were most active preparing the kick-off meeting, the first web page and contents, the recruitment of the researchers, securing the material budget needed for the next four years and drafting the consortium with the rest of the partners. All researcher were hired and started their appointment from February to September 2014.
Once all the members of the team were at CERN, the training work package prepared a series of courses in general aspects of metrology, team building, making presentations, communication or French. Each ESR researched and studied the state of the art in their respective subjects. Researchers were in close contact with their hosting universities and joined their doctoral cursus. The first PACMAN workshop took place at the end of this period. It offered the possibility to the students to present their subject, the analysis of the state of the art and their research study proposal in front of experts in their field.
After the first year, each researcher started working on its individual set-ups for validating their measurement principles and expected accuracy through simulations and measurements. The full team agreed on the choice of a common wire (a Copper Beryllium wire with a diameter of 0.1 mm), which was characterized by ESR1.1 and evaluated by the other researchers for feasibility. In order to reach the major milestone of the project: the measurements on the Final PACMAN Alignment Bench (FPAB), dedicated meetings of preparation and integration of all the systems developed started in May 2015. The FPAB is the assembly of a quadrupole and a 15 GHz BPM. It is used to validate the methods developed to find the reference axes of the components using a stretched wire, and the methods to determine the position of this wire w.r.t. external alignment targets.
The FPAB was ready in June 2016, where compatibility tests between the systems took place successfully. At the same time, the second PACMAN workshop was organized in Debrecen (HU). The students had the opportunity to present the results of the methods developed on individual set-up in front of experts.
Three slots of measurements on the FPAB were organized in the metrology lab: the 1st one in July 2016 to study the repeatability of measuring methods, the 2nd one in November 2016, integrating the FSI system, and the third one in March 2017, just after the 3rd PACMAN workshop. This final workshop was a very nice forum to present the results, to evaluate the remaining tests for the 3rd slot of measurements in metrology lab and to extrapolate the results on other projects.
Here is a summary of the main results achieved in the project:
- The magnetic axis of a quadrupole, the electric center of a 15 GHz BPM and the electro-magnetic center of an Accelerating Structure from the CLIC project is found within a sub-micrometric repeatability, using a Coper Beryllium wire (ESR2.1 ESR4.1 and ESR4.2).
- The absolute position of both axes w.r.t external alignment targets (fiducialisation) is measured at an accuracy below 5 µm using CMM and FSI measurements, below 10 µm using micro-triangulation. Both FSI and micro-triangulation can replace CMM to perform portable micrometric measurements of components (ESR1.1 ESR1.2 and ESR1.3).
- The position of quadrupole and BPM reference axes is known within an uncertainty of measurement below 5 µm (ESR2.1 ESR4.1).
- The resolution of the 15 GHz BPM was demonstrated to be below 12 nm using a stretched wire instead of a beam (ESR4.1).
- An uncertainty budgeting methodology has been proposed, capable of determining accurately the alignment measurement uncertainty for the thermal compensation (ESR3.1).
- A specific seismic sensor integrating 4 different types of sub-nanometre transducers was designed, assembled and validated. The multi-pass Michelson interferometer demonstrated a resolution of 6.5 pm, 4 times better than what is required for PACMAN (ESR3.2).
- Two versions of miniaturized PCB rotating coils for magnetic measurements were manufactured and tested (among which a 26 layers PCB: the smallest ever manufactured). The results were successful as the sensors showed high performance (ESR2.2).
- The nano-positioning system supporting the quadrupole/BPM assembly was upgraded. It was demonstrated that it can control the assembly according to 4 degrees of freedom, by decentralized strategy, including 0.25 nm stepping achievable in vertical direction (ESR3.3).
- A high-precision contactless sensor to be plugged on the CMM head in order to measure the wire has been designed and is under assembly.
Lectures from world-known experts have been organized to put the work of PACMAN researchers in perspective against an international effort on metrology and alignment (R. Ruland, J. di Marco, W. Bich). PACMAN technical achievements have been presented in international conferences. A knowledge transfer and intellectual property workshop was held in October 2015 in which experts in partner institutions and outsiders alike discussed different concepts and approaches to intellectual property. PACMAN researchers followed the workshop with a practical session on examples from how to generate intellectual property.
The first tangible asset generated by the PACMAN network is the solid connections tied between the consortium partners. The common interest of all partners in the technologies at stake as well as the deeper knowledge of each other through the researchers and dedicated meetings has generated further collaborations aside from PACMAN.
For the young researchers, the network has offered the possibility to meet the most relevant professional on their field. Some have already collaborated with laboratories outside PACMAN enlarging their professional network and increasing considerably their visibility and employability.
Besides training ten researchers in key technologies around precision, metrology or microwaves, the technologies that PACMAN is working on have direct application on almost any large scale technological project requiring very precise alignment of their individual components. We have mentioned large telescopes in our motivation but similar techniques are also applicable to microscopy, telecommunications, aerospace as well as other accelerators like light sources or medical applications.
At last, the impact in society is more difficult to measure at this stage of the project. Efforts have been made to promote science and more particularly women in science. A program of outreach activities took place in schools. PACMAN female researchers were involved in various outreach activities like “expand your horizons” or “shadowing a scientist” to motivate high school girls to follow careers in science. Members of the PACMAN network regularly communicated to the public through public science events, local newspapers or promotional videos.

ETHICS: CERN confirms that the scientific programme has not given rise to ethical issues during the project lifetime.

Figure 2 (please see attachment: Figure 2:students picture)