CERN Accelerating science

First experimental results from the CLEAR facility at CERN

The plasma lens experiment after being fully installed in the CLEAR beamline (Credit: Wilfrid Farabolini -

The continuous development of high gradient technologies (e.g. X-band, THz radiation, plasma acceleration) makes compact linear electron accelerators attractive for many applications, e.g.such as photon sources (Free Electron Lasers and Inverse Compton), medical application, and components irradiation studies.

Linear accelerators are also the only viable solution for electron-positron colliders at the high-energy frontier. In this case, high gradient technologies allow for cost optimisation and/or for maximizing the energy reach of such machines.

The CERN Linear Electron Accelerator for Research (CLEAR) facility at CERN was set up to expand the testing capabilities of those ideas and technologies and to provide on top the possibility to perform direct measurement with beam of machine components and training of young scientists.

The new CLEAR facility at CERN started its operation in fall 2017. CLEAR results from the conversion of the probe beam line of the former CLIC Test Facility (CTF3) into a new testbed for general accelerator R&D and component studies for existing and possible future accelerator applications, such as X-band structures, plasma and THz technology, nm- and fs-resolution beam instrumentation, sub-ps bunches production, but also for investigating possible use of electron beams for medical purposes or electrical component sensitivity to radiation.

The hardware modifications implemented in 2017 to the existing infrastructure allowed to provide stable and reliable electron beams with energies between 60 and 220 MeV in single or multi bunch configuration at 1.5 GHz.

CLEAR inherited not only the equipment, but also the experience of from operating the previous CTF3 facility: the first beam was set up in August 2017 and, after only a few weeks of commissioning, users could take the first beams to perform experiments in September.

The first CLEAR beam was used for the continuation of the irradiation tests performed on the Very energetic Electron facility for Space Planetary Exploration missions in harsh Radiative environments (VESPER), which was set up at the end of the CALIFES beamline already during the CTF3 era.

VESPER was initially set up to characterise electronic components for the operation in a Jovian environment – as foreseen in the JUpiter Icy Moon Explorer mission (JUICE) of ESA, in which trapped electrons of energies up to several hundred MeVs are present with very large fluxes.

Initial measurements showed the first experimental evidence of electron-induced single event upsets (SEU) on electronic components, pointing to the necessity of extending such an investigation to different electron energies. The CLEAR flexibility allowed to continue the study, showing a dependency of SEU cross-section with energy. Instead, no dependency was observed on radiation flux, suggesting that such components do not suffer from prompt dose effects.

A wider range of devices has also been tested, showing a strong dependency on the device process technology. Preliminary test on a set of memories sensitive to latch-up, a type of short circuit which disrupts the proper functioning of the memory, has also shown that electrons can cause destructive events. Further tests on 16 nm FinFET technology devices were performed by ESA and their contractors IROC in March 2018 and the data are now being analysed.

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The CLEAR beam line seen by the final dump. (Credit: Davide Gamba -

The scope of VESPER was extended to dosimetry for medical applications.

Recent advances in compact high-gradient accelerator technology, largely prompted by the CLIC study, renewed the interest in using very-high energy electrons (VHEE) in the 50 – 250 MeV energy range for radiotherapy of deep-seated tumours.

Understanding the dosimetry of such beams is essential in order to assess their viability for treatment. For this reason a group from the University of Manchester carried out studies in the VESPER installation on energy deposition using a set of EBT3 Gafchromic films submerged in water. The measured dose deposition profile was in agreement with Monte Carlo tracking simulations within 5%. At the same time, the possible aberration of crossing in-homogenous bodies was investigated by measuring the longitudinal dose profiles with and without inserts of various density material. The results confirmed the expectation from simulations that electron beam are relatively unaffected by both high-density and low-density media.

The obtained results indicated that VHEE has the potential to be a reliable mode of radiotherapy for treating tumors also in highly inhomogeneous and mobile regions such as lungs.

Further studies on the dose distribution of a converging beam as opposed to a parallel wide beam, and possibly on multi-angle irradiation are planned for the future.

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View of the VESPER test stand set up for irradiation of electronics tests. (Credit: Davide Gamba -

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Water phantom being installed on the VESPER test stand by Agnese Lagzda – Manchester University. (Credit: Kyrre Ness Sjobaek -

CLEAR opened the possibility of exploring also new accelerator technologies, one being active plasma lenses which are a promising technology for strongly focusing particle beams. Their compact size is a plus for potential use in novel accelerators. However, transverse field uniformity and beam excitation of plasma wake-fields may turn out to be significant limitations.

Lead by the University of Oslo, a collaboration between CERN, DESY and Oxford University was set up to develop a novel low-cost, scalable plasma lens. The developed setup consists of a 1 mm diameter, 15 mm long sapphire capillary installed in the middle of a 20x20x20 cm3 aluminum vacuum chamber. The capillary is filled with He or Ar at a controllable pressure. The gas is ionized by a 500 A peak current discharge with a duration of up to a few hundred ns, provided by a 20 kV spark-gap compact Marx bank generator. The longitudinal discharge current is responsible as well for the transverse focusing force in both transverse planes.

The experimental set-up was installed in the CLEAR beamline in September 2017 and after a fast commissioning it was possible to show a clear focusing effect. Extensive measurements were taken during December 2017 and March 2018. Transverse position scans of a pencil beam revealed gradients as high as 350 T/m, which would be compatible with its use for a staged plasma accelerator. More studies are now being conducted for measuring the uniformity of the field and beam emittance preservation also employing different gas species.

Moreover, evidence of non-linear self-focusing at relatively high bunch charge (∼50 pC/bunch) was observed when the beam goes throw the plasma after the discharge. This opens another branch of possible studies on passive plasma lenses that will be further developed.

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Overview of the CLEAR plasma lens setup. The actual plasma lens, a 1 mm diameter, 15 mm long sapphire capillary, is installed inside the cubic vacuum chamber. (Credit: Kyrre Ness Sjobaek -

Another technology being explored at CLEAR is the possibility of producing terahertz radiation (1 THz corresponds to 4 meV photon energy, or 300 µm radiation wavelength). This technology has a strong impact in many areas of research, spanning the quantum control of materials, plasmonics, and tunable optical devices based on Dirac-electron systems to technological applications such as medical imaging and security.

The aim at CLEAR is to characterize a LINAC-based THz source, exploiting relativistic electron bunches which emit coherent radiation in the THz domain. For such a source sub-picosecond electron bunches are needed. This triggered a study and optimisation of the CLEAR injector in collaboration with the “Laboratoire de l'Accélérateur Linéaire” (LAL), thanks to which sub-ps bunches down to 200 fs rms have been demonstrated in the machine, paving the way to the THz radiation generation.

The current studies at CLEAR are focused on the production of (sub-)THz radiation by Coherent Transition Radiation (CTR), i.e. making the electron beams passing through thin metal foils and collecting the emitted radiation. With this technique, a peak power of about 1 MW at 0.3 THz have been measured, in agreement with theoretical expectations.

Further experimental tests have been started for producing THz radiation by Coherent Smith-Purcell Radiation (CSPR) targets, where the electron beam passes nearby a periodic structure, emitting radiation at harmonics of its period.

At the same time, investigations are ongoing for producing THz radiation using the Coherent Cherenkov Radiation (CCR) mechanism.

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Set up for studies on CTR, CSPR and CCR installed in CLEAR. (Credit: Alessandro Curcio -

CLEAR allows to continue the R&D for CLIC technologies, for example by measuring the resolution of CLIC cavity Beam Position Monitor prototypes and by verifying the behavior of the Wake Field Monitor installed on the present design of the CLIC accelerating structures.

Additionally, CLEAR serves as unique opportunity for fast verification of beam instrumentation, e.g. it was possible to perform first calibration of the scintillator screen used in the electron spectrometer of the AWAKE experiment.

Finally, CLEAR offers also a unique playground for young accelerator physics. During march 2018 part of the students from the Joint Universities Accelerator School (JUAS) had the opportunity of spending one day at the facility performing hands on experiments.

Interview by Romain Muller (CERN)
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An interview with Anders Unnervik, Head of the Procurement and Industrial Services Group at CERN

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More bang from your beam: reimagining X-ray conversion

Team HADDPHI, from Hungary, represented by Balazs Ujvari, Berta Korcsmaros, David Baranyai and Balazs Gyongyosi, worked on the challenge of making x-ray conversion more effective. (Image: CERN)

The X-rays used for radiation therapy, medical imaging and many other applications are produced by slamming an electron beam into a piece of metal. Most of the energy from the beam goes into heat, and the X-rays that are released have a wide energy spectrum. Only a small fraction of the X-rays actually end up being useful, e.g. for irradiating a tumour.

The challenge is therefore to make X-ray conversion more efficient, reducing the size and cost of medical accelerators used for radiation therapy. If the spectrum could be made narrower, then it would allow better targeting of tumours with less radiation delivered to healthy tissues. Narrow-band X-ray sources also have applications in medical imaging, non-destructive testing and security.

To solve this challenge and a series of others CERN organised a Hackathon for students and young professionals in April 2018 targeted to the medical field. RadiaBeam set the challenge of reimagining X-ray conversion and the selected team named HADDPHI (for Hardware Development Debrecen PHysics Institute) was given access to relevant CERN technologies.

Why is a hackathon organised by CERN focusing on MedTech? Early activities at CERN relating to medical applications date back to the 1970s. In light of the significant growth in these activities, in 2017, CERN published a formal medical applications strategy. The Medtech:Hack was then initiated to explore new ways of developing viable applications in the field.

So coming back to the RadiaBeam challenge: how to make X-ray conversion more efficient, reducing the size and cost of medical accelerators used for radiation therapy?

Before joining the Medtech:Hack, the HADDPHI team saw a presentation about Crystal Clear collaboration and were very impressed how High Energy Physics (HEP) can support medical applications. Though their exposure to the accelerators’ community was limited, they seized the opportunity to be exposed to more experts through the hack process.

From the CERN technologies, they selected the GEANT4 based on their past experience starting with simulations seemed a good idea. Furthermore, GEANT4 can be used to simulate one setup and its reliability has been proven in the field of particle physics. Finding the optimal configuration amongst millions needs the development of a framework that can manage this search. This could be performed thanks to the flexibility of the GEANT4 software.

So what is the solution the HADDPHI came up with?

Using GEANT to find the best parameters for an existing or paperboard X-ray devices for radiation therapy. It is key to find the optimal configuration amongst 100 to 200 parameters to render the X-ray spectrum as sharp as possible for X-ray therapy. They also investigated how to use the Timepix detector to monitor the beam created and therefore identify the corresponding delivered treatment to the patients.

When asked about their experience during the Hackathon, Berta Korcsmaros one of the team members stated: “We are electric engineers and physicists, we've never thought about business plan. Talking about how to bring a solution to the market was very new and very exciting to us. The Hackathon support team helped us a lot with making a good presentation. As well although we knew what CERN was doing, it was such a great experience to see how the Research and Development is done in real life.”

Indeed Timepix and GEANT4 experts explained to the team how to optimise the parameters to improve the Timepix detector and to use GEANT4 for the dedicated simulation to be performed. Also the Challenge Owner, Salime Boucher the CEO of RadiaBeam helped the team with benchmarking their proposed solution against state-of-the-art devices and this is him who introduced the team to the GEANT4 simulations. Moreover, for him, “The HADDPHI team came up with an interesting idea that I had not thought of, which was the use of the Timepix detector for realtime diagnosis of the beam energy. Currently, there is no direct way of measuring electron beam energy in medical linear accelerators. Instead we use indirect methods, such as the depth-dose profile of the resulting X-rays in water.”

With regards to the next steps, the team still has simulation work to do to find the optimal configuration for the accelerator generating the X-ray beam. In parallel, there will be the need to validate the simulation through detection measurements of the beam.

As Salime outlined: “At RadiaBeam we are in constant production of linear accelerators for medical and industrial applications. It would be quite easy for us to try out new X-ray converter geometries on one of our existing accelerators, so an experiment could be accomplished in just a few months. However, we probably would want to iterate between experiment and simulations a few times. ”

An interesting challenge ahead that will look familiar to any entrepreneurs trying to bring innovation out there!



Nicholas Sammut (University of Malta)
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25th edition of Joint Universities Accelerator School
13 Mar 2018

25th edition of Joint Universities Accelerator School

Twenty-five years of training accelerator scientists and going from strength to strength

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25th edition of Joint Universities Accelerator School

 Invented at the turn of the 20th century, at the same time as modern physics was reinventing the concept of the particle, particle accelerators developed as the workhorses of nuclear and particle physics to become the largest scientific instruments ever built by man. Today, they also constitute essential tools for the study of condensed matter and biomolecules, and find numerous societal applications in medical diagnostics and treatment, the polymer and electronic component industries, public security and food and health product safety.

The science and the technology of accelerators are specific domains of physics and engineering in their own right. They must be taught as such, along with their latest developments, to the future designers, builders and operators of these strange machines.  

This is precisely what the Joint Universities Accelerator School (JUAS) has been doing each year since 1994 at ESI-Archamps.  Two specialised 5-week courses are proposed to Master and Doctoral students, as well as young professionals from industry or research centres. The courses are delivered by a faculty comprising some 50 experts from academia, research facilities and industries active in the field. The curriculum is overseen by the Advisory Board in which JUAS’ 16 partner universities are represented. Both courses are concluded by exams enabling partner universities to attribute ECTS and/or doctoral credits to their participating students.  

In all, more than 1000 students have been trained at JUAS since its creation.

Frédérick Bordry, CERN Director of Accelerators and Technology celebrating the 25th edition of JUAS at ESI in February  (Image: ESI-Archamps)

JUAS employs an innovative pedagogical approach, with a unique mix of lectures, tutorials, seminars, group workshops, laboratory visits and practical sessions. The latter include for some students the opportunity to take part in machine development sessions on real accelerators in operation – this year on the synchrotrons of ESRF in Grenoble and on the linear accelerator CLEAR at CERN.  Students also spend two days at the Paul Scherrer Institut near Zürich and a full day at Bergoz Instrumentation. In the words of Jacinta Yap, a PhD student at the University of Liverpool who attended JUAS 2017, “JUAS has been a really great opportunity to learn all about accelerators in a condensed amount of time. I think the biggest take-away for me is that I’m at the beginning of my PhD and so it’s really great to learn about these fundamentals so early on.”

Practical sessions at CERN (Image: ESI-Archamps)

Also to be mentioned is the involvement of JUAS in the production of a MOOC on accelerators, in the framework of the “Training, Communication and Outreach” work package of the H2020 project ARIES.

JUAS is synonymous with diversity: a stimulating mix of physicists and engineers, students and more experienced scientists coming from some 20 countries in Europe, Asia and America. It is worth noting that one third of the 2018 students in Course 1 are female. Such diversity creates exciting opportunities for international and intercultural exchange, and prepares the students for flexible career paths in an increasingly globalised world.

The success of JUAS is in great part due to its intrinsic voluntary nature. This is apparent in the way academic institutions, laboratories and industrial companies allow their staff to teach at JUAS, grant access to their premises and equipment and provide financial support. Likewise the personal commitment of all those involved in running the School. It is only through this voluntary action that we can maintain high standards of teaching while keeping fees to a minimum. In this respect, the true value of JUAS greatly exceeds its financial budget. Our heartfelt thanks to all our partners.

Philippe Lebrun, JUAS Director celebrating the 25th edition of the school with Hermann Schmickler, Director of the CERN Accelerator School and Louis Rinolfi, former JUAS Director (Image: ESI-Archamp)

Long live JUAS … for at least the next 25 years!

Header imageStudents and faculty from JUAS 2018’s second module on the technology and applications of particle accelerators at ESI-Archamps (France) (Image: ESI-Archamps)

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