CERN Accelerating science

Improving access to FEL facilities through the CompactLight project

Group photo from CompactLight's Midterm Review Meeting. (Image: CompactLight)

 

The H2020 design study CompactLight gathered in Helsinki for its Midterm Review Meeting, a full review of the project held halfway through its duration, at the end of the project’s Month 18. CompactLight is an international collaboration that brings together world-leading experts in the fields of accelerator technologies and FEL radiation, with the goal of designing a new generation of Free-Electron Laser (FEL) facilities. These aim to be more compact, more power-efficient, and cheaper in construction and operation, compared to present-day facilities, based on conventional technologies. FELs are among the most powerful tools for investigating matter, but have been chronically overbooked by the users, due to the small number of existing facilities owing to their huge costs. By making FEL facilities cheaper, and thus affordable for institutions with less financial resources, the project aims to contribute to the larger diffusion of these light sources.

The key concept of the project is to combine cutting-edge technology for each different component of a FEL light source into a single machine. One of technologies used by CompactLight is the high-gradient normal conductive Radio-Frequency (RF) acceleration in the X band, developed at CERN within the context of the Compact Linear Collider (CLIC) study, as well as innovative undulator technologies developed by leading institutes like KIT and STFC.

A group of 24 academic and industrial partners bring the necessary expertise to fulfil the project’s goals. The collaboration, coordinated by Elettra Sincrotrone Trieste, involves eleven public European research institutes (Elettra, CERN, STFC, IASA, INFN, ENEA, ALBA-CELLS, CNRS, KIT, PSI, CSIC), eight European universities (Uppsala, Ankara-IAT, Lanchester, Eindhoven, Roma-Sapienza, Helsinki-HIP, Free University of Amsterdam, Strathclyde), two European industrial partners (Kyma and VDL), and three extra-European partners (SINAP, University of Melbourne and ANSTO-AS).

The successful meeting in Helsinki provided clear evidence of the large progress in defining the machine parameters and of the advancements made by the partners in the design of each single subsystem, since the Annual Meeting in Barcelona in December 2018. Options for a very compact beam injector in the S-, C- and X- band of frequencies have been presented and discussed. The definition of a standardised unit for the X-band linac is progressing, a component that could even be used as a stand-alone element for smaller projects, for instance, a university-scale Compton source, or smaller FELs for special applications that can be constructed and operated with smaller budgets. New concept undulators like superconductive undulators and cryogenic permanent-magnet undulators, as well as exotic schemes, like microwave undulators, are also under investigation.

A session dedicated to industrial partners, rarely occurring at such an early stage of a design study, demonstrated the great interest the project is gathering. World-leading industries attended the conference: CPI, Canon (ex-Toshiba), Thales, Scandinova, and Jema displayed their latest developments and carefully studied the new directions suggested by the CompactLight collaboration. Accelerator physicists and industrial partners gathered to exchange experiences, technology requirements, and the new machine parameters, to identify where industry must look at in the future.

The large demand for high-quality X-rays sources and the technological developments pushed by CompactLight make the project particularly appealing, not just for the accelerator community and for the numerous X-rays users, but also for European and worldwide industries. The industry session demonstrated that, despite its young age, CompactLight is already showing an impact in the field today. The success of CompactLight, determined by the promising results obtained so far, will affirm X-band technology as a new standard for accelerator-based facilities and advance undulators to the next generation of compact photon sources. A new generation of compact X-band-based accelerators and light sources is approaching, across and beyond Europe.

The next meeting will be held in Istanbul in January 2020. At the time, the collaboration is expected to demonstrate further progress and a concrete idea of the final CompactLight design.

Panagiotis Charitos (CERN)
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Matthew Streeter (Imperial College London)
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Towards single-cycle attosecond light from accelerators

Header image (Figure 1): Technological breakthroughs in light generation and selected applications enabled by qualitatively new light source capabilities (for image sources see below*).

Methods for generating light pulses that are much shorter and brighter than currently available has been set out in a new paper by an international collaboration of accelerator scientists [1]. Figure 1 shows how throughout history the breakthroughs in light generation have revolutionized our ability to study smaller and smaller objects, from microcrystals to viruses and even to individual atoms. The discovery of X-rays at the end of the 19th century enabled diffraction imaging at the atomic scale while, with the invention of synchrotrons in the 1940’s, the photon flux became sufficient to capture the diffraction patterns of nanocrystals in the 1970’s. The time resolution at the atomic scale from these X-ray sources was, however, severely lacking.  Meanwhile, the progress in conventional laser technology, born in the 1960’s, together with the invention of chirped pulse amplification (CPA) developed in the 1980’s, now enables lasers to generate sufficient intensity to drive High-Harmonic Generation (HHG) in gases which can output attosecond duration pulses of light. However, while the femtosecond barrier was broken by laser technology and HHG, the spatial and temporal resolution potentially offered by accelerator-based X-ray sources, remains beyond their reach.

The Free-Electron Laser (FEL) is a cutting-edge, accelerator-based instrument that has the potential to provide simultaneous access to the spatial and temporal resolution of the atomic world. In a FEL, ultra-short electron bunches from an accelerator are passed through a long undulator magnet to generate coherent light. Recently, scientists from SLAC demonstrated the first generation of attosecond hard X-ray pulses, using the Linac Coherent Light Source. Now, as described in the review article by Alan Mak et al. [1], researchers are proposing developments that will make the FEL a fully coherent, single-cycle (attosecond) X-ray laser. The new concepts build upon a strong nexus between linear accelerators, FELs and quantum lasers, to produce extreme attosecond pulses with controllable waveforms.


Figure 2: The left plot in the panel (a) shows the minimum pulse duration attained over time with various demonstrated (blue) and potential (red) technologies while the right plot of the same panel depicts typical temporal waveforms. For simplicity, the period of the carrier is taken to be the same. The panel (b) presents the state-of-the-art of the pulse energy achievable by short-pulse light sources. The HHG and novel undulator concepts deliver few-cycle light pulses whereas the FEL sources shown in the figure deliver light pulses of significantly more than a few cycles. Adapted from [1].

The need for the development of a new attosecond technology is motivated by the diminishing progress in the generation of short pulses with conventional lasers, as depicted in Fig. 2a. The combination of CPA and HHG in gas allowed laser technology to break the femtosecond barrier in the 2000’s, but the initial rapid progress in pulse duration reduction has since levelled off. Another issue with conventional lasers, is that the pulse energies, critical in attosecond science, decrease rapidly as the pulses get shorter as shown in Fig. 2b. On the other hand, the attosecond regime is shown in simulations to be accessible via methods based on coherent radiation from undulators [1]. Moreover, it is seen that the pulse energy of undulator-based attosecond sources may exceed the pulse energy of the equivalent conventional laser sources by three orders of magnitude for the same pulse duration. The sub 50-attosecond, high-energy light pulse generation predicted from undulator-based technology, can therefore open up and provide access to the uncharted territory of the fastest time scales in atoms. 
 


Figure 3: Sudden radiation damage upon ionization in bio-relevant molecules and in DNA, in particular, is related to the electron-hole dynamics occurring on the sub-femtosecond scale. 

An important scientific application of such intense attosecond pulses is the study of electron flow from one region of a molecule to another, so called charge migration, which is a fundamental process in biology. As illustrated in Fig. 3, intense radiation can induce charge migration that leads to DNA and cell damage. Detailed investigations of the mechanism of charge migration are essential for understanding the processes that result in biological malfunction. Such knowledge can be obtained using the high temporal and spatial resolution offered by the proposed developments in undulator attosecond technology.

The authors of the review article have recently expanded their collaboration to form the LUSIA consortium (Towards Attosecond SIngle-cycle Undulator Light). The objectives of the consortium are to shift the paradigm of FEL pulses from long multi-cycle output, towards tailored single-cycle pulses. They aim to conduct proof-of-principle experiments leading to the development of a new enabling technology for attosecond science.


References

  • [1] Alan Mak et al “Attosecond single-cycle undulator light: a review.” Reports on Progress in Physics 82 (2019) 025901

 

* Copyright of images used in Figure 1 from left to right.

Top row:

1. Wikipedia article “Laser,” credits to David Monniaux - Kastler-Brossel Laboratory at Paris VI: Pierre et Marie Curie; 
2. Wikipedia article “Synchrotron,” credits to EPSIM 3D/JF Santarelli, Synchrotron Soleill
3. The Eurpean XFEL: https://www.xfel.eu/facility/overview/index_eng.html
4. Own artistic work
5. Own artistic work

Bottom row:

1. Google Commons.
2. Janos Hajdu “Diffraction before destruction,” talk at the Nobel Symposium on Free Electron Laser Research, 2015.
3. https://lcls.slac.stanford.edu/multimedia, “LCLS: The Linac Coherent Light Source at SLAC.”
4. N. Saito et al. "Attosecond streaking measurement of extreme ultraviolet pulses using a long-wavelength electric field." Scientific reports 6 (2016): 35594. Licensed under a Creative Commons Attribution 4.0 International License.
5. Wikipedia Commons: category “atomic orbitals.”

Marco Zanetti (INFN & Univ. Padua), Frank Zimmermann (CERN)
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Different techniques of emittance measurements for SLS and FELs

The Topical Workshop on Emittance Measurements for Synchrotron Light Sources and FELs held at ALBA-CELLS in January 2018 brought together experts working on emittance measurements for electron machines, like Synchrotron Light Sources (SLS) and Free Electron Lasers (FEL). The workshop presented the status of the different techniques and discussed the challenges that this community is facing for the next generation of ultra-low emittance machines.

The emittance itself is not a direct measurable parameter and most accelerators monitor its surrogate: the electron beam size. In the case of SLS, the preferred techniques to infer the beam size are based on the analysis of the synchrotron radiation due to its non-destructive nature. In order to overcome the diffraction limit, direct imaging techniques like pinhole cameras or Compound Refractive Lenses use the x-ray part of the synchrotron radiation to avoid the diffraction limit. These techniques were reviewed and compared by L. Bobb (Diamond) and F. Ewald (ESRF), and it was concluded that they can be used down to the ~4µm level.

Other SLS like KEK or Max-IV use techniques based on the analysis of the synchrotron light coherence, like the double-slit interferometry presented by T. Mitsuhashi (KEK), or the polarization methods shown by A. Andersson (Max-IV). These techniques analyze the visible part of the synchrotron light, but in order to measure beam sizes below the ~2µm level, the system should be adapted to smaller wavelengths, like ultra-violet or even x-rays. This was the topic of A. Snigirev's (IKBF) presentation, highlighting the challenges of performing diffraction or interferometry in the x-rays part.

On the other hand, the beam sizes in FELs are measured through the interaction with obstacles in the electron beam trajectory, which often detrimentally affect the electron beam. Such is the case of Optical Transition Radiation (OTR) screens, which can be used for direct beam imaging or to produce Diffraction Radiation Interferometry, as shown by E. Chiadroni (INFN). Other obstacles like Wire Scanners (reviewed by K. Wittenburg from DESY) are nowadays getting thinner (down to the 1µm level) using lithography and electroplating, which improves the method resolution, thus allowing to measure beam sizes down to the 500nm level (S. Borrelli, SLS).  This level of resolution can also be achieved by using a laser wire, where the electron beam does not interact with a solid (metallic) object, but a “light pencil”, as shown by P. Karataek (JAI). However, this solution involves an important team of experts to maintain and operate the technique.

To measure the ultra-low emittance of the future SLS and FELs requires significant knowhow, while a new machine may need to combine different techniques, as presented by B. Yang (ANL) for the APS upgrade, whose design of the emittance measurement system already combines an x-ray pinhole camera, a diffractometer, and an x-ray interferometry. Perhaps this type of designs are the new path for the emittance measurement techniques. Finally, the workshop set forth new ideas like the beam size measurements using Heterodyne Speckle Fields or Cherenkov radiation, as well as a review of the methods used in other accelerators like hadron colliders or laser plasma accelerators.

The diagnostics experts on emittance measurements for SLS and FELs will strongly benefit from all the synergies between the different communities.

 

Header image: Group picture from the Topical Workshop on Emittance Measurements for SLS and FELs (Image credit: ALBA-CELLS)

Chris Edmonds (University of Liverpool)
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