The conversion of the CALIFES beamline of CTF3 into the “CERN Linear Electron Accelerator for Research” (CLEAR) facility was approved in December 2016. The primary focus for CLEAR is general accelerator R&D and component studies for existing and possible future accelerator applications. This includes studies for high gradient acceleration methods, e.g. for CLIC and plasma technology, and prototyping and validation of accelerator components, e.g. for the HL-LHC upgrade. The facility also provides irradiation test capabilities for characterisation of electronic components and for medical applications. A description of the facility with details on the achievable beam parameters, and the status and plans are presented.

/pdf/the-clear-user-facility-at-cern-44xi6nwdei.pdf

The CLEAR user facility at CERN

The compact linear accelerator for research and applications (CLARA) is an ultrabright electron beam test facility being developed at STFC Daresbury Laboratory. The ultimate aim of CLARA is to test advanced free electron laser (FEL) schemes that can later be implemented on existing and future short-wavelength FELs. In addition, CLARA is a unique facility to provide a high-quality electron beam to test novel concepts and ideas in a wide range of disciplines and to function as a technology demonstrator for a future United Kingdom x-ray FEL facility. CLARA is being built in three phases; the first phase, or front end (FE), comprises an S-band rf photoinjector, a linac, and an S-bend merging with the existing versatile electron linear accelerator beam line; the second phase will complete the acceleration to full beam energy of 250 MeV and also incorporate a separate beam line for use of electrons at 250 MeV; and the third phase will include the FEL section. The CLARA FE was commissioned during 2018, and the facility was later made available for user experiments. Significant advancements have been made in developing high-level software and a simulation framework for start-to-end simulations. The high-level software has been successfully used for unmanned rf conditioning and for characterization of the electron beam. This paper describes the design of the CLARA FE, performance of technical systems, high-level software developments, preliminary results of measured beam parameters, and plans for improvements and upgrades. © 2020 authors. Published by the American Physical Society.

Design, specifications, and first beam measurements of the compact linear accelerator for research and applications front end

Understanding the effects of rf breakdown in high-gradient accelerator structures on the accelerated beam is an extremely relevant aspect in the development of the Compact Linear Collider (CLIC) and is one of the main issues addressed at the Two-beam Test Stand at the CLIC Test Facility 3 at CERN During a rf breakdown high currents are generated causing parasitic magnetic fields that interact with the accelerated beam affecting its orbit The beam energy is also affected because the power is partly reflected and partly absorbed thus reducing the available energy to accelerate the beam We discuss here measurements of such effects observed on an electron beam accelerated in a CLIC prototype structure Measurements of the trajectory of bunch trains on a nanosecond time scale showed fast changes in correspondence of breakdown that we compare with measurements of the relative beam spots on a scintillating screen We identify different breakdown scenarios for which we offer an explanation based also on measurements of the power at the input and output ports of the accelerator structure Finally we present the distribution of the magnitude of the observed changes in the beam position and we discuss its correlation with rf power and breakdown location in the accelerator structure

/pdf/effects-of-rf-breakdown-on-the-beam-in-the-compact-linear-34nlxy2387.pdf

Effects of rf breakdown on the beam in the Compact Linear Collider prototype accelerator structure

The CLIC/CTF3 collaboration is studying the feasibility of a multi-TeV electron-positron collider, the so-called CLIC: Compact LInear Collider. The idea is to use a two-beam acceleration scheme at an RF frequency around 12 GHz. In this scheme, the RF power to accelerate the main beam is produced by a high current drive beam. To demonstrate the feasibility of this scheme, the CTF3 facility (CLIC Test Facility 3) has been constructed at CERN. Recently, the complex to generate the drive beam has been successfully commissioned producing a beam with a current 28 A. This beam is now being used to test the power production and main beam acceleration according to the two-beam scheme. The results of the test facility provide vital input for the CLIC conceptual design report that is presently being prepared.

/pdf/the-clic-feasibility-demonstration-in-ctf3-36rz1cg2r6.pdf

The CLIC feasibility demonstration in CTF3

The thesis presents phase stability studies for the Compact Linear Collider (CLIC) and focuses in particular on CLIC Drive Beam longitudinal phase stabilisation. This topic constitutes one of the main feasibility challenges for CLIC construction and is an essential component of the current CLIC stabilisation campaign. The studies are divided into two large interrelated sections: the simulation studies for the CLIC Drive Beam stability, and measurements, data analysis and simulations of the CLIC Test Facility (CTF3) Drive Beam phase errors. A dedicated software tool has been developed for a step-by-step analysis of the error propagation through the CLIC Drive Beam. It uses realistic RF potential and beam loading amplitude functions for the Drive and Main Beam accelerating structures, complete models of the recombination scheme and compressor chicane as well as of further CLIC Drive Beam modules. The tool has been tested extensively and its functionality has been verified. The phase error propagation at CLIC has been analysed and the critical phase error frequencies have been identified. The impact of planned error filtering and stabilisation systems for the Drive Beam bunch charge and longitudinal phase has been simulated and the optimal specifications for these systems, such as bandwidth and latency time, have been calculated and discussed. It has been found that a realistic feed-forward system could sufficiently reduce the longitudinal phase error of the Drive Beam, hence verifying that a satisfactory CLIC luminosity recovery system is possible to develop. Alternative designs of the Drive Beam accelerator, the recombination scheme and the phase signal distribution system have also been analysed. Measurements of the CTF3 Drive Beam phase have been performed. The source of the phase and energy errors at CTF3 has been determined. The performance of the phase feed-forward system prototype for CTF3 has been simulated. The prototype's specifications have been defined so that it will provide a sufficient test of the feed-forward correction principle. The prototype based on the defined specifications is currently in development and is to be installed at CTF3 in the second half of 2013.

CLIC drive beam phase stabilisation

The Compact Linear Collider (CLIC) study proposes a multi-TeV, high luminosity, electron-positron linear collider in order to fulfill the current need for a lepton collider. The study has been started in the late 80s at CERN and currently is a joint effort with a collaboration of 40 institutes. An innovative scheme of high peak RF power production for the high accelerating gradient has been proposed for CLIC. The so called "two-beam scheme" consists of two beams that are running parallel to each other. One of the beams is to be accelerated for the collision experiments and called "the main beam". The second beam of the CLIC scheme is "the drive beam" and will be employed for the power production. The quality of the main beam acceleration depends on the stability of the power that is generated by the drive beam. Therefore, the optimization of the drive beam production with the proper time structure and within the required beam dynamics tolerances is one of the most important accelerator physics aspects of the project. Currently in the conceptual level, the baseline design of the drive beam injector consists of a thermionic gun. This electron source has to be combined with a sub-harmonic bunching system in order to provide the required time structure of the drive beam. However, a big disadvantage of this scheme is the parasitic satellite bunches that are produced due to the sub-harmonic bunching system. PHIN photoinjector has been raised as another option in order to replace the existing thermionic gun of CLIC test facility (CTF3) and to form the bases of a source for the CLIC drive beam. The PHIN project is in the framework of the European CARE (Coordinated Accelerator Research in Europe) program.

http://iopscience.iop.org/article/10.1088/1742-6596/347/1/012036/pdf

PHIN photo-injector as the CLIC drive beam source

The probe beam Linac, CALIFES, of the CLIC Test Facility (CTF3) has been developed by CEA Saclay, LAL Orsay and CERN to deliver trains of short bunches (0.75 ps) spaced by 0.667 ns at an energy around 170 MeV with a charge of 0.6 nC to the TBTS (Two-beam Test Stand) intended to test the high gradient CLIC 12 GHz accelerating structures. Based on 3 former LEP Injector Linac (LIL) accelerating structures and on a newly developed RF photo-injector, the whole accelerator is powered with a single 3 GHz klystron delivering pulses of 45 MW during 5.5 μs to a RF pulse compression cavity and a network of waveguides, splitters, phase-shifters and an attenuator. We relate here results collected during the various commissioning and operation periods which gave stable beam characteristics delivered to the TBTS with performances close to nominal. Progress has been made in the laser system to improve the beam charge and stability, in the space charge compensation to optimize the emittance, in RF pulse shape for energy and energy spread. The installation of a specially developed RF power phase shifter for the first accelerating structure used in velocity bunching allows the control of the bunch length.

/pdf/ctf3-probe-beam-linac-commissioning-and-operations-qgarz28hrq.pdf

CTF3 probe beam LINAC commissioning and operations

The CLEX building in the CTF3 facility is the place where essential experiments are performed to validate the Two-Beam Acceleration scheme upon which the CLIC project relies. The Drive Beam enters the CLEX hall after being recombined in the Delay loop and the Combiner Ring in intense beam trains of 24 A - 120 MeV lasting 140 ns and bunched at 12 GHz, although other beam parameters are also accessible. This beam is then decelerated in dedicated structures installed in the Test Beam Line (TBL) and in the Two-Beam Test Stand (TBTS) aimed at delivering bursts of 12 GHz RF power. In the TBTS this power is used to generate a high accelerating gradient of 100 MV/m in specially designed accelerating structures. To assess the performances of these structures a probe beam is used, produced by a second Linac. We report here various experiments conducted in the TBTS making use of the versatility of the probe beam and of dedicated diagnostics.

Two Beam Test Stand Experiments in the CTF3 Facility

A photoinjector, PHIN (PHotoINjector), has been realized at CERN by a joint effort of several institutes within the European Coordinated Accelerator Research in Europe program. The test facility has been installed and commissioned at CERN with the aim to demonstrate the beam parameters needed for the CLIC Test Facility 3 (CTF3). This beam is unique with respect to its long bunch train and high average charge per bunch requirements. The nominal beam for CTF3 consists of 1908 bunches each having a 2.33 nC charge and a bunch frequency of 1.5 GHz. Thus, a total charge of similar to 4.4 mu C has to be extracted and accelerated. The stability of the intensity and the beam parameters along this exceptionally high average current train is crucial for the correct functioning of the CLIC drive beam scheme. Consequently, extensive time-resolved measurements of the transverse and longitudinal beam parameters have been developed, optimized, and performed. The shot-to-shot intensity stability has been studied in detail for the electron and the laser beams, simultaneously. The PHIN photoinjector has been commissioned between 2008 and 2010 during intermittent operations. This paper reports on the obtained results in order to demonstrate the feasibility and the stability of the required beam parameters.

Production of long bunch trains with 4.5 μ C total charge using a photoinjector

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Papers

PHIN photo-injector as the CLIC drive beam source

CTF3 probe beam LINAC commissioning and operations

Two Beam Test Stand Experiments in the CTF3 Facility

Production of long bunch trains with 4.5 μ C total charge using a photoinjector