The CERN Linear Electron Accelerator for Research (CLEAR) has now finished its second year of operation, providing a testbed for new accelerator technologies and a versatile radiation source. Hosting a varied experimental program, this beamline provides a flexible test facility for users both internal and external to CERN, as well as being an excellent accelerator physics training ground. The energy can be varied between 60 and 220 MeV, bunch length between 1 and 4 ps, bunch charge in the range 10 pC to 2 nC, and number of bunches in the range 1 to 200, at a repetition rate of 0.8 to 10 Hz. The status of the facility with an overview of the recent experimental results is presented.

/pdf/status-of-the-clear-electron-beam-user-facility-at-cern-3n91svwavc.pdf

Status of the CLEAR Electron Beam User Facility at CERN

In July 2016, the first X-band test facility operating with two interwoven RF pulses produced from 6 MW klystron was commissioned at CERN. Outputting up to 49 MW after pulse compression [1], the new test stand allows testing of two structures concurrently with a combined repetition rate of up to 400 Hz. Commissioning of two of the four lines has been completed and testing of high gradient accelerating structures for the Compact Linear Collider has commenced. Operations have been ongoing for more than a year, where dedicated control algorithms have been developed to conditioning the structure and to keep the pulse compressors tuned. Significant progress has been made in understanding the conditioning of two structures that are sharing an interlock and vacuum system. The higher pulse repetition rate has demonstrated a significantly reduction in the time required to condition accelerating structures.

High Power and High Repetition Rate X-band Power Source Using Multiple Klystrons

ii Abstract (in Finnish) iiiin Finnish) iii Abstract (in Swedish) ivin Swedish) iv

Vacuum arc localization in CLIC prototype radio frequency accelerating structures

In order to build the Compact LInear Collider (CLIC), accelerating structures reaching extremely high accelerating gradients are needed. Such structures have been built and tested using normal-conducting copper, powered by X-band RF power and reaching gradients of 100 MV/m and above. One phenomenon that must be avoided in order to reliably reach such gradients, is vacuum arcs or “breakdowns”. This can be accomplished by carefully designing the structure geometry such that high surface fields and large local power flows are avoided. The research presented in this thesis presents a method for optimizing the geometry of accelerating structures so that these breakdowns are made less likely, allowing the structure to operate reliably at high gradients. This was done primarilly based on a phenomenological scaling model, which predicted the maximum gradient as a function of the break down rate, pulse length, and field distribution in the structure. The model is written in such a way that it allows direct comparison of different criteria, i.e. the peak electric field, the peak local power flow or “modified Poynting vector” Sc, and the global power flow per iris circumference. Using this method, a set of highly optimized accelerating cells were created, as was a C++ library capable of estimating the performance of an RF structure based on these accelerating cells. This library was used in the rebaselinging of the CLIC machine. In addition to this, the thesis also presents a particle in cell simulation of the initial stages of a vacuum arc. This model follows the development of the arc from a small field emitter into a multi-ampere discharge, tracking the evolution of the plasma and the circuit, as well as the interaction between the plasma and the surface. From this model, our understanding of the importance of the ion bombardment under the plasma sheath was improved, especially the ion energy distribution and its effect on sputtering vs. the effective sputtering yields required for the arc to sustain itself and grow.

Avoiding vacuum arcs in high gradient normal conducting RF structures

Dogan, Mustafa (Dogus Author) -- Conference full title: 5th International Particle Accelerator Conference, IPAC 2014; International Congress Center DresdenDresden; Germany; 15 June 2014 through 20 June 2014.

Conceptual design of an X-FEL facility using CLIC X-band accelerating structure

A new klystron based X-band rf power source operating at 11994 GHz has been installed and started to be commissioned at CERN in collaboration with CEA Saclay and SLAC for CLIC accelerating structure tests The system comprises a solid state high voltage modulator, an XL5 klystron developed by SLAC, a cavity based SLED type pulse compressor, the necessary low level rf system including rf diagnostics and interlocks and the surrounding vacuum, cooling and controls infrastructure The system is designed to produce up to 50 MW rf pulses of 1500 ns pulse width and 50 Hz repetition rate After pulse compression, up to 100 MW of rf power at 250 ns pulse width will be available in the structure test bunker This paper describes in more detail this setup and the process of commissioning which is necessary to arrive at the design performance

/pdf/commissioning-of-the-first-klystron-based-x-band-power-nofr6gdjc6.pdf

Commissioning of the first klystron-based x-band power source at cern

The objective of this work is the design of the optical bench for the RIIFS (Rugged Imaging Infrared Fourier Spectrometer) project, a miniaturized FTS developed for future planetary missions. The considered design requirements allow the usage of the proposed design for general space missions. Two different design solutions are presented and compared as the result of a combination between topology optimization and parametric design. Mass minimization and high stiffnesses are the design drivers for the two case studies whose ranking is based on quasi-static and modal finite element analyses.

RIIFS spectrometer optical bench design

This paper summarizes the design implementation and validation of the measurement setup for the thermal characterization of the thermally insulating mountings of the MAJIS instrument. The challenges of the measurement derive from the low value of the conductance to be measured, in the range of few mW/K, the wide temperature range to be investigated, from 100 to 300 K, the need of operating under vacuum. The developed configuration exploits the guarded plate configuration to reduce the uncertainty on the parasitic heat fluxes, multilayer insulators to minimize the radiative heat exchange, a liquid nitrogen loop for the cooling of the heat sink. The setup has been built and validated with tests on a specimen to assess the repeatability of the measurement. With the characterization of different specimens from the same production batch the setup has allowed also assessing the repeatability of the manufacturing process.

Thermal conductance measurement of the MAJIS insulating mountings down to cryogenic temperature

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Commissioning of the first klystron-based x-band power source at cern

RIIFS spectrometer optical bench design

Thermal conductance measurement of the MAJIS insulating mountings down to cryogenic temperature