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

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 large electro-magnetic fields are generated and produce parasitic magnetic fields which interact with the accelerated beam affecting its orbit and energy. 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 which 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 orbit and we discuss its correlation with RF power and breakdown location in the accelerator structure.Comment: 10 pages, 8 figures. We replace the previous version of the article with this one, in which we extend our discussion on the experimental set-up and on the interpretation of our measurements, on the basis of the inputs received during the review process for publication on Phys. Rev. Special Topics - Accelerators and Beams. The essence of our conclusions remain unchange

CLIC 2008 Parameters

This note presents the CLIC parameter set as of beginning 2008 and describes the different subsystems,pointing out how the design of the different components is driven. This design emerged from an updated understanding of limitations for normal conducting accelerating structures, which led to a new optimised design for the CLIC 12 GHz accelerating structure. The structure parameters and improvements in other sub-systems have resulted in a major revision of the parameters. The overall layout and efficiencies for CLIC with this updated parameter-set are presented

Studies of vacuum discharges in the CLIC accelerating structure

The Compact Linear Collider (CLIC), is one of the proposed successors to the Large Hadron Collider (LHC) accelerator at the European Organization for Nuclear Research. (CERN). In CLIC, particles are accelerated by very strong electric fields. Unfortunately, large electric fields may lead to vacuum discharges, which in turn can affect the particle beam by disrupting the flow of energy inside the accelerating structure and deteriorate the performance of the accelerator. Studies of the physics of vacuum discharges and its effect on the beam are crucial for the realization of the CLIC accelerator. The objective of this master thesis is to improve the knowledge of how vacuum discharges occur and what can be done to prevent them. The project was done together with the CLIC group at Uppsala University. They have their diagnostic equipment, Uppsala/CLIC X-band spectrometer (UCXS), at the High Gradient Test Stand for research on RF-structures X-Box2, where also the accelerating structure is located. The spectrometer consists of a collimator, a dipole and a fluorescent screen, which receives images from electrons that come from discharges. These images have been analysed, together with Radio Frequency (RF)-signal data from X-Box2, to find discharge positions inside the accelerating structure. The achieved results are consistent with previous analyses used with the help of the RF signals. We see that there is a possibility to use images to study the geometrical shape of the discharges. Longitudinal positioning of the discharges with the help of images, cannot be done. While longitudinal positioning cannot be done, transversal have had success and can be found. Images have also been seen with multiple features. It is hard to accredit one image to a single breakdown and a hypothesis is that there are more than one breakdown during a pulse, creating multiple features

CLIC Wake Field Monitor as a detuned Cavity Beam Position Monitor: Explanation of center offset between TE and TM channels in the TD26 structure

The Wake Field Monitor (WFM) system installed on the CLIC prototype accelerating structure in CERN Linear Accelerator for Research (CLEAR) has two channels for each horizontal/vertical plane, operating at different frequencies. When moving the beam relative to the aperture of the structure, a disagreement is observed between the center position of the structure as measured with the two channels in each plane. This is a challenge for the planned use of WFMs in the Compact Linear Collider (CLIC), where they will be used to measure the center offset between the accelerating structures and the beam. Through a mixture of simulations and measurements, we have discovered a potential mechanism for this, which is discussed along with implications for improving position resolution near the structure center, and the possibility determination of the sign of the beam offset.Comment: 16 pages, 20 figure

Measurements of wakefields and bunch length with beam in linear electron accelerators: a case study at the CLEAR facility.

The thesis focuses on the study of two phenomena, both connected to transverse electromagnetic forces, affecting the particles inside an accelerator. The first study regards the undesired transverse kick given to the particles by the so-called wakefields, electromagnetic fields generated by the interaction of the particles themselves with the surrounding environment (e.g., the vacuum beam pipe of an accelerator, or the inner copper surface of an accelerating structure). In the thesis, a novel method for assessing the transverse kick induced by wakefields is proposed. This method is used to study the wakefields excited by the beam in the latest prototype of the high gradient accelerating structure developed for future compact accelerators, in the framework of the CLIC study at CERN. The experimental results are compared with simulations, and the comparison shows a good agreement. The results clearly indicate that the dominant effect in this case is given by the short-range part of the wakefield. The investigation explored also the often neglected dependence of the wakefield kick on the length of the particle bunch. The second study also regards the interaction of a particle bunch with a transverse electromagnetic field, but in this case the field is intentionally applied in order to perform a measurement of the bunch length. Firstly, an overview of the standard measurement technique is presented. In this layout two novel measurement methods that allows to obtain additional information on the bunch properties are presented. Such extension allows in particular to estimate the energy chirp and energy spread and to obtain information about the correlations between particle position, angular divergences, and energy. Later, an improved layout for performing the measurement is presented. Such layout does not alter the conventional measurement properties and, at the same time, gives the flexibility to directly measure the correlations between particle position, divergences, and energy. Besides, using the improved layout it is possible to enhance the method's metrological performance improving the resolution and the uncertainty. A detailed study of these two topics is presented, by comparing the result obtained with the two layouts. All derived theoretical models were benchmarked with simulation and experimental measures performed on the CLEAR linear electron accelerator at CERN. A satisfying agreement was found in all cases

Critical Issues in Linear Colliders

Linear colliders (LC) on the energy 0.5-1 TeV are considered as the next step in the particle physics. High acceleration gradients, small beam sizes, precision tolerances, beam collision effects are main problems for linear colliders. In this paper we discuss physics motivation, parameters and status of current LC projects, e+e-, gamma-gamma and gamma-electron modes of operation, physical limitations on the energy and luminosity. Present technologies allow to reach energies about 5 TeV with adequate luminosities. Advanced technique based on plasma and laser method of acceleration can provide much higher accelerating gradients, however, perspectives of these methods for high energy colliders are still under big question. Linear colliders with energies above 10 TeV are hard for any acceleration technology. Speculations on possibility of PeV linear colliders based on ponderomotive laser acceleration are just not serious and contain several mistakes on conceptual level. It is shown that due to radiation in the transverse laser field, methods of acceleration based on laser bunch ``pressure'' do not work at high energies.Comment: 16 pages, Latex, 1 figure, Talk at Workshop on Quantum Aspects of Beam Physics and Other Critical Issues of Beams in Physics and Astrophysics, January 7-11, 2003, Hiroshima University, Higashi-Hiroshima, Japan. To be published by World Scientifi

Transverse deflecting cavities

Transverse deflecting cavities are used for a number of applications in modern accelerators. In this paper we discuss the fields of these cavities, some of their applications, and some important aspects of their design.Comment: 11 pages, contribution to the CAS - CERN Accelerator School: Specialised Course on RF for Accelerators; 8 - 17 Jun 2010, Ebeltoft, Denmar

A primary electron beam facility at CERN -- eSPS Conceptual design report

The design of a primary electron beam facility at CERN is described. The study has been carried out within the framework of the wider Physics Beyond Colliders study. It re-enables the Super Proton Synchrotron (SPS) as an electron accelerator, and leverages the development invested in Compact Linear Collider (CLIC) technology for its injector and as an accelerator research and development infrastructure. The facility would be relevant for several of the key priorities in the 2020 update of the European Strategy for Particle Physics, such as an electron-positron Higgs factory, accelerator R\&D, dark sector physics, and neutrino physics. In addition, it could serve experiments in nuclear physics. The electron beam delivered by this facility would provide access to light dark matter production significantly beyond the targets predicted by a thermal dark matter origin, and for natures of dark matter particles that are not accessible by direct detection experiments. It would also enable electro-nuclear measurements crucial for precise modelling the energy dependence of neutrino-nucleus interactions, which is needed to precisely measure neutrino oscillations as a function of energy. The implementation of the facility is the natural next step in the development of X-band high-gradient acceleration technology, a key technology for compact and cost-effective electron/positron linacs. It would also become the only facility with multi-GeV drive bunches and truly independent electron witness bunches for plasma wakefield acceleration. A second phase capable to deliver positron witness bunches would make it a complete facility for plasma wakefield collider studies. [...

A Study of the Beam Physics in the CLIC Drive Beam Decelerator

CLIC is a study for a Multi-TeV e+e- linear collider, in which the rf power for the main linacs is extracted from 100 ampere electron drive beams, by the use of specially designed power extraction structures. Up to 90% of the beam energy is extracted from the drive beams along one kilometer long decelerator sectors, rendering the beam transport challenging. We have identified two major challenges for robust beam transport: the significant transverse wakes in the power extraction structures, and the large energy spread induced by the power extraction process. By beam dynamics studies we have qualified power extraction structure designs, leading to the present CLIC baseline structure in which the transverse wakes are sufficiently mitigated. We have further shown that the beam energy spread induced by the deceleration implies that standard 1-to-1 correction might not ensure satisfactory drive beam transport. As alternative, we propose a decelerator orbit correction scheme based on dispersion-free steering and exploiting the structure beam loading. By simulation the proposed scheme shows excellent performance, assuming sufficient beam position monitor resolution. We have performed successful demonstrations of similar orbit correction schemes in the linac of the CLIC Test Facility 3. The results of the beam dynamics studies have lead to specifications for decelerator instrumentation and magnets, described in detail in this w ork. The first prototype of the baseline power extraction structure has been tested with beam in the Two-beam Test Stand in the CLIC Experiment Area, where a field recirculator has been installed to boost power production. We have derived formulae for rf power production and voltage, including a simple model describing power extraction with recirculation. The model has been applied to the first Two-beam Test Stand experimental result. We compare the measured rf power, phase and energy loss with reconstructed signals based on beam intensity measurements, and a good agreement between the measurements and the reconstruction is shown