High-Gradient issues in S-band RF Acceleration Structure for Hadrontherapy accelerators and Radio Frequency Quadrupoles

La investigación de estructuras de microondas de alto gradiente en aceleradores lineales es un tema clave en instalaciones de física de altas energías, y en particular en sistemas de física médica basados en hadronterapia. Esta Tesis Doctoral se centra en el estudio de los problemas que surgen al emplear sistemas de alto gradiente en estructuras aceleradoras de radiofrecuencia no superconductoras. Uno de los efectos que aparecen durante la operación de alta potencia en estos sistemas son los arcos de vacío de RF, llamados breakdown. Esto ocurre debido a los altos campos electromagnéticos de RF que aparecen en las estructuras aceleradoras. Tales eventos tienen un impacto en el rendimiento de la estructura y en la calidad del sistema. Los breakdown generan daños en la superficie que pueden desintonizar la frecuencia de resonancia de la estructura, y en consecuencia cambiar el avance de fase entre las cavidades aceleradoras, lo cual es importante para sincronizar la estructura con el haz. El logro de un alto gradiente en las estructuras de un acelerador lineal es un tema fundamental clave en una instalación aceleradora de hadronterapia, por lo que debe ser estudiado con detalle. Tesis incluye una visión general de los aceleradores de partículas. Se presenta una introducción general de la teoría de RF y los conceptos básicos relacionados con aceleradores lineales. Se dan algunos ejemplos de los diseños de estructuras de aceleración, cuyo análisis cuyo análisis se llevó a cabo. Los principales parámetros y los problemas relacionados con el breakdown también se discuten en detalle. También se describen las ventajas que ofrece la tecnología de alto gradiente para aplicaciones médicas, junto con un resumen de las actividades actuales en el desarrollo de aceleradores lineales de terapia con hadrones. Se describen con detalle los experimentos de alta potencia en un prototipo de 3 GHz para la terapia de hadrones que se han llevado a cabo en el CERN Sbox. Las pruebas se han realizado con éxito, y se ha determinado el límite máximo de operación de un acelerador lineal en términos de los campos máximos que se pueden alcanzar. Además del acondicionamiento, se han realizado otras mediciones. Las técnicas de análisis de datos utilizadas para la localización de breakdown en esta estructura se presentan en detalle en comparación con los resultados del post-procesamiento. También se realiza una investigación estadística de los eventos de ruptura para comprender los procesos subyacentes de estos eventos. Los datos recopilados se compararon con los resultados de los prototipos de RF de la banda X de CERN-CLIC para estimar el rendimiento de la estructura. La descripción de la configuración de alta potencia, y las actualizaciones también se describen. Se presenta un primer estudio sistemático de eventos de ruptura en sistemas RFQ. Se han estudiado dos RFQ diseñadas para diferentes frecuencias y aplicaciones. Se discuten las características de diseño de estas estructuras. También se presenta el comportamiento a largo plazo de ambas RFQ. Se discuten los principales cambios observados durante los eventos de ruptura, y se propone una técnica de identificación de la ubicación de la ruptura basada en estas observaciones. Estos estudios han servido para predecir los rendimientos de la estructura. Las principales conclusiones de esta Tesis Doctoral se presentan. El objetivo final de la investigación general es comprender el fenómeno del breakdown para maximizar el rendimiento de este tipo de estructuras, y confirmar la capacidad de usar tecnologías de alto gradiente para diversas aplicaciones, especialmente en Física Médica.The achievement of high-gradient in the linac structures is a key issue in design facility for high energy physics and other applications. This thesis is focused on the study of the high-gradient limitations in the normal-conducting radio-frequency accelerating structures. One of the effects that could appear during high power operation of the accelerating structures is the RF vacuum arcs or breakdowns. This occurs due to the high electromagnetic fields in the structure. Such events have an impact on the structure performance and the quality of the accelerated beam. Breakdowns lead to surface damage that can detune the resonant frequency of the structure and change the phase advance between cells which is important for synchronizing the structure with the beam. Therefore, this phenomenon requires a detailed study on the dedicated developed systems. A general introduction of the RF theory and the basic concepts related to linear accelerators is given in this thesis. Some examples of the accelerating structure designs are given, the analysis of which will be carried out in subsequent chapters. The main limiting quantities and the issues related to RF breakdown phenomena are also discussed in detail. The advantages that high gradient technology offers for medical application and a summary of the current activities in the development of high gradient hadron therapy linacs are also described. The study of BD phenomenon is focused in two kinds of RF structures: a 3 GHz high gradient structure and 4-vane RFQs designed for applications in hadron therapy linacs. The results of the breakdown experiments are presented together with the instruments and facilities used. The high power tests on a 3 GHz prototype of accelerating structure for hadron therapy have been successfully performed at the CERN Sbox, and the maximum operation limit of a linac in terms of maximum achievable fields and breakdown probability have been determined. The conditioning and another measurements have been performed. The data analysis techniques used for the breakdowns localization in this structure are presented in detail in comparison with post-processing results. A statistical investigation of the breakdown occurrences is also made to understand the underlying processes of these events. The data collected have been compared with CLIC X-band RF prototypes results in order to estimate the structure performance. The description of the high-power set-up and the updates are described. A first systematic study of breakdown events in RFQs is presented. Two RFQs designed for different frequencies and applications are considered. The design features and the relative merits of these structures are discussed. The long term behaviour of the RFQs is also presented. The main changes observed during breakdown events are discussed and a technique of identification of breakdown location is proposed based on these observations. The experimental work presented in this thesis is carried out in order to compare the physics of breakdowns occurring in the high-gradient accelerating structures and RFQs. These studies have been applied to determine the breakdown limitations and to predict the structure performances. The final goal of the overall breakdown research is to understand the phenomenon, to maximize the structure performance and to confirm the ability of using high-gradient technologies for various applications

H- Beam formation simulation in negative ion source for CERN's Linac4 accelerator

The caesiated surface negative ion source is the first element of CERN's LINAC4 a linear injector designed to accelerate negative hydrogen ions to 160 MeV. The IS03 ion source is operated at 35 mA beam intensity and reliably feeds CERN's accelerator chain, H- ions are generated via plasma volume and caesiated molybdenum plasma electrode surface mechanisms. Studying the beam extraction region of this H- ion source is essential for optimizing the H- production. The 3D Particle-in-cell Monte Carlo code ONIX (Orsay Negative Ion eXtraction), written to study H- beam formation processes in neutral injectors for fusion, has been adapted to single aperture accelerator H- sources. The code was modified to match the conditions of the beam formation and extraction regions of the Linac4 H- source. A set of parameters was chosen to characterize the plasma and to match the specific volume and surface production modes. Simulated results of the extraction regions are presented and benchmarked with experimental results obtained at the Linac4 test stand

High-Gradient RF laboratory at IFIC for medical applications

General interest has been shown over the last years for compact and more affordable facilities for hadron-therapy. The High-Gradient (HG) know-how and technology for normal-conducting accelerating RF (Radio-Frequency) electron linac (linear accelerator) structures recently developed for projects such as CLIC (CERN), has raised the achievable accelerating gradient from 20-30 MV/m up to 100-120 MV/m. This gain has come through a better understanding of the high-power RF vacuum arcs or breakdowns (BD) phenomena, the development of quantitative HG RF design methods and refinements in fabrication techniques. This can allow for more compact linacs also for protons, which is potentially important in the new trend in hadron-therapy of using linacs able to provide protons of 70-230 MeV or light ions of 100-400 MeV/u. Linacs are of particular interest for medical applications because they can provide a high degree of flexibility for treatment, such as running at 100-400 Hz pulse rate and pulse-to-pulse beam energy (and intensity) variations. This kind of accelerator is very well suited to treat moving organs with 4D multi-painting spot scanning technique. HG operation is limited by the BD phenomena and is characterized by the BD-Rate. New fresh structures initially operate at a reduced performance and must be conditioned through extended high-power rf operation until the maximum operational gradient is reached. This process is a time consuming, and consequently costly task (> 350 million pulses) which is important to understand and reduce. The IFIC HG-RF laboratory is designed to host a high-power and high-repetition rate facility for testing S-Band (2.9985 GHz) normal-conducting RF structures. This facility will allow the development, RF conditioning and studies of the BD phenomena in HG structures.General interest has been shown over the last years for compact and more affordable facilities for hadron-therapy. The High-Gradient (HG) know-how and technology for normal-conducting accelerating RF (Radio-Frequency) electron linac (linear accelerator) structures recently developed for projects such as CLIC (CERN), has raised the achievable accelerating gradient from 20-30 MV/m up to 100-120 MV/m. This gain has come through a better understanding of the high-power RF vacuum arcs or breakdowns (BD) phenomena, the development of quantitative HG RF design methods and refinements in fabrication techniques. This can allow for more compact linacs also for protons, which is potentially important in the new trend in hadron-therapy of using linacs able to provide protons of 70-230 MeV or light ions of 100-400 MeV/u. Linacs are of particular interest for medical applications because they can provide a high degree of flexibility for treatment, such as running at 100-400 Hz pulse rate and pulse-to-pulse beam energy (and intensity) variations. This kind of accelerator is very well suited to treat moving organs with 4D multi-painting spot scanning technique. HG operation is limited by the BD phenomena and is characterized by the BD-Rate. New fresh structures initially operate at a reduced performance and must be conditioned through extended high-power rf operation until the maximum operational gradient is reached. This process is a time consuming, and consequently costly task (> 350 million pulses) which is important to understand and reduce. The IFIC HG-RF laboratory is designed to host a high-power and high-repetition rate facility for testing S-Band (2.9985 GHz) normal-conducting RF structures. This facility will allow the development, RF conditioning and studies of the BD phenomena in HG structures

High-Power Test of Two Prototype X-band Accelerating Structures Based on SwissFEL Fabrication Technology

This article presents the design, construction, and high-power test of two $X$-band radio frequency (RF) accelerating structures built as part of a collaboration between CERN and the Paul Scherrer Institute (PSI) for the compact linear collider (CLIC) study. The structures are a modified 'tuning-free' variant of an existing CERN design and were assembled using Swiss free electron laser (SwissFEL) production methods. The purpose of the study is two-fold. The first objective is to validate the RF properties and high-power performance of the tuning-free, vacuum brazed PSI technology. The second objective is to study the structures' high-gradient behavior to provide insight into the breakdown and conditioning phenomena as they apply to high-field devices in general. Low-power RF measurements showed that the structure field profiles were close to the design values, and both structures were conditioned to accelerating gradients in excess of 100 MV/m in CERN's high-gradient test facility. Measurements performed during the second structure test suggest that the breakdown rate (BDR) scales strongly with the accelerating gradient, with the best fit being a power law relation with an exponent of 31.14. In both cases, the test results indicate that stable, high-gradient operation is possible with tuning-free, vacuum brazed structures of this kind

High Gradient Issues in S-band RF acceleration structure and Radio Frequency Quadrupoles for hadron therapy accelerators

he achievement of high-gradient in the linac structures is a key issue in design facility for the high energy physics and other applications. This thesis is focused on the study of the high-gradient limitations in the normal-conducting radio-frequency accelerating structures. One of the effects that could appear during high power operation of the accelerating structures is the RF vacuum arcs or breakdowns. This occurs due to the high electromagnetic fields in the structure. Such events have an impact on the structure performance and the quality of the accelerated beam. Therefore, this phenomenon requires a detailed study on the dedicated developed systems. A general introduction of the RF theory and the basic concepts related to linear accelerators are given in this thesis. The main limiting quantities and the issues related to RF breakdown phenomena are discussed in detail. The advantages, that HG technology offers for medical application, and a summary of the current activities in the development of HG hadron therapy linacs are also described. Afterwards, the results of the breakdown experiments are presented together with the instruments and facilities used. The high power tests on a 3 GHz prototype of accelerating structure for hadron therapy have been successfully performed at the CERN Sbox, and the maximum operation limit of a linac in terms of maximum achievable fields and breakdown probability have been determined. Apart from the conditioning, another measurements have been performed. The data analysis techniques used for the breakdowns localization in this structure are presented in detail in comparison with post-processing results. A statistical investigation of the breakdown occurrences is also made to understand the underlying processes of these events. The data collected have been compared with CLIC X-band RF prototypes results in order to estimate the structure performance. A first systematic study of breakdown events in RFQs ..

Preliminary Simulation of CERN’s Linac4 H⁻ Source Beam Formation

Linac4 is the new (H⁻) linear injector of CERN’s accelerator complex. This contribution describes the modelling activities required to get insight into H⁻ beam formation processes and their impact on beam properties. The simulation region starts from a homogeneous hydrogen plasma, the plasma then expands through the magnetic filter field. H⁻ ions and electrons are electrostatically extracted through the meniscus (line of separation between the plasma and the extracted beam) and eventually accelerated. The physics is simulated via the 3D PIC code ONIX. This code, originally dedicated to ITER’s neutral injector sources, has been modified to match single aperture sources. A new type of boundary condition is described, as well as the field distribution and geometry of the standard IS03 and a dedicated proto-type of CERN’s Linac4 H⁻ source. A plasma electrode prototype designed to provide metallic boundary conditions was produced and tested. This plasma electrode geometry enables Optical Emission Spectroscopy in the region closest to meniscus. A set of plasma parameters was chosen as input characterizing the plasma. Preliminary simulation results of beam formation region are presented

Start-to-End Beam Dynamics Simulations for PRAE

International audienceThe PRAE project (Platform for Research and Applications with Electrons) aims at creating a multidisciplinary R&D facility in the Orsay campus gathering various scientific communities involved in radiobiology, subatomic physics, instrumentation and particle accelerators around an electron accelerator delivering a high-performance beam with energy up to 70 MeV and later 140 MeV, in order to perform a series of unique measurements and future challenging R&D. In this paper we report the first start-to-end simulations from the RF gun, going through the linac and finally to the different experimental platforms. The beam dynamics simulations have been performed using a concatenation of codes. In particular for the linac the RF-Track code recently developed at CERN will be used and benchmarked. The different working points have been analysed in order to minimise the transverse emittance and the beam energy spread including space charge effects at low electron energies

Design and Construction of a High-Gradient RF Lab at IFIC-Valencia

International audienceThe IFIC High-Gradient (HG) Radio Frequency (RF) laboratory is designed to host a high-power infrastructure for testing HG S-band normal-conducting RF accelerating structures and has been under construction since 2016. The main objective of the facility is to develop HG S-band accelerating structures and to contribute to the study of HG phenomena. A particular focus is RF structures for medical hadron therapy applications. The design of the laboratory has been made through collaboration between the IFIC and the CLIC RF group at CERN. The layout is inspired by the scheme of the Xbox-3 test facility at CERN, and it has been adapted to S-band frequency. In this paper we describe the design and construction status of such a facility

Construction and Commissioning of the S-Band High-Gradient RF Laboratory at IFIC

International audienceAn S-Band High-Gradient (HG) Radio Frequency (RF) laboratory is under construction and commissioning at IFIC. The purpose of the laboratory is to perform investigations of high-gradient phenomena and to develop normal-conducting RF technology, with special focus on RF systems for hadron-therapy. The layout of the facility is derived from the scheme of the Xbox-3 test facility at CERN* and uses medium peak-power (7.5 MW) and high repetition rate (400 Hz) klystrons, whose RF output is combined to drive two testing slots to the required power. The design and construction of the various components of the system started in 2016 and has been completed. The installation and commissioning of the laboratory is progressing, with first results expected before mid 2018. The technical characteristics of the different elements of the system and the commissioning status together with preliminary results are described

High Power Conditioning of X-Band RF Components

As part of the effort to qualify CLIC accelerating struc-tures prototypes, new X-band test facilities have been built and commissioned at CERN in the last years. In this context, a number of RF components have been designed and manufactured aiming at stable operation above 50 MW peak power and several kW of average power. All of them have been tested now in the X-band facility at CERN either as part of the facility or in dedicated tests. Here, we describe shortly the main design and manufac-turing steps for each component, the testing and eventual conditioning as well as the final performance they achieved