Beam Secondary Shower Acquisition Design for the CERN High Accuracy Wire Scanner

Abstract

The LHC injectors upgrade (LIU) project aims to boost the LHC luminosity by doubling the beam brightness with the construction of the new LINAC4, the first linear accelerator on the LHC chain. The brighter beams require upgrades on the full injector chain to deliver low emittance beams for the future High-Luminosity LHC (H-LHC). Thus, new and more precise beam instrumentation is under development to operate on this new scenario. These upgrades include the development of a new beam wire scanners generation (LIU-BWS), interceptive beam profile monitors used for the beam emittance calculation. Wire scanners determine the transverse beam profile by crossing with a carbon wire (30 um) through the particle beam. The beam profile is inferred from the intensity of the shower of secondary particles, scattered from beam-wire interaction, and the wire position. The current BWS generation features high operational complexity and its performance is in part limited by their secondary shower detectors and acquisition systems. They are traditionally based on scintillators attached to a Photo-Multiplier tube (PMT) through optical filters. These detectors require tuning according to the beam energy and intensity prior to a measurement to not saturate the readout electronics, located on the surface buildings. Under these circumstances, many configurations lead to a poor SNR and very reduced resolution, directly affecting the measurement reliability. In addition, bunch-by-bunch profile measurements are degraded by the use of long coaxial lines, which reduce the system bandwidth leading to bunch pile-up. This thesis covers the design of an upgraded secondary shower acquisition system for the LIU-BWS. This includes the study of a novel detector technology for BWS, based on polycrystalline Chemical Vapour Deposited (pCVD) diamond, and the implementation of two acquisition system prototypes. This work reviews operational acquisition systems to identify their limitations and shows advanced particle physics simulations with FLUKA for better understanding of the secondary particles shower behaviour around the beam pipe. Simulations, along with a study of the different beams in each machine, leaded to the estimation of the required dynamics per accelerator, and an optimised placement of the upgraded detectors. To cope with the injectors working points, the acquisition systems implemented performed high dynamic range signal acquisition and digitisation in the tunnel with a radiation-hard front-end nearby the detector, digital data is afterwards transmitted to the counting room through a 4.8Gbps optical link. This novel schema not only allowed low-noise measurements, but also avoided the bandwidth restrictions imposed by long coaxial lines, and greatly simplified the scanner operation. The upgraded design investigates two approaches to cover a dynamic of about 6 orders of magnitude: a single-channel system with logarithmic encoding, and a multi-channel system with different gains per channel. Prototypes of both schemas were fully developed, characterised on laboratory and successfully tested on SPS and PSB under different operating conditions. The evaluation of the acquisition systems during beam tests allowed the study of the LIU-BWS mechanical performance and comparative the measurements with operational systems. pCVD diamond detectors, with a typical active area of 1cm2 were systematically evaluated as BWS detectors. This document analyses the results from several measurement campaigns on SPS over its energy and intensity boundaries (5e9 - 1.1e11 protons per bunch and 26 - 450 GeV). The SPS results suggest a potential application on LHC beam wire scanners.

La operación de la actual generación de beam wire scanners (BWS) en el CERN es compleja y la precisión de su medida está parcialmente limitada por sus detectores de partículas secundarias y sistemas de adquisición. Estos detectores se basan en centelladores orgánicos, filtros de densidad neutra seleccionables y tubos foto-multiplicadores. Antes de cada medida se debe acomodar su punto de trabajo del detector en función de las condiciones del haz (intensidad, tamaño estimado y energía) para evitar saturar la electrónica de adquisición, localizada en superficie. En esta situación, muchas configuraciones implican una pobre relación señal ruido (SNR) y una resolución limitada, afectando directamente a la fiabilidad de la medida del perfil transversal del haz. Además, la restricción en el ancho de banda, impuesto por los largos cables (hasta 250m) empleados para la transmisión de señal analógica, provoca el solape de los pulsos provenientes de cada bunch separados por 25ns, y compromete la medida del perfil de bunches individuales. Esta tesis cubre el diseño de nuevos sistemas de adquisición de partículas secundarias para una nueva generación de BWS en el CERN desarrollada en el marco del proyecto “LHC Injectors Upgrade” (LIU). Este trabajo revisa los sistemas de adquisición operacionales para identificar sus limitaciones y muestra avanzadas simulaciones de física de partículas para caracterizar la lluvia de secundarias alrededor del escáner en diferentes escenarios operacionales. Estas simulaciones junto con un estudio de los haces típicos empleados en cada acelerador determinan un rango dinámico requerido de hasta 1e6 y a su vez proporcionan una localización óptima para los detectores. El nuevo diseño investiga en el uso de una innovadora tecnología de detectores para BWS basada en el uso de detectores de diamante poli-cristalino (pCVD) y propone el uso de sistemas de adquisición de alto rango dinámico en las inmediaciones de los detectores. Este nuevo esquema se basa en integradores a 40Mhz con digitalización en el túnel empleando un front- end tolerante a la radiación y transmisión óptica digital a 4.8Gbps hasta la electrónica de superficie. Se contemplan dos estrategias para cubrir un alto rango dinámico: un sistema mono- canal con codificación logarítmica o un sistema multi-canal con diferentes ganancias por canal. Prototipos de ambos esquemas han sido completamente desarrollados, caracterizados en laboratorio y evaluados en PSB y SPS con diferentes haces. El uso de estos sistemas no solo permite medidas de bajo ruido, sino que también evita el solape de la señal pulsada proveniente de cada bunch y simplifica significativamente la operación del escáner.