A novel optical fibre analysis system for particle accelerators

在粒子加速器中发生的一些过程，例如从光束中逸出的杂散粒子（光束损耗），可能会损坏或照射机器。这可能会阻碍或阻止加速器的功能，并且辐射延迟了机器的维护，因为它产生的任何残留放射性都必须在工作开始之前衰减到安全水平。粒子加速器用户设施通常旨在最大限度地减少机器停机时间，为设施用户提供最大的可用光束时间。由于损坏的机器需要停机维修，而高辐照水平延长了机器停机时间，因此预防损坏和辐照对于实现这一目标非常重要。通常，这是通过监测波束损耗来实现的，以确保其发生在安全水平。局部探测器，如电离室（ic）通常用于监测光束损失。然而，他们每个人只能单独监控一小块区域；因此，利用集成电路精确确定波束损耗的位置是不可行的。[2]射频加速腔内有时会产生火花（射频击穿）。这会损坏腔面和速调管放大器，因为传入的射频功率被反射[3]，[4]。传统的射频击穿探测器会随着击穿[5]时电流或压力的增加而增加。由于这些检测器在击穿发生后才作出反应，因此现有的减轻损害的方法仅限于反应性措施。一种解决方案是通过基于光纤的探测器，通过这些事件产生的二次粒子阵雨来检测光束损失和射频击穿。这些相对论性粒子通过光纤产生切伦科夫光，这种光传播到光电探测器。然后使用飞行时间计算来确定损失位置[6]。这些探测器是非侵入性的，可以提供对机器的连续覆盖；同时监测波束损耗和射频击穿；快速的响应时间（大约几纳秒）；而事件定位分辨率在几厘米量级（适用于当前版本的探测器系统）。与集成电路不同，光纤对磁场和x射线不敏感。这使得光纤可以定位在许多其他探测器无法定位的区域，比如像波动体[7]这样的磁性结构内部。光纤技术已经在各种加速器上取得了成功，在这些加速器上，它已经证明了足够的能力来保护单个加速器部分[8,9]甚至整个加速器[10]；除了成功监测RF腔[4]，[11]中的暗电流和击穿。该项目将开发和测试新版本的光纤探测器系统，并展示其作为能量回收线性加速器（ERLs）的机器保护系统的光束损耗和射频击穿分辨率的能力。erl是一种新型加速器，通过从已使用的高能束流中提取能量，然后将其转移到新的低能量束流中，从而减少加速器的能量需求。旧粒子束和新粒子束（粒子束的子段）经常在ERL内同时循环。当代的例子包括康奈尔大学的CBETA，以及欧洲核子研究中心提出的LHeC对撞机。该应用面临的主要挑战包括：从单个快速重复波束中区分波束损耗信号，例如通常在erl中产生的波束；并区分这些束的能量，以衡量每个束通过ERL的进展。本项目还旨在将机器学习引入到obm系统[3]，[14]中，用于机器损坏事件的预测和预防。这就允许实施主动而不是被动的干预方法。测试将在不同的机构进行，如CLARA（达斯伯里实验室）、CLEAR（欧洲核子研究中心）和CBETA（康奈尔大学）。

Some processes occurring within particle accelerators, such as stray particles escaping from the beam (beam loss), can damage or irradiate the machine. This can impede or prevent the function of the accelerator, and the irradiation delays machine maintenance as any residual radioactivity it creates must decay to safe levels before work can begin.Particle accelerator user facilities commonly aim to minimise machine downtime to offer maximum availability of beam time to facility users. Since a damaged machine requires downtime for repairs [1], and high irradiation levels extend the machine downtime, prevention of both damage and irradiation is important to this goal. Typically, this is performed through monitoring of beam losses to ensure its occurrences are kept to safe levels.Localised detectors such as ionisation chambers (ICs) are commonly used to monitor beam losses. However, they can each individually only monitor a small area; hence it is unfeasible to precisely determine locations of beam loss using ICs. [2] RF accelerating cavities can sometimes undergo sparking within them (RF breakdown). This damages the cavity surface and also the klystron amplifiers as the incoming RF power is reflected [3], [4]. Conventional RF breakdown detectors follow the increase in current or pressure that accompanies a breakdown [5]. Since these detectors respond after the onset of breakdown, the available damage mitigation methods are limited to reactive measures.A solution is offered through optical fibre-based detectors to detect beam losses and RF breakdowns through the showers of secondary particles these events produce. Passage of these relativistic particles through an optical fibre produces Cherenkov light, which propagates to photodetectors. A time-of-flight calculation is then used to determine the loss location [6]. These detectors are non-invasive which can offer, continuous coverage of a machine; simultaneous monitoring of both beam losses and RF breakdowns; a fast response time (on the order of a few nanoseconds); and event location resolution on the order of a few cm (for the current version of the detector system). Unlike ICs, optical fibres are insensitive to magnetic fields and X-rays. This allows fibres to be positioned in areas where many other detectors cannot, such as inside magnetic structures like undulators [7].Optical fibre technology has been successful on a variety of accelerators, where it has demonstrated sufficient capability to protect individual accelerator sections [8, 9] or even entire accelerators [10]; in addition to successfully monitoring dark current and breakdowns in RF cavities [4], [11]. This project will develop and benchmark the new version of the optical fibre detector system and demonstrate its capabilities for beam loss and RF breakdown resolution as a machine protection system for energy recovery linear accelerators (ERLs).ERLs are a type of novel accelerator that seek to decrease accelerator energy demands by extracting energy from its used, high-energy beams and subsequently transferring it to fresh, low-energy beams. Both used and fresh particle bunches (sub-segments of beams) often circulate simultaneously within the ERL. Contemporary examples include CBETA at Cornell University [12], and the LHeC collider proposed by CERN [13]. Key challenges for this application include distinguishing beam loss signals from individual bunches of fast-repeating beams, such as those typically produced in ERLs; and discriminating between the energies of these bunches to gauge the progress of each bunch through the ERL.This project also aims to introduce machine learning to the oBLM system [3], [14], for the prediction and prevention of machine damaging events. This allows for the implementation of proactive rather than reactive intervention methods. Testing will be carried out at various facilities, such as CLARA (Daresbury Laboratory), CLEAR (CERN), and CBETA (Cornell).