Optical transition radiation diagnostics for low energy ion beams

跃迁辐射（TR）是指带电粒子穿过光学不均匀区域（如两种介电性质不同的介质之间的界面）时所发射的电磁辐射。跃迁辐射在高能粒子探测、相干辐射源、光束诊断等方面都有应用。随着材料科学和纳米技术的发展，近年来人们对人工工程材料塑造跃迁辐射特性进行了一些研究。电荷分布诊断是加速器运行和优化的基本诊断要求，通常称为横切面的束流剖面和纵切面的束流电荷或电流监测。目前已有几种成熟的方法，如二次电子监测仪（SEM）网格和光束诱导荧光（BIF），用于横向光束轮廓测量，然而，随着加速器和光束动力学的快速发展，这些方法在重离子束背景下的快速辐射硬诊断的适用性受到质疑。它们受到被测轮廓的分辨率、信号产生过程的基本限制以及测量仪器对波束轮廓的影响的限制。基于光跃迁辐射（OTR，即可见范围内的TR）的光束诊断于1973年提出，并在大约50年的时间里，由于其具有光产率高、实验装置简单、可以使用不同波长等优点而受到了广泛的关注。辐射产率和跃迁辐射的极化程度与入射角和观测角度、光束能量和光束轮廓、目标材料和表面特性（如粗糙度）、发射辐射波长等的关系仍然是许多研究的主题，研究结果被用于改进基于OTR的光束诊断技术。了解跃迁辐射的形成机制、性质以及灵活塑造它的方法，为光束诊断仪器以及未来加速器和对撞机的粒子计数器提供了新的可能性。OTR在重离子束诊断中的应用有其自身的机遇和挑战。跃迁辐射的主要特征之一是观测平面的高度极化。有趣的是，粗糙目标的总产率和极化程度有显著差异。除了预期的观测平面线性极化跃迁辐射外，粗糙表面和非相对论性离子束的非极化辐射也有重要组成部分。理论模型的建立和对粗糙表面OTR的更深入的理解为新的光束诊断技术的发展提供了一条有希望的道路。这个项目旨在通过实验和模拟来验证这种理解并发展新的见解。本项目由STFC CDT LIV共同资助。德国INNO和GSI-FAIR。这项工作将在英国的Cockcroft研究所和德国的GSI进行。

Transition radiation (TR) refers to the emission of electromagnetic radiation when a charged particle moves across an optically inhomogeneous region like the interface between two media with different dielectric properties. Transition radiation have applications in detection of high-energy particles, coherent radiation sources, beam diagnosis etc. With advances in material science and nanotechnol_24ogy, there are some recent studies on shaping the properties of Transition Radiation by artificially engineered materials. Charge distribution diagnostics, typically called beam profiling in transverse plane and beam charge or current monitoring in longitudinal plane, are fundamental diagnostic requirements for accelerator operation and optimization. There are several established methods like secondary electron monitor (SEM) grids as well as beam induced fluorescence (BIF) for transverse beam profile measurement, however, with rapid advances in accelerators and beam dynamics, the suitability of these methods is under question for fast radiation-hard diagnostics in context of heavy ion beams. They are limited by resolution of the measured profiles, fundamental limits on signal generation processes and the effect of measurement apparatus on beam profile. Optical transition radiation (OTR, i.e. TR in visible range) based beam diagnostics was proposed in 1973 and over around 50 years, have gathered a lot of attention due to some advantages like high light yield, simple experimental setup, possibility of working with different wavelengths. Dependence of the radiation yield and the degree of polarization of transition radiation on angle of incidence and angle of observation, beam energy and beam profile, target material and surface characteristics (e.g. roughness), wavelength of emitted radiation etc. continues to be the topic of numerous studies and the findings are exploited to improve OTR based beam diagnostic techniques. Understanding of mechanism of formation of the transition radiation, its properties, and ways to flexibly shape it offers novel possibilities in beam diagnostics instrumentation as well as particle counters for future accelerators and colliders. Application of OTR for heavy ion beam diagnostics comes with its own set of opportunities and challenges. One of the main characteristics of transition radiation is the high degree of polarization in the plane of observation. Interestingly, overall yield and degree of polarization is significantly different for rough targets. In addition to expected linearly polarized transition radiation in the plane of observation, a significant component of unpolarized radiation is reported with rough surface and non-relativistic ion beams. A theoretical model and deeper understanding of OTR from rough surfaces looks a promising way forward to new beam diagnostic technique. This project aims to do that with experiments and simulations to verify this understanding and develop new insights. This project is jointly funded by STFC CDT LIV.INNO and GSI-FAIR, Germany. The work will be carried out at the Cockcroft Institute, U.K and GSI, Germany.