Secondary electron yield measurements of anti-multipacting surfaces for accelerators
Secondary electron yield measurements of anti-multipacting surfaces for accelerators
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2016
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通讯作者：
Sihui Wang
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作者：
Sihui Wang
电子云是一种有害的效应，它限制了高强度、短束团间距的正电荷粒子束加速器的性能。然而，如果加速器腔室表面的二次电子产额（SEY）小于1，则可以充分抑制由束感生二次碰撞引起的电子云。通常，SEY通过两种方式降低：表面化学改性和表面粗糙度工程。这个博士项目的目标是系统地研究SEY作为各种表面相关参数的函数，例如表面化学和表面形态，以及粒子加速器的常见处理方法（如束管烘烤和表面处理）的效果，最终旨在设计具有低SEY的表面以减轻电子云。在这项工作中，过渡金属及其涂层和激光处理表面作为退火处理和电子轰击的功能进行了研究。采用直流磁控溅射法制备了过渡金属薄膜，并进行了进一步的测试.本文的前两章介绍了电子发射效应的研究现状，包括电子发射的过程、影响电子发射的因素以及低SEY材料的研究实例。第三章介绍了本研究中所采用的SEY测量和表面研究的实验方法。第四章详细介绍了自行搭建的SEY测量装置。第五章研究了过渡金属及其涂层和非蒸散型吸气剂（NEG）涂层。所有的样品已经通过SEY测量进行了表征，它们的表面形貌用扫描电子显微镜（SEM）和它们的化学用X射线光电子能谱（XPS）进行了研究。研究了不同的表面处理方法，如电子束预处理、真空热处理等。例如，Ti、Zr、V和Hf的最大SEY（δmax）分别为2.30、2.31、1.72和2.45。在7.9×10- 3C mm-2的剂量后，Ti的δmax下降到1.19。在6.4×10- 3C·mm-2、1.3×10-3和5.2×10- 3C·mm-2剂量下，Zr、V和Hf的δmax分别下降到1.27、1.48和1.40。在加热到350 ℃持续2.5小时后，体Ti的SEY分别下降到1.21和1.40。由于所有散装样品具有平坦的表面，因此没有形态差异。因此，SEY的降低被认为是电子轰击后在表面上生长薄石墨膜和退火后去除表面上的污染物的结果。本文的第六章是关于激光处理表面的。激光照射可以将高反射性金属转化为黑色或深色金属。从SEM结果来看，纳秒脉冲激光辐照改性的金属表面形成了高度组织化的金字塔表面微观结构，这增加了表面粗糙度。因此，激光处理后的原始表面的δmax可以小于1，这可以避免电子云现象。本章研究了不同的激光处理参数，如功率、开口距离、不同气氛等对SEY的影响。同时，对激光处理后的表面进行了不同的表面处理，如电子处理和热处理，并用XPS进行了研究。例如，在空气中，开口距离为50、60和80 μm的接收态I型的δmax分别为0.75、0.75和0.80。在加热至250 ℃ 2小时后，在所有情况下，δmax分别下降至0.59、0.60、0.62。由于特殊的金字塔结构增加了表面的粗糙度，所有接收样品的SEY都小于1。热处理后，SEY进一步降低。这是通过去除表面上的污染物引起的。总之，本研究在很大程度上提高了知识的电子云缓解技术的表面工程的真空室。一方面，表面处理可以改变表面化学，例如通过电子条件在表面上产生图形碳层和通过热处理去除表面顶部的污染层。另一方面，通过设计表面粗糙度，SEY可以非常低。这两种方法都允许达到小于1的δmax。首次展示了电子云激光处理表面的效率，这引起了人们对现有和未来粒子加速器的这种新技术应用的极大兴趣。
Electron cloud is an unwanted effect limiting the performance of particle accelerators with positively charged particle beams of high-intensity and short bunch spacing. However, electron cloud caused by beam induced multipacting can be sufficiently suppressed if the secondary electron yield (SEY) of accelerator chamber surface is lower than unity. Usually, the SEY is reduced by two ways: modification of surface chemistry and engineering the surface roughness. The objective of this PhD project is a systematic study of SEY as a function of various surface related parameters such as surface chemistry and surface morphology, as well as an effect of such common treatments for particle accelerators as beam pipe bakeout and surface conditioning with a beam, ultimately aiming to engineer the surfaces with low SEY for the electron cloud mitigation. In this work, transition metals and their coatings and laser treated surface were studied as a function of annealing treatment and electron bombardment. The transition metal thin films have been prepared by DC magnetron sputtering for further test. In the first two Chapter of this thesis, the literature review on electron emission effect is introduced, which includes the process of the electron emission, the influence factor and examples of low SEY materials. In the third Chapter, the experimental methods for SEY measurements and surface investigation used in this work are described. In Chapter 4, the SEY measurement setup which is built by myself are introduced in detail. In Chapter 5 transition metals and their coatings and non-evaporable getter (NEG) coatings have been studied. All the samples have been characterized by SEY measurements, their surface morphology was analysed with Scanning Electron Microscopy (SEM) and their chemistry was studied with X-ray Photoelectron Spectroscopy (XPS). Different surface treatments such as conditioning by electron beam, thermal treatment under vacuum on the sample surfaces have been investigated. For example, the maximum SEY (δmax) of as-received Ti, Zr, V and Hf were 2.30, 2.31, 1.72 and 2.45, respectively. After a dose of 7.9×10-3 C mm-2, δmax of Ti drops to 1.19. δmax for Zr, V and Hf drop to 1.27, 1.48 and 1.40 after doses of 6.4×10-3 Cmm-2, 1.3×10-3 and 5.2×10-3 Cmm-2, respectively. After heating to 350 ⁰C for 2.5 hours, the SEY of bulk Ti has dropped to 1.21 and 1.40, respectively. As the all bulk samples have a flat surface, there are no difference of morphology. So this reduction of SEY is believed to be a consequence of the growth of a thin graphitic film on the surface after electron bombardment and the removal of the contaminations on the surface after annealing. Chapter 6 of this thesis is about the laser treated surface. Laser irradiation can transform highly reflective metals to black or dark coloured metal. From SEM results, metal surfaces modified by a nanosecond pulsed laser irradiation form a highly organised pyramid surface microstructures, which increase the surface roughness. Due to this reason, δmax of as-received laser treated surface could be lower than 1, which can avoid the electron cloud phenomenon. In this Chapter, the influence of different laser treatment parameters, such as power, hatch distance, different atmospheres on SEY has been investigated. Meanwhile, different surface treatments such as electron conditioning and thermal treatments are studied on the laser treated surface with the investigation of XPS. For example, the δmax of as-received type I with hatch distance 50, 60 and 80 μm in Air are 0.75, 0.75 and 0.80, respectively. After heating to 250 oC for 2 hours, in all case the δmax drop to 0.59, 0.60, 0.62, respectively. The SEYs of all as-received samples are lower than 1 due to the increasing the roughness on the surface by the special pyramid structure. After thermal treatment, the SEY reduces even further. This is caused by removing the contaminations on the surfaces. In conclusion, the present study has largely improved the knowledge of the electron cloud mitigation techniques by surface engineering of vacuum chambers. On the one hand, the surface treatments can modify the surface chemistry, such as the produce the graphic carbon layer on the surface by electron condition and the removal the contamination layer on the top of the surface by thermal treatment. On the other hand, the SEY could be critically low by engineering the surface roughness. Both methods allow reaching δmax less than unity. The efficiency of laser treated surface for e-cloud was demonstrated for a first time leading to a great interest to this new technology application for existing and future particle accelerators.