Secondary electron yield measurements of anti-multipacting surfaces for accelerators
thesisposted on 22.11.2016, 14:31 by Sihui Wang
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.