The electron cloud effect (ECE) has posed serious challenges in recent high-intensity proton and positron rings [1]. One of the applicable solutions is preparing a surface with a low secondary electron yield (SEY) on the inner wall of beam pipes. In this study, we used a commercial method called “thermal spray”, where copper powder was melted and sprayed onto an aluminum substrate to create a rough surface, and was investigated for the first time as a method for producing a low SEY surface [2]. Figure 1(a) is a schematic of the thermal spraying used in this study. After an electron exposure of ≈ 1 ×10-1 C/mm2 at an energy of 350 eV, the lowest δmax (the maximum SEY within scanning) of the copper thermal spray (T.S.) coating reached ≈ 0.7.
The Emax (the incident electron energy corresponding to δmax) of T.S. coating was found to be inversely related to the surface temperature during spraying. The roughness parameters and surface composition were measured to clarify the key factors affecting SEY. In addition, to check the applicability of T.S. coating in accelerators, its outgassing rate, adhesive strength, impedance and dust generation rate were measured as a reference.
Finally, an aluminum beam pipe with a T.S. coating was produced and installed in the positron ring of SuperKEKB to measure the electron density around the beam. The measured electron densities were compared with those obtained from other beam pipes with different inner surfaces, and also investigated using a simulation code.
The results show that the outgassing rate and adhesive strength of T.S. coating were acceptable. However, the amount of dust and impedance were not inconsiderable. The measured electron densities of the T.S.-coated beam pipe was comparable with that of the TiN-coated beam pipe even under the influence of the uncoated aluminum screen, as shown in Fig. 1(b-c). Therefore, the T.S. coating can be considered as a candidate technology for reducing ECE, while there are still room for improvement. This study can provide a new and useful information for researchers in this field in developing a low-SEY coating on beam pipes [3].
References:
[1] G. Rumolo and G. Iadarola, Proc. CAS-CERN Accelerator School: Intensity Limitations in Particle Beams, 411 (2015).
[2] M. Yao et al, Vacuum, 207, 111619 (2023).
[3] M. Yao, PhD thesis, Accel. Sci. Dept., SOKENDAI (2023).
