Niobium cavities are important components in modern particle accelerators based on superconducting radio frequency (SRF) technology. The interior of SRF cavities are cleaned and polished in order to produce high accelerating field and low power dissipation on the cavity wall. Current polishing methods, buffered chemical polishing (BCP) and electropolishing (EP), have their advantages and limitations. We seek to improve current methods and explore laser polishing (LP) as a greener alternative of chemical methods.;The topography and removal rate of BCP at different conditions (duration, temperature, sample orientation, flow rate) was studied with optical microscopy, scanning electron microscopy (SEM), and electron backscatter diffraction (EBSD). Differential etching on different crystal orientations is the main contributor to fine grain niobium BCP topography, with gas evolution playing a secondary role. The surface of single crystal and bi-crystal niobium is smooth even after heavy BCP. The topography of fine grain niobium depends on total removal. The removal rate increases with temperature and surface acid flow rate within the rage of 0~20 °C, with chemical reaction being the possible dominate rate control mechanism. Surface flow helps to regulate temperature and avoid gas accumulation on the surface.;The effect of surface flow rate on niobium EP was studied with optical microscopy, atomic force microscopy (AFM), and power spectral density (PSD) analysis. Within the range of 0~3.7 cm/s, no significant difference was found on the removal rate and the macro roughness. Possible improvement on the micro roughness with increased surface flow rate was observed.;The effect of fluence and pulse accumulation on niobium topography during LP was studied with optical microscopy, SEM, AFM, and PSD analysis. Polishing on micro scale was achieved within fluence range of 0.57~0.90 J/cm 2, with pulse accumulation adjusted accordingly. Larger area treatment was proved possible by overlapping laser tracks at proper ratio. Comparison of topography and PSD indicates that LP smooths the surface in a way similar to EP. The optimized LP parameters were applied to different types of niobium surfaces representing different stages in cavity fabrication. LP reduces the sharpness on rough surfaces effectively, while doing no harm to smooth surfaces. Secondary ion mass spectrometer (SIMS) analysis showed that LP reduces the oxide layer slightly and no contamination occurred from LP. EBSD showed no significant change on crystal structure after LP.
展开▼
机译：铌腔是基于超导射频（SRF）技术的现代粒子加速器中的重要组件。 SRF腔的内部经过清洁和抛光，以在腔壁上产生高加速场和低功耗。当前的抛光方法，缓冲化学抛光（BCP）和电抛光（EP）具有其优点和局限性。我们寻求改进现有方法，并探索激光抛光（LP）作为化学方法的绿色替代方法;;通过光学显微镜，扫描仪研究了在不同条件（持续时间，温度，样品方向，流速）下BCP的形貌和去除速率电子显微镜（SEM）和电子反向散射衍射（EBSD）。在不同晶体取向上的差异蚀刻是细晶粒铌BCP形貌的主要贡献者，而气体逸出起次要作用。即使经过大量的BCP处理，单晶和双晶铌的表面也很光滑。细晶粒铌的形貌取决于总去除量。在0〜20°C范围内，去除率随温度和表面酸流量的增加而增加，化学反应可能是主要的速率控制机制。表面流有助于调节温度，避免气体在表面积聚。通过光学显微镜，原子力显微镜（AFM）和功率谱密度（PSD）分析研究了表面流速对铌EP的影响。在0〜3.7 cm / s的范围内，去除率和宏观粗糙度无明显差异。观察到随着表面流速的增加，显微粗糙度可能得到改善。;利用光学显微镜，SEM，AFM和PSD分析研究了液流和脉冲积累对LP时铌形貌的影响。在0.57〜0.90 J / cm 2的注量范围内实现了微尺度抛光，并相应地调整了脉冲累积。通过以适当的比例重叠激光轨迹，证明可以进行更大面积的处理。地形和PSD的比较表明LP以类似于EP的方式使表面光滑。将优化的LP参数应用于代表腔体制造不同阶段的不同类型的铌表面。 LP有效地降低了粗糙表面上的清晰度，同时又不损害光滑表面。二次离子质谱仪（SIMS）分析表明，LP略微减少了氧化层，并且LP没有污染。 LP后，EBSD的晶体结构无明显变化。
展开▼
