1.School of Mechanical and Automotive Engineering, South China University of Technology, Guangzhou 510641, China 2.Department of Physics and Materials Science, City University of Hong Kong, Hong Kong 999077, China 3.Huan Qiu Project Management (Beijing) Co. Ltd., Beijing 100029, China
Fund Project:Project supported by the National Natural Science Foundation of China (Grant Nos. 52005187, 51705157, 51905177), the Basic and Applied Basic Research Foundation of Guangdong Province, China (Grant No. 2019A1515110065), the China Postdoctoral Science Foundation (Grant No. 2019M662909), and the Fundamental Research Fund for the Central Universities, China (Grant No. 2019MS063)
Received Date:20 August 2020
Accepted Date:26 October 2020
Available Online:19 March 2021
Published Online:05 April 2021
Abstract:In order to solve the problems of unstable discharge, low deposition rate and large difference in ionization rate between different targets in high power impulse magnetron sputtering, a novel cylindrical cathode with annular magnetic field based on hollow cathode effect is proposed, which can be used to produce ion beam with high ionization rate, high plasma density and no large particles. However, the traditional channel structure could not guarantee its high efficiency and uniform heat dissipation. The sealing ring may be damaged by ablation due to high power density, which restricts the further improvement of power density. Therefore, it is necessary to optimize the design of the channel structure. SolidWorks flow simulation software is used to simulate the cooling channel of plasma source. The influence of water hole structure parameters on cooling effect is analyzed, including distribution angle, hole number, diameter and inlet hole height. And the channel structure parameters are optimized. The results show that the increasing of the circumferential distribution range of the water hole is beneficial to the uniformity of heat dissipation, ensuring a large temperature difference between cooling water and copper sleeve, and strengthening heat exchange. The water inlet hole set in the upper layer of the structure is conducive to alleviating the temperature stratification phenomenon of the cooling water, so that the copper sleeve and sealing ring are in good cooling condition. Appropriately reducing the aperture is beneficial to increasing the cooling water jet velocity, enhancing the jet impact effect, and then increasing the turbulence degree, strengthening the heat transfer and improving the heat transfer efficiency. By systematically studying the influencing factors, the optimized cooling flow field structure of cylindrical cathode with an annular magnetic field is obtained. The distribution angle is 30°, the number of holes is 6, the aperture is 4 mm, and the height of water inlet hole is 36 mm. The optimized channel structure can improve the utilization rate of cooling water, obtaining better cooling effect at the same flow rate, and improving the discharge stability of the plasma source, which provides a basis for designing the cooling structure of the cylindrical cathode with annular magnetic field. Keywords:cylindrical cathode with annular magnetic field/ numerical simulation/ cooling/ optimized design for the flow channel structure
表2不同出入水孔分布角度、孔数时的胶圈最高温度 (℃) Table2.Maximum temperature of aprons with different distribution angles and number of holes (℃).
针对表2结果, 结合不同孔分布角度时的冷却水温度切面进行分析, 如图5所示, 随着分布角度的增大, 整体水温趋向均匀, 局部的高温区域温度降低且面积逐渐减少直至消失. 局部高温会减弱该区域的换热效果, 而初始冷却水与铜套表面的较大温差有利于强化传热, 因此当入水孔分布跨度更广时, 铜套表面接触初始冷却水的范围更广, 换热效果更好, 且冷却更加均匀. 部分冷却水从最两端的出水孔流出, 在腔体中的停留时间短, 水温相对较低, 因此换热充分、温度较高的冷却水不会在中央出水孔处大量富集, 避免了局部高温的产生. 图 5 不同孔分布角度的出水孔高度(40 mm)冷却水温度切面 Figure5. Water temperature section with outlet height (40 mm) of different hole distribution angles.
铜套表面温度更加直观地反映了冷却水的冷却效果, 与胶圈最高温度有着相同的变化趋势, 铜套温度降低有利于减少传递给胶圈的热量, 使胶圈温度降低. 图6为分布角为30°时不同出入水孔数量时的出水侧铜套表面温度, 随着孔数的增加铜套表面温度明显降低, 且上端接近胶圈的位置降温更明显. 孔数量的增加同样增大了初始冷却水的分布范围, 使冷却更加均匀, 减少局部高温. 图 6 不同出入水孔数量的铜套表面温度 Figure6. Surface temperature of copper sleeve with different number of holes.
23.2.出入水孔孔径对冷却效果的影响 -->
3.2.出入水孔孔径对冷却效果的影响
根据前一节模拟分析得到的最优冷却效果, 采用分布角度为30°的6孔冷却结构, 保持入水孔高度32 mm、出水孔高度40 mm不变, 对孔径大小为3, 4, 5, 6和7 mm五种结构的模型进行模拟计算. 不同入水端压力下的胶圈最高温度如图7所示, 在实际工况下(入口水压5.0 × 105 Pa), 胶圈最高温度随着入水孔孔径增大呈现先减小后增大再减小的趋势, 在孔径4 mm时实现最优的冷却效果, 胶圈稳态最高温度为46.71 ℃. 当入水端压力恒定时, 随着孔径改变冷却水流速与流量都会发生变化, 此时系统冷却效果受到流量与流速的综合影响. 由图8可见, 冷却水经过入水孔发生射流, 冲击铜套壁面, 射流速度随孔径减小而增大, 冲击面积也随之增大. 射流冲击作用是强化局部换热的有效方法, 提高射流速度有利于增强冷却水的湍流度, 提高对流传热系数, 同时加剧对壁面的冲击, 冲击面积的增大有利于扩大换热强化区域范围, 进而提升换热效果. 因此, 胶圈最高温度呈现随着孔径减小而降低的趋势. 图 7 不同水压下出入水孔孔径对胶圈温度的影响 Figure7. Influence of hole diameter on apron temperature under different water pressure.
图 8 出入水孔孔径对入水孔射流速度的影响 Figure8. Influence of hole diameter on water jet velocity at water inlet.