Amidst the lush lychee groves in Dongguan, Guangdong Province, stands a world-class "neutron observation station". It is the China Spallation Neutron Source (CSNS), the largest single-investment large scientific facility ever built in China to date. Large scientific facilities are an important part of the national scientific and technological infrastructure platform. They refer to large-scale facilities that are completed through large-scale investment and engineering construction, and after completion, achieve important scientific and technological goals through long-term stable operation and continuous scientific and technological activities. Familiar examples include the Five-hundred-meter Aperture Spherical Radio Telescope (FAST), the Shanghai Synchrotron Radiation Facility, and the High Energy Photon Source (HEPS), all of which belong to large scientific facilities. One might ask, why do we need to build large scientific facilities like the spallation neutron source? A National Heavyweight Put simply, a pulsed spallation neutron source is a large "super microscope" – a scientific research device that uses neutron scattering to probe the microstructure and movement of matter, with exceptional capabilities in studying neutron properties. We all know that there are many things in the world invisible to the naked eye. To further explore the world, scientists invented optical microscopes, followed by ones with higher resolution capable of observing smaller cells and viruses. Before the invention of pulsed spallation neutron sources, people relied on these microscopes to study the microscopic world – such as bacteria and cells – that cannot be directly seen with the naked eye. However, pulsed spallation neutron sources can outperform all microscopes, revealing an even more microscopic world. They can even clearly observe how a drop of water in a plant's root is transported to its branches and leaves step by step. Why can pulsed spallation neutron sources probe the microstructure and movement of matter with such precision? Neutrons are uncharged particles, and they do not use X-ray synchrotron radiation. When the atomic nuclei of a substance interact with neutrons, not all neutrons pass directly through; some are deflected or scattered when attempting to pass through the nuclei. At this point, the spectrometer of the pulsed spallation neutron source comes into play, capturing the flight trajectories and then deducing the internal structure of the atomic nuclei for researchers to study. For example, when scientists study underwater combustible ice, the extreme pressure at great depths requires thick-walled containers. Light elements such as nitrogen, carbon, and hydrogen in the ice are difficult to detect, but only pulsed spallation neutron sources can penetrate the containers, are sensitive to these light elements, and clearly distinguish the structure of combustible ice. Today, China's pulsed spallation neutron source technology is applied in various fields, including testing the safety of high-speed trains and aircraft, and cancer treatment. Additionally, it plays a significant role in driving the development of disciplines such as new nuclear energy development, national defense research, materials science, physics, chemistry, nanoscience, life sciences, and medicine. Therefore, it is well-deserved to be called a "national heavyweight". A Visit to the China Spallation Neutron Source Equipment Every year, the China Spallation Neutron Source takes a "summer vacation" – a shutdown period lasting one and a half to two months. During this time, researchers perform "maintenance" on the facility. The China Spallation Neutron Source is a national major scientific infrastructure approved and supported by the National Development and Reform Commission, with the Institute of High Energy Physics, Chinese Academy of Sciences as its legal entity. This facility makes China the fourth country in the world, after the United Kingdom, the United States, and Japan, to possess a pulsed spallation neutron source. Visually Striking: Colorful "Black Technology" Spallation neutron sources are often likened to "super microscopes" because they use neutrons produced by accelerating protons into a target to explore the microstructure of matter. Its core – the accelerator system – lies underground like a coiled dragon. Seventeen meters underground, air conditioning and fresh air systems keep the originally humid air dry. Walking along the bright green corridor, one enters a colorful world of "black technology". The yellow components are drift tube linac systems that make particles "race"; the blue ones are quadrupole magnets that focus particles into a beam; the red ones are dipole magnets that make particles "turn" at a 15-degree angle... They first form a long chain and then enclose a large ring. The chain part is the linear accelerator, and the ring part is the rapid cycling synchrotron. Despite their bulky appearance, these devices require installation precision at the 10-micron to hundred-micron level, allowing protons – tiny particles in nature – to be controlled and accelerated as required. When operating, the rapid cycling accelerator "receives" 25 batches of negatively charged hydrogen ions waiting to be accelerated every second, like a tourist bus. Each batch of negative hydrogen ions "boards" and is converted into protons, which travel approximately 20,000 laps along the rapid cycling synchrotron in 0.02 seconds, reaching a speed of 0.92 times the speed of light. Subsequently, the near-light-speed proton beam rushes toward a heavy metal target like "micro-bullets", smashing the atomic nuclei of the metal target. Scientists then use special devices to slow down the uncharged neutrons from the "debris" and introduce them into various spectrometers. The spectrometers, located near the accelerator tunnel, are also colorful. The first phase of the China Spallation Neutron Source includes three spectrometers: a general powder diffractometer with a green casing, a small-angle neutron scattering spectrometer, and a multi-functional reflectometer with a blue casing. Over the past four years, the China Spallation Neutron Source has also collaborated with universities and research institutions in the Guangdong-Hong Kong-Macau Greater Bay Area to build several more spectrometers to meet the needs of national and local research institutions and enterprises. Red, green, blue, yellow... With the target station at the center, the completed and under-construction spectrometers stretch out in all directions, making the China Spallation Neutron Source look like a blooming seven-color flower. "The localization rate of our equipment exceeds 90%," Chen Yanwei, Director of the Spallation Neutron Source Science Center and Deputy Director of the Institute of High Energy Physics, Chinese Academy of Sciences, told *China Science Daily*. Nearly 100 cooperative units nationwide have completed the development and mass production of various equipment for the facility, with many reaching international leading or advanced levels. 5000, 97%, 800, 122%... Practicality: The "Superpowers" of the Super Microscope At the China Spallation Neutron Source, researchers like to let the numbers speak. One number they are most proud of is "5000". Here, time is measured in hours rather than years, months, or days. "We provide neutron beam time for experiments for 5000 hours each year," Chen Yanwei said. 5000 hours means that the China Spallation Neutron Source generates neutrons and conducts experiments most of the year (over 8700 hours). "Among other international spallation neutron sources, those in the UK and Japan typically provide around 4000 hours of neutron beam time annually," Chen Yanwei added. Another proud number is "97%". "From 2020 to 2021, our actual operation efficiency exceeded 97%, a level unmatched by other spallation neutron sources worldwide," said Wang Sheng, Deputy Director of the Spallation Neutron Source Science Center and researcher at the Institute of High Energy Physics, Chinese Academy of Sciences. Actual operation efficiency is the ratio of the actual operation time to the planned operation time of the spallation neutron source. A higher number indicates lower failure rates and better stability in planned operation. When describing the operational achievements of the China Spallation Neutron Source, they prefer to use the number of research projects. "In four years, the China Spallation Neutron Source has operated in 8 open rounds, completing over 800 projects, with key support for machine time for national major demand projects," Chen Yanwei said. Facing the world's scientific and technological frontier, they have conducted research on super steel, molecular sieve adsorbents, and quantum materials. Facing the main economic battlefield, they have provided important support to high-tech enterprises and research institutions such as the Central Iron and Steel Research Institute, Guodian Power Development Co., Ltd., and China National Petroleum Corporation in fields such as high-performance chips, batteries, materials, and stress detection. Facing national major needs, they have completed stress testing of aerospace engine blades and verified the welding process of the "Striver" deep-sea submersible. Facing people's life and health, in August 2020, they successfully developed China's first boron neutron capture therapy experimental device with complete independent intellectual property rights, which began installation at Dongguan People's Hospital at the end of July this year. Good data and achievements have led to a snowballing increase in users. Chen Yanwei noted that the number of registered users has exceeded 3800, and the number of project applications in the 2021-2022 period increased by 122% year-on-year, with a project approval rate of 29%. Increasing power, optimizing performance, adding terminals, promoting interdisciplinary research... Forward Planning: Future "Small Goals" The growing demand for machine time and the escalating technological competition have made upgrading the China Spallation Neutron Source a practical necessity. Researchers reserved space for upgrades during the engineering design phase, enabling direct upgrades based on the first-phase project. Chen Yanwei said that the China Spallation Neutron Source has completed all first-phase design indicators. In February 2020, the target beam power reached the design indicator of 100 kW, one and a half years ahead of schedule; in February 2022, the target beam power reached 125 kW, exceeding the design indicator by 25%, with stable and efficient operation, significantly improving the facility's performance. Increasing the target beam power will generate more neutrons in the same time, shortening experiment duration and improving sample resolution. "It's like taking photos in strong daylight – the exposure time is shorter than at night, and the photos are clearer," Chen Yanwei explained. One of the researchers' future "small goals" is to increase the target beam power to 500 kW, making the neutron source brighter. Additionally, Liang Tianjiao, Deputy Director of the Spallation Neutron Source Science Center, noted that after upgrading, the China Spallation Neutron Source is expected to cover most areas of user demand, meet more structural and dynamic characterization at various scales, and provide stronger support for interdisciplinary research. CSNS and Vacuum Technology The construction of the CSNS large scientific facility involves many disciplines, among which vacuum technology is one of the most important basic disciplines. Below, we systematically explain the important role of the vacuum system in this facility based on the division of CSNS sections. According to CSNS physical requirements, the working pressures of each section of the vacuum system are as follows: - IS and LEBT: 2.0×10-3 Pa - RFQ and MEBT: 1.0×10-5 Pa - DTL: 1.0×10-5 Pa - LRBT & RTBT: 1.0×10-5 Pa - RCS: 5.0×10-6 Pa 1. Ion Source and Low-Energy Beam Transport Line (IS & LEBT) The ion source is the particle generation device of the accelerator. CSNS uses a cesium-added negative hydrogen surface source – a Penning ion source. To ensure the stable generation of negative hydrogen ion (H-) beams, a piezoelectric valve injects hydrogen into the ion source cavity at a frequency of 25Hz at a rate of 10sccm (1.69×10-2Pa·m³/s). Under the action of the ion source's electromagnetic field, a 20mA H- ion current is generated. Negative hydrogen ions are extracted through a small hole. To reduce the stripping loss of H- beams in vacuum, the dynamic vacuum pressure needs to be around 2×10-3Pa. To achieve this vacuum level, the ion source is pumped by two 2000L/s turbomolecular pumps, each equipped with an 8L/s scroll pump, achieving a dynamic vacuum of 2.5×10-3 Pa. The low-energy beam transport line matches the beam extracted from the ion source to the downstream RFQ, with a length of only 1.68m. The LEBT vacuum pipe is made of 304 stainless steel, and its surface outgassing rate is very low after vacuum pretreatment, so its gas source mainly comes from hydrogen in the ion source. The LEBT beam pipe has a small aperture and is equipped with an 800L/s turbomolecular pump unit, which can reduce the impact of hydrogen from the ion source on the RFQ vacuum system through differential pumping. 2. Radio Frequency Quadrupole Accelerator (RFQ) and Medium-Energy Beam Transport Line (MEBT) The radio frequency quadrupole accelerator focuses, bunches, and accelerates the beam, effectively controlling beam emittance growth and increasing beam energy. It requires high processing precision, has many welds, and complex field tuning. The RFQ cavity is made of oxygen-free copper, with an average diameter of about 350mm. Due to its special four-wing electrode structure, the conductance is very limited. To obtain effective pumping speed, each face of the RFQ has a CF150 flange pumping port, and the four pumping ports are connected in parallel, pumped by both ion pumps and turbomolecular pumps. Three 1000L/s ion pumps and two 500L/s turbomolecular pump units are used simultaneously, achieving a working pressure of less than 1×10-5Pa. The medium-energy beam transport line (MEBT) matches the beam to the next accelerator section to reduce beam loss. It consists of bunching cavities, beam measurement elements, and vacuum components. The MEBT vacuum pipe is made of stainless steel, with a total length of 3.03m and an inner diameter of 50mm, with variable-diameter vacuum chambers in special locations based on beam envelope and magnet inner diameter. The two bunching cavities in this section are pumped by two 200L/s ion pumps, and two 100L/s ion pumps are installed on the beam measurement equipment. 3. Drift Tube Linac (DTL) The drift tube linac has high acceleration efficiency, and the quadrupole lenses in the drift tubes provide good beam focusing, effectively controlling beam emittance growth. The DTL consists of four independent physical cavities, each containing 3 process cavities, with a total length of about 34m and a cavity diameter of 490mm. To achieve a dynamic vacuum pressure of less than 1×10-5 Pa, each process cavity is equipped with two 1000L/s ion pumps, and each physical cavity is configured with two 500L/s turbomolecular pump units. 4. LRBT The main line of the LRBT transport line is approximately 197m long, whose main task is to transport the beam pre-accelerated by the linear accelerator (LINAC) to the RCS injection stripping foil. There is also a branch transport line (LDBT) leading to three beam dump stations, with a total length of about 42m. The LRBT is divided into 9 sections by 9 all-metal pneumatic gate valves, each section equipped with corresponding roughing valves and vent valves. Vacuum measurement uses cold cathode gauges, and some sections are equipped with residual gas analyzers to monitor system operation. Each section is independent; if a section is exposed to the atmosphere or its vacuum deteriorates, the valve for that section can be closed manually or automatically to prevent other sections from being affected. One 100L/s ion pump is installed every 6m on average in the LRBT. Assuming a thermal outgassing rate of 1.33×10-11Pa·m³/s·cm², calculations using a program developed by CERN (European Organization for Nuclear Research) show a pressure distribution curve for 5 sections, with an average pressure of 5×10-6Pa. 5. RTBT The main line of the RTBT transport line (from the RCS extraction beam to the spallation target window) is about 145m long, whose main task is to transport the high-power proton beam extracted from the RCS ring to the target station. There is also a branch transport line (RDBT) leading to the beam dump station, with a total length of about 37m. The inner diameter of the RTBT vacuum chamber is mainly 168mm. The RTBT has 6 all-metal pneumatic gate valves, dividing the vacuum system into 6 independent sections. A DN200 fast valve is installed 25m before the spallation target; if the proton beam window or the inflatable bellows sealing device for remote disassembly leaks, the fast valve can close within 40ms to prevent other sections from being exposed to the atmosphere. Since the first 25m near the beam window is a high-radiation area inaccessible to staff, no vacuum acquisition or measurement equipment is installed in this section within the shielding wall; the entire section is pumped by a 1000L/s turbomolecular pump outside the shielding wall. Other sections are equipped with one 200L/s ion pump every 6m on average, achieving an average pressure of 8×10-6Pa. 6. Rapid Cycling Synchrotron (RCS) The rapid cycling synchrotron accepts negative hydrogen ions from the linear accelerator and converts them into protons through a stripping foil. First, protons accumulate in the RCS ring to increase the pulse current; then they are accelerated, with energy increasing from 80MeV to 1.6GeV; finally, high-energy proton beams are extracted from the ring. The RCS has a total length of about 228m, divided into 8 sections by 8 all-metal gate valves. Each section is equipped with pre-pumping valves and vent valves, and vacuum is pre-pumped using mobile turbomolecular pump units; for long sections, 3 turbomolecular pump units are used simultaneously. Vacuum measurement also uses cold cathode gauges, and some sections are equipped with residual gas analyzers to monitor system operation. A total of 41 ion pumps with pumping speeds of 300-1000L/s are installed in the RCS, achieving an average pressure better than 5×10-6Pa. Since the magnetic fields of the dipole and quadrupole magnets in the RCS change rapidly, the vacuum chambers in these magnets must limit eddy currents