Secondary electron emission (SEE) is the process of electron emission from the surface of a solid material when it is bombarded by energized electrons. This phenomenon has significant implications in various fields such as high-power oscillators^[1-3], accelerators^[4-5], and aircrafts^[5], as it can cause microdischarge effects, electron cloud effects, and electric effects, leading to instability and reduced device lifespan. Researchers in accelerator, physics, and synchrotron radiation laboratories have been focusing on finding ways to inhibit SEE on material surfaces, and coatings have emerged as an effective solution for inhibiting SEE on dielectric surfaces^[6]. In a study conducted by Trucchi D.M. et al.^[7-8], diamond films were prepared using chemical vapor deposition, and the influence of grain orientation and crystal quality on SEE performance was investigated. They proposed a mathematical expression to calculate grain orientation and its corresponding relationship with the SEE coefficient. Liu Yu et al.^[9] coated a 40 nm-thick Ta-C film on a substrate using vacuum sputtering coating and found that the coated surface had a significantly smaller SEE coefficient compared to the uncoated surface. Wei Qiang et al.^[10] prepared MgO films and MgO/Au composite films on stainless steel substrates using magnetron sputtering. They discovered that the degradation rate of SEE materials was correlated with the conductivity of the coating. Their study also revealed the underlying mechanism behind the SEE-induced degradation of dielectric films. Although coatings have proven effective in inhibiting SEE, the coating process requires a controlled environment and may occur high costs. Additionally, controlling the thickness of some films can be challenging. Excessively thick coatings are prone to detachment, while excessively thin coatings may not effectively reduce the SEE rate. A recent approach proposed by scholars is the use of laser etching to restrain SEE on material surfaces. Valizadeh R et al.^[11] processed laser-etched copper, aluminum, and stainless steel materials, and observed a significant decrease in the secondary electron yield on the surface of these materials. This laser etching process successfully reduced the SEE rate on all three materials. The application of laser etching to beam components in FCC-hh at CERN greatly reduced the risk of electron clouds in particle accelerators. Wang Yi-gang et al.^[12] used laser etching to create a regular, nearly triangular micrometer groove structure on a copper surface, achieving a secondary electron yield of less than 1. Laser etching offers advantages over coating and other techniques as it can be performed in-situ in air, with lower cost and higher repeatability. The surfaces treated with laser etching show good stability. Laser etching is suitable for treating the surfaces of commonly used materials in accelerators. However, when attempting to fabricate micro-trap structures with a large depth-to-diameter ratio, laser etching may lead to ablation on the substrate surface and the production of slags.

Laser-assisted water jet technology is a new technique that combines laser beams and water jets to remove materials from a surface^[13-14]. This technology involves using a laser to heat and soften the material, followed by a water jet to wash away the softened materials, resulting in a smooth cut, a small heat affected area, and a high-quality surface. By increasing the laser power, the depth of the groove created also increases. The water jet helps to remove any slags and cools the area, preventing the groove width from enlarging significantly. Therefore, this technology is capable of fabricating micro-grooves with a large depth-to-diameter ratio. Compared to other techniques such as water jet-guided laser cutting, laser-assisted water jet technology utilizes higher water jet pressure, a smaller nozzle-to-target distance, and smaller nozzle offset^[15]. This allows for more precise and controlled cutting. Several studies have applied laser-assisted water jet technology to various materials. Hui Qing-zhi et al.^[16] used this technology to optimize the process parameters for 4H-SiC and produced a micro-channel with a depth-width ratio of 3∶1. Feng Shao-chuan et al.^[17] constructed a micro-groove structure with a depth-width ratio of 4∶1 on the surface of single-crystal SiC. Zhang Chen-xu et al.^[18] successfully produced a micro-groove with a large depth-width ratio on the surface of alumina ceramic materials. It has been proved that micro-grooves on stainless steel surfaces can significantly reduce SEE^[11]. However, the currently used methods^[19-20] for processing stainless steel, such as laser etching, abrasive water jet, and fiber laser underwater cutting, have limitations. Laser etching with nanosecond UV lasers can achieve precise control of micro-groove size, but the material easily oxidizes and shows liquefaction and splashing effects. Abrasive water jet methods can modify the surface morphology and properties of stainless steel but cannot produce micro-grooves with a high depth-width ratio. Fiber laser underwater cutting can cut steel plates of different thicknesses but is not suitable for producing micro-grooves on stainless steel surfaces. Considering the advantages of laser-assisted water jet technology in creating micro-grooves with a large depth-to-diameter ratio, we applied it to process 316L stainless steel. It provides more methods for the production of micro-trap structures on material surfaces. Li Cheng-yuan et al.^[21] conducted a study on the processing of 316L stainless steel using a nanosecond UV laser. They were able to achieve precise control over the size of micro-grooves. However, a major drawback they encountered was the oxidation of the material, which increased nonlinearly with the number of scans. Additionally, the processed area exhibited liquefaction and splashing, while the unprocessed portion was covered in molten slags that had splashed from the processed area. Duan Ren-hui et al.^[22] attempted to modify the surface morphology and properties of 316L stainless steel using the abrasive water jet method. Unfortunately, they were unable to produce micro-grooves with a high depth-width ratio on the material’s surface. Li Qian et al.^[23] cut stainless steel plates using the fiber laser underwater cutting technology. They explored various aspects related to the process, such as heat transfer between stainless steel and the aqueous medium, phase change patterns, temperature distribution on the cutting surface, and the mechanism behind the formation of surface morphology. However, while the fiber laser underwater cutting technology proved effective for cutting steel plates of different thicknesses, it did not meet the requirements for producing micro-grooves on the surface of stainless steel. Currently, there are few studies that apply laser-assisted water jet technology to treat 316L stainless steel. Taking into account the limitations of other methods and the advantages of laser-assisted water jet technology, particularly its ability to construct micro-grooves with a large depth-to-diameter ratio, this study employed the latter to process stainless steel, aiming to explore additional methods for producing micro-trap structures on material surfaces.

The effects of various parameters including laser repetition frequency, pulse duration, average power, water jet pressure, repeat times, nozzle offset, nozzle-to-target distance, focal position, offset distance between grooves, and processing speed were systematically analyzed. The study aimed to investigate the influence of those parameters on the morphology, groove depth, groove width, and the width of the ridge between grooves on the stainless steel surface. Additionally, the machining process of a pound sign-shaped trap structure was optimized in this study. We have tested the SEE coefficients of the "well" structure. The obtained results offer valuable insights for the development of micro-trap structures on material surfaces. Furthermore, laser-assisted water jet processing technology exhibits great potential in combating SEE on metal surfaces.

The test material is 316L stainless steel with 10 mm×10 mm×1 mm. The tensile strength of this material is 515 MPa at room temperature, the yield strength is 205 MPa, the elongation is 30%, and the shrinkage rate is 50%. Before and after processing, the workpiece was ultrasonic cleaned in ethanol solution for ten minutes. Fig. 1 shows the experiment and principle diagram.

The micro-grooving of 316 L stainless steel surfaces was processed by laser-assisted water jet equipment. The equipment includes a nanosecond pulsed fiber laser (RFL-P100M), a precision motion control system, a compound cutting head and a water jet booster in four parts. The laser is a Gaussian beam with a wavelength of 1064 nm and a maximum average power of 104 W. The processing area of the equipment is 300 mm×300 mm with a sensitivity of 1 μm. The water jet booster is driven by pressurized air with a maximum jet pressure of 40 MPa. The composite cutting head consists of a laser head, a water jet nozzle and a three-axis precision platform. The laser head and the water jet nozzle are fixed on the three-axis precision platform at the same time. The three-axis precision platform can adjust the angle and position between the laser head and the water jet nozzle, with an accuracy of 1 μm and the nozzle inner diameter of 300 μm. According to the preliminary experiment, the repetition frequencies of 490 kHz, 315 kHz and 100 kHz which can process the target groove are selected for the experiment. After processing, the groove depth, groove width, groove spacing and surface topography were characterized by VK-X200 laser confocal microscope (CLSM) to obtain the optimal parameter combination. The SEE coefficient is measured by irradiating the sample with a single pulse electron beam and using the collector electrode method. Test sample size is 10 mm × 10 mm, vacuum, energy range is from 50 eV to 2 keV. The specific parameters selected for the processing test are shown in Table 1.

When varying the repetitive frequency (either 490 kHz or 315 kHz), it was found that micro-grooves with discernible depth and width could only be observed when the power was adjusted to 40 W. The relationship between groove depth, width, and jet pressure at 40 W can be seen in Figure 2. Interestingly, micro-grooves produced at 315 kHz had a greater depth-width ratio compared to those at 490 kHz. Specifically, at 490 kHz, increasing the jet pressure from 4 MPa to 14 MPa resulted in a decrease in groove depth from 74.36 μm to 66.07 μm, and groove width from 45.52 μm to 42.9 μm. However, at 315 kHz, increasing the jet pressure from 4 MPa to 14 MPa led to a decrease in groove depth from 95.74 μm to 65.33 μm, and groove width from 44.39 μm to 41.11 μm. The rise in jet pressure led to a strengthening of the cooling effect, which in turn weakened the impact of the laser. Consequently, there was a decrease in the amount of material removed, resulting in a downward trend for both groove depth and width.

Figure 3 illustrates the changes in groove depth and width with power at a frequency of 100 kHz. Firstly, it was observed that the groove depth increased as the power increased. However, the rate of increase in groove depth was lower at a jet pressure of 4 MPa compared to other jet pressures. Furthermore, the rate of increase in groove depth remained constant as the jet pressure increased from 6 MPa to 14 MPa. For instance, when the jet pressure was set to 4 MPa, the groove depth increased from 77.57 μm to 120.41 μm as the laser power increased from 60 W to 100W. Similarly, at a jet pressure of 14 MPa, the groove depth increased from 59.72 μm to 122.4 μm as the laser power increased from 60W to 100W. Secondly, the groove width also increased with the rising power. The rate of increase in the groove width remained consistent across jet pressures ranging from 4 MPa to 14 MPa. At a laser power of 100 W, the largest groove width of 71.35 μm was observed at a jet pressure of 14 MPa. Additionally, the material removal rate increased with the rising power at the same jet pressure, resulting in an overall increase in groove depth and width. This increase was particularly significant.

The changing trend of groove depth and width with repeat times is depicted in Figure 4 at a frequency of 100 kHz. The groove depth showed minimal variation with an increase in repeat times, regardless of the jet pressure. Conversely, when the jet pressure remained unchanged, the groove width increased to a varying degree as the number of repeat times increased. However, the groove width decreased with an increase in jet pressure from 4 MPa to 12 MPa, even at the same number of repeat times. Therefore, the curves displayed a slight increase in groove depth and a significant increase in groove width with an increasing number of repeat times. The laser-assisted water jet technology primarily removes materials through laser ablation, and the focal position of the laser remains constant with an increasing number of repeat times if the laser is at a fixed distance from the workpiece. Thus, the repeat times have no effect on the groove depth. As the number of repeat times increases, the area of laser radiation expands, resulting in an increase in the groove width.

The Figures 5 and 6 demonstrate the variations in groove depth and width and the surface morphology of the material when the nozzle offset is 0 and the nozzle-to-target distance is 0.6 mm. It can be observed that the micro-groove depth increased as the power increased, while the jet pressure remained constant. Interestingly, the groove depth was found to be larger at a low processing speed compared to a high processing speed, when the power and jet pressure were kept constant. This can be attributed to the fact that at a low processing speed, the workpiece received a higher amount of energy from the laser in a given time interval. Furthermore, for the same power and processing speed, the groove depth exhibited a significant decrease as the jet pressure increased, especially when the jet pressure exceeded 16 MPa. This decline can be attributed to the enhanced cooling effect of the jet at higher pressures, which ultimately reduces the laser's effect. In conditions where the processing speed is 1 mm/s, the jet pressure exceeds 18 MPa, and the power is below 40 W, or when the processing speed is 2 mm/s, the jet pressure exceeds 16 MPa, and the power is below 40 W, the groove depth is consistently smaller than 50 μm. Similarly, when the processing speed is increased to 3 mm/s, the groove depth remains smaller than 50 μm at jet pressures exceeding 16 MPa, regardless of the specific pressure value.

The micro-groove width increased proportionally with power when the jet pressure remained constant. Interestingly, when comparing processing speeds, the groove width was found to be larger at lower speeds than at higher speeds under consistent conditions. Additionally, as the processing speed increased, the distribution of groove width values became more scattered. In specific conditions where the processing speed was set at 1 mm/s, the jet pressure exceeded 16 MPa, and the power was below 30 W, or when the processing speed was 2 mm/s and the jet pressure was above 16 MPa, the groove width remained smaller than 50 μm. However, when the processing speed was set at 3 mm/s, the groove width sharply decreased as the jet pressure increased to surpass 16 MPa with the same power value. To visually demonstrate the surface morphologies under different conditions, Figure 7 (color online) illustrates the corresponding images. Figure 7(a) depicts the material surface without any micro-grooves. In Figure 7(b), numerous slags are observed on both sides of the micro-grooves. Figure 7(c) displays the surface morphology where the micro-grooves are incomplete and discontinuous. Finally, Figure 7(d) showcases micro-groove structures with excellent morphology, exhibiting a large depth-width ratio. Specifically, the depth measures 194.99 μm, and the width measures 101.17 μm.

The Figure 8 demonstrates the evolving trend of groove depth at the nozzle offset of 0 and the focal position of 0.035 mm. Under the same power and jet pressure, as the processing speed increased, there was a decrease in the groove depth until it reached a plateau. The rate of decrease in groove depth diminished with higher processing speeds. Conversely, with the same processing speed and jet pressure, the groove depth increased as the power increased. At the same power and processing speed, the groove depth diminished as the jet pressure increased, with the maximum depth achieved at a jet pressure of 4 MPa. Therefore, it is possible to obtain deep grooves under conditions of low processing speed, high laser power, and low jet pressure.

Figure 9 presents the change in groove width with the jet pressure at the nozzle offset of 0 and the focal position of 0.035 mm. When keeping the power and processing speed constant, the groove width diminished as the jet pressure increased, reaching its peak at 4 MPa. With the same power and jet pressure, the groove width decreased as the processing speed increased. The rate of decrease in groove width remained relatively constant despite increasing processing speeds. Finally, at the same jet pressure and processing speed, the groove width expanded with the power increment.

After careful parameter optimization, we successfully created pound sign-shaped micro-grooves using three different combinations of repetitive frequency and pulse duration: 100 kHz, 490 kHz, and 315 kHz. The morphology of these grooves is illustrated in Figure 10 (color online). However, we observed that the quality of the micro-grooves produced at 100 kHz was unsatisfactory, as it exhibited spiny bulges at the bottom. On the other hand, the micro-grooves generated at 490 kHz and 315 kHz displayed a regular morphology with smooth bottoms and edges. For the micro-groove created at 100 kHz, which featured the highest depth-width ratio, the depth measured 91.4 μm, while the width stood at 48.77 μm. The micro-groove crafted at 490 kHz showcased a depth of 78.57 μm and a width of 41.11 μm, again, the maximum depth-width ratio among all the variations. Lastly, the micro-groove formed at 315 kHz stood out with a depth of 95.74 μm and a width of 41.44 μm, also achieving the highest depth-width ratio. Comparing the three parameter combinations, we found that the micro-grooves created at 315 kHz exhibited the largest depth-width ratio and showcased the most regular morphology, with smooth bottoms and edges. The micro-grooves crafted at 490 kHz displayed a slightly higher depth-width ratio and a more regular shape compared to those formed at 100 kHz. The micro-grooves produced at 100 kHz displayed the lowest depth-width ratio of 1.87 while failing to achieve the desired pound sign shape. Furthermore, in forming the pound sign shape, micro-grooves with different spacing deviated and intersected. Interestingly, we observed that the inner ridge width averaged around 4-5 μm smaller than the outer ridge width. By carefully analyzing and comparing the results, we concluded that the 315 kHz parameter combination yielded the best results, with the micro-grooves exhibiting the highest depth-width ratio, the most regular morphology, and the smoothest bottom and edges. The findings from this study provide valuable insights for further applications and optimizations in micro-groove fabrication techniques.

Baced on the above optimization results, the 3 mm×3 mm pond sign-shaped structure was processed by using the 315 kHz parameter with the highest aspect ratio and the best morphology, as shown in Figure 11 (color online). The SEE coefficient of 316L stainless steel before and after processing was tested. Under the same energy, the SEE coefficient of the "well" structure is higher than that of the unprocessed surface. During the process of increasing energy from 50 eV to 2 keV, the SEE coefficient before and after processing showed the same trend of first increasing and then decreasing, and the increase rate was greater than the decrease rate, as shown in Figure 12. Within the range of 50−500 eV and 1700−2000 eV, there is a significant difference in the SEE coefficient before and after processing. When the energy is 250 eV, the SEE coefficient after processing decreases from 2.20 to 1.81. The SEE is suppressed to the great extent.

The laser-assisted water jet method was employed to process 316L stainless steel in this study. By optimizing various parameters, such as laser repetition frequency, pulse duration, average power, water jet pressure, repeat times, nozzle offset, focal position, offset distance between grooves, and processing speed, well-defined pound sign-shaped trap structures of consistent size were successfully fabricated. The research also examined the impact of these parameters on the stainless steel surface morphology, groove depth, groove width, and the overall morphology of the pound sign-shaped trap structures. The results revealed that the number of repeat times had minimal influence on the groove depth while significantly affecting the groove width when the laser power and jet pressure were kept constant. Moreover, when the laser power and repeat times were held steady, increasing the jet pressure led to a decrease in both groove depth and width. Conversely, at a fixed processing speed and jet pressure, groove depth and width increased with the rising laser power. It was evident that by carefully optimizing the parameters, micro-trap structures with superior quality and a large depth-width ratio can be achieved. Through the test, the SEE coefficient of the pround sign structure is reduced by 0.5 at most. The SEE coefficient is reduced from 2.20 to 1.81 at 250 eV. The SEE on that surface of the stainless steel is greatly inhibit. The application of the laser-assisted water jet technique to construct micro-trap structures on the surface of 316L stainless steel, a critical component of particle accelerators, holds significant promise in addressing the issue of SEE on metal surfaces. As SEE is a prominent challenge in the development of the next-generation accelerators, this method offers a potential solution.