Design and experimental research of UHV flanges for the Hefei Advanced Light Facility

The Hefei Advanced Light Facility (HALF) is a diffrac-tion-limited storage ring (DLSR) light source based on the compact multi-bend achromat (MBA) lattice. There-fore, the gaps between those focusing magnets are small. The commonly used ConFlat® flange, with a large axis dimension, is not suitable for the compact lattice in HALF. In this work, a stainless steel tapered flange fas-tened by a chain clamp has been designed for its smaller axis dimension. Two types of sealing structures are used, which are knife-edge and spring-energized metal C-ring structures, respectively. The copper gaskets with and without silver coating are used for knife-edge flange, respectively. Besides, the spring-energized metal C-ring is manufactured by SUS 304 with a tin layer of 50 μm. These flanges and chain clamps were made of SUS 304, and their vacuum properties were tested. The results indi-cate that these UHV flanges can meet the demands for the vacuum system of HALF.


Introduction
The Hefei Advanced Light Facility under construction is a DLSR light source, using MBA lattice.The number of bending, quadrupole and sextupole magnets is larger than the double-bend achromat (DBA) and triple-bend achromat (TBA) lattice in third-generation light sources.Besides, the straight section was designed as long as possible.Therefore, the gaps between these focusing magnets are thin, which greatly limits the axis dimension of vacuum components.The commonly used CF flange was not suitable in this case, and a chain clamp flange with a thinner size was designed to be applied in space limited section.
The chain clamp flange, also called the quick discon-nect vacuum flange and proposed by Vacom® [1], is wide-ly used at CERN.Some EVAC chain clamp flanges have been widely used in the UHV system at Brookhaven Na-tional Laboratory [2].In 1993, a chain clamp flange sealed with a Helicoflex resilient metal seal was proposed for SSC and LHC [3].A large-sized chain clamp flange has been used in CSNS [4].Besides, in the DLSR vacuum system, the chain clamp flange has been applied [5][6].We have noticed that some commercial vendors offer chain clamp flanges, such as VACOM, EVAC, Kurt J.Lesker, JPHE, and so on.However, the cost of this flange compo-nent is pretty high.The funding for HALF is limited, so the R&D of the chain clamp flange has been listed in our pre-research project.The knife-edge structure of the CF flange has been widely used for several decades.This structure has high stability during bakeout and cryogenic conditions.Therefore, the knife edge was chosen as one type of seal-ing method.Considering the coupling impedance of flanges, the spring-energized metal Cring and sealing grooves were applied as the second type of vacuum seal-ing method.The sealing was achieved through the com-pression of the spring-energized metal C-ring, so the two flanges can fit together well to eliminate the gap.
In this paper, the chain clamp flanges with two types of sealing methods have been designed for HALF.The seal-ing performance was experimentally tested.The stainless steel SUS 304 was used to manufacture flanges and chain clamps.For knife-edge structures, the leakages for Oxy-gen-Free Copper (OFC) gaskets with and without silver coating are tested, respectively.The spring-energized metal Cring is made of SUS 304.These flanges have been mounted and unmounted five times to see their reliability.The sealing performance after bakeout cycles and in cryogenics liquid nitrogen was also tested.

Chain clamp knife-edge flange
The chain clamp knife-edge flange is modified from the CF flange and KF flange.Both the flange and the chain clamp are made of SUS 304.The gasket material is OFC.The gasket is soft with respect to the knife edge so that the vacuum seal can be acquired at a proper fas-tening torque.The surface roughness (Ra) of the knife edge and the copper gasket after CNC machining is under 0.8 and 1.6 microns, respectively.The flange taper angle is 15°.The design parameter of the knife edge was modified from the CF35 flange.Figure 1 depicts an example of the chain clamp knife-edge flang.The chain clamp consists of four links, three straps, seven pins, and a mod-ified M6 bolt.The inner radius of the gasket is modified to fit the inner profile of the vacuum chamber, reducing coupling impedance.

RF-shielded Structure of spring-energized metal C-ring
The gap between the two flanges is one of the main sources of coupling impedance.Although the impedance caused by a UHV flange is negligible, the number of UHV flanges used in the storage ring is large, which causes considerable coupling impedance.Although some modi-fications have been done to the CF flange to achieve a zero gap, the structure is complex and less reliable for UHV sealing.In this work, the spring-energized metal C-ring was chosen.According to its sealing principle, the sealing property of this metal C-ring is directly related to its compression.By controlling the compression, the vacuum seal can be acquired while two flange surfaces contact each other.Figure 2 shows the design principle of the sealing structure.
In order to achieve a zero gap, either the flange or gas-ket should be modified [7][8].For a modified flange, its ends near the vacuum chamber contact each other.For a modified gasket, the gasket ends at its inner radius should contact the flanges along the circumference of the vacu-um chamber's inner radius.However, such contacts can share part of the sealing force from the knife edge, which is bad for UHV sealing.With regard to the spring-energized metal C-ring, it contains an inner spring, which has good rebound resilience.Its sealing properties are highly related to the load-compression curve of the metal C-ring.Therefore, the groove depth must be carefully chosen to guarantee sufficient compression.Even if the flange contact each other, the inner spring can hold the sealing force and ensure UHV sealing.

Repeated sealing test
The chain clamp knife-edge flange and chain clamp flange sealed with spring-energized metal C-ring were manufactured for sealing performance tests.Both of these two kinds of flanges are made of SUS 304.The knife-edge flange was repeatedly sealed five times using copper gaskets with and without silver coating, respectively.And the flange sealed with spring-energized metal C-ring was also repeatedly sealed five times.During these tests, the fastening torque and detector readings were recorded.Figure 3 shows the test results of these two types of flanges.The vacuum seal was achieved at a fastening torque of approximately 5-9 N m for the flange sealed with the copper gasket, as shown in figure 3 (a).And the vacuum seal was achieved at a fastening torque of approximately 7-10 N m for the flange sealed with the copper gasket coated with silver, as shown in figure 3 (b).Besides, the vacuum seal was achieved at a fastening torque of ap-proximately 5-7 N m for the flange sealed with spring-energized metal C-ring, as shown in figure 3 (c).The test-ed results reveal that all the leak rates of these three tests are below 1×10 -12 mbar•L/s, which is the minimum reading of the helium mass spectrometer leak detector.In addi-tion, the fastening torques of the knife-edge flange are slightly higher than the flange sealed with a metal C-ring.The two types of copper gaskets were sealed with the same flange.And the

Bakeout cycle and liquid nitrogen tests
In order to testify the reliability of chain clamp flanges, the knife-edge flange with copper gasket has been baked and put in liquid nitrogen.For bakeout experiments, a total of 15 bakeout cycles were carried out, and the flange was heated from room temperature (about 25 ℃) to 300 ℃, holding for one hour at the highest temperature.And the flange was tested by injecting gaseous helium into the junction and putting the flange into the plastic bag filled with gaseous helium.After the first, third, sev-enth, and fifteenth bakeout cycles, the helium test was performed.For the cryogenic experiment, the tested junction was put in the liquid nitrogen, and then the flange was taken out when the liquid was off the boil.The helium test was performed after the flange returned to room temperature.A total of seven cryogenic tests were carried out.Table 1 summarizes the test results of bakeout and cryogenic tests.The leak rates for all the tests are below 1×10 -10 mbar•L/s, which meets the vacuum system demands.

Finite element analysis
The finite element analysis was applied for the knife edge flange to investigate the stress condition.In order to enhance computing efficiency, a 1/4 model of chain clamp flange was used.A force load of 11.8 kN, corresponding to the fastening torque of 10 N m, is applied to the clamp.The simulation results of stress contours are shown in figure 4. The stress mainly concentrates on the knife edge, where the sealing happens.The stress on the clamp is slightly lower than that of the flange.

Conclusion
The chain clamp flange has been designed to suit the compact lattice of HALF.A sealing structure using the spring-energized metal C-ring was proposed to reduce the flange gap, which was the main reason for coupling impedance.The chain clamp knife-edge flanges and chain clamp flange sealed with springenergized metal C-ring were manufactured and tested.The results indicated that two types of chain clamp flanges could be repeatedly sealed.Besides, no leak happened under bakeout cycles and in liquid nitrogen for the chain clamp stainless steel flange.The tin coating is used for the spring-energized metal C-ring in the repeated sealing test, and the results show good sealing properties.However, the melting point of tin is about 232 ℃, which is not suitable for high-temperature bakeout.We have already ordered spring-energized metal C-rings with silver coatings.Further investigation on vacuum properties will be performed.In addition, future work will also focus on the measurement of coupling impedance for the RF shield flange and its structural development.

Figure 1 .
Figure 1.Drawings of the chain clamp knife-edge flange.

Figure 2 .
Figure 2. The design principle of a low-impedance flange applying the spring-energized metal C-ring.

Figure 3 .
Figure 3. Measured leak rates as a function of the fastening torque of the two chain clamp flanges, (a) is the copper gasket, (b) is the copper gasket coated with sliver, and (c) is the spring-energized metal Cring.

Figure 4 .
Figure 4. Stress contours of the finite element model.

Table 1 .
Leak rate tested results of the knife edge flange using the copper gasket.