Modification of the Vacuum System of Lanzhou Heavy Ion Accelerator Front Beam

Table 1 Front beam line and distribution Situation Segmental pipe distribution length Internal surface area volume cloth Pump condition (mm) Valve 1 Valve 2 Valve 2 Valve 4 Cryogenic pump B 4 Valve 4 from STR 4 Valve 6 Chilled pump H Ion pump 5 is located at device B1 ((:, 0,, ​​and can be determined according to the degree of vacuum required for retrofitting, and the required modification value of the gas load is given. Table 3 shows the measured value of the gas load before the modification and the required value after the modification.

Table 3 Gas load distribution of the vacuum system in the front beamline (The required value of the X-section measurement transformation is shown in Table 3. The difference between the in-section measurement value and the transformation requirement value is relatively large. This section is the focus of the transformation. For the sake of more intuition, you can Analysis: The gas load of the segment is much higher than that of other segments.The reasons are: the gas load of the stripper loaded with STRCHIRFL, and the STR is a large gas source; (2) the leakage phenomenon; (3) the layout of the vacuum pump is not Reasonably needs adjustment. In order to understand the source of gas load, residual gas mass spectrometry analysis was performed on stage I and the results are shown.

The mass spectrometry results show that the residual gas contains a large proportion of water vapor, nitrogen, oxygen, and carbon dioxide. This shows that the system still has a more serious leakage phenomenon. It can be seen that the comprehensive and systematic leak detection and trapping is a must-do work. The higher composition of the riftt gas indicates that the et pumping capacity of the ion pump has decreased, and the pump core needs to be cleaned or replaced. To this end, appropriate measures will be taken to help improve the vacuum in this section.

5 Pressure gradient of the vacuum system in the front beamline Generally, the common vacuum system has a large conductance in the vacuum chamber, and the pressure gradient formed in the vacuum chamber can be neglected. However, for the front beam vacuum system, the conductance is small, and the pressure gradient formed inside the tube determines the ultimate vacuum that the vacuum system can achieve. The pressure gradient in the vacuum chamber for a uniform gas load (primarily the surface of the material) is determined by the following equation as the area of ​​the vacuum chamber (cm2).

As can be seen from the above formula, the farther from the pump port, the smaller the conductance C, the greater the area A of the gas outlet, the greater the pressure difference; conversely, the smaller the ~.

Therefore, the reform requirements for the pump outlet pressure and the center plane pressure of the two pumps are proposed, that is, the pressure difference is controlled within a certain range. As an elongated vacuum system, the front beam line has a very small conductance, and the pressure gradient formed in the line determines the ultimate vacuum that the vacuum system can reach. The calculation of the pressure distribution uses a segmented calculation method, which is to calculate the pressure distribution separately for the 5 sections of pipelines separated by ultra-high vacuum valves. By formula Q/S (Q is the gas load measured in this segment, S is the effective pumping speed under the joint action of several vacuum pumps in the segment), the corresponding pressure value can be obtained. List of calculation results 4. Table 4 Pressure distribution before the transformation of the front toe line vacuum system (X106Pa) Code location (mm) Pressure code location (mm) Pressure code location (mm) Pressure shows the vacuum of each vacuum pump and the center plane of the two pumps. The difference between the actual value and the required value of the retrofit. Among them, the BC two pump center plane pressure is too high, this is because of the result of the load STR; at the same time, because under normal circumstances, one of the two pumps A, B is in standby state (AB distance is very close, at the same time work waste of resources), However, the distance between the two BC pumps is relatively long and no good pumping effect can be obtained. C~G pump outlet pressure is high, which is in line with the deviation of the gas load provided in the situation, so the key pipeline transformation should be addressed.

6 The reform measures and discussion of the vacuum system of the front beam line pipe We can see from the P==Q/Se that reducing the plant T can take 2 measures, namely reducing the total system A gas body load or the large effective pumping speed fork 6.1 increasing the effective pumping beam Se or optimize the performance and layout of the vacuum pump to install the two original ion pump R, S in the original pipe, to improve the vacuum condition of the in-section pipeline. This requires the processing of the pumping chamber and can be improved on the basis of the original pipeline.

Adjust the position of cryogenic pump B. Because the STR is a large-loaded gas source, the b pump is too far away, and indirectly increases the gas load of the n-stage pipeline; of course, the AB2 cryogenic pump works at the same time in close range, so that the vacuum condition of the I-stage pipeline is good, but it is undoubtedly Waste of resources. In actual work, one station is usually working normally and the other station is in standby mode.

Vacuum pump maintenance and updates. Cryopumps re-adhere to activated carbon; chillers and compressors are serviced and overhauled to meet power requirements. It is hoped that the pressure distribution index of the vacuum system before the pumping speed can reach 1500L-iT for N2 and 1000L-fi.1 for H2, which is the vacuum pump port test requirement value. For the two-pump central plane ion pump, the pump core and pump body should be cleaned, and the aging pump core should be replaced to maintain its performance. Refer to the test requirement value; X. is the length of the distance SFC, and the unit is mm. Molecular pump unit ultrasonic cleaning, replacement of pump oil.

6.2 Decrease the system total gas load Clean the inner surface of the front beam line. The front beam line pipe is removed piece by piece and sent to the special manufacturer for electrochemical cleaning and polishing. After electrochemical treatment, the parts are put into 2% to 5% ammonia water and neutralized for 15 to 20 seconds and then washed with water, and anhydrous ethanol is dehydrated. ,drying. The surface of the component that should not be disassembled has also been thoroughly scrubbed. Specific methods: first wipe with carbon tetrachloride, and then rub with acetone, then wipe with anhydrous ethanol, dehydration, after installation with a tungsten halogen lamp for baking. After the cleaning treatment, the total air load of the system can be reduced by at least 1/2, that is, it can reach 2. 6X103Pal'+k which includes the leakage part).

Grind and re-machine the sealing surface. All sealing flanges must undergo a rigorous inspection. All surfaces with low surface finish must be ground. Anything that needs rework must be processed in time to meet the sealing requirements. All sealed rubber strips were replaced with fluoroelastomers with a low outgassing rate. Replace the seal. Replace the seal between the two flat flanges with a rubber plate and seal with an O-ring. Metal seals can be sealed with metal. Check all vacuum gauges and valves, repair sealing surfaces, remove contaminants.

Comprehensive and systematic leak detection. From the mass spectrometry analysis, the system can be judged to be leaky. This needs to increase the effective sensitivity of the leak detector in the work, and it must be strictly monitored during the installation and debugging of the front beam line transformation to conduct a comprehensive and systematic leak detection; during the operation and use, the possible leakage of the vacuum system will be monitored. The n-segment leak is the most serious and is the focus of leak detection.

Vacuum baking. After the system is reinstalled, take baking measures. Considering the temperature to which the fluorine rubber seal can withstand, it can be baked with a heating tape to about 100C.

7 The reform of the front beamline vacuum system for the front beamline vacuum system was implemented from June 20, 2001 to September 10, 2001. The indexes after the transformation are as follows (measured on November 26, 2001): Total gas load. Before the transformation, the gas load measurement of the toe-line vacuum system was 6.1×103 PaLs, and the measured value after modification was 103 Pa°LfL. Table 5 lists the gas load values ​​after reconstruction.

Table 5 Gas load test results of the vacuum system after modification The surface gas output rate. Before the transformation, the surface gas exit rate of the front beam system vacuum system is 1. As shown in the figure, the B pump is moved to the dotted line; the original pipeline is filled with R and S 2 ion pumps (the dotted line shows the R and S pumps). In consideration of the reconstruction purpose of the buncher B1, the The cryogenic pump H at this location; the other pump positions remain unchanged.

List of pressure values ​​before transformation of each section of the front beam 6. Table 6 Pressure distribution of the vacuum system before the transformation of the toeline (X1(T6Pa) code location (mm) pressure code location (mm) pressure code location (mm) pressure is given The pressure distribution after the reform: For the key transformation areas, that is, the front beam vacuum system loaded by the vacuum pump C~G, the vacuum degree at the pump port and the vacuum level at the center plane of the two pumps have been significantly improved. Less than 5X1 (T6Pa two pump center plane pressure is less than 1X10-5Pa to meet the transformation requirements.

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