Ion trap is a device that uses the interaction between charge and electromagnetic field to restrain the movement of charged particles. It is often used in atomic spectroscopy research to improve the accuracy of spectral measurement. In order to study certain ions, the ion trap must be placed in an ultra-high vacuum (better than 5. 10-10Pa) environment; in addition, the thermal motion of the molecules is closely related to the temperature, and the lower the temperature, the less violent the molecular motion. In order to weaken the movement ability of the studied ions, the ion trap must be placed in a low temperature (less than 4.2K) environment. That is, the ultra-low temperature ultra-high vacuum environment is indispensable for the ion trap device. We designed an ion trap low temperature ultra-high vacuum system, which is simple to operate and highly reliable. After performance testing, all indicators are better than the technical indicators.
2 Technical specifications; (2) vacuum chamber vacuum working temperature: better than 6.0x10-10Pa (3) ion trap outer surface temperature: less than 4.2K. 3 system composition system consists of vacuum chamber, vacuum pumping system (including Dry pump, molecular pump, ion pump and titanium sublimation pump compound pump), pulse tube refrigerator, temperature monitoring and control equipment, vacuum measurement and display equipment and ultra-high vacuum manual valve and other components.
3.1 Vacuum Chamber The vacuum chamber main function is to provide ultra-high vacuum and low temperature environment for ion trap work. As shown in the figure, the vacuum chamber is divided into two upper and lower sections, and the upper and lower chambers are connected by CF200 flanges. The upper cylinder of the vacuum chamber is arranged with two electrical feedthrough interfaces for measurement of temperature and vacuum and energization of the ion trap. The seal type is CF16 knife-edge seal. The cold head of the refrigerator is connected to the upper section of the vacuum chamber, which is limited by the cold head structure of the refrigerator. Here, the seal can only select the “0†ring and the material is fluorine rubber. The ion trap is attached to the bottom of the secondary cold head of the refrigerator, with an indium sheet between the two to reduce the thermal contact resistance. The radiation protection screen is connected to a cold head of the refrigerator to reduce the radiation leakage heat m. The radiation protection screen material is copper, and the whole nickel plating treatment. The lower part of the vacuum chamber is equipped with a vacuum extraction port, a resistance gauge tube interface, an ionization gauge tube interface, and an optical window interface. The seals of these interfaces are in different sizes of CF flange seals. The vacuum pumping port connects four ultra-high vacuum manual Valves through the CF40 four-way valve. They are used to separate or communicate with the vacuum chamber body, and to isolate or communicate with the dry pump and molecular pump unit, and to cut off with the ion pump and the titanium sublimation pump compound pump. Connected, disconnected or connected with the atmosphere.
Vacuum cavity three-dimensional design. 2 Vacuum evacuation system The vacuum evacuation system includes a pre-evacuation system and a main evacuation system. V is connected to the vacuum chamber through CF40 metal bellows. In order to ensure that the entire vacuum system is clean and oil-free in the entire area, the pre-evacuation system consists of a dry pump (Agilent) and a turbomolecular pump (Agilent). A solenoid valve is installed between the dry pump and the molecular pump to prevent the vacuum system from being suddenly powered off. The atmosphere enters the molecular pump and damages the high speed rotating molecular pump blades. Since the vacuum chamber is equipped with a refrigerator cold head, the heating temperature of the apparatus cannot be higher than 100°C, thereby lowering the baking temperature of the entire system. The molecular pump in the design is only used as a pre-pumping pump for the main pumping system. The opening pressure of the main pumping system is 5.0X10-6Pa, and the effective pumping speed of the pre-evacuation system required by the vacuum chamber body is calculated to select the pumping speed and limit pressure of the turbomolecular pump.
The relationship between the compression ratio of the turbo-molecular pump is as follows: From the above equation, the relationship between the compression ratio of the turbo-molecular pump and the molecular mass of the gas is the greater the mass of the gas molecule under the conditions of constant rotation speed and geometric ratio of the pump ratio. The greater the compression ratio, the smaller it is. For some light-mass gases, its compression ratio is small. From the limit pressure of turbo-molecular pump (Qiao) 0: From (1) and (2), we can see: PfiP/e, the higher the mass of the gas, the lower the partial pressure obtained, and vice versa, the lower the mass of the gas. The higher the partial pressure it gets. Therefore, in a vacuum system pumped by a turbomolecular pump, the main components of the residual gas are some light gases, mainly hydrogen. At the same time, in the ultra-high vacuum system, the gaseous components emitted by the material are also mainly hydrogen 3 , which limits the improvement of ultimate vacuum. In the vacuum obtaining equipment, the titanium sublimation pump is an oil-free vacuum pump with a simple structure, low cost, easy to use, and high pumping speed for hydrogen and other active gases. Titanium sublimation pump is selected to make up for turbo molecular pump with small capacity for hydrogen pumping. Disadvantages, while taking into account the influence of the inert gas with small molecular mass on the ultimate vacuum of the vacuum system, the sputtering ion pump has a large pumping speed for the inert gas. The ion pump and titanium sublimation pump compound pump are selected as the main pumping system of the vacuum system.
In order to minimize the consumption of titanium to extend the service life of titanium sublimators, the starting pressure of the main pumping system in the equipment is 5.0x106Pa. The pre-evacuation system is used to discharge a large number of gas loads and air in the vacuum chamber and the compound pump space. The residual moisture. The main pumping system is mainly used for exhausting in the ultra-high vacuum stage.
3.3 Temperature Measurement and Control The Model 336 temperature controller is calibrated with the calibrated DT670 diode temperature sensor to measure the temperature of the wall surface of the ion trap. The heating resistor is placed on the wall surface of the ion trap to control the temperature of the ion trap.
3.4 Vacuum Measurement According to the ultimate vacuum requirements, select the Agilent convectTorr model for low vacuum measurement with a range of 1.0X10 -1 ~105Pa. Select the Agilent UHV-24P model for high vacuum measurement with a range of 6.7x10 - 10 vacuum Meter for display. Both regulators are calibrated before use.
3.5 Pulse Tube Refrigerator According to the radiation heat leakage of the vacuum chamber, considering the influence of vibration on the work of the ion trap, select the PT415-RM type pulse tube refrigerator to reduce the temperature of the ion trap. The minimum temperature of the refrigerator under no load can reach 2.8K. Volume index is 1. 4 system assembly, debugging and results analysis 4.1 system assembly System assembly in a clean room, before the vacuum chamber assembly, all stainless steel parts in accordance with ultra-high vacuum system cleaning process cleaning, and placed in the oven to bake Degassing, heating temperature is 250°C, duration is 24h. TIG welding is used for all welding on the vacuum chamber, and the weld is leak checked with a helium mass spectrometer leak detector to ensure that each weld and each flange is sealed The leak rate is less than 1.0x10-8Pa L/s, ensuring that the leak rate of the entire system is less than L/s. The ambient temperature is around 20C. After the preparations are checked, the test is started.
4.2 Debugging process In the debugging process, the ultimate vacuum of the vacuum chamber was measured under different conditions.
The pre-evacuation system is turned on, the main pumping is not turned on, and the system is not baked After the assembly of the system is completed, all parts are not baked out, the purge valve is closed, the remaining valves are opened, and the dry pump is started, when the vacuum chamber vacuum is low Open the molecular pump at 1Pa, the vacuum degree of the vacuum chamber of 2h is 8.0x10-4Pa, the vacuum degree of the vacuum chamber of 8h is 9.0x10-6Pa, the vacuum degree of 12h is -6Pa, and the ultimate vacuum degree of the vacuum chamber under this condition is 5.0x10-6Pa. is the actual vacuum measurement curve after the molecular pump is turned on.
The pre-evacuation system is turned on, the main pumping system is turned on, and the system is not baked. On the basis of the above tests, the ion pump and the titanium sublimation pump are turned on. The sublimation current is 40 A. The sublimation current is once every 4 hours, and the sublimation time is 3 minutes. After 12 hours, the vacuum The ultimate vacuum of the chamber reaches 4.1×10-7Pa. It is the curve of the vacuum degree of the vacuum chamber after opening the main pumping system.
The pre-evacuation system is turned on, the main pumping system is turned on, and the system bakeout and degassing is based on the (2) test. The vacuum chamber and the evacuation pipeline are evenly wound around the heating belt, and the high-purity nitrogen gas is slowly charged into the vacuum chamber. The pressure in the vacuum chamber is slightly higher than the atmospheric pressure. The temperature increase rate is controlled by a pressure regulator. The temperature of the ion trap inside the vacuum chamber slowly rises to about 90C and maintains this temperature. Then the dry pump is turned on to vacuum the entire vacuum system. This process is maintained. The temperature on the evacuated pipe is about 150C. The heating device of the ion pump and titanium sublimation pump compound pump is used to maintain its temperature at about 200°C. After 24 hours, the molecular pump and the king pumping system are turned on. In this process, the titanium is sublimated. The pump titanium wire and ionization gauge tube were degassed H, and the heating was stopped after 24 hours. The system ultimate vacuum reached 1.9×10-8 Pa, meeting the technical specifications.
The degree of vacuum in the vacuum system reached 1.9x10-8Pa, the pulse tube refrigerator was turned on, and the ion trap temperature was monitored by a temperature controller. The chiller was turned on for 100 minutes and the ion trap temperature reached a low temperature of 3.9K to meet the specification requirements. At the same time, the ultimate vacuum degree of the vacuum system reached 5.0X1010Pa. As shown in the figure, the ion trap temperature changes with the opening time of the refrigerator. Shown as the system vacuum degree changes with the refrigerator opening time curve.
Ion Trap Temperature vs. chiller On Time Curve System Vacuum Degree vs. chiller On Time 4.3 Results Analysis Ion pump and titanium sublimation pump compound pump in the system play a significant role in improving the vacuum degree of the system. Only the dry pump is used. Pre-extraction systems with turbomolecular pumps do not meet the ultimate vacuum requirements; vacuum system baking is essential for obtaining ultra-high vacuum. 5. To achieve the best degassing effect, the dry pump is turned on. In the state, the baking time is not less than 24 hours, then the molecular pump is turned on, and the ion pump and the titanium sublimation pump compound pump continue to pump the vacuum system for 24 hours, and the titanium sublimation pump titanium wire and the ionization gauge tube are performed during the heating process. In addition to gas, after the system was returned to room temperature after the heating was stopped for 24 hours, the system reached the ultimate vacuum.
During the commissioning process, the oxygen-free copper mats (CF200 to CF16) of various types of CF flanges that have been used were used twice and leaked by the helium mass spectrometer leak detector. The flanges were used for the second time. Pre-tightening force is greater than the first time, and it can guarantee the leakage rate requirement of 1.0X10-8Pa L/s.
From the point of view, the chiller cools rapidly between the normal temperature state of chiller to 6.1K after 55 minutes, and the cooling rate of 3.9K after the lapse of 60 minutes after 4.2K to 75 minutes is slow, indicating that with the decrease of the system temperature, the vascular tube The cooling capacity of the refrigerator decreases. From the point of view, in the 15min before the chiller is turned on, the degree of vacuum is deteriorated, indicating that the temperature of the chiller cold head and ion trap is not low enough to condense the adsorbed gas. Instead, due to the period of time just after the chiller is turned on, the compressed gas is compressed. Heat causes the vacuum to deteriorate. Combining with the above, the pulse tube refrigerator is not only used as a cold source in the system, but also has the function of condensation adsorption. After the vacuum system reaches the ultimate vacuum, the physical process of condensing and adsorbing gas using the surface of the refrigerator cold head and ion trap is utilized. Get and maintain a higher degree of vacuum in the system. Vacuum system from the ultimate temperature of 1.9X10-8Pa ultimate vacuum to 5.0X10-10Pa ultimate vacuum at low temperature, increased by nearly two orders of magnitude, indicating that the temperature is an important factor affecting the ultimate vacuum vacuum system.
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