RITU-cave vacuum guide FYSS320 group reports Ulrika Jakobsson, Steffen Ketelhut, Pauli Peura, Aki Puurunen, Panu Ruotsalainen Supervisor - Juha Uusitalo Revised May 2010 edition (v. 1.4)
Contents 1 Vacuum system of RITU 2 1.1 Vacuum pumps.......................... 2 1.1.1 Rotary vane pumps.................... 2 1.1.2 Turbomolecular pumps.................. 3 1.1.3 Roots type of pumps................... 5 1.1.4 Scroll pump........................ 5 1.2 Pressure gauges.......................... 6 1.2.1 Pirani sensors....................... 6 1.2.2 Penning sensors...................... 7 1.3 Structure of the pumping system................ 8 1.3.1 RITU in differential pumping mode.......... 8 1.3.2 RITU in gas window mode................ 9 2 Differential pumping 10 2.1 Pumping down RITU, target chamber and beam line..... 10 2.2 Venting RITU, target chamber and beam line......... 12 3 Window system 14 3.1 Pumping down the beam line in window mode......... 14 3.2 Opening the target chamber in window mode......... 16 4 The focal plane vacuum setup 18 4.1 Operation of valves and gauges................. 19 4.2 Pumping down the focal plane.................. 19 4.3 Turning on the gas counter.................... 20 4.4 Turning off the gas counter.................... 22 4.5 Venting the focal plane...................... 22 1
Chapter 1 Vacuum system of RITU 1.1 Vacuum pumps The vacuum in RITU, target chamber and in the beam line in front of the target chamber is generated by a combination of different type of vacuum pumps. The pumps used in the rough vacuum region and for pre-pumping are mainly rotary vane pumps. In the high vacuum region turbomolecular pumps are most commonly used. In the next sections all the pumps used in the RITU-cave and their working principles are briefly introduced. 1.1.1 Rotary vane pumps Single and two-stage rotary vane pumps are mainly used in rough and medium vacuum range. They can run as stand alone units or be utilized as backing pumps for Roots or turbomolecular pumps. Normally rotary vane pumps are used for creating a rough pre-vacuum ( 10 2 mbar) e.g. for turbomolecular pumps. A schematic diagram of a rotary vane pump is illustrated in figure 1.1. Gas from the vacuum chamber comes to the suction chamber through a vacuum connection (9). The gas will be compressed by the rotor (3) and vane (4) in the compression chamber (2). Finally the gas will exit through the exhaust (6). The compression chamber is sealed by an oil-covered valve (7) and on the other hand by the vanes of the rotor which are pushed towards the pump cylinder by the centrifugal force (1). One working cycle takes one full turn and during it two suction and two compression steps take place. The purpose of the oil is to lubricate the pump and in addition to seal the surface of the pump cylinder. 2
Figure 1.1: Function diagram of a rotary vane vacuum pump of single and two-stage design. 1 Pump cylinder; 2 Compression chamber; 3 Rotor; 4 Vane; 5 Gas ballast inlet; 6 Exhaust; 7 Valve; 8 Oil level; 9 Vacuum connection; 10 Connecting passage To reach a lower ultimate pressure, two-stage rotary vane pumps are used. The rotary vane pumps used in RITU-cave are: Pfeiffer-Balzers, DUO 016 B, two-stage RVP (works as a backing pump for the RITU s turbomolecular pump. This type of pump is also used as a backing pump for the furthest beam line turbomolecular pump, fig. 4.1 P1) Pfeiffer-Balzers, DUO 20, direct drive RVP (works as a backing pump for the turbomolecular pump which is located in front of the target chamber, fig. 4.1 P3) Pfeiffer-Balzers, DUO 10, direct drive RVP (works as a roughing pump for the target chamber and beam line, fig. 4.1 P7.) 1.1.2 Turbomolecular pumps Turbomolecular pumps belong basically into the group of mechanical pumps. The working region extends however to much lower pressures, approximately even down to 10 10 mbar. Turbomolecular pumps consist of turbinetype blades rotating between blades of the stator. The gas molecules enter through the inlet and after that the blades of the rotor hit the molecules. This changes the momentum of the 3
Figure 1.2: Schematic drawing of the axial turbomolecular pump. molecules and causes them to move into the gas transfer holes in the stator. After that the molecules come to the next stage where they again collide with the blades of the rotor. This same process is repeated until the molecules finally go outwards through the exhaust. To make the gas molecules go downward, the speed of the rotor needs to be of the same order of magnitude as the thermal speed of the molecules i.e. (100 500) m/s. This naturally sets quite remarkable requirements for the mechanical structure of the pump. A schematic drawing of a turbomolecular pump is illustrated in figure 1.2 Turbomolecular pumps used in RITU-cave are: Pfeiffer-Balzers TPH 240 (works as RITU s turbo, the same kind of turbo is also installed in the beam line, fig. 4.1 T4) Pfeifer-Balzers TMH 1001 P (this turbo is nearest to the target chamber, fig. 4.1 T3) Pfeiffer-Balzers TP H 520 (this turbo is further upstream in the beam line, fig. 4.1 T1) 4
Figure 1.3: The working principle of the Roots pump 1.1.3 Roots type of pumps The Roots pump is normally used together with the rotary vane pump, especially when higher pumping speed and lower end-pressure is desired. Roots pumps consist of two eight-shaped rotors which rotate in opposite directions. The rotors do not touch each other or the pump walls so they are not subject to abrasion. The structure and the working principle of the Roots pump is illustrated in figure 1.3. As the rotors turn, the right-hand side rotor absorbs the air from the entry line into the volume between the rotor and the pump wall (fig. 1.3, 1.). When the rotors are at 45, the pump communicates with the inlet valve and closes it (fig. 1.3, 2.). As the rotors turn further, the absorbed air is compressed and taken out from the pump (fig. 1.3, 3. and 4.). The Roots pumping station used is Edwards Mechanical Booster EH 1200. 1.1.4 Scroll pump A scroll pump uses two interleaved spiral-like vanes to pump gases. In the ordinary scroll compressor, one of the scrolls is made stationary and the other 5
Figure 1.4: The working principle of the scroll pump is subject to an orbital movement with respect to the stationary scroll to effect the compression. The working principle of the scroll pump is illustrated in figure 1.4. 1.2 Pressure gauges In the RITU vacuum system the pressure range is from 10 7 mbar to air pressure. This range is covered by three different kinds of pressure gauges: mechanical pressure gauge, pirani gauge and penning gauge. The mechanical pressure gauge is a direct measurement gauge with a measuring range of (10 1000) mbar. Pirani and penning gauges are indirect measurement gauges which cover the range from 100 mbar to 10 7 mbar. 1.2.1 Pirani sensors Pirani gauge is based on thermal conductivity of the gas. The head of the gauge consists of a electrically heated metal wire placed in a vacuum system. In a molecular flow region (that is when the mean free path of the molecules in vacuum is larger than the separation of the surfaces) the rate of heat 6
conduction between the two surfaces by gas molecules is proportional to the pressure. As gas molecules are removed the heat flow from the wire is reduced and it heats up. The resistance of the wire is proportional to its temperature. The gas pressure can be determined by measuring changes in resistance. The measuring range of the Pirani gauge is 10 3 mbar 100 mbar. 1.2.2 Penning sensors Penning gauge is based on ionization of the gas. The Penning gauge is a cold cathode ionization gauge with parallel E and B fields. The pressure is computed from the anode current. The measuring range of the Penning gauge is 10 8 mbar 10 2 mbar. 7
1.3 Structure of the pumping system In this section, the structure of the vacuum system of RITU, target chamber and beam line is described. RITU can be operated in two kind of vacuum modes, differential pumping and gas window modes, which both are presented in the text. 1.3.1 RITU in differential pumping mode Differential pumping was introduced to get rid off the gas window which was a limiting factor in experiments where high beam intensities were used. Gas window namely caused e.g. scattering and energy straggling of the primary beam and also increased the counting rate of the Ge-detectors at the target area. RITU s vacuum system is illustrated in figure 4.1. On the beam line side there are three turbomolecular pumps which each have two-stage rotary vane pumps as backing pumps. Nearest to the target chamber there are Roots pump station which is an essential part of the differential pumping system. The volume of RITU is pumped by one turbo pump. Different kind of pressure gauges (mechanical, pirani and penning gauges) are located to the strategic places in the vacuum system to monitor the pressure from different parts of the system. The helium gas is released to the RITU from a helium bottle standing next to the separator. The rate of helium input and pressure (typically 0,3-1,0 mbar) inside RITU is controlled by a needle valve connected to the bottle. The Roots pump station in front of the target chamber pumps most of the helium coming from RITU through a 10-12 mm collimator. Roots pump station consists of 1020 m 3 /h Roots pump which is pre-pumped by two-stage rotary vane pump. Roots pump is used because it can take most of the volume flow of helium quickly. Nevertheless it doesn t have the cabability to reach pressures lower than of the order 10 5 mbar. That is why there is an effective turbomolecular pump (S(N 2 )=880 l/s) right after the Roots pump station. The turbo station pumps the beam line down to the 10 6 mbar - 10 7 mbar pressure region with two additional, smaller turbos. Now the constant flow of helium through RITU keeps the impurities inside the dipole chamber to a minimum. Helium flow through the target chamber helps also cooling down the target foil which allows measurements with higher beam intensities. 8
1.3.2 RITU in gas window mode Previously different vacuum and pressure regions were separated from each other with carbon or nickel foils. Nowadays it might still be necessary to use this system for example when we have detectors inside the target chamber and we have to keep the target chamber in as good vacuum as possible. The figure 4.2 illustrates the vacuum system when the RITU is operated in the gas window mode. In the gas window mode, a gas window is placed behind the target chamber to completely separate the beam line vacuum and the RITU s helium from each other. On the beam line side, vacuum is pumped by turbomolecular pumps as in the differential pumping mode. Now, however, the use of Roots pump station is unnecessary. In the RITU side, helium is still flowing through the separator but the mechanism is quite different. The helium is released to the dipole chamber through the needle valve and the mass-flow-meter (MFM). Now we need an additional pump (e.g rotary vane pump) to take the helium out of the system. Between the pump and the dipole chamber there is a butterfly valve which communicates with the MFM. The butterfly valve thereby opens and closes according to the MFM reading. For this reason gas flow and pressure inside RITU can be held constant. In the following chapter it is explained in detail what the procedures of the vacuum pumping in RITU, target chamber and beam line are. Because there are two vacuum modes in which RITU can be operated, two kinds of procedures are introduced according to the different vacuum modes. 9
Chapter 2 Differential pumping In this chapter it is explained how to pump down the beam line and the target chamber and set up a differential pumping to RITU. The notations for different parts of the vacuum system (valves, pumps, etc.) used in the text, correspond to the notations used in figure 4.1. 2.1 Pumping down RITU, target chamber and beam line Let s assume that the beam line and the target chamber are in air pressure (e.g. after alignment of the beam line and target) and RITU is in vacuum (as it normally is when not used). Now the valve in front of the first quadrupole (VR6) is closed. The whole volume of RITU is pumped by turbomolecular pump (Pfeiffer-Balzers TP H 240, T4 in fig. 4.1) and the valve VR7 is open. RITU is also separated from the focal plane detector system with VR8. On the beam line side, the valve beyond the wall of the cave (V14) is closed. All the valves (V5, V6, V7 and V8) which connect the three turbomolecular pumps (T1, T2 and T3) and the Roots pump (R1) to the beam line are also closed. The vacuum pumping should be done as follows: 1. First one can pump the volume between valves V14 and VR4. This is done by the rotary vane pump (e.g. Pfeiffer DUO 10, P7 in fig. 4.1). 2. Make sure that the nitrogen venting valve V4 and the valves V1, V2 and V3 are closed. After that start the pump and wait a few seconds before opening slowly valve V2. (Now it is not necessary to use the 10
needle valve V3 because there are no gas windows implemented. But if there is a danger to break the target foil, then it s reasonable to use the needle valve.) 3. One can see the pressure level dropping from the mechanical gauge. After it has reached the limit of its scale start monitor the pressure level from the Pirani gauge (lowest digital display below the beam line). Wait until the pressure level comes down to 10 2 mbar. This will take a few minutes. 4. Next one can open turbo pump s valve V7 and close valve V2. Shut down the roughing pump (P7) and vent it with N 2 by opening V4. Then close valves V1 and V4. Open also valve V14 for the turbo pump behind the wall of the RITU-cave. It has to be opened from the control room and it s called VR1 in the control system. Open V6. 5. Monitor the pressure level at the beginning with the Pirani gauge, then with the Penning gauge (one has to switch the sensor from the display, fig. 4.4, unit M29). Wait until the pressure drops down to 10 6 mbar. 6. The next thing is to start the Roots pump station. First start Roots pre-pump P6 by pressing green button on the side of the pump. Check the pressure of the Roots with the Pirani gauge, if it s round 10 2 mbar you can open the valve V8. Now everything is clear on the beam line side. 7. Check the pressure level in RITU (fig. 4.3, unit F), it should be 10 6 mbar, and open valve VR6 (the button is on the white box which is located between dipole and the second quadrupole). Then close the valve VR7 (this valve is operated from the same place as VR6) to separate the turbo from RITU. Now RITU is being pumped only by the beam line turbo and Roots pumps. Remember to close the Penning gauge located at the dipole chamber. 8. The next thing is to adjust the desired helium pressure inside RITU. First, open the helium bottle s main valve which is on top of the bottle. There are two additional valves from which one should open the smaller one. The bigger valve is a pressure regulator valve and it s normally already adjusted so don t touch it. 9. Check that the mass-flow-meters bypass valve V10 is open and the V11 closed. 11
10. Next open the needle valve V9. It s basically a two-stage valve with on/off type of trimmer (upper yellow ring) and needle valve part (lower yellow ring). First open the on/off trimmer and start opening needle valve gradually. 11. Check from unit B in fig. 4.3 (Balzers TPG-300) that the pressure in the beam line does not exceed 10 5 mbar. The pressure is mesured with Penning sensor located in the beam line between valves V6 and V7. 12. Monitor the RITU helium pressure from the black MKS 600 series unit (4.3, D) and adjust the pressure with the aid of the needle valve V9. 13. If the focal plane is ready for use (check instructions from chapter 4), open the valve VR8. 14. Now RITU runs in the differential pumping mode and one can open the valve V14 from the control room to get the beam to the target (valve V14 is VR1 in control room). 2.2 Venting RITU, target chamber and beam line Assume that RITU is running in the differential pumping mode and one wants to vent the whole system e.g because of the target foil change. Then following procedures should be performed: 1. If the gas counter is ON, DON T CONTINUE WITH THIS! Separate the focal plane detector system from RITU by closing valve VR8. 2. Close all the valves from the helium bottle which allow helium to flow to RITU (see items 8 and 10 in the previous section) and open valve VR7 which connects the turbo to RITU. At the same time one can close VR6 to separate RITU from the beam line (look item 7 in the previous section to see where to operate these valves). 3. Next close valve VR4 to separate target chamber from the beam line, close also V8 and stop the Roots pump station (If one is just changing the target it s not necessary to stop Roots pump). Close V14 (VR1 in control system) from the control room. This can be done for safety reasons but is not necessary. 12
4. Start venting the target chamber with N 2. Open valve V4 and start slowly opening needle valve V3. 5. Check the pressures from RITU and beam line (meters B and F in fig. 4.3) between valves VR4 and V14 to make sure that the valves don t leak. 6. If everything looks OK, one can open the needle valve V3 and continue venting the target chamber. When the pressure is high enough, open V2. If one wants to break the beam line in parts e.g for the target alignment, one must vent the beam line from VR4 to V14. Then 1. Close the valves V5, V6 and V7 to separate the turbos from the beam line. Make sure that the V14 is closed (from the cyclotron control room, VR1 in the system)! 2. There is another N 2 hose near the furthest turbo T1, which can be used to vent the beam line. The valve for this is V16. 3. VR4 can be opened. 4. Note: Remember to check that all Penning gauges are turned off before venting!! 13
Chapter 3 Window system In this chapter it is explained how to operate the RITU vacuum system in the window mode. Figure 4.2 shows the setup of RITU and beamline when using the window mode. The valve, turbo etc. numbering will refer to figure 4.2 in this chapter. In this chapter it is assumed that the beam line is already in vacuum. Instructions for pumping down the beam line can be found from chapter 2. Generally good speed for the change of vacuum pressure when pumping down or venting is 1 mbar / s. 3.1 Pumping down the beam line in window mode Let s assume that the target chamber is in air pressure and RITU is in vacuum. Now the valve VR6 in front of the first quadrupole is closed as is also valve VR4 which separates the beam line from target chamber. The whole volume of RITU is pumped by turbomolecular pump (Pfeiffer-Balzers TP H 240, T4 in fig. 4.2) and the valve VR7 is open. RITU is also separated from the focal plane detector system with VR8. The bypass valve V15 is open. The vacuum pumping should now be done according to the following procedure: 1. Close valves V1, V2, V3 and V4. 2. Start the prepump P7. 3. Check that the bypass valve V15 is open. 14
4. Slowly open the needle valve V3 to start carefully going down in pressure. Look at the mechanical pressure gauge for the rate of pressure change. Good rule of thumb is that the rate of change sould be about 1 mbar / s. 5. When opening the needle valve V3 does not decrease the pressure anymore like in the beginning, you can carefully open valve V2. You can look for the pressure also from the Pirani gauge when the pressure has dropped below 1 mbar. 6. When the pressure is at 10 2 mbar, open VR4. 7. Close valves V2 & V3. 8. Turn off prepump P7 and use the N 2 valve V4 to vent the the prepump. After venting close valves V1 & V4. 9. Open VR6. (white boxes below Q2 close to the RITU dipole for VR4 & VR6 operation). 10. If needed, focal plane can be filled with isobuthane at this point. Look for instructions from chapter 4. 11. Close the window system bypass valve V15. 12. Check that valve V12 closest to the butterfly valve is closed. Open other valves between the butterfly valve and the prepump P5. 13. Check that prepump P5 is on. If not, turn it on. 14. Put on the butterfly valve V13 by pushing the "open" button of MKS 600 series control unit (fig. 4.3, unit D, button "O"). 15. Close VR7, and check that the pressure reading of meter M27 (fig. 4.3, unit F) inside RITU dipole chamber goes up. 16. Open the valve V12 of the prepump P5 (if the dipole chamber pressure goes up very quickly at this point, the prepump P5 is not on or is not pumping properly). Now only the prepump pumps the dipole chamber. 17. Turn off the penning pressuge gauge from RITU dipole chamber (fig. 4.3, unit F) to prevent it from breaking when the pressure goes up. Change the display from Penning gauge to Pirani (from sensor B1 to A1 on unit F). 15
18. He-bottle: Leave the biggest valve untouched. Open the main valve of the He bottle and the small needle valve. 19. Open the He-bottle needle valve V9 (yellow valve, has text "balzers", NOT the valve just below this one) to let the helium in to RITU. 20. Put on the MKS Type 246 mass-flow meter from the on-off flip switch (fig. 4.3, unit E). Look immediantely that the beamline pressure at the the top-rightmost pressure meter is not going up. If pressure goes up, the window might be broken and you have to close the yellow He-bottle needle valve V9. 21. Select "Set point A" preset setting from the MKS 600 series control unit (fig. 4.3, unit D, button A). 22. You can check that the butterfly valve V13 is working by listening that it is making "crack-crack-crack" noise. 23. Check that the MKS meter is operating at the middle area on it s scale (fig. 4.3, unit D, look at the bottom part of the display). 24. Check the pressure meter readings to ensure that there are no leaks in the system (check fig. 4.3, units B & F and fig. 4.4, unit M29). 3.2 Opening the target chamber in window mode Now RITU has He gas inside and the beam line is in vacuum, so the gas has to be removed and the beam line to be isolated. 1. Close the yellow needle valve V9 by turning the lower part of the valve. Don t touch the middle part of the valve. 2. Turn the MKS Type 246 massflow meter off from flip-switch (fig. 4.3, unit E). 3. Turn on the butterfly valve V13 by pushing the "open" button from the middle of the black MKS meter. 4. He-bottle: Leave the biggest valve untouched. Close the main valve of the He bottle and the small needle valve. 5. When the pressure at RITU is at 10 2 mbar (figure 4.3, unit F) open the VR7 valve for RITU turbo. 16
6. Close valve V12. This ensures that oil from prepump P5 is not pumped from the prepump into RITU. 7. Change the pressure meter from Pirani to Penning (change from sensor A1 to B1), and check that the pressure is really going down (figure 4.3, unit F). 8. When pressure has gone down to 10 6 mbar, it s time to carefully open the bypass valve V15 of the window system between the target position and RITU. 9. Close valves VR4 and VR6. If the roots pump R1 is used, close the valve V8 to the roots pump also. 10. Check that valves V2 and V3 are closed. 11. Open valve V4. 12. Using needle valve V3 start to vent the target chamber slowly with N 2 gas. After you see that the pressure at Pirani gauge (meter station M29, figure 4.4) is going up, check that beamline and RITU pressures are not going up. Good rule of thumb is that the rate of change sould be about 1 mbar / s. 13. If pressures at beamline and RITU are ok (fig. 4.3, unit F and fig. 4.4, unit M29), continue opening the needle valve V3 slowly. Check pressures from Pirani gauge and eventually from mechanical pressure gauge. Desired speed for the pressure change is about 1 mbars 1. 14. When the target area is in air pressure, target chamber can be opened and i.e. target can be changed. 17
Chapter 4 The focal plane vacuum setup The GREAT chamber is pumped by a turbo pumpo P2. When the gas counter is not in use, it pumps the whole volume of the chamber. A turbo pump needs a vacuum of 10 2 mbar to start pumping, therefore a scroll pump P3 is used as a prepump (a second prepump P4 has also earlier been used for faster pumping). Another scroll pump P1 is used as a backing pump for P2. The turbo and the backing pump are constantly kept running. When not in use, they are separated from the rest of the chamber by gate valve VR9. A Pirani gauge is used to monitor the pressure between the turbo and its backing pump, just to make sure the backing pump and the turbo are working properly. Another Pirani gauge is set in the pumping line of the prepump. It is mainly used to follow the pumping. A Penning gauge monitors the chamber at low pressures, when the Pirani gauges are out of range. When the gas counter is used, isobutane is lead into the volume separated from the rest of the chamber by thin mylar windows. The isobutane is then circulated with the help of P3. Now the bypasses BP1 and BP2 are closed and the turbo only pumps the volume behind the gas counter. The volume adjacent to RITU is connected to the RITU vacuum system by opening gate valve VR8. The mylar windows can easily break when stressed by too high pressure changes, thus the GREAT chamber must be vented and pumped slowly. Two baratron meters are set to the gas inlet. One monitors the line in vacuum mode (it is used during pumping and venting) and the other one when gas is led into the gas counter. The chamber is vented through a needle valve using a nitrogen line. Nitrogen is a dry gas and therefore prevents moisture from being built up in the chamber. RITU itself is rarely vented, so VR8 is only kept open during 18
measurements. Instructions on how to pump down and vent the GREAT chamber are given in the following chapters. Included is also how to turn on and off the gas counter and how to handle the vacuum system when doing it. 4.1 Operation of valves and gauges The upper part of Fig. 4.5 shows a drawing of the vacuum setup at the focal plane, the lower one a drawing of the control rack next to the focal plane. The two gate valves VR8 (to RITU) and VR9 (to turbo pump P2) are pneumatic valves. They are controlled by two switches at the back of RITU. The piezo valve V6 is adjusted by unit C at the control rack, it regulates the gas flow. All other valves (V1-V5, V7, BP1, and BP2) are operated manually, V5 with a handle at unit D of the control rack. The setup includes three kinds of gauges: Diaphragm (or Betatron) gauges, Pirani gauge, and Penning gauges. They are read out by following equipment: - G1, G2, and G4 at unit A of the Control rack. - G3 has an analogue reading at the gauge head, and is not in use anymore. - G5 at the analogue reading of unit D. - G6 at unit C (reading is in 10 2 mbar or Pa). - G7 at unit B. - G8 and G9 are read out by a unit in the back of the Control rack. They are not in use anymore. 4.2 Pumping down the focal plane Before starting to pump down the focal plane, make sure that the chamber is fully sealed; Gate valves VR8 (to RITU) and VR9 (to the focal plane turbo P2) are closed and the gas counter bypasses BP1 and BP2 are open. Check that the "full range cable is connected to the back of screen C. 1. Check that needle valve V5 to prepump P3 is closed and that the needle valve V2 to the venting nitrogen line is closed. 19
2. Turn on prepump P3, or check that it is already on. If it needs turning on, just connect it to normal electricity. 3. Open valve V5 just slightly. The needle on screen D (connected to G5) should shoot up rapidly. 4. Turn to screen C (that is connected to G7). The pressure starts to drop slowly. Try to keep the pumping speed as slow as 1 mbar/s. Follow the change in pressure and open V5 carefully a little bit more whenever the pumping seems to slow down. At some point you need to turn to screen A (upper reading, connected to G2) to follow the pressure. 5. When the pressure has reached 10 1 mbar, turn on the roughing pump P4 underneath the chamber next to the turbo prepump. Plug the power cable in a free spot on the socket next to where the turbo prepump is connected. Wait a few minutes. Open up V1 slowly and close V5. Wait for the pressure to reach 10 2 mbar. Open up VR9 to the turbo. Now you should see a rapid drop in pressure to 10 3 mbar. 6. The final pressure of the focal plane is around 10 5 mbar. 4.3 Turning on the gas counter Now the whole GREAT chamber (and of course also RITU) should be in vacuum, and the gate valve VR8 to RITU should be closed. This also means that the gas counter bypasses BP1 and BP2 should be open. The valves V4, V5 and V6 are closed. 1. OBS! Check that the Transtruder (Baratron) with range from 0 mbar to 10 mbar (NOT 0 mbar - 1000 mbar!) IS connected. Connect the cable "10 mbar" to the back of screen C. OBS! 2. Open gate valve VR8 to RITU. Check that the pressure in RITU stays at good vacuum, (10 5 10 6 ) mbar (10 4 mbar is alarming). This can be seen from the monitor on top of the dipole chamber. 3. Close bypasses BP1 and BP2. The gas counter is now isolated from the rest of the GREAT chamber. 4. Check that the prepump P3 behind the rack is on and working properly, valve V5 is closed and the reading of screen D is out of range. 20
5. Open the blue gas bottle under the rack, V7. It can be fully opened. 6. Go to the control unit C of the gas inlet. Turn on the monitoring from the power switch, if it is not on already. (a) Put the input voltage to 10 V. (b) Check that the potentiometer on the left of screen C is closed. It does not show zero, but also will not turn further counter clockwise. (c) Turn on the control of the piezo valve V6 in the gas inlet by turning the switch on the right of the operating board from closed to auto. (d) By turning the "set point" potentiometer on the left slightly clockwise, raise the pressure at the gas inlet seen on screen C. Also open the needle valve V5 to pump P3 slightly. 7. Now gas should be flowing through the system and the pressure reading of C rises slightly. Check that he pressure in the dipole chamber does not rise above 10 5 mbar (i.e. that the mylar windows of the gas counter are not leaking). 8. If everything looks ok, set the pressure of the gas flow to 3.5 mbar (see stage 5.(d) above). That is 350 Pa on screen C. Don t be too hasty, the potentiometer is quite stiff, and the gas pressure fluctuates. While approaching the final pressure value, occasionally stop turning to let the pressure settle. Finally check that it stays at 350 Pa for a while. 9. Open the gas outlet valve V5 as well, to bring the pressure of the outlet to 0.2 mbar on screen D. 10. Turn on the gas counter. The electronics is in the upper bin in the rack. (a) Turn on the power of the bin with the gas counter. (b) Turn on the power of the gas counter. This is done by turning the switch next to the uppermost potentiometer from disabled to 1 kv. (c) If ovl LED is on, press the "reset overload" button. (d) Start biasing up the gas counter by turning the uppermost potentiometer. Check that the lights on the timing unit (silvery card next to the bias supply) don t flash repeatedly. If they do, take the voltage down and try again. Finally set the voltage to 470 V. 21
4.4 Turning off the gas counter Turning off the gas counter is essentially done in reversed order to turning it on. 1. Take down the voltage of the gas counter and switch off the high voltage power of the card (switch between potentiometer from 1 kv to disabled and press off on the right). Turn off the power of the bin. 2. Turn the "set point" potentiometer on the left of screen C to zero, or as close as it goes. 3. Turn off the piezo valve by switching from auto to closed on the right of screen C. 4. Close V7 at the isobutane bottle. 5. Open needle valve V5 fully to pump out the gas from the gas counter volume. You can see the pressure in the gas inlet drop on screen C. 6. When the pressure has dropped, open the bypasses. Open first BP2 to RITU and then BP1 to the focal plane side. 7. Close the V5 so the turbo won t pump in dirt through P3. 8. Close VR8 to RITU. 4.5 Venting the focal plane Before starting to vent you must make sure that gate valve VR8 to RITU is closed. If it is still open, close it before doing anything else! At this point also needle valve V5 to the prepump should be closed and the gas counter bypasses BP1 and BP2 should be open. Check that the "full range cable at the back of screen C is connected. If not, connect it! 1. Close VR9 to the focal plane turbo. You should see the pressure of the focal plane rising to 10 3 mbar on screen A (upper reading connected to G2) as the turbo does not pump the chamber anymore. To make sure VR8 is properly closed, you can check that the pressure in the dipole chamber stays at (10 5 10 6 ) mbar. 22
2. Start venting slowly by turning the needle valve V2 to the nitrogen line. Keep an eye on the pressure on screens A and C (connected to G2 and G7). The venting speed should be the same as the pumping speed, 1 mbar/s. 3. Follow the change in pressure and open the needle valve carefully whenever the venting seems to slow down. 4. As the chamber has reached air pressure, remember to close the needle valve to the nitrogen line! 5. If the meter on screen C is still on, you can turn it off. 23
Figure 4.1: RITU and beamline vacuum system in the differential mode. 24
Figure 4.2: RITU and beamline vacuum system in the window mode. 25
Figure 4.3: The measurement equipment control units which are located on top of the RITU dipole magnet. 26
Figure 4.4: Target chamber measurement equipment control units. These are located close to the three beam line quadrupoles. 27
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