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Cold leak tests of LHC beam screens
Vacuum 84 (2010) 293–297
Contents lists available at ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
Cold leak tests of LHC beam screens C. Collomb-Patton, P. Cruikshank, B. Jenninger*, N. Kos CERN, CH-1211 Geneva 23, Switzerland
a b s t r a c t Keywords: Beam screen LHC Cryogenic leak test
In order to guide the high energy proton beams inside its two 27 km long vacuum rings, the Large Hadron Collider (LHC) at CERN, Geneva, makes use of superconducting technology to create the required magnetic fields. More than 4000 beam screens, cooled at 7–20 K, are inserted inside the 1.9 K beam vacuum tubes to intercept beam-induced heat loads and to provide dynamic vacuum stability. As extremely high helium leak tightness is required, all beam screens have been leak tested under cold conditions in a dedicated test stand prior to their installation. After describing the beam screen design and its functions, this report focuses on the cold leak test sequence and discusses the results. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction In the Large Hadron Collider (LHC) at CERN, Geneva, two high intensity proton beams will collide at extremely high energy. Superconducting technology is required to produce the necessary magnetic field to guide the beams. Its main components are more than 1700 superconducting bending and focusing magnets, most of them cooled to 1.9 K [1]. Apart from the experimental areas, the beams are circulating in separate apertures, with an integral length of the cryogenic beam vacuum system of about 45 km. The bunch of protons, moving at practically speed of light, will cause beam-induced heat deposition and gas desorption. To reduce the influence of these effects, beam screens have been inserted into the beam vacuum tubes of the cryo-magnets. Beam screens are helium cooled shields at an operating temperature between 7 K and 20 K. Pumping slots allow definitive cryosorption of molecules on the 1.9 K cold bore surface, where they are protected against beam-induced desorption. In this way the dynamic beam vacuum stability is improved [1].
The different types of beam screens vary in both cross-section and length. The most common ones are the 2464 main dipole beam screens with a length of 15.3 m (Fig. 2). Helium leaks in the cooling circuit would increase the beam–gas interactions. Then, the lost protons will deposit their energy directly in the superconducting coils of the magnets, which may lead to a resistive transition (quench). The influence of a helium leak into the cold beam vacuum has been evaluated [3]. The maximum tolerable leak rate is reported to be in the order of 5 108 Pa m3 s1 at operation conditions. This leak tightness level prevents from the risk of a magnet quench between yearly scheduled shut downs. The beam vacua of the 2.8 km long cryo-magnet chains (arcs) are not sectorised. A repair of a leak in this zone would require the warm-up of the whole arc to room temperature. The down time of the LHC would be in the order of months and the costs for one warm-up and re-cooling would have been comparable with the estimated costs for cold testing all beam screens. For this reason it has been decided to cold test all beam screens close to operation conditions before their installation in the magnets. This test follows the factory leak tests at room temperature.
2. Beam screen requirements The beam screens are made of a high manganese stainless steel grade specially developed for this application. Two cooling tubes have been attached by laser spot-welds [2] (Fig. 1). At their extremities, feedthroughs are welded on the cooling tubes. All helium to beam vacuum welds are partially penetrated to reduce the risk of leaks.
* Corresponding author. Tel.: þ41 22 767 3417; fax: þ41 22 767 5100. E-mail address: [email protected] (B. Jenninger). 0042-207X/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2009.04.030
3. Test objectives The cryogenic tests aimed to reveal weaknesses of the laser spot-welds by applying thermal constraints. To avoid risks related to the storage, the tests were scheduled right before the installation. The leak tightness level at operation conditions has to be scaled to the test conditions to take into account the flow dependence with temperature in the leak. Applying the law of Haagen–Poiseuille, a leak measured at 100 K and 2 bar, would increase by a factor 200
294
C. Collomb-Patton et al. / Vacuum 84 (2010) 293–297
Fig. 1. Beam screen components.
for the most unfavourable operating conditions (7 K and 3.6 bar, see Fig. 3). The evolution of cold helium leaks with temperature has been studied on tubular and orifice-like leaks [4,5]. The measured increase in leak rates was between 10 and 80 at 1 bar for a temperature decrease from 77 K to 4.2 K liquid helium. For the same parameters, the law of Hagen–Poiseuille predicts a factor of 500. Applying this law for scaling from test to operation conditions is therefore a conservative approach. For a test at 100 K and 2 bar the required sensitivity is 2 1010 Pa m3 s1, which is measureable with commercially-available leak detectors. Although a leak tightness level has been given, it was decided to reject any beam screen showing a leak since it could degrade with time. An average number of 30 tested beam screens per week was required in order to follow the LHC magnet installation schedule.
4. Set-up The beam screens are inserted in batches inside a cryostat and cooled with helium through the cooling tubes. The closed circuit flow of helium is driven by a membrane compressor (Fig. 4). The helium passes through a liquid nitrogen heat-exchanger and enters the beam screens at 85 K. The thermal shield is directly cooled with liquid nitrogen. The beam screen test stand and preparation zones are shown in Fig. 5. The helium signal is continuously recorded with a mass spectrometer leak detector connected behind a mobile turbomolecular pumping station which pumps cryostat vacuum envelope. The response time for a helium signal is few seconds, and the system time constant is about 40 s. The connection to the cryostat helium supply and the return loops has been made by automatic orbital welds, which have been cut off after the test.
Fig. 2. Installed beam screen inside magnet.
C. Collomb-Patton et al. / Vacuum 84 (2010) 293–297
1.E-07 Gas 1bar Liquid 1bar
Flow [Pa.m3s-1]
3.6 bar
1.E-08
Operation conditions
1.E-09 Test conditions Leak dimensions: D = 180 nm, L = 1 mm
1.E-10
Classical Leak test at RT
1.E-11 1
10
100
1000
Temperature [K] Fig. 3. Evolution of helium leak rates as function of temperature and pressure.
To reduce the helium background signal inside the cryostat, resulting from the outgassing and permeation, well below the required limit, an all-metal assembly is mandatory. 5. Test sequence The main steps of the test sequence are as follows. The indicated times correspond to the recordings during a test (Fig. 6). a. Evacuation of cryostat (9:40). b. Leak detection on cryostat envelope, two helium rinsing cycles (10:30). c. Start cool-down (11:00). d. At 95 K the cooling is stopped, followed by two pressure cycles from 0–2.5 bar (abs.) inside the cooling tubes. The pressures are hold 10 min each time (23:05). e. Isolate cryostat vacuum from pumping station. At that moment, the recorded helium signal drops immediately, in general about 4 1012 Pa m3 s1. This drop was very helpful for the interpretation of the results. It confirmed that the
295
helium signal from the cryostat was correctly measured during the pressure cycles and no valve between leak detector and cryostat was accidentally closed (23:45). f. Few seconds later, breaking the vacuum with nitrogen to accelerate the warm-up. In this way the beam screens could be warmed up to room temperature less than 10 h. g. Open the cryostat, remove cold tested beam screens, and insert new beam screens to be tested (9:56). The beam screens of the next batch have been prepared in parallel with an ongoing test, which included orbital welding to the helium supply and return loops and leak testing before insertion in the test bench. A whole test cycle lasted 24 h, including exchanging beam screens, pump down, leak detection and rinsing the helium circuit. The cool-down, pressure cycles and the warm-up have been realised in automatic mode. 6. Interpretation of the results The results have been checked for the appearance or presence of a helium leak before opening the cryostat: An internal helium leak on the cooling circuit would have been shown a strong correlation with the helium pressure cycles, as their duration was much longer than the 40 s system time constant. An external helium leakage into the test hall, would increase the helium concentration in the ambient air, hence would increase the residual of the leak detector. But it would not affect the size of the signal drop when isolating the cryostat. Leaks on the cryostat vacuum envelope increase the signal drop, but would not correlate with the pressure cycles. 7. Results No leak has been identified on cooling tube attachment welds. This confirms the correct design and it credits the follow-up and quality control of the beam screen manufacturing.
GHe
Water bath heater
P FM
Membrane Compressor P
P
P
Cryostat T
LN2
T P
Dewar
Beam screens
P
L
Leak detector
Heat exchanger
GN2 P
Pumping station Fig. 4. Cryogenic flow and vacuum scheme of the test stand.
296
C. Collomb-Patton et al. / Vacuum 84 (2010) 293–297
Fig. 5. Photographs of test stand and preparation zone.
Fig. 6. Temperatures, pressures, helium signal and vacuum.
A Few leaks were found after the pump down. Most of these leaks appeared to be one of the temporary orbital welds. In particular one leak opened up during the cool-down. It was found to be on a repair of an orbital weld by testing it for a second time. In all these cases, the whole batches were retested after cutting out the leaking weld and rewelding it. Two leaks were due to the fatigue of the flexible hoses of the front flange. 8. Discussion Due to the temperature variations in the test hall, parallel activities using helium or leakages to ambient air, the helium signal often drifted during a test. These drifts, however, did not reduce the
sensitivity of the tests, since they remained usually within the 1011 Pa m3 s1 range. As the helium inlet and outlet were on the same side of a beam screen, it was necessary to verify that no significant thermal short would impede an effective cooling of the opposite extremity. To verify the effectiveness of the cooling, the mean temperature of the beam screens at the end of the cool-down has been calculated from the integrated heat evacuated with the helium flow. Thermal losses from the cryostat have been taken into account. At the end of a cool-down to a helium outlet of 95 K, the mean beam screen temperature was less than 125 K. The thermal constraints on the laser spot-welds were not homogeneous along a beam screen, but always higher than the LHC
C. Collomb-Patton et al. / Vacuum 84 (2010) 293–297
cool-down scenario, where the beam screens are passively cooled by thermal radiation from 290 to 150 K. Other methods of testing the beam screens have been considered, such as helium accumulation [6]. The high helium residual decreased the sensitivity of this method makes the results difficult to interpret. Acknowledgement The authors gratefully acknowledge the CERN Cryolab team for the realisation of the prototype cryostat, Olivier Pirotte (AT-CRG) and his engineering team for the cryogenic support on the upgrade and
297
cryogenic infrastructure. Not to forget also Adriaan Rijllart (AB–CO) and his team for the support on the automation of the set-up. References [1] Brunig O, Collier P, Lebrun P, Myers S, Ostojic R, Poole J, et al., editors. LHC Design Report CERN-2004-003-v1. Geneva: CERN; 2004. 548 p. [2] Cruikshank P, Artoos K, Bertinelli F, Brunet JC, Calder R, Campedel C, et al. 17th Particle Accelerator Conference, Vancouver, Canada; 1997. p. 3586–8. [3] Baglin V. Vacuum 2007;81:803–7. [4] Mollary ML, Laumer HW. Nucl Instrum Methods 1980;177:481–4. [5] Sinharoy S, Lange WJ. J Vac Sci Technol 1982;20:978–81. [6] Billard F, Hilleret N. CERN Vacuum Technical Notes 01-06 and 03-06, CERN EDMS-Nr: 382340.
Contents lists available at ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
Cold leak tests of LHC beam screens C. Collomb-Patton, P. Cruikshank, B. Jenninger*, N. Kos CERN, CH-1211 Geneva 23, Switzerland
a b s t r a c t Keywords: Beam screen LHC Cryogenic leak test
In order to guide the high energy proton beams inside its two 27 km long vacuum rings, the Large Hadron Collider (LHC) at CERN, Geneva, makes use of superconducting technology to create the required magnetic fields. More than 4000 beam screens, cooled at 7–20 K, are inserted inside the 1.9 K beam vacuum tubes to intercept beam-induced heat loads and to provide dynamic vacuum stability. As extremely high helium leak tightness is required, all beam screens have been leak tested under cold conditions in a dedicated test stand prior to their installation. After describing the beam screen design and its functions, this report focuses on the cold leak test sequence and discusses the results. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction In the Large Hadron Collider (LHC) at CERN, Geneva, two high intensity proton beams will collide at extremely high energy. Superconducting technology is required to produce the necessary magnetic field to guide the beams. Its main components are more than 1700 superconducting bending and focusing magnets, most of them cooled to 1.9 K [1]. Apart from the experimental areas, the beams are circulating in separate apertures, with an integral length of the cryogenic beam vacuum system of about 45 km. The bunch of protons, moving at practically speed of light, will cause beam-induced heat deposition and gas desorption. To reduce the influence of these effects, beam screens have been inserted into the beam vacuum tubes of the cryo-magnets. Beam screens are helium cooled shields at an operating temperature between 7 K and 20 K. Pumping slots allow definitive cryosorption of molecules on the 1.9 K cold bore surface, where they are protected against beam-induced desorption. In this way the dynamic beam vacuum stability is improved [1].
The different types of beam screens vary in both cross-section and length. The most common ones are the 2464 main dipole beam screens with a length of 15.3 m (Fig. 2). Helium leaks in the cooling circuit would increase the beam–gas interactions. Then, the lost protons will deposit their energy directly in the superconducting coils of the magnets, which may lead to a resistive transition (quench). The influence of a helium leak into the cold beam vacuum has been evaluated [3]. The maximum tolerable leak rate is reported to be in the order of 5 108 Pa m3 s1 at operation conditions. This leak tightness level prevents from the risk of a magnet quench between yearly scheduled shut downs. The beam vacua of the 2.8 km long cryo-magnet chains (arcs) are not sectorised. A repair of a leak in this zone would require the warm-up of the whole arc to room temperature. The down time of the LHC would be in the order of months and the costs for one warm-up and re-cooling would have been comparable with the estimated costs for cold testing all beam screens. For this reason it has been decided to cold test all beam screens close to operation conditions before their installation in the magnets. This test follows the factory leak tests at room temperature.
2. Beam screen requirements The beam screens are made of a high manganese stainless steel grade specially developed for this application. Two cooling tubes have been attached by laser spot-welds [2] (Fig. 1). At their extremities, feedthroughs are welded on the cooling tubes. All helium to beam vacuum welds are partially penetrated to reduce the risk of leaks.
* Corresponding author. Tel.: þ41 22 767 3417; fax: þ41 22 767 5100. E-mail address: [email protected] (B. Jenninger). 0042-207X/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2009.04.030
3. Test objectives The cryogenic tests aimed to reveal weaknesses of the laser spot-welds by applying thermal constraints. To avoid risks related to the storage, the tests were scheduled right before the installation. The leak tightness level at operation conditions has to be scaled to the test conditions to take into account the flow dependence with temperature in the leak. Applying the law of Haagen–Poiseuille, a leak measured at 100 K and 2 bar, would increase by a factor 200
294
C. Collomb-Patton et al. / Vacuum 84 (2010) 293–297
Fig. 1. Beam screen components.
for the most unfavourable operating conditions (7 K and 3.6 bar, see Fig. 3). The evolution of cold helium leaks with temperature has been studied on tubular and orifice-like leaks [4,5]. The measured increase in leak rates was between 10 and 80 at 1 bar for a temperature decrease from 77 K to 4.2 K liquid helium. For the same parameters, the law of Hagen–Poiseuille predicts a factor of 500. Applying this law for scaling from test to operation conditions is therefore a conservative approach. For a test at 100 K and 2 bar the required sensitivity is 2 1010 Pa m3 s1, which is measureable with commercially-available leak detectors. Although a leak tightness level has been given, it was decided to reject any beam screen showing a leak since it could degrade with time. An average number of 30 tested beam screens per week was required in order to follow the LHC magnet installation schedule.
4. Set-up The beam screens are inserted in batches inside a cryostat and cooled with helium through the cooling tubes. The closed circuit flow of helium is driven by a membrane compressor (Fig. 4). The helium passes through a liquid nitrogen heat-exchanger and enters the beam screens at 85 K. The thermal shield is directly cooled with liquid nitrogen. The beam screen test stand and preparation zones are shown in Fig. 5. The helium signal is continuously recorded with a mass spectrometer leak detector connected behind a mobile turbomolecular pumping station which pumps cryostat vacuum envelope. The response time for a helium signal is few seconds, and the system time constant is about 40 s. The connection to the cryostat helium supply and the return loops has been made by automatic orbital welds, which have been cut off after the test.
Fig. 2. Installed beam screen inside magnet.
C. Collomb-Patton et al. / Vacuum 84 (2010) 293–297
1.E-07 Gas 1bar Liquid 1bar
Flow [Pa.m3s-1]
3.6 bar
1.E-08
Operation conditions
1.E-09 Test conditions Leak dimensions: D = 180 nm, L = 1 mm
1.E-10
Classical Leak test at RT
1.E-11 1
10
100
1000
Temperature [K] Fig. 3. Evolution of helium leak rates as function of temperature and pressure.
To reduce the helium background signal inside the cryostat, resulting from the outgassing and permeation, well below the required limit, an all-metal assembly is mandatory. 5. Test sequence The main steps of the test sequence are as follows. The indicated times correspond to the recordings during a test (Fig. 6). a. Evacuation of cryostat (9:40). b. Leak detection on cryostat envelope, two helium rinsing cycles (10:30). c. Start cool-down (11:00). d. At 95 K the cooling is stopped, followed by two pressure cycles from 0–2.5 bar (abs.) inside the cooling tubes. The pressures are hold 10 min each time (23:05). e. Isolate cryostat vacuum from pumping station. At that moment, the recorded helium signal drops immediately, in general about 4 1012 Pa m3 s1. This drop was very helpful for the interpretation of the results. It confirmed that the
295
helium signal from the cryostat was correctly measured during the pressure cycles and no valve between leak detector and cryostat was accidentally closed (23:45). f. Few seconds later, breaking the vacuum with nitrogen to accelerate the warm-up. In this way the beam screens could be warmed up to room temperature less than 10 h. g. Open the cryostat, remove cold tested beam screens, and insert new beam screens to be tested (9:56). The beam screens of the next batch have been prepared in parallel with an ongoing test, which included orbital welding to the helium supply and return loops and leak testing before insertion in the test bench. A whole test cycle lasted 24 h, including exchanging beam screens, pump down, leak detection and rinsing the helium circuit. The cool-down, pressure cycles and the warm-up have been realised in automatic mode. 6. Interpretation of the results The results have been checked for the appearance or presence of a helium leak before opening the cryostat: An internal helium leak on the cooling circuit would have been shown a strong correlation with the helium pressure cycles, as their duration was much longer than the 40 s system time constant. An external helium leakage into the test hall, would increase the helium concentration in the ambient air, hence would increase the residual of the leak detector. But it would not affect the size of the signal drop when isolating the cryostat. Leaks on the cryostat vacuum envelope increase the signal drop, but would not correlate with the pressure cycles. 7. Results No leak has been identified on cooling tube attachment welds. This confirms the correct design and it credits the follow-up and quality control of the beam screen manufacturing.
GHe
Water bath heater
P FM
Membrane Compressor P
P
P
Cryostat T
LN2
T P
Dewar
Beam screens
P
L
Leak detector
Heat exchanger
GN2 P
Pumping station Fig. 4. Cryogenic flow and vacuum scheme of the test stand.
296
C. Collomb-Patton et al. / Vacuum 84 (2010) 293–297
Fig. 5. Photographs of test stand and preparation zone.
Fig. 6. Temperatures, pressures, helium signal and vacuum.
A Few leaks were found after the pump down. Most of these leaks appeared to be one of the temporary orbital welds. In particular one leak opened up during the cool-down. It was found to be on a repair of an orbital weld by testing it for a second time. In all these cases, the whole batches were retested after cutting out the leaking weld and rewelding it. Two leaks were due to the fatigue of the flexible hoses of the front flange. 8. Discussion Due to the temperature variations in the test hall, parallel activities using helium or leakages to ambient air, the helium signal often drifted during a test. These drifts, however, did not reduce the
sensitivity of the tests, since they remained usually within the 1011 Pa m3 s1 range. As the helium inlet and outlet were on the same side of a beam screen, it was necessary to verify that no significant thermal short would impede an effective cooling of the opposite extremity. To verify the effectiveness of the cooling, the mean temperature of the beam screens at the end of the cool-down has been calculated from the integrated heat evacuated with the helium flow. Thermal losses from the cryostat have been taken into account. At the end of a cool-down to a helium outlet of 95 K, the mean beam screen temperature was less than 125 K. The thermal constraints on the laser spot-welds were not homogeneous along a beam screen, but always higher than the LHC
C. Collomb-Patton et al. / Vacuum 84 (2010) 293–297
cool-down scenario, where the beam screens are passively cooled by thermal radiation from 290 to 150 K. Other methods of testing the beam screens have been considered, such as helium accumulation [6]. The high helium residual decreased the sensitivity of this method makes the results difficult to interpret. Acknowledgement The authors gratefully acknowledge the CERN Cryolab team for the realisation of the prototype cryostat, Olivier Pirotte (AT-CRG) and his engineering team for the cryogenic support on the upgrade and
297
cryogenic infrastructure. Not to forget also Adriaan Rijllart (AB–CO) and his team for the support on the automation of the set-up. References [1] Brunig O, Collier P, Lebrun P, Myers S, Ostojic R, Poole J, et al., editors. LHC Design Report CERN-2004-003-v1. Geneva: CERN; 2004. 548 p. [2] Cruikshank P, Artoos K, Bertinelli F, Brunet JC, Calder R, Campedel C, et al. 17th Particle Accelerator Conference, Vancouver, Canada; 1997. p. 3586–8. [3] Baglin V. Vacuum 2007;81:803–7. [4] Mollary ML, Laumer HW. Nucl Instrum Methods 1980;177:481–4. [5] Sinharoy S, Lange WJ. J Vac Sci Technol 1982;20:978–81. [6] Billard F, Hilleret N. CERN Vacuum Technical Notes 01-06 and 03-06, CERN EDMS-Nr: 382340.