Long Term Stability of the LHC Superconducting Cryodipoles After Outdoor Storage
Summary (3 min read)
Introduction
- The main superconducting dipoles for the LHC are being stored outdoors for periods from a few weeks to several years after conditioning with dry nitrogen gas.
- A dedicated task force was established to study all aspects of long term behaviour of the stored cryodipoles, with particular emphasis on electrical and vacuum integrity, quench training behaviour, magnetic field quality, performance of the thermal insulation, mechanical stability of magnet shape and of the interface between cold mass and cryostat, degradation of materials and welds.
- In particular, one specifically selected cryodipole stored outdoors for more than one year, was re-tested at cold.
- In addition, various tests have been carried out on the cryodipole assembly and on the most critical subcomponents to study aspects such as the hygrothermal behaviour of the supporting system and the possible oxidation of the Multi Layer Insulation reflective films.
- This paper summarizes the main investigations carried out and their results.
I. LHC CRYODIPOLE GLOBAL DESCRIPTION
- The dipole cold masses are cooled by superfluid helium down to 1.9 K.
- An Authors are within CERN, Geneva, Switzerland Manuscript received September 20, 2005.
- No cryodipole components were explicitly designed to meet external storage environmental effects.
- A dedicated task force was established to study the Dipole Long Term Stability (DLTS) and determine the effects of long term outdoor storage on magnet electrical insulation and continuity, field harmonics, quench training memory, geometry stability, degradation of materials and welds.
II. STORAGE TIME AND CONDITIONS
- The dipole cold masses are delivered at CERN from three manufacturers and are first stored outdoors.
- The magnet is then assembled into its cryostat and the performances of each individual dipole are assessed under cryogenic operating conditions.
- After the cold tests, a second outdoor storage period occurs.
- The cleaning of the beam tubes and subsequent insertion of the beam screens are performed before lowering down the cryomagnet into the 27-km circumference tunnel.
- The distribution of the storage time for both the cold masses and the already assembled cryodipoles, at the start of the DLTS investigation early 2005, is summarized in Fig.
III. ELECTRICAL INSULATION AND CONTINUITY
- Two samples of 33 units were built to compare the electrical insulation of dipole cold masses stored outdoors for long periods, between 5 and 14 months, and for short ones, below 10 days.
- The leakage currents flowing in-between main insulated magnet components were measured at 1.9 K with maximum applied voltages of 2.7 kV and 3.1 kV as a function of the electrical circuits.
- Furthermore, a complete electrical check is performed on each cryomagnet before their installation in the tunnel in order to ensure the electrical system integrity.
- Up to now, more than 80 cryodipoles stored outdoors for periods up to one year have already passed this check successfully.
- Finally, the IFS was designed to sustain external storage conditions, and specific conditioning procedures are applied that ensure the IFS functionality in the long-term.
IV. FIELD HARMONICS
- The extended duration of outdoor storage was questioned to eventually enhance the creep in the coil structure [2].
- Induced relaxation of the coil pre-stress would perturb the coil geometry, resulting in non-negligible variations in the magnet field quality.
- The effect is thus modest but not negligible.
- Measurements of the field quality at CERN show that the offsets between warm and cold field measurements in b3 are stable within ±0.5 units, regardless of the time of storage of the magnets.
- The results are very similar for b5 and b7 with stability within ±0.2 and ±0.1 units respectively.
V. QUENCH TRAINING
- Similarly, the relaxation of the coil pre-stress in the cold mass could cause quench training degradation.
- It was then cold tested for a second time after one year of outdoor storage.
- Quench training results are presented in Fig. 6.
- For a firm statement concerning the whole population of LHC superconducting magnets, a statistical study is required.
VI. GEOMETRY AND ALIGNMENT
- The geometry stability of the LHC cryodipoles has been assessed by comparing their geometry between two stages; before outdoor storage once they are assembled and cold tested; and after storage when the beam screens are inserted into the beam tubes.
- Measurements of 329 cryodipoles, that were stored outdoors for periods varying between one month and two years, were used to do this analysis.
- Statistically, more than 95% of the cryomagnets remained stable within +/-0.5 mm both horizontally and vertically.
- The mean and the standard deviation of the movements of the ends of the dipoles have been calculated using windows sliding over storage time.
- No indication of long term trends has been detected.
VII. DEGRADATION OF MATERIALS
- A series of cold masses and assembled cryodipoles stored outdoors for periods of the order of one year has been inspected for potential degradation [4], with the aim to identify the components susceptible to degrade and assess the nature and importance of the degradations as well as their effects on thermal, structural, and vacuum performance of the magnets.
- The excessive penetration of a weld between the beam tube ultra high vacuum and the helium pressurized cold mass was specifically monitored.
- The lack of back gas shielding during this welding operation associated to a full penetration resulted in a corroded aspect of the weld root visible after some storage time.
- Visual inspections of every LHC cryomagnet were thus carried out and further X-rays and repair will be performed, when necessary, to ensure that only magnets adequate for LHC operation will be installed in the machine.
- It should be noted that this issue is coming from a cold mass manufacturing non conformity, and is thus only indirectly linked to the DLTS work.
A. Cryodipole components, cold mass welds and bellows
- Further results from the inspections are summarized in the following.
- A few decilitres of liquid water were found in a cold mass heat exchanger line because of poor storage conditioning.
- Since one of the three producers used cast 304 H instead of cast 304 L for the cold feet pad, particular attention was paid to the possible sensitization of the Heat Affected Zones (HAZ) of the welds of this component.
- Ageing tests of welds after removing rust stains were further carried out.
- The vacuum enclosure was found leak tight, regardless of the outdoor storage time.
C. Thermal insulation
- The air moisture in the vacuum vessel during outdoor storage might oxidize the 40 nm aluminium coating deposited on each side of the MLI layers.
- The thickness of the aluminium coat on each MLI polyester layer was determined by measuring the electrical resistivity of the sheet, and the thermal performance of two superimposed 15 layers blankets was measured with a heatmeter.
- Similarly, the overall MLI thermal performances have been degraded by 17% with respect to the test performed on a new reference sample.
- The GFRE support posts were mechanically tested before and after hygrothermal treatment.
- This was shown not to affect the alignment stability of the magnets in the LHC machine.
VIII. CONCLUSION AND RISK ASSESSMENT
- The long-term behaviour of the LHC main superconducting dipoles stored outdoors has been investigated, with a focus on electrical integrity, magnetic field quality, quench training, geometry, degradation of materials and welds, performance of the thermal insulation and interface between the cold mass and the cryostat.
- The analysis has shown that dipole cryomagnets remain functionally unaffected or weakly affected by long-term outdoor storage, even if such a constraint was not considered during the design phase.
- Thanks to the inspection work done, a manufacturing non conformity in a cold mass critical weld was monitored and treated, and the quality control of the storage conditions was improved.
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Citations
16 citations
Cites background from "Long Term Stability of the LHC Supe..."
...geted MB for the study of the long term stability did not reveal a significant drift of the quench performance after about 1 year of storage [28]....
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11 citations
Cites result from "Long Term Stability of the LHC Supe..."
...The observed behaviour is contradictory to the conclusion drawn in [1], namely that long term storage does not affect the quench behaviour of the cryodipoles....
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1 citations
Cites result from "Long Term Stability of the LHC Supe..."
...These results also validate the performance of the LHC magnet cryostats after long term (up to 3 years) outdoor storage prior to installation [11] and shows that installation and interconnection activities have not degraded in any significant way the thermal performance of the cryostats....
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References
7 citations
"Long Term Stability of the LHC Supe..." refers methods in this paper
...A mechanical model of the dipole cross-section, previously developed and validated with experimental data [3], was used to evaluate the influence of coil pre-stress on field quality....
[...]
2 citations
"Long Term Stability of the LHC Supe..." refers background in this paper
...The extended duration of outdoor storage was questioned to eventually enhance the creep in the coil structure [2]....
[...]
Related Papers (5)
Frequently Asked Questions (18)
Q2. What are the contributions mentioned in the paper "Long term stability of the lhc superconducting cryodipoles after outdoor storage" ?
A dedicated task force was established to study all aspects of long term behaviour of the stored cryodipoles, with particular emphasis on electrical and vacuum integrity, quench training behaviour, magnetic field quality, performance of the thermal insulation, mechanical stability of magnet shape and of the interface between cold mass and cryostat, degradation of materials and welds. This paper summarizes the main investigations carried out and their results.
Q3. How many kVs are used to measure the leakage currents?
The leakage currents flowing in-between main insulated magnet components were measured at 1.9 K with maximum applied voltages of 2.7 kV and 3.1 kV as a function of theelectrical circuits.
Q4. What are the main risks of the PTFE® coated surface?
The main risks have been identified as, first, a degradation of the friction coefficient of the PTFE® coated surface allowing sliding of the extremity support posts on the cryostat; and, second, a decrease of the stiffness of the GFRE support posts eventually degrading the alignment stability of the LHC magnets in the machine.
Q5. What is the procedure for lowering down the cryomagnet?
The cleaning of the beam tubes and subsequent insertion of the beam screens are performed before lowering down the cryomagnet into the 27-km circumference tunnel.
Q6. What is the effect of air moisture on the MLI?
The air moisture in the vacuum vessel during outdoor storage might oxidize the 40 nm aluminium coating deposited on each side of the MLI layers.
Q7. What is the effect of the coil pre-stress on field quality?
Measurements of the field quality at CERN show that the offsets between warm and cold field measurements in b3 are stable within ±0.5 units, regardless of the time of storage of the magnets.
Q8. What is the storage condition of the cold masses and cryodipoles?
2. The conditioning of the cold masses and cryodipoles includes protection of electrical cabling, installation of leak tight covers on both magnet and vacuum vessel extremities and pressurization of the cold mass with 1.2 bar of gaseous nitrogen.
Q9. What is the way to test the vacuum enclosures?
VacuumCondensation and re-evaporation cycles in the vacuum enclosures are possible during outdoor storage; the oxidation of the vacuum system was therefore investigated.
Q10. What are the main characteristics of the LHC main superconducting dipoles stored outdoors?
The long-term behaviour of the LHC main superconducting dipoles stored outdoors has been investigated, with a focus on electrical integrity, magnetic field quality, quench training, geometry, degradation of materials and welds, performance of the thermal insulation and interface between the cold mass and the cryostat.
Q11. How was the position of the cold mass extremities in the cryostat studied?
The position change of the cold mass extremities within the cryostat has been studied, as it is representative of both the corrector magnet alignment and the sagitta change of the dipole.
Q12. How many leaks were found in the vacuum enclosure?
Magnets stored outside for a period of one year that had already been equipped with beam screens and cooling circuits were inspected, and 3 of them were leak tested.
Q13. What was the result of the inspection work done?
Thanks to the inspection work done, a manufacturing non conformity in a cold mass critical weld was monitored and treated, and the quality control of the storage conditions was improved.
Q14. What is the main reason for the degradation of dipole cryomagnets?
The analysis has shown that dipole cryomagnets remain functionally unaffected or weakly affected by long-term outdoor storage, even if such a constraint was not considered during the design phase.
Q15. How long did the MLI have been stored in an unprotected environment?
The thermal performance of a production sample that was stored in an unprotected environment for a period of more than two and a half years has been analyzed and tested [5].
Q16. What is the effect of the hygrothermal treatment on the GFRE?
A specific hygrothermal ageing treatment of both GFRE supports posts and low-friction centering pieces has been defined, based on a series of humidity absorption and desorption cycles on samples of the GFRE composite material.
Q17. How is the geometry of the LHC cryodipoles assessed?
The geometry stability of the LHC cryodipoles has been assessed by comparing their geometry between two stages; before outdoor storage once they are assembled and cold tested; and after storage when the beam screens are inserted into the beam tubes.
Q18. What is the effect of coil pre-stress on field quality?
A mechanical model of the dipole cross-section, previously developed and validated with experimental data [3], was used to evaluate the influence of coil pre-stress on field quality.