WHAT REPAIR ACTIVITIES CAN BE DONE TODAY ON A LOCALLY WARMED-UP SUB-SECTOR

P. Cruikshank, CERN, Geneva, Switzerland

Technology Department, Vacuum, Surfaces & Coatings Group

Abstract

Interventions to repair components inside LHC cryostats can only be performed with the system at room temperature. The architecture of the cryogenic and vacuum system permits local warm-up of the 2.8 km arc cryostat. The method, prerequisites, means, constraints, risks and time to locally warm-up a sub-sector are compared to a complete arc warm-up, including the steps to get back to 1.9 K powering. Configuration variants in each arc sector and sub-sector are considered and their impact on the local warm-up procedure. Typical repairs such as diode exchange, helium leak repair and splice consolidation will be presented, including the recent experience of local warm-up in sectors 8-1 and 2-3 to repair DFBA flexible hoses. The necessity and impact of beam vacuum venting during local warm-up will be explained and reviewed.

introduction

Each 2.8 km continuous cryostat houses 4500 tonnes of cold mass and many thousands of components at cryogenic temperature. From an early stage in the conceptual design of the LHC project, the down-time and cost benefits of locally warming only part of the arc to execute repairs were identified. Reduction of the number of thermal cycles also minimises risks to component and circuit integrity. Cryogenic and vacuum sectorisation of the LHC arcs was implemented during the design phase of the LHC following inputs from the Sectorisation Working Group [1].

LOCAL warm-up SCENARIOS

Baseline

Sectorisation of the arcs applies only to the insulation vacuum and cryogenics systems, in particular the magnet cold masses. The two beam vacuum apertures are continuous along the 2.8 km arc and are equipped with room temperature vacuum valves at their extremities.

During a typical forced warm-up, warm helium is fed via the helium distribution line (QRL) to the magnet cold masses. The configuration of cryogenic valves and so-called bus-bar plugs at short straight sections (SSS) permits local warming of some cold masses whilst other remain cold. Longitudinal vacuum barriers in the SSS are positioned to coincide with the bus-bar plug positions and transverse vacuum barriers are positioned in each jumper. The cryogenic and vacuum sectorisation of the arc cryostat can be seen in figure 1. The standard vacuum and cryogenic subsector is 214 m long, equivalent to 2 machine cells. A number of variants exist: the dispersion suppressor subsectors at the arc extremities are 170 m long, the mid-arc cryogenic subsector is 321 m long (3 cells) but its insulation vacuum volume is subdivided into 214m and 107 m lengths.

Figure 1: Cryogenic and vacuum sectorisation of the arc

Baseline local warm-up is restricted to interventions in the insulation vacuum or on the cold mass helium volume. Typical interventions on magnet interconnections include repairs on the cold mass volume such as diode, busbar, splice, helium leak or instrumentation feedthrough, and repairs in the insulation vacuum such as air leak, temperature sensor, BPM cable, feedthrough flange, alignment check or MLI inspection. Interventions on helium circuits or vacuum volumes which extend outside of the locally warmed subsectors (beam vacuum, thermal shield, beam screen, heat exchanger) are excluded as the cold and warm zones cannot be isolated from each other.

Figure 2: Baseline warm-up scenario

The warm-up scenario is shown in figure 2. The intervention zone is in the central subsector, n. To ensure that there is no water condensation at the intervention subsector extremities following venting and exposure to tunnel air, thermal buffer subsectors, n-1 and n+1, to the left and right are also warmed to room temperature but remain under vacuum.

Baseline revisited

Following a full thermal cycling of the arc 7-8, the RF fingers of several plug-in modules (PIM) of the beam vacuum system collapsed into the beam aperture. The PIM Working Group concluded that the RF finger problems could not be excluded in any of the LHC arcs [2]. When applying the baseline warm-up scenario, PIMs could collapse in any of the 16 interconnections of the intervention subsector or any of the 32 interconnections of the buffer subsectors. The new challenge is 3 fold: how to identify collapsed PIMs, how to repair PIMs in a 2.8 km beam vacuum system which is only locally warmed and how to access PIMs in the evacuated buffer zones.

In the revisited local warm-up scenario, the zone warmed to room temperature is limited to the intervention subsector. A simplified cryogenic scheme of the subsector is shown in figure 3. The 2 cell subsector contains 16 cold masses (3 cells and 24 cold masses for the mid-arc subsector), and the SSS cold masses at the subsector extremities are integral with the vacuum barriers. The SSS cold mass to the left is part of the chain of the 16 cold masses that is warmed to room temperature. The SSS cold mass on the right cannot be warmed via the QRL without also warming at least one cell of magnets in the inaccessible buffer zone to the right.

Figure 3: Simplified cryogenic scheme of subsector

As the buffer subsectors to the left and right of the intervention subsector remain cold, part of the vacuum barrier, thermal shield and SSS cold mass to the right will be at cryogenic temperatures. Subsequent venting to tunnel air will result in condensates such as water, oxygen, nitrogen. Ensuring that all surfaces are > 90 K will eliminate 02 and N2 condensation at atmospheric pressure. Venting with dry air and subsequent containment of the dry air at the intervention subsector extremities will ensure low levels of water contamination.

Figure 4: Full scale test on SSS523 in SM18

Proposals were made via the PIM Working Group to force warm this single SSS using the beam tubes as longitudinal heat exchangers, with nitrogen gas being fed in and out via the SSS pumping pipes [3]. The warm-up proposal was validated in 2 full-scale tests on SSS523 in SM18 – see figure 4. Warm-up from 80 to 290 K took ~ 6 days, with a maximum heat transfer of 2.5 kW [4]. Stress measurements at the cold mass/cold bore interface indicate that no plastic deformations are induced. However this unconventional usage of the beam vacuum system carries many risks for its integrity; including potential contamination from dust, water and other condensable gases or oil from recirculation pump.

PIM behaviour of this SSS during warm-up to room temperature was analysed [5]. The non-accessible PIM, type QQBI, would undergo 16.9 mm of compression, which could result in residual deformation to non-conforming RF fingers. Checking the condition of this PIM during a local warm-up is strongly recommended. Keeping the SSS cold mass at cryogenic temperature will reduce the compression and hence the risk of PIM failure.

Inspection of PIMs

A full sector warm-up or a local subsector warm-up may cause PIMs to fail. A single RF finger in the beam aperture would have a large impact. Following studies by the PIM working group, the maximum warm-up temperature which avoids any PIM failure is 130K [5].

Invasive inspection methods, typically by endoscope, require venting and opening of both 2.8 km beam tubes. Contamination of the vacuum system, in particular with condensable gases in the cold zones must be avoided. Laboratory measurements [6] show that the beam vacuum system can be protected from gases which may enter at the temporary opening, and back-stream to towards the cold surfaces. A 0.5 m/s outflow of nitrogen gas from a 50 mm cold bore is sufficient to avoid helium back-streaming > 0.5 m. Extrapolation of this result implies that other circuits that extend outside the locally warmed zone may be protected in the same way, opening the possibility to perform repairs on the beam screen, heat exchanger and thermal shield lines, and therefore, potentially, the replacement of a cryomagnet [7]. .

Non-invasive methods using x-ray imaging have been extensively used on LEP and LHC RF shielded bellows to check finger geometries. With an LHC cryomagnet interconnection fully open, standard x-ray methods can be used to check PIM integrity. However, checking of a subsector would require the opening of 16 interconnects. Other interconnects in the inaccessible subsector, in particular the QQBI interconnect on the SSS to the right of the intervention subsector cannot be checked in this way. A tomograph has been studied, developed and recently procured[8] in order to check PIM integrity with cryomagnet interconnections closed. The inspection technique can be made with the beam or insulation vacuum system evacuated and at cryogenic temperatures. Elimination of arc beam vacuum venting avoids complex venting procedures and contamination risks.

Figure 5: Tomography image of a damaged RF finger
Revised baseline

Warm the intervention subsector to room temperature

Suppress the warming of buffer subsectors to room temperature; warm the 107 m machine cell to the right of the intervention subsector to ~ 90 K.

Vent insulation vacuum with dry air from the compressed air distribution line.

Open the interconnection at the repair location.

Contain dry air at the sub-sector extremities to avoid water condensation on cold surfaces. The longitudinal position of these anti-condensation pockets is determined by the repair location.

Monitor cryostat wall temperature near subsector extremities and apply heaters if required to maintain external surfaces above air dew point.

PIM integrity checks:

The 16 interconnects in the intervention subsector and the QQQBI interconnect at the right hand SSS must be checked

By tomography – interconnects are checked as soon as the forced warming is complete.

By endoscopy – beam vacuum is vented with a non-condensable gas, via the SSS pumping pipes, in the room temperature zone. With a small overpressure in the beam vacuum, 2 PIMs at an opened interconnect are cut and removed. Gas outflow is maintained to avoid H2O back-streaming.

PIM repairs:

All PIM repairs require venting and PIM cutting as mentioned above. Precise control of gas overpressure in the beam vacuum is required during PIM re-welding.

LOCAL warm-up experience

Two local warm-ups were made in 2007 in the sector 4-5 to repair leaking DFBA flexible hoses. Both warm-ups were made in accordance with the baseline. As each intervention subsector was at the extremity of the arc, warm-up of only one buffer subsector was required. The PIM problem had not been evoked at that time. Following the DFBA repairs, the subsectors were cooled down without checking of the PIMs.

In 2009, 3 further local warm-ups were performed. During the cool-down of sector 6-7 an electrical short circuit occurred. With a minimum temperature of 250 K in the arc, PIM damage due to thermal cycling could be excluded. A single 214 m subsector was re-warmed to room temperature to perform the repairs. No precautions were taken to avoid condensation on the cold surfaces at the subsector extremities.

Over a period of days, the sectors 2-3 and 8-1 both suffered helium to insulation vacuum leaks at a DFBA flexible hose. The revised baseline was adopted for the local warm-up. The warm-up procedure was significantly simplified as the intervention subsector was at the right extremity of the arc ie no cold SSS to the right. PIM inspection was performed by endoscopy as the tomography was not available. With minimum cold bore temperatures ~ 50 K, the beam vacuum was vented to ultra-pure neon via a NEG filter. PIMs were cut in both beam lines at the same QQBI interconnect. A minimum overpressure of 2 mbar was maintained to avoid back streaming. The PIM cutting position was chosen with the following constraints: allow visual inspection of the 170 m intervention subsector with a 100 m endoscope; chose an interconnect with high risk of PIM failure; enable installation of anti-condensation pocket; allow upstream feed of neon at a room temperature SSS; minimise the number of interconnects to be opened. No PIM failures were observed. The intervention on the beam vacuum, including its re-pumping, was longer than the DFBA flexible repair. PIM inspection with the tomograph could reduce the total intervention time. Based upon the experience gained, detailed intervention procedures have been prepared.

Full versus LOCAL warm-up

All interventions involving local or full sector warm-up are primarily aimed at performing a repair and getting the LHC back into operation. Considering that only 8% of the arc is warmed up in the revised baseline, the time gain for cryogenic activities in comparison to a full sector warm-up and cool down is only 5 days due to mass flow limitations into the intervention subsector. However, as only a small part of the arc is thermally cycled, there are much reduced test programmes for electrical quality assurance (ELQA) and powering. Table 1 show the critical path activities for a typical intervention to repair a helium leak on a cold mass circuit. The time gain of local warm-up compared to full sector warm-up is > 2 weeks, 52.5 days compared with 69 days, assuming no secondary problems in the thermally cycled zone eg PIM or electrical fault.

Table 1: Critical path activities to repair a helium leak

summary

Local warm-up of the arc has always been part of the LHC baseline to permit local repairs. However the baseline has been revised to deal with risk of PIM failure. The revised baseline further reduces the zone which is thermally cycled to room temperature and hence reduces the probability of electrical degradations.

Warming of the SSS an the intervention sub-sector extremity using the cold bores as heat exchangers has been validated in full-scale tests in SM18.

Retro-diffusion tests have been performed to confirm that beam vacuum contamination by gas back streaming can be eliminated. The results imply that repairs could be performed on other non-sectorised LHC circuits.

A dedicated tomograph has been developed to check PIM integrity in the thermally cycled zone without venting of beam vacuum or opening of magnet interconnections.

Several local warm-ups have been performed on LHC arcs. Valuable experience has been gained on warm-up procedures with respect to cryogenics, beam vacuum venting with neon, PIM inspection by endoscopy, PIM replacement, and application of anti-condensation barriers.

The potential time gain comparing local warm-up with full sector warm-up is > 2 weeks.

Acknowledgements

The author acknowledges the extensive work performed by the Sectorisation Working Group over 10 years ago, the members of the PIM Working Group chaired by P. Strubin, and many discussions and inputs from L. Tavian and S. Claudet. Many thanks to A. Poncet who has been a strong driving force behind the revised baseline. B. Jenninger played a key role to organise and execute SSS warm-up measurements together with M. Guinchard and A. Siemko. B. Jenninger and A. Sinturel initiated and performed the backstreaming tests. F. Caspers and L. Williams created the reality of the new tomograph. Many thanks to colleagues in the TE and EN departments who contributed to and participated in local warm-up interventions.

References

[1]	M. Bona et al., LHC Project report 60, Cryogenic and Vacuum Sectorisation of the LHC Arcs, September 1996.

[2]	D.Ramos, “Modelling of the RF-Shield Sliding Contact Fingers for the LHC Cryogenic Bean Vacuum Interconnects using Implicit and Explicit Finite Element Formulations”, EPAC08, Genoa, June 2008.

[3]	C. Garion and A. Poncet, “Sectorisation et interventions pour réparation avec réchauffage partiel <<revisited>>”, EDMS, March 2008.

[4]	B. Jenninger, “SSS523 Forced Warm-up Tests in SM18”, EDMS 1069354, November 2008.

[5]	D.Ramos, Consequences to the PIM contact fingers resulting from partial warm-up scenario under study for the intervention in an LHC arc subsector, EDMS 978654, January 2008.

[6]	B. Jenninger and A. Sinturel, “Helium backstreaming into LHC beam aperture”, EDMS 1065598, March 2010.

[7]	S.Claudet, “Can we change a magnet without warming-up a full arc?”, Chamonix Workshop 2010, Chamonix, January 2010.

[8]	L.Williams, Technical Specification for the supply of a mobilex-ray comuted tomography system for the inspection of the interconnection regions in the LHC, EDMS 979133, November 2008.

Poole

2010-03-28

NICE