Experience with LEP (and LHC) cryo-modules

Workshop on cryogenic and vacuum sectorisations of the SPLO.Brunner – November ’09

The LEP RF System• Made up of:

– 288 SC cavities (272 Nb/Cu, 16 bulk Nb) ≈ 1600m2

– 72 cryomodules ≈ 850m– total acc. voltage ≈ 3650MV

• Located in 200m long straight sections on each side of the four IPs

• One cryogenic plant per IP operating at 4.5K

• LHC RF system: 16 cavities (4 modules) installed at P4

The LEP RF System

IP4,8

≈200m ≈200m

The LEP RF Cavities• Frequency: 352.2MHz• Operating temperature: 4.5K• Operating field: 6MV/m• Number of cells: 4• He pressure sensitivity: < 10Hz/mbar• Qo at operating field (4.5K): >3.2 109

• Qo at low field (4.5K): >6.4 109

• LHC cavities: 400.8MHz, 1 cell

The LEP RF Modules (1)• Four cavities are grouped together in on common cryostat of about 12m length (≈8000kg)• Smallest unit treated independently for cooling and control

– Common gas collector– Communicating liquid space

• LHe is fed in the first cavity and gas is evacuated from the last one– Each cavity is housed in a He vessel containing 180 liters of LHe (i.e. 720 l/module)– Operating pressure: 1350mbar

• Smallest unit treated independently for vacuum:– Common beam vacuum (one sector valve on each side)– Common insulation vacuum

The LEP RF Modules (2)• RF power:

– Common klystron for eight cavities -> RF interlock– Common power converter for two klystrons (sixteen cavities) -> HV interlock

• Safety: the RF modules are protected against:– He pressure >1450mBar =>RF interlock – He pressure >1550mBar => HV interlock – He pressure >1600mBar => Beam dump

– four safety valves (one per cavity) opening at 2bars: to protect against small air leak in the cryostat– four 80mm diameter rupture disks with a 3bars breaking pressure: to protect against breakdown

of the beam vacuum or major air leak

• LHC RF modules: shorter (8 meters), less LHe (320l/module), lower breaking pressures: 1.8 and 2.3bars (less robust He vessel)

LEP/LHC: Module length limited to four cavities

• Transport:– SC modules must be transported under vacuum

• Beam vacuum• Cryostat vacuum (mechanical stability)

• Clean rooms:– Several activities require opening of beam vacuum (installation of power couplers, HOM

couplers, etc) => must be done in a clean room– SM18 clean room (class10): 15 meter long

• Safety:– LHe volume

• Alignment of cavities in cryostat?• RF power distribution, control, interlock, etc

LEP/LHC Cavities acceptance tests• Field emission is one of the main field limits of SC cavities because of the increased cryogenic load,

possible quenching and radiation• Although every precaution was taken to avoid contamination and surface defect during fabrication,

some field emission was nevertheless observed in most cavities at fields well below the specified maximum.

• LEP (and LHC) acceptance tests at low power (with a matched coupler):

– Before being assembled into a module, the cavities were individually conditioned to full-field, and their Q0(E) curve was measured (vertical test stand). o 40-50% of the cavities had to be high pressure rinsed with

ultra pure watero 15-25% of the cavities had to be recoated

– After assembly of the 4-cavity module, before mounting the main power and HOM couplers, the cavities were cooled down and tested again and the Q0(E) curve was re-measured. When necessary, the cavity was reconditioned using standard, pulsed , or eventually helium processing.

– Before installation on the SC cavities, the main couplers were RF-conditioned. (two by two, at room temperature)

High power conditioning• LEP (and LHC) acceptance tests at high power:

– After the main (and HOM) couplers were main (and HOM) couplers were mounted onto the cavities, before installation in the machine, the fully equipped modules were tested and conditioned to full field in a high power test stando Mainly necessary to condition the power couplerso However a few cavities degraded somewhat during the final stage of installation:

Max field limited to 4-5MV/m and more aggressive conditioning was necessaryFor about 10 cavities, He processing had to be used to recover the cavitiesTwo (out of the 288 cavities) had to be dismantled and rinsed (high pressure rinsing with ultra pure water)

– Once installed in the tunnel the modules were again conditioned. This procedure was repeated after every warm up of a module or when the performance of a module showed significant deterioration.

– He processing was often the only way to recover a degraded cavity

He processing• The cavities were filled to ≈10-5bar with He gas• The combination of the application of RF and field emission caused ionization and

bombardment of the emitter by the He ions• Drawbacks:

• Introducing He gas in cold cavities in a delicate (and potentially dangerous) operation

• Full module (vacuum sub-sector) affected• Danger in case of arcing during processing: lower Qo->better matching-> arc

sustained-> possible damaged to Nb surface or RF window• Presence of gaseous He in the cavity required modification to the interlock system• After He processing: complete warm up of the module to room temperature, long

vacuum pumping (>24hours) before cooling down again

LEP Experience: operation (1)• Context: the quest for higher gradients:

• From 1998 to 2000, a large campaign was launched to push the Nb/Cu cavities for higher gradients

• Intensive processing was used to reached an average value of 7.5MV/m

• Degradation of cavities:• No strong/permanent degradation of cavities was observed after warm up of the

modules • Only standard processing was necessary to get back to nominal field

• Continuous reconditioning of degraded cavities was necessary. Several cavities degraded during operation (field limited due to high radiation level or He pressure spikes or He level interlocks)

• Most of them could be recovered by standard processing• ≈10-15 required He processing (safe in-situ He processing would have been welcomed)• 1 (or 2?) could not be recovered in situ (Nb surface damaged) and had to be removed

from the tunnel, dismantled, recoated and re-installed the following shutdown.

LEP Experience: operation (2)• 1 module was damaged by vacuum incident (adjacent subsector vented with vacuum

sector valves opened) and had to be removed, dismantled, rinsed and re-installed the following year.

• Continuous reconditioning of degraded cavities was necessary and safe in-situ He processing would have been welcomed

• During LEP operation, they were no Helium blowouts or failure of the ceramic RF windows

• A better mechanical damping of structural resonances of the cavity system would have helped against ponderomotive instabilities

• A variable coupler would have allowed:• Equalizing cavity fields• Optimization of RF power• In situ measurements of Q0(E) curves

LHC Experience: operation• Rupture disk broken (Sept ‘09)

• Due to cryo operator hiccup• No damage to cavity nor to cryostat• Relatively long recovery process: complete warm up, long He flush to remove

humidity…

• No degradation of cavities was observed after warm up of the modules• Only standard processing was necessary to get back to nominal field

Dark currents/radiation

• Radiation emitted from the module is due to dark current of field emitted electrons• These electrons can potentially be accelerated by the field in the cavity in which they are

produced and also by the field of the neighboring cavities (LEP: >50MeV)• In LEP several sector valves were severely damaged during conditioning

• Sector valve were kept opened during conditioning• LHC: electron stoppers have been installed on each side of the RF zone to protect

the rest of the machine• Radiation levels are monitored and interlocked

LEP Experience: Reliability• Typically:

• ≈ 35 interlock per RF power system• ≈ 30 interlock per cavity

In LEP, the total number of interlock was >> 10’000 In LHC, it is about 1000 In SPL is would be over 12’500 (2 cavity per klystron scenario)

• LEP: higher RF voltage shorter MTBT • In ‘99: mean time between trips ≈ 25minutes• Recovery time:

• Few minutes for RF trips• Up to 20minutes for HV trips (klystrons heater)

• about 50% spurious trips• LHC RF interlock system more stable…no

operational experience yet…

Conclusions• Several parameters to be taken into account in the vacuum and cryogenic sectorization:

• Transport and handling• Several transports to tests place, clean room• Transport and installation in tunnel• Alignment

• Clean rooms activities• Safety (max LHe volume)

• Recovery time in case of incident (rupture disc broken by e.g.)• Radiation during conditioning• He processing• Repair (removal of a module from tunnel)• Potential incident:

• Vacuum incident (power coupler broken)