Safety Review: RF Safety Review: RF IssuesIssues

Derun LiDerun Li

Absorber Safety ReviewAbsorber Safety ReviewDecember 9-10, 2003December 9-10, 2003

Lawrence Berkeley National LaboratoryLawrence Berkeley National LaboratoryBerkeley, CA 94720Berkeley, CA 94720

OutlinesOutlines

• Introduction of RF cavities for MICEIntroduction of RF cavities for MICE• RF modules sit between absorbers RF modules sit between absorbers

– Integral part of the whole cooling channel: Integral part of the whole cooling channel: interfaces of mechanical, vacuum, and interfaces of mechanical, vacuum, and sharing magnetic forces sharing magnetic forces safetysafety

– Operational issues Operational issues

• Safety concerns and plansSafety concerns and plans– RF safety relatedRF safety related– Absorbers relatedAbsorbers related

• SummarySummary

RF Cavities for MICE RF Cavities for MICE • Introduction of RF cavities in MICE Introduction of RF cavities in MICE

cooling channelcooling channel– Eight cavities at 201.25 MHzEight cavities at 201.25 MHz– Operating at ~ 8 MV/m with ~ 1 MW peak Operating at ~ 8 MV/m with ~ 1 MW peak

power (~ 1 kW average power) per cavitypower (~ 1 kW average power) per cavity– Cavity phases can be adjusted independentlyCavity phases can be adjusted independently– Thin Be windows to terminate beam irisesThin Be windows to terminate beam irises

•Higher shunt impedanceHigher shunt impedance

•Lower peak surface field versus open iris Lower peak surface field versus open iris structuresstructures

MICE Cooling ChannelMICE Cooling ChannelEight 201.25 MHZ RF cavities

Three Absorber Modules

For each cooling channel ½ For each cooling channel ½ cellcell• Four RF cavities sit in between absorbersFour RF cavities sit in between absorbers

– Interfaces (vacuum seals & magnetic forces)Interfaces (vacuum seals & magnetic forces)– No vacuum windows for RF modulesNo vacuum windows for RF modules– No differential pressures on the curved thin Be No differential pressures on the curved thin Be

windows: windows: 0.38-mm thick and 21-cm radius0.38-mm thick and 21-cm radius– Absorber is an integral part of the RF moduleAbsorber is an integral part of the RF module

•Mechanical stability (interface + magnetic forces)Mechanical stability (interface + magnetic forces)•Vacuum seals for RF operations (may include RF Vacuum seals for RF operations (may include RF

conditioning)conditioning)

201.25 MHz cavity concept 201.25 MHz cavity concept

Spinning of half shells and e-beam welding

Water coolingchannels

Be window

Safety concerns on RF Safety concerns on RF operation operation • Standard Standard RF interlockRF interlock to preclude people being in to preclude people being in

the experimental area during operation and the experimental area during operation and conditioning conditioning

• RF cavity conditioning and operations RF cavity conditioning and operations – With and without magnetic field With and without magnetic field – MultipactoringMultipactoring– Sparking, dark currents, x-ray radiationSparking, dark currents, x-ray radiation

• Coaxial RF loop couplers Coaxial RF loop couplers – May be conditioned separately May be conditioned separately – Multipactoring or sparking in the coaxial couplerMultipactoring or sparking in the coaxial coupler– With and without magnetic field With and without magnetic field – Coaxial ceramic windowsCoaxial ceramic windows

• Isolate vacuum from air or pressurized gas while allowing RF Isolate vacuum from air or pressurized gas while allowing RF to propagate without reflection to propagate without reflection

Impacts on absorber safety and Impacts on absorber safety and plansplans• Sparking, multipactoring & high RF fields produceSparking, multipactoring & high RF fields produce

– More dark currents More dark currents – Extra thermal load to absorbers (estimated to be less than a Extra thermal load to absorbers (estimated to be less than a

few watts)few watts)– Plan: monitor and minimize sparking rates (standard practice)Plan: monitor and minimize sparking rates (standard practice)

• vacuum gauge and arc detectorsvacuum gauge and arc detectors

• If the thin Be window breaks (cracks)If the thin Be window breaks (cracks)– No shock wave to the absorberNo shock wave to the absorber– No vacuum load to the absorberNo vacuum load to the absorber– RF power shuts down in less than ms (SW cavity)RF power shuts down in less than ms (SW cavity)– Be hazard (unlikely to produce Be powders) requires ES&H Be hazard (unlikely to produce Be powders) requires ES&H

personnel at RAL to check and clean the systempersonnel at RAL to check and clean the system • Vacuum manifold to protect Be windows from having Vacuum manifold to protect Be windows from having

differential pressures differential pressures

Concept to protect Be Concept to protect Be windowswindows

RF Cavity RF Cavity

Vacuum Port

PUMP

Thin Be window

Impact of RF window Impact of RF window breaking breaking

• If a coaxial ceramic window breaks:If a coaxial ceramic window breaks:– Pressurized gas sucks into the cavity (slowly)Pressurized gas sucks into the cavity (slowly)– Unlike a shock wave, it usually takes many Unlike a shock wave, it usually takes many

seconds to minutes; can be detected easily seconds to minutes; can be detected easily – To protect the Be windows: vacuum manifold To protect the Be windows: vacuum manifold

to equalize the vacuum inside and outside of to equalize the vacuum inside and outside of the cavity in the modulethe cavity in the module

– To prevent oxygen gets into the system from To prevent oxygen gets into the system from RF transmission lines, we plan to use RF transmission lines, we plan to use pressurized nitrogen gaspressurized nitrogen gas

Impact of water pipe Impact of water pipe breaking breaking

• If water cooling pipe breaks If water cooling pipe breaks – One piece of pipe with joints outside the vacuum One piece of pipe with joints outside the vacuum

vesselvessel– Unlikely water burst, but pinhole leakage (slow)Unlikely water burst, but pinhole leakage (slow)– Vacuum gauge picks up first, not water flow meters Vacuum gauge picks up first, not water flow meters – Be windows are protected by the vacuum manifoldBe windows are protected by the vacuum manifold

• We plan to haveWe plan to have– Water valves and vessel pressure release valveWater valves and vessel pressure release valve– Interlock with vacuumInterlock with vacuum

• Close both inlet and outlet of water valves Close both inlet and outlet of water valves

• Turn off RF powerTurn off RF power

SummarySummary• Cavity conditioning and operation produce dark currents and Cavity conditioning and operation produce dark currents and

x-rays x-rays extra thermal load to absorbers at a few watts extra thermal load to absorbers at a few watts ((smallsmall))

• Sparking is Sparking is contained insidecontained inside the RF cavities the RF cavities• Be windows are protected by vacuum manifold, ES&H Be windows are protected by vacuum manifold, ES&H

personnel’s guidance at RAL in the worst cases (unlikely)personnel’s guidance at RAL in the worst cases (unlikely)• RF window cracking is a relatively slow process, pressurized RF window cracking is a relatively slow process, pressurized

nitrogen gas not only helps to increase RF breakdown nitrogen gas not only helps to increase RF breakdown voltage, but also prevents oxygen from the systemvoltage, but also prevents oxygen from the system

• RF interlock to preclude people in the experimental area RF interlock to preclude people in the experimental area during operationsduring operations

• Operation with strong magnetic fields may make everything Operation with strong magnetic fields may make everything worse worse