Safety Review: RF Issues Derun Li Absorber Safety Review December 9-10, 2003 Lawrence Berkeley National Laboratory Berkeley, CA 94720

Absorber Safety Review, LBNL, Dec. 9-10, 2003 Outlines Introduction of RF cavities for MICE Introduction of RF cavities for MICE RF modules sit between absorbers RF modules sit between absorbers –Integral part of the whole cooling channel: interfaces of mechanical, vacuum, and sharing magnetic forces  safety –Operational issues Safety concerns and plans Safety concerns and plans –RF safety related –Absorbers related Summary Summary

Absorber Safety Review, LBNL, Dec. 9-10, 2003 RF Cavities for MICE Introduction of RF cavities in MICE cooling channel Introduction of RF cavities in MICE cooling channel –Eight cavities at MHz –Operating at ~ 8 MV/m with ~ 1 MW peak power (~ 1 kW average power) per cavity –Cavity phases can be adjusted independently –Thin Be windows to terminate beam irises Higher shunt impedance Higher shunt impedance Lower peak surface field versus open iris structures Lower peak surface field versus open iris structures

Absorber Safety Review, LBNL, Dec. 9-10, 2003 MICE Cooling Channel Eight MHZ RF cavities Three Absorber Modules

Absorber Safety Review, LBNL, Dec. 9-10, 2003 For each cooling channel ½ cell Four RF cavities sit in between absorbers Four RF cavities sit in between absorbers –Interfaces (vacuum seals & magnetic forces) –No vacuum windows for RF modules –No differential pressures on the curved thin Be windows: 0.38-mm thick and 21-cm radius –Absorber 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 conditioning) Vacuum seals for RF operations (may include RF conditioning)

Absorber Safety Review, LBNL, Dec. 9-10, MHz cavity concept Spinning of half shells and e-beam welding Water cooling channels Be window

Absorber Safety Review, LBNL, Dec. 9-10, 2003 Safety concerns on RF operation Standard RF interlock to preclude people being in the experimental area during operation and conditioning Standard RF interlock to preclude people being in the experimental area during operation and conditioning RF cavity conditioning and operations RF cavity conditioning and operations –With and without magnetic field –Multipactoring –Sparking, dark currents, x-ray radiation Coaxial RF loop couplers Coaxial RF loop couplers –May be conditioned separately –Multipactoring or sparking in the coaxial coupler –With and without magnetic field –Coaxial ceramic windows Isolate vacuum from air or pressurized gas while allowing RF to propagate without reflection Isolate vacuum from air or pressurized gas while allowing RF to propagate without reflection

Absorber Safety Review, LBNL, Dec. 9-10, 2003 Impacts on absorber safety and plans Sparking, multipactoring & high RF fields produce Sparking, multipactoring & high RF fields produce –More dark currents –Extra thermal load to absorbers (estimated to be less than a few watts) –Plan: monitor and minimize sparking rates (standard practice) vacuum gauge and arc detectors vacuum gauge and arc detectors If the thin Be window breaks (cracks) If the thin Be window breaks (cracks) –No shock wave to the absorber –No vacuum load to the absorber –RF power shuts down in less than ms (SW cavity) –Be hazard (unlikely to produce Be powders) requires ES&H personnel at RAL to check and clean the system Vacuum manifold to protect Be windows from having differential pressures Vacuum manifold to protect Be windows from having differential pressures

Absorber Safety Review, LBNL, Dec. 9-10, 2003 Concept to protect Be windows RF Cavity Vacuum Port PUMP Thin Be window

Absorber Safety Review, LBNL, Dec. 9-10, 2003 Impact of RF window breaking If a coaxial ceramic window breaks: If a coaxial ceramic window breaks: –Pressurized gas sucks into the cavity (slowly) –Unlike a shock wave, it usually takes many seconds to minutes; can be detected easily –To protect the Be windows: vacuum manifold to equalize the vacuum inside and outside of the cavity in the module –To prevent oxygen gets into the system from RF transmission lines, we plan to use pressurized nitrogen gas

Absorber Safety Review, LBNL, Dec. 9-10, 2003 Impact of water pipe breaking If water cooling pipe breaks If water cooling pipe breaks –One piece of pipe with joints outside the vacuum vessel –Unlikely water burst, but pinhole leakage (slow) –Vacuum gauge picks up first, not water flow meters –Be windows are protected by the vacuum manifold We plan to have We plan to have –Water valves and vessel pressure release valve –Interlock with vacuum Close both inlet and outlet of water valves Close both inlet and outlet of water valves Turn off RF power Turn off RF power

Absorber Safety Review, LBNL, Dec. 9-10, 2003 Summary Cavity conditioning and operation produce dark currents and x-rays  extra thermal load to absorbers at a few watts (small) Cavity conditioning and operation produce dark currents and x-rays  extra thermal load to absorbers at a few watts (small) Sparking is contained inside the RF cavities Sparking is contained inside the RF cavities Be windows are protected by vacuum manifold, ES&H personnel’s guidance at RAL in the worst cases (unlikely) Be windows are protected by vacuum manifold, ES&H personnel’s guidance at RAL in the worst cases (unlikely) RF window cracking is a relatively slow process, pressurized nitrogen gas not only helps to increase RF breakdown voltage, but also prevents oxygen from the system RF window cracking is a relatively slow process, pressurized nitrogen gas not only helps to increase RF breakdown voltage, but also prevents oxygen from the system RF interlock to preclude people in the experimental area during operations RF interlock to preclude people in the experimental area during operations Operation with strong magnetic fields may make everything worse Operation with strong magnetic fields may make everything worse