Safety Review: RF Issues
Derun Li
Absorber Safety Review
December 9-10, 2003
Lawrence Berkeley National Laboratory
Berkeley, CA 94720
Outlines
• Introduction of RF cavities for MICE
• 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
– RF safety related
– Absorbers related
• Summary
Absorber Safety Review, LBNL, Dec. 9-10, 2003
RF Cavities for MICE
• Introduction of RF cavities in MICE cooling
channel
– Eight cavities at 201.25 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
• Lower peak surface field versus open iris structures
Absorber Safety Review, LBNL, Dec. 9-10, 2003
MICE Cooling Channel
Eight 201.25 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
– 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)
• Vacuum seals for RF operations (may include RF
conditioning)
Absorber Safety Review, LBNL, Dec. 9-10, 2003
201.25 MHz cavity concept
Be window
Spinning of half
shells and e-beam
welding
Water cooling
channels
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
• RF cavity conditioning and operations
– With and without magnetic field
– Multipactoring
– Sparking, dark currents, x-ray radiation
• 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
Absorber Safety Review, LBNL, Dec. 9-10, 2003
Impacts on absorber safety and plans
• 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
• 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
Absorber Safety Review, LBNL, Dec. 9-10, 2003
Concept to protect Be windows
Thin Be window
RF Cavity
RF Cavity
Vacuum Port
PUMP
Absorber Safety Review, LBNL, Dec. 9-10, 2003
Impact of RF window breaking
• 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
–
–
–
–
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
– Water valves and vessel pressure release valve
– Interlock with vacuum
• Close both inlet and outlet of water valves
• 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)
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)
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
Operation with strong magnetic fields may make everything
worse
Absorber Safety Review, LBNL, Dec. 9-10, 2003