Status of the MuCool Cavity Prototype

1
Status of the MuCoolCavity Prototype
MICE Collaboration Meeting October 21, 2005

• Steve Virostek
• Lawrence Berkeley National Laboratory

2
Progress Since Last Year

• Leak check of all e-beam welds was performed
• Cooling tubes were TIG brazed to cavity exterior
• A final mechanical buff of the interior was
  completed
• Cavity interior surfaces were electropolished
• RF couplers were fabricated and conditioned
• First 201 MHz beryllium window delivered to LBNL
• Cavity manual tuner and support stand were
  fabricated
• Final assembly and leak check performed at J-Lab
• Cavity was shipped to the MTA, assembled and leak
  checked

3
RF Cavity Overview

• Two 6 mm thick copper shells are e-beam welded
  together at equator to form cavity
• Cavity half-shells are formed from annealed, flat
  plate using a spinning technique
• Separate copper nose piece rings are e-beam
  welded to cavity aperture
• RF and vacuum ports are formed by pulling a die
  through a hole cut across the equator weld
• Externally brazed tubes provide cooling
• Two thin, pre-curved beryllium windows are to be
  mounted on cavity aperature
• Cavity inside surfaces are finished by
  mechanically buffing and electropolishing

4
Shell Spinning Measurement

• A 6.35 mm OFHC copper sheet is spun against a
  pre-machined form to generate half-shells
• Shell outer edges are trimmed as specified after
  spinning
• Half-shell profiles at various cross-sections are
  measured using a portable CMM
• Half-shell is placed on a copper sheet and
  frequency is determined using low level RF
• Comparisons are made between RF analysis of
  measured shape and frequency check

5
Cavity Stiffener Ring

• Hard copper stiffener ring is e-beam welded on
  inside and outside edges to the outside of the
  cavity shell
• Rings initially provide for safe handling the
  half-shells
• Rings are turned on a lathe after being welded on
  to provide an accurate reference for subsequent
  operations
• Rings provide an interface for tuner mechanisms
  and for mounting the finished cavity to the
  support structure
• Load tests on sample welds indicate the e-beam
  weld strength is much higher than required for
  tuning

6
Nose Hole Equator Joint Machining

• Nose hole is cut with a numerically controlled
  horizontal mill using the stiffener ring for
  reference
• Special aluminum fixturing was designed to hold
  the half shells
• Hole detail includes a 1 mm lip to register the
  nose piece ring
• Shell lip is cut using a numerically controlled
  horizontal mill with a right angle head adaptor
• A close fitting aluminum disc supported the lip
  during machining
• Lip includes a chamfered step that mates with the
  opposing shell to prevent slipping prior to and
  during e-beam welding

7
Shell Cleaning and Buffing

• Shells are cleaned prior to welding operations by
  rotating them through a chemical bath
• Some scratches and dents created on the inside
  surfaces of the shells during spinning
• Cavity surfaces smoothed out mechanically with an
  abrasive buffing wheel

8
Cavity Equator Weld

• Cavity and fixture system is mounted and
  assembled on a plate and placed on the welder
  sliding table
• External structural weld is near full penetration
  and is achieved in three offset passes
• A final cosmetic/vacuum weld is performed on the
  inside of the joint with the cavity mounted on a
  horizontal rotary table
• Inside equator weld is smoothed using an abrasive
  wheel

9
Nose Fabrication Welding

• Nose piece ring is OFHC copper with a stainless
  steel support ring brazed to the roughed out
  copper ring
• SS ring bolt circles allow attaching Be window
  and cover plate
• After brazing, copper is machined to final shape
• Cavity is placed in welder chamber horizontally
  on a rotary table with fixturing holding nose
  piece ring
• Similar to the equator weld, nose piece weld
  consists of 3 offset external welds and an inside
  cosmetic/vacuum pass
• Inside nose weld are smoothed and weld blow
  through is removed using an abrasive wheel

10
Cavity Port Forming Welding

• Local annealing only (to preserve cavity overall
  strength is) achieved by repeatedly passing a
  diffuse e-beam around port
• Port pulling tool is used in a horizontal
  orientation, and a weld prep is machined into the
  port lip using an NC mill
• Cavity is held vertically in welder chamber on a
  fixture that facilitates 90 degree rotations
• Structural and vacuum weld is made with a single
  inside pass
• Prototype cavity uses a stainless flange with a
  TIG welded copper insert (flange must hold
  vacuum)
• MICE cavity will use an all copper flange for RF
  sealing only

11
Vacuum Leak Check

• After ports are attached, a vacuum leak check of
  all welds (equator, nose pieces 2, stiffener
  rings 2, ports 4, port flange inserts 4) is
  performed
• Conflat flanges on ports allow easy blank-off and
  leak detector attachment
• Nose openings are covered by a plate with an
  O-ring
• An internal support is required to prevent the
  external atmospheric pressure from collapsing
  cavity

12
Cooling Tube Weld

• Cavity cooling is achieved by TIG brazing a 9.5
  mm diameter copper tube to the exterior in an
  alternating skip pattern
• Four turns (1 circuit) per cavity side step
  inward near to the stiffener ring OD and then
  step back out to the equator
• Cooling tube spacing is approximately 10 cm
• Fittings are brazed to the tube ends and tacked
  to the cavity
• Nominal flow rate is expected to be 3 gpm per
  circuit

13
Final Interior Buffing

• Final interior buffing of cavity is performed to
  ensure the surfaces are ready for
  electropolishing
• Less buffing needed near equator where fields are
  lower
• An automated process of buffing was developed
  using a rotary buffing wheel and a cavity
  rotation fixture
• Some local hand work required to clean up some
  areas
• A series of pads with graduated coarseness was
  used
• Goal was scratch depth shallow enough for EP
  removal

14
Interior Surface Electropolish

• After buffing, cavity underwent a chemical
  cleaning process
• Test bars with various degrees of buffing were
  run through an electropolish process
• Cavity was rotated with a U-shaped electrode
  fixed in place
• Initial polish failed due to depletion of the
  solution, and rebuffing was required
• 2nd EP successfully removed scratches in high
  field regions
• Final process is a high pressure water rinse of
  cavity surface

15
Cavity RF Couplers

• Coupling loops were fabricated using standard
  copper co-ax
• Most coupler parts were joined by torch brazing
  vacuum leaks were found in two of the outer
  conductor joints
• Coupling loop contains an integrated cooling tube
• The coupler was designed to mate with an SNS
  style RF window manufactured by Toshiba
• High power conditioning performed at SNS (ORNL)
• The MICE coupler will require a bellows
  connection interface with the outer vacuum vessel
  ports

16
201 MHz Beryllium Windows

• Each cavity will require a pair of 0.38 mm thick
  pre-curved beryllium windows with TiN coating
• Double-curved shape prevents buckling caused by
  thermal expansion due to RF heating
• Thermally induced deflections are predictable
• A die is applied at high temperature to form
  window
• Copper frames are brazed to beryllium windows in
  a subsequent process

17
Shipment to the MTA at FNAL

• System assembly included tuner plates, port
  blank-offs, diagnostic spool, window cover
  plates, gate valve and window pumpout tubes
• Final leak check conducted prior to shipping
• Cavity was back-filled with nitrogen in its
  assembled state and packaged in a custom made
  crate for shipping to the MTA

18
Final Assembly at the MTA

• Cavity assembly was mounted on the support and
  couplers were installed in a portable clean room
• Dummy copper windows (flat) are used initially
• Couplers were set and frequency was measured
• Bakeout system hardware was installed
• System is now leak tight and ready for bake