High Gradient RF Cavity R

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High Gradient RF Cavity RD for Muon Cooling
Channels

• Derun Li
• Beam Electrodynamics Group
• Center for Beam Physics
• Lawrence Berkeley National Laboratory
• DOE High Energy Physics Review
• February 18-19, 2004

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Outline

• Introduction
• High Gradient RF Cavity RD Activities
• 805 MHz cavity
• 201 MHz cavity
• Be window RD
• LBNL Responsibility for MICE
• RF and coupling coil module
• Cooling channel integration
• Summary

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Introduction

• Muon beam is unstable, and has short life-time (
  2 ?s at rest)
• Muon beam is created with LARGE 6-D phase space
• Muon beam manipulation must be done quickly,
  including cooling
• Demand for highest possible RF gradient cavities
• Technical challenges
• Requirements of RF cavity for a muon cooling
  channel
• High cavity shunt impedance and high accelerating
  gradient
• Gradient at 201 MHz 17 MV/m Kilpatrick 15
  MV/m
• Gradient at 805 MHz 30 MV/m Kilpatrick 26
  MV/m
• Normal-conducting strong external magnetic
  fields preclude use of superconducting RF
• Practical implementation for muons RF cavity
  with irises terminated by thin low-Z material
  windows
• Higher shunt impedance
• Lower ratio of peak surface field to accelerating
  field
• Independent phase control, higher transit-time
  factor
• We started with experimental RD at 805 MHz cavity

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805 MHz Cavity Test Setup at Lab G of FNAL
Up to 12 MW peak power
SC Solenoid
805 MHz cavity inside the solenoid
Input RF Waveguide
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Collaboration on the 805 MHz Cavity Tests at Lab
G, Fermilab
LBNL 805 MHz cavity had been under test for
nearly two years at Lab G, Fermilab

• Exceeded its design gradient of 30 MV/m at no
  magnetic field and reached up to 40 MV/m
• Thin Be windows with and without TiN-coated
  surface have been tested versus magnetic fields
  up to 4 Tesla
• No surface damage was found on the Be windows
• Little multipacting was observed accelerating
  gradient limit is a function of the external
  magnetic field

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201 MHz Cavity Concept
Extruding ports
Spinning of half shells using thin Cu sheets and
e-beam welding to join the shells
Water cooling channels
Cavity design accommodates different windows
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The Cavity Body Profile
Spherical section at the equator to ease
addition of ports ( 6o) Elliptical-like (two
circles) nose to reduce peak surface field
Stiffener ring
2o tilt angle
6-mm Cu sheet permits spinning technique and
mechanical tuners similar to SCRF ones
De-mountable pre-curved Be windows to terminate
RF fields at the iris
Bolted Be window
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The Cavity Parameters

• The cavity design parameters
• Frequency 201.25 MHz
• ß 0.87
• Shunt impedance (VT2/P) 22 M?/m
• Quality factor (Q0) 53,000
• Be window radius and thickness 21-cm and 0.38-mm
• Nominal parameters for cooling channel in a
  neutrino factory
• Up to 17 MV/m peak accelerating field
• Peak input RF power 4.6 MW per cavity (85 of
  Q0, 3t filling)
• Average power dissipation per cavity 8.4 kW
• Average power dissipation per Be window 100
  watts

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Spinning _at_ ACME
An example of using spinning technique !
Spinning tools
Spinning a bowl
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RF CMM Measurements at LBNL
3 CMM scans per half shell conducted at 0o, 45o,
90o, respectively.
Measured frequency 196.97 MHz (simulated
frequency 197.32 MHz)
CMM scans, RF frequency and Q measurements of
half shells Cu tape for better RF contacts.
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E-Beam Welding
Stiffener ring
Preparation for e-beam welding of the stiffener
ring (left) after the e-beam Welding (above)
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Extruding Tests at JLab
Successful extruding recently!
Extruding tests on a flat Cu plate going through
e-beam joint
Possible improvement Anneal around the extruding
area or Combination between pilot hole
dimensions and lid heights,
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201 MHz cavity status

• Four half shells have been formed by spinning
• Cu stiffener rings were e-beam welded to two half
  shells
• The shells were mechanically cleaned at JLab
• Shells are ready for machining prior to e beam
  welding of equator joint
• Equator weld fixturing has been fabricated at
  LBNL
• Cavity nose piece rings (Univ. of Mississippi)
  have been brazed at LBNL
• Conceptual design of RF loop coupler
• E-beam welding of equator joint
• Extruding four ports
• Chemical cleaning and electro-polishing of the
  cavity
• Pre-curved Be windows
• The cavity should be ready for test in MTA at
  Fermilab this fall

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Be Windows RD

• Ideal windows
• Transparent to muon beams
• Perfect electric boundary to RF field
• No detuning of cavity frequency
• Engineering solutions
• Pre-stressed flat Be (low-Z) windows
• Pre-curved Be windows
• Grids

Window profile evolutions
A pre-curved Be window 0.25 mm thick and 21 cm
in radius
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Curved S.S. Windows
Succeeded in the S.S. window with Cu frame for
805 MHz cavity
Pre-formed at room temperature by holding foil
edge then brazing the Cu frames
A finished curved S.S. window with brazed Cu
frame
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Curved Be Window
Failed in forming Be window at room temperature
The curved Be windows can be formed at higher
temperature at Brush- Wellman Company. Purchase
order has been placed and the window will be
ready soon.
The Be foil cracked during forming at room
temperature (LBNL)
A successfully formed Be foil (Feb. 5th 2004
at Brush-Wellman)
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Components for MICE

• LBNL responsibility for MICE RF and coupling
  coil module
• Eight 201 MHz RF cavities with Be windows for
  the cooling channel (D. Li)
• SC magnet (M. Green) and system integration (S.
  Virostek)

Solenoid
Magnet
Cooling Channel
Envelope
Coupling coils
RF Cavity
Cavities
Raft
Absorber
Tuning
Bellows
Eight 201 MHz cavities
LH absorber
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Summary

• LBNL provides a leadership role in muon-beam RD
• RF cavity RD
• Be window RD
• SC magnets and system integration
• Expertise that LBNL provides to MC is well
  recognized
• 805 MHz cavity performs well at Lab G and
  provides valuable operation experience and
  experimental data that will benefit accelerator
  community
• 805 MHz cavity and the SC magnet will be moved to
  MTA, FNAL
• LBNL plays leading role and is responsible for
  eight 201 MHz RF cavities, two SC magnets and
  system integration
• 201 MHz test cavity fabrication progressing well
  ready for testing this fall