Distributed versus Lumped Coupling Magnets

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Distributed versus LumpedCoupling Magnets

• Michael A. Green and Soren Prestemon
• Lawrence Berkeley Laboratory, Berkeley CA 94720,
  USA
• Marco Apollonio and Holger Witte
• Oxford University Physics, Oxford OX1-3RH, UK

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Why would on want to change the design of the
RFCC magnet?

• One would like to reduce the magnetic field at
  the coil so that the coupling magnet temperature
  margin is increased.
• One would like to reduce the stored energy of the
  magnet, in order to improve quench protection.
• One would like to reduce the field at the HTS
  leads and at the cooler. The high temperature
  end of the HTS lead is most affected by field.
  This affects the position of the cooler and leads.

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Lumped Coupling Magnet(the baseline design with
new tracker)
Lumped Coupling Magnet
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Field on Axis of MICE from Lumped Coupling Magnet
in the Flip Mode
Muon average momentum 240 MeV/c
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Magnetic Field from MICE with a Lumped Coupling
Magnet in the Flip Mode
p 240 MeV/c
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Distributed Coupling Magnet
Distributed Coupling Magnet
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Field on the Axis of MICE from Distributed
Coupling Magnet in the Flip Mode
Muon average momentum 240 MeV/c
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Magnetic Field from MICE with a Distributed
Coupling Magnet in the Flip Mode
p 240 MeV/c
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On Axis Magnetic Field for MICE with the Lumped
and Optimized Extended Coupling Magnet
The green is the field for the lumped case. The
red is the first optimized extended case. The
blue below is the on axis field change from the
lumped case.
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MICE Channel Beta Function with Lumped and
Optimized Extended Coupling Magnet
The black is the TRD case beta. The green is the
beta for the lumped case. The red is the beta
for the optimized extended coupling coil case.
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Lumped and Extended Coil Fields
p 240 MeV/c
RF Iris
RF Iris
RF Iris
RF Iris
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Magnet Parameters for both types ofCoupling
Magnets
Muon average momentum 240 MeV/c
b 420 mm
b 320 mm
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Magnet Module Forces both Types of Coupling
Magnets
Muon average momentum 240 MeV/c
B 420 mm
B 320 mm
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Lumped and Extended Coupling MagnetLoad Lines
and Temperature Margin
Lumped Temperature Margin 0.6 K _at_ 240
MeV/c Extended Temperature Margin 2.4 K _at_ 240
MeV/c
Both cases are in the flip mode.
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Magnetic Field at z 1.375 m versus Radius
Muon average momentum 240 MeV/c
Lumped coil outer R 841 mm Distributed coil
outer R 762 mm
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What happens when the beam b in the AFC is
increased to the nominal 420 mm?

• The current in the coupling coil must increase
  about 15 percent to about 242 A. The magnet
  temperature margin is about 2 K at 240 MeV/c.
• The current in the focusing magnet will decrease
  about 21 percent to 199 A. The temperature
  margin for the magnet is 1.6 K at 240 MeV/c.
• The forces on the magnet cold mass will change
  and become closer to the nominal case with the
  lumped coupling magnet. The force changes are
  dictated by the tracker tuning coils.
• The peak b in the cavities and tracker will go
  down to the value for the TRD case.

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Benefits from a Change of the Design of the RFCC
magnet

• The peak magnetic field at the coil is reduced by
  a factor of two. The magnet temperature margin
  is increased by nearly 1.4 K.
• The magnet stored energy is reduced by 45
  percent.
• The field on the outside of the coils is reduced.
  This is important for coolers, leads and vacuum
  pumps.
• The cryostat outside diameter is reduced.
• Less current is needed for the AFC coils, thus
  their temperature margin is increased about 1 K.

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Problems caused by a change the design of the
RFCC magnet

• Two of the RF couplers, two cavity tuners (8 to
  12) 25 to 50 mm F ports) must pass through the
  magnet, which complicates the cryostat design.
• The RFCC vacuum pumping ports may be a little too
  long, which will reduce the conductance.
• The magnet and the RF vacuum vessel will have to
  be fabricated as a single unit.
• The cost of fabricating the magnet and its
  cryostat is increased, but the cost of the RF
  cavity vacuum vessel is eliminated. We must get
  vendor budgetary quotes to see how much the cost
  is increased.

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Potential Issues brought on by a change the
design of the RFCC magnet

• The magnetic field within the the cavities is
  more uniform than in the lumped coil case. The
  field on the outside of the center cavities is
  lower. The field on the outside of the end
  cavities may be higher. The effect of the
  distribution is unknown. In all cavities the
  field is more parallel to the cavity axis.
• The magnetic field in the RF couplers is
  different in both magnitude and direction than in
  the lumped case. For the two center cavities the
  field is lower and perpendicular to the coupler
  axis. For the two end cavities the field is
  parallel to the coupler axis.

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A Cross-section through RF coupler Port of a
Distributed Coupling Magnet that can be Built
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Where do we go from here?

• It is clear that lengthening the coupling coil
  has benefits, but there are also problems caused
  by the extended coupling coil presented here.
• We should stay with the baseline magnet design
  which uses a single coupling coil. It is worth
  the extra effort to lengthen this coil to
  increase the temperature margins for both the
  coupling and AFC magnets. Lengthening the coil
  will reduce the field on the outside of the
  magnet.
• The magnet must fit through the MICE hall door.