Minimizing the RF fields

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• Minimizing the RF fields
• on the superconducting surface of an SRF cavity
• by optimizing its shape
• REU 2009

Valery D Shemelin, LEPP, Cornell University for
David Stark, University Minnesota Twin Cities
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Superconducting accelerating cavity what are
they for?
A key component of the modern particle
accelerator is the device that imparts energy to
the charged particles.
Single cell and multicell elliptical cavities.
Radio frequency (RF) cavities are used to
accelerate charged particles to high speeds. The
electric field inside the cavity kicks a charged
particle passing through it. A large speed is
attained after the particle travels through many
cavities. A typical cavity generates a potential
of over 1 million volts. If we tried to do the
same thing using ordinary 9 volt batteries, for
example, we would need more than 100,000
batteries. A conventional copper cavity requires
a lot of power to produce a high potential. The
power being dissipated in the cavity walls can be
as high as 1 million watts. As a result, copper
cavities cannot operate continuously at high
potential superconducting radio frequency (SRF)
cavities must be used for this purpose. The power
dissipation in an SRF cavity is about 100,000
times smaller than in a copper cavity. Special
materials and extremely cold temperatures are
needed to make a cavity superconducting. An SRF
cavity made of niobium is operated at a
temperature of about -456 F or 2 Kelvin (K).
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What accelerating cavities does the world want
today?

• ILC wants high gradient (31.5 MV/m is the base
  line of the project, but 40, 50 MV/m who will
  mind?)
• ERL wants moderate gradients (20) but low losses
  (refrigerator)
• What are the issues for each application?
• How do we design the cell to make these issues
  less problems?

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Comparison of some cells
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Fields in the cavity

• Epk/Eacc and Hpk/Eacc define maximal fields on
  the surface of the cavity. They are responsible
  for X-radiation Epk, and thermal breakdown
  (quench) Hpk.

Fields in a reentrant cavity
TESLA and Reentrant cavity
Electric
and Magnetic field

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their electric field along the profile line
The best shape (just aesthetically) should have a
more regular dependences for these curves
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and magnetic field on the profile line
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Case of the reentrant cavity elliptic and a
6-arc curve
The reentrant cell with 20 higher electric
field and 10  lower magnetic field than in TESLA
cell, shape and electric field along the profile
line.
We can divide the iris ellipse into two parts (or
more but it will be too complicated) and deal
with them separately.
However, I could not make a flat H-field profile
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Attempt to flatten the H field
Upper part of this shape consists of straight
segments
This ugly shape gives the best result for de
20
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THE TASK

• Find the shape having Epk/Eacc 2.4
• (20 to TESLA, TESLA has 2.0),
• and minimum Hpk/Eacc.
• We had the world record with Hpk/Eacc 37.8
  Oe/(MV/m),
• -10 relative to TESLA.
• I hope that we can have -15 . Even -12 would
  be a good result.

This is a computational work. The work is
supposed to be done with a SuperLANS 2D computer
code, designated for designing of axially
symmetric RF devices, their fields, losses and
other Figures of Merit.