SC TW ACCELERATING STRUCTURE FOR ILC

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SC TW ACCELERATING STRUCTURE FOR ILC
SC Traveling Wave Accelerating Structure for ILC
P. Avrakhov 1, A. Kanareykin1, S. Kazakov2,
N. Solyak3, V. Yakovlev3, W.Gai.
1Euclid Techlabs LLC, Rockville, MD 2KEK,
Tsukuba, Japan 3FNAL, Batavia, USA
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Motivation

• ILC Main Linac contains 2?7920 9-cell 1.3 GHz SC
  cavities.
• Loaded acceleration gradient is 31.5 MeV/m.
• Main Linac length is 2?11 km.
• Total length of RF structure is 2?8224 m, or 75
  of the main linac length.
• ILC cost reduction is one of the most important
  problems.
• One of the ways to reduce the length of the ILC
  is to increase the acceleration gradient.

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Gradient Limitations

• The accelerating gradient in a SC structure is
  limited mainly by quench, i.e, by the maximum
  surface RF magnetic field.
• Techniques developed to increase the gradient
• Development of surface processing in order to
  avoid the field enhancement caused by surface
  microstructure. A recently developed
  electropolishing technique allows microtips less
  than 0.1 micrometers.
• Improvement of niobium material. For example,
  large grain and mono-crystal materials are
  currently being considered.
• Improvement of the structure shape in order to
  decrease surface magnetic field for a given
  accelerating gradient.

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Surface RF Magnetic Field

• There are two ways to decrease the surface RF
  magnetic field
• Homogeneous RF magnetic field distribution over
  the cavity surface
• - Reentrant structure, Cornell
• - Low-Loss structure, DESY and
• - Ichiro structure, KEK.
• Improvement of the beam interaction with the
  structure increased transit time factor.
• The maximal gradient achieved in the one-cell
  cavity is 54 MeV/m for an aperture of 70 mm and
  59 MeV/m for 60 mm (Reentrant, H. Padamsee et al).

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Cavity Geometries
(a) (b)
( c) The cavity geometry and RF magnetic
field pattern in the TESLA cavity (a), Low-Loss
cavity (b) and Re-Entrant cavity (c).
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Standing Wave SC 9-cell RF cavities.
Problems

• Small transit time factor T (TEacc/Eaverage,
  Eacc acceleration gradient, Eaverage average
  field over the cell gap). Note, that if
  acceleration gradient is limited by maximum
  surface RF magnetic field, Eacc T. The higher T
  is, the higher the acceleration gradient! For SW
  ?-structure T0.7 .
• Poor stability of the field distribution with
  respect to small geometrical perturbations
• dE df/k?N2,
• where dE is the maximal field perturbation,
  df is the cell resonance frequency perturbation,
  k is the coupling coefficient, and N is the
  number of cells. Note strong (quadratic) increase
  of the field perturbation with the number of
  cells.
• -The field perturbation gives the field
  enhancement in the structure and limits the
  acceleration gradient
• - The field perturbation limits the number
  of the cells in the structure that leads in
  turn to
• small length of the structure (9 cells for ILC)
  
• large number of input couplers and HOM dampers
• great number of gaps between the structures and
  thus,
• effective decrease in the acceleration gradient.
  

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Trapped Modes
c. Trapped modes. If the cells of the structure
have the same length, the field in the end cells
is the same as in the regular cells only for the
operating mode. For all other modes the maximal
field may occur not in the end cells, but in the
regular cells. It may happen that the field in
the end cells is small, preventing high-order
mode (HOM) extraction the so-called trapped
modes.
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SC Traveling Wave Structure
Alternative structures should be discussed and
developed! Our proposed alternative approach a
superconducting traveling wave acceleration
structure (STWA) Benefits Higher transit time
factor (T1) higher acceleration gradient for
the same surface RF magnetic field. For an ideal
structure with a small aperture
Tsin(?/2)/(?/2) (? is
phase advance per cell) and the acceleration
gradient gain compared to SW ? -structure is
Gain E
?/E? 2/??sin(?/2)
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SCTW Structure Advantages (1)
1. Higher Gradient. The gain in the accelerating
gradient of the traveling wave accelerating
structure relative to a standing wave ?-structure
versus the phase advance per cell for the ideal
case. For ? ?/2 (90?) the gain is v2, or 42
(!) - for the ideal structure, of course.
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SCTW Structure Advantages (2)

• 2. Stability of the field distribution along the
  structure with respect to geometrical
  perturbations. This permits
• a much longer structure length (if technology
  allows), up to the length of a cryostat (10 m)
• a much smaller number of input couplers, at most
  two couplers per cryostat
• no gaps between short cavities (for the ILC there
  is a 280 mm gap between each 1 m long 9-cell
  cavity). This results in an additional effective
  gradient increase of 27!
• 3. No trapped modes for the lower dipole mode
  passband ! Two HOM dampers for a long structure.
• Note, that the STWA structure has the same
  behavior in the case of breakdown as the SW
  structure (J. Haimson, HG meeting, SLAC-2007),
  i.e., in the case of breakdown the power from the
  source is reflected from the structure but not
  dissipated in the structure destroying the walls.

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SCTW Structure Advantages. (3)

• Pay-off
• The STWA has a negligibly small RF field
  attenuation, and thus high power feedback is
  necessary. Technology needs to be developed to
  fabricate and process long SC structures with a
  feedback waveguide
• High-power coupler could be required to feed a
  long SC accelerating structure (the number is
  defined by the length).
• Two tuners are necessary instead of one for the
  SW structure in order to
• - compensate for microphonics and Lorentz
  force and
• - provide good VSWR ratio in the structure
  and feedback waveguide.

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SC Feedback Structure (1)

• Traveling wave structures with feedback
  waveguides have a long history.
• First acceleration structures were TW structures
  with feedback! (R.B. Shersby-Harvi and L.B.
  Mullett, 1949).
• First SC acceleration structures were TW
  structures with feedback (R.B. Neal, 1968)

TW structure with feedback (R.B. Neal, 1968)
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SC Feedback Structure (2)
3. First suggestion to use STWA for SC linear
collider in order to increase
acceleration gradient by decrease of the
electric field on the aperture to avoid
breakdown (N. Solyak, 1998)
SC TW structure for linear collider with the
feedback waveguide excited through directional
coupler. The structure is optimized in order to
decrease the aperture electric field (N. Solyak,
1998).
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SC Feedback Structure (3)
4. Suggestion to use a STWA structure with small
phase advance per cell in order to minimize
surface RF magnetic field to eliminate quenches
that limit the acceleration gradient in SC
structures. Two-coupler concept without high
power bridge (V. Yakovlev, 2001).
The Resonant Loop of the Superconducting
Traveling Wave Accelerating Structure powered
with two TESLA TTF-III 250 kW couplers spaced at
(n1/4)??wg (V. Yakovlev, 2001) .
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We propose the development of a STWA structure
for ILC

• The following problems are under investigation
• Optimization of the STWA structure cells in order
  to minimize surface
• magnetic field without sacrificing surface
  RF field on the aperture.
• Optimization of the end coupler that couples the
  structure to the feedback
• waveguide.
• Optimization of the feedback waveguide.
• Investigation of structure stability versus
  geometrical perturbation
• and determination of the maximal possible
  structure length.
• Investigation of the structure excitation and
  tuning.
• Investigation of the possibility of dipole mode
  trapping in the structure.
• Preliminary engineering design of the structure
  has been performed.
• Development of the strategy for structure
  development.
• A one-cell cavity with feedback waveguide for
  preliminary HG tests was
• developed and ideas for mechanical
  preliminary design have been explored.

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SC TRAVELING WAVE ACCELERATING STRUCTURE FOR ILC
Schematic of an example of a traveling wave
structure with a feedback waveguide and feedback
couplers. The input coupler is not shown. Above
The gain in accelerating gradient versus phase
advance per cell. The aperture is 60 mm, the
diaphragm thickness is 11.5 mm wide, and the
surface magnetic and electric fields are the same
as for the Reentrant structure .Left gain in the
gradient of the TW structure compared to a SW,
ideal case.
The cavity geometry and RF magnetic field pattern
in the superconducting TW accelerating (STWA)
cavity. The TESLA, Low-Loss and Re-Entrant
cavities are presented above for comparison.
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Cavity Shape 24 gain
The cell shape has been optimized to reach the
maximum accelerating gradient while keeping the
magnitude of surface magnetic and electric fields
less than the experimentally verified limits.
1050 phase advance 24 gradient gain
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Field Flatness 26 gain
TTF (p) STWA (105º)
Coupling () 1.88 3.344
Nc per 1 m 9 15
Unflatness(Nc, k, df/f)
Unflatness(Lcavity 1m) 5 0.65
Unflatness(Lcavity 2m) 15.8 1.0
Unflatness(Lcavity 4m) 30.5 1.5
Unflatness(Lcavity 8m) gt 50 2.26
Unflatness(Lcavity 16m) - 3.42
The stability of the ?/2-mode gives the
possibility of using long accelerating
structures. It allows further accelerating
gradient increase of 26 - see the gap of 283 mm
at TESLA.
Field unflatness for p and p/2 structures
Ideal gain 50
Flatness vs. phase advance
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The End Couplers
Electric (a) and magnetic (b) fields in the
coupling section
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Optimized Coupling Section
Magnetic field of the 18-cell SCTW cavity with
the optimized coupling section.
Passband of the 18-cell SCTW structure with
couplers.
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Optimization of the Feedback Waveguide.
Magnetic field enhancement in the waveguide
caused by the bend. Rin is the internal bend
radius.
Wave reflection from the bend.
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Waveguide Height
10 mm
Taking into account a possible field enhancement
in the waveguide bend and coaxial coupler
elements, the 20 mm height was chosen.
20 mm
30 mm
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Structure Excitation and Tuning

• TW structure tuning
• - two independent tuners are necessary in order
  to tune both partial standing wave modes to the
  resonance
• - main tuner that compensates for the structure
  frequency deviation caused by microphonics,
  Lorentz force, etc
• - special matcher in the feedback waveguide
  that compensates reflections from the
  structure-waveguide coupler, bends, etc.
• The field distribution inside the ring is
  critically sensitive to the reflection
  coefficient of the matcher.
• Coupling reflection should be adjusted to -40 dB
  or VSWR 1.021
• Tolerances are tough but comparable with those
  achieved in the TESLA cavity

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Model of the resonant TW ring excited by two
couplers
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Two couplers for the excitation of the resonant
ring containing the SC TW
Backward wave 5 10 power of nominal
Parameter Tolerance Padditional () Uback /Uforward ()
?L/L Waveguide Loop Length 3.2310-6 10 0.026
?L2/L2 Length between Couplers 3.710-2 5 5
?L4/L4 Length between Tuner and Section 2.5910-5 0.25 5
?k/k Input Couplers Coupling 0.0475 0.95 5
?f Input Couplers Phase Shift 2.86 deg. 0.12 5
??/? Tuner Reflection 4.0710-4 0.25 5
?Qext/Qext Loaded Q Factor 0.33 10 0
?f0 - Resonant Frequency Detuning 106 Hz 10 1.17
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The resonant ring model single coupler and a
tuner
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Model of the resonant TW ring excited by one
coupler
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Multipactor, simple analysis
Multipactor near the cavity equator (V.
Shemelin).
Rc, Req- geometrical parameters, B0 RF magnetic
field near the cavity equator. For the
considered TW structure, p0.9 no multipactoring
at any M!
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Multipactor
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High Order Modes
Dispersion curves for the first six dipole modes
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High Order Modes in TW structure. Mode Damping
Transmission S12 for the lowest dipole
dispersion curve for 9 cells
Transmission S12 for the second dipole and
second monopole modes
Transmission for higher frequencies is good
enough to extract the modes from the structure
(HOM couplers)
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Development Strategy
In first part of the project, we propose to
demonstrate high gradient operation and conduct
RF field measurements for a single-cell cavity
experiment that is customary for experimental
high power testing of all new types of SC
cavities like the Reentrant, Low-Loss or Ichiro
designs proposed especially for the ILC
application. The single test cavity will have
the same geometry as full-sized STWA and feedback
waveguide. This experiment will establish a
baseline for characterization of the proposed
methods and technology and will validate the
suitability of the STWA structure concept for
potential ILC applications.
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Problems to be solved in the first stage
?refinement of the single-cell cavity and
feedback waveguide electrical parameters in order
to achieve the same ratio of the RF field in the
feedback waveguide to the field in the cavity as
those in the full-sized STWA structure. ?
conceptual design to be done of the single-cell
cavity with feedback waveguide. ? engineering
design development of the single test cavity to
be done by AES Inc. ?fabrication of two or
three single-cell test cavities by AES Inc. More
than one cavity will be necessary to reduce
uncontrollable negative factors that may
influence the high-gradient test results, again a
common practice in SC cavity development.
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Problems to be solved on the first stage (2)
? full surface processing of a single test
cavity with feedback to be carried out at the
FNAL SC surface processing facility ? high
gradient testing of the single test cavity at the
FNAL vertical cryomodule ? theoretical analysis
and computer modeling for a) tuning parameters,
b) HOM damping, c) high-power input, and d) beam
loading issues
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One-cell cavity with feedback waveguide
The magnetic field distribution in the test
cavity. The field in the feedback waveguide is
about 60 of the field in the cavity.
(a) a layout of the single-cell STWA test cavity
with feedback waveguide.
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Symmetrized Cavity
Initial
New
General dimensions of the cavity and the
waveguide in mm. The waveguide width is 160 mm.
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Single-cell cavity parameter refinements
initial Symmetrized
H (Magnetic Field) Asymmetry 5 0
Ratio of H field in the WG to H field in cavity, 60 50
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The Sequence of Cavity Manufacturing
AES, Inc.
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SC Traveling Wave Structure Studies

1. Shape Optimization of Cells of a Superconducting
   TW Accelerating Structure.
2. Parameter Optimizations of the Rectangular
   Feedback Waveguide.
3. Design and Development of the Coupling Section.
4. Superconducting TW Accelerating Structure
   Parameters.
5. Flatness Studies for the SC TW Structure.
6. Modeling of the TW Regime and Tuning.
7. Multipactoring Performance Analysis.
8. HOM Modes Simulations and Damping of Long Range
   Wakefields.
9. Engineering Aspects of Superconducting TW
   Structure Fabrication.