SC TW ACCELERATING STRUCTURE FOR ILC. SC Traveling Wave Accelerating Structure for ILC. P. Avrakhov 1 , A. Kanareykin 1 , S. Kazakov 2 , N. Solyak 3 , V. Yakovlev 3 , W.Gai. 1 Euclid Techlabs LLC, Rockville, MD 2 KEK, Tsukuba, Japan 3 FNAL, Batavia, USA. Motivation.
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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
Surface RF Magnetic Field
- Reentrant structure, Cornell;
- Low-Loss structure, DESY;and
- Ichiro structure, KEK.
(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).
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.
Alternative structures should be discussed and developed!
Our proposed alternative approach – a superconducting traveling wave acceleration structure (STWA)
Higher transit time factor (T~1) – higher acceleration gradient for the same surface RF magnetic field. For an ideal structure with a small aperture
( is phase advance per cell) and the acceleration gradient gain compared to SW -structure is
Gain = E /E = 2/sin(/2)
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 √2, or 42% (!) - for the ideal structure, of course.
- compensate for microphonics and Lorentz force and
- provide good VSWR ratio in the structure and feedback waveguide.
TW structure with feedback (R.B. Neal, 1968)
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).
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 (n+1/4)wg (V. Yakovlev, 2001) .
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.
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
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.
Ideal gain ~ 50 %
Field unflatness for π and ~π/2 structures
Flatness vs. phase advance
Electric (a) and magnetic (b) fields in the
Magnetic field of the 18-cell SCTW cavity with the optimized coupling section.
Passband of the 18-cell SCTW structure with couplers.
Magnetic field enhancement in the waveguide caused by the bend. Rin is the internal bend radius.
Wave reflection from the bend.
Taking into account a possible field enhancement in the waveguide bend and coaxial coupler elements, the 20 mm height was chosen.
- 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.
Two couplers for the excitation of the resonant ring containing the SC TW
Backward wave – 5%
10% power of nominal
– containing the SC TW- S-matrix developed with HFSS with the couplers and beam loading taken in account
of the six-pole (Altman)
P inputThe resonant ring model: single coupler and a tuner
Multipactor, simple analysis containing the SC TW
Multipactor near the cavity “equator” (V. Shemelin).
Rc, Req- geometrical parameters, B0 –RF magnetic field near the cavity “equator”.
For the considered TW structure, p=0.9:
no multipactoring at any M!
Dispersion curves for the first six dipole modes
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)
Development Strategy containing the SC TW
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 structureconcept for potential ILC applications.
Problems to be solved in the first stage containing the SC TW
●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.
Problems to be solved on the first stage (2) containing the SC TW
● 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
One-cell cavity with feedback waveguide containing the SC TW
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.
Symmetrized Cavity containing the SC TW
General dimensions of the cavity and the waveguide in mm. The waveguide width is 160 mm.
Single-cell cavity parameter refinements containing the SC TW
The Sequence of Cavity Manufacturing containing the SC TW
SC Traveling Wave Structure Studies containing the SC TW