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Modifications to a TESLA cavity for CW high-current operations

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Modifications to a TESLA cavity for CW high-current operations

Steve Lidia

Center for Beam Physics

E.O. Lawrence Berkeley National Laboratory

With contributions from C. Beard, M. Cordwell,

D. Li, P. McIntosh, E. Wooldridge

Steve Lidia

E.O. Lawrence Berkeley National Laboratory

- Motivation
- Modifications to 7-cell TTF cavity
- Optimization to end cell geometry
- Pass-band behavior
- External coupling benchmarking

Steve Lidia

E.O. Lawrence Berkeley National Laboratory

- High rep-rate (kHz - CW) SRF linacs are called for in many new accelerator designs (eg. ERLs, high average power FELs, etc.)
- Many proven cavity/cryomodule designs work at low rep-rate (~10 Hz) and low average beam currents (~100 µA)
- New cavity/cryomodules are designed to better handle:
- High accelerating gradients (>~15 MV/m)
- Higher average beam currents (>~100mA)
- Larger HOM power dissipation

- We are participating in a CCLRC/Cornell/Stanford/LBNL/FZR Rossendorf collaboration to build a next generation CW SCRF linac.

Steve Lidia

E.O. Lawrence Berkeley National Laboratory

Steve Lidia

E.O. Lawrence Berkeley National Laboratory

Optimized for low average current

103.3mm inner cell equatorial radius

104.8mm outer cell equatorial radius

Steve Lidia

E.O. Lawrence Berkeley National Laboratory

Central 5 cells + inner half cells

remain identical to TTF 7-cell design

End caps modified

to balance field

New high-power coaxial coupler and beampipe

Steve Lidia

E.O. Lawrence Berkeley National Laboratory

Cornell high-power coupler

Steve Lidia

E.O. Lawrence Berkeley National Laboratory

- The end cell cups and beam tubes are redesigned to provide sufficient field-flatness (< 0.5% rms), minimize surface fields, provide desired coupling in conjunction with the input coupler and propagation of HOMs to dampers.
- Parametric models were created in Microwave Studio to allow easy variation of parameters and geometry generation.
- Early studies indicated that the end cup slope provides very useful ‘knobs’ for optimization.

Steve Lidia

E.O. Lawrence Berkeley National Laboratory

1

104.8mm

35mm

39mm

123.1 mm

C

L

Input Coupler

Elliptical

Circular

New

Existing

Steve Lidia

E.O. Lawrence Berkeley National Laboratory

2

53mm

104.8mm

C

L

Existing

New

Steve Lidia

E.O. Lawrence Berkeley National Laboratory

Peak-Cell-Amplitude RMS/Average

%

29.125°

~0.3%

End cell slope, 2 [°]

Steve Lidia

E.O. Lawrence Berkeley National Laboratory

/Avg = 0.3%

Average

Steve Lidia

E.O. Lawrence Berkeley National Laboratory

E-field

f = 1300.75 GHz

Qext = 1 107

Surface fields

Emax/Eacc = 2.9

Hmax/Eacc = 59.5 Oe/MV/m

H-field

Steve Lidia

E.O. Lawrence Berkeley National Laboratory

-mode

fNearest neighbor ~1.2 MHz

fPi-Zero ~23 MHz

0-mode

Steve Lidia

E.O. Lawrence Berkeley National Laboratory

Zero mode

Pi mode

Steve Lidia

E.O. Lawrence Berkeley National Laboratory

Optimum position

Steve Lidia

E.O. Lawrence Berkeley National Laboratory

- ‘Standard’ methods (eg. Kroll-Yu, Balleyguier, etc.) work well with low Qext structures, but lose accuracy with high Qext.
- Microwave Studio has facilities for calculating Qext within a frequency domain simulation environment. Is it correct?
- Time domain methods that observe field decay require excessively long simulation times for high-Q structures.
- Time domain methods observing the field buildup can provide high accuracy with relatively short simulation times.
- Multiple modes can complicate matters . . .
- Simple models are useful to benchmark techniques.

Steve Lidia

E.O. Lawrence Berkeley National Laboratory

TM010 mode

1.3 GHz

Coaxial coupler

Axial modal

electric field

Frequency domain parameters relate on-axis peak field, voltage, stored energy, etc. for mode.

Steve Lidia

E.O. Lawrence Berkeley National Laboratory

Field probes:

Electric

Magnetic

Forward wave voltage

Cavity probe response

cosrest

t cos(rest+0)

Steve Lidia

E.O. Lawrence Berkeley National Laboratory

Circuit equation for

mode voltage evolution

Factor out fast oscillation,

and drive on-resonance

Integrate from t=0 to t=T,

(constant amplitude drive)

Short duration, 0T << 2QL

Relate to measured quantities

Steve Lidia

E.O. Lawrence Berkeley National Laboratory

Eigenmode parameters

Time domain results

Excellent agreement for single modes!

Microwave Studio simulations

Steve Lidia

E.O. Lawrence Berkeley National Laboratory

- Work to redesign proven L-band SRF cavity technology for ERL application is underway.
- Simple changes to the cavity end cell geometry permit the addition of high power input couplers while guaranteeing field flatness specifications.
- We have found time domain methods to easily calculate external Q-factors for high-Q cavities, without unreasonably expensive computational runs.
- Near term work will focus on HOM modeling and loss- and kick- factor calculation.

Steve Lidia

E.O. Lawrence Berkeley National Laboratory

Steve Lidia

E.O. Lawrence Berkeley National Laboratory