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. Outline. Motivation Modifications to 7-cell TTF cavity

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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

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


Outline

Outline

  • 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


Motivation

Motivation

  • 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


Erlp prototype cryomodule

ERLP Prototype Cryomodule

Steve Lidia

E.O. Lawrence Berkeley National Laboratory


Existing ttf 7 cell design

Existing TTF 7-cell Design

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


Modifications to ttf 7 cell cavity

Modifications to TTF 7-Cell Cavity

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


Input couplers

Cornell high-power coupler

Input Couplers

Steve Lidia

E.O. Lawrence Berkeley National Laboratory


Cavity geometry optimization

Cavity Geometry Optimization

  • 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


End cell parameterization

1

104.8mm

35mm

39mm

123.1 mm

C

L

End Cell Parameterization

Input Coupler

Elliptical

Circular

New

Existing

Steve Lidia

E.O. Lawrence Berkeley National Laboratory


End cell parameterization cont d

2

53mm

104.8mm

C

L

End Cell Parameterization, cont’d.

Existing

New

Steve Lidia

E.O. Lawrence Berkeley National Laboratory


Optimization of end cell slope

Peak-Cell-Amplitude RMS/Average

%

29.125°

~0.3%

End cell slope, 2 [°]

Optimization of End Cell Slope

Steve Lidia

E.O. Lawrence Berkeley National Laboratory


Mode axial e field profile

-Mode Axial E-field Profile

/Avg = 0.3%

Average

Steve Lidia

E.O. Lawrence Berkeley National Laboratory


Mode fields

-Mode Fields

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


Coupled modes in tm 010 pass band

-mode

fNearest neighbor ~1.2 MHz

fPi-Zero ~23 MHz

0-mode

Coupled Modes in TM010 Pass-band

Steve Lidia

E.O. Lawrence Berkeley National Laboratory


External coupling to tm 010 pass band

External Coupling to TM010 Pass Band

Zero mode

Pi mode

Steve Lidia

E.O. Lawrence Berkeley National Laboratory


Variation of q ext with coupler position

Optimum position

Variation of Qext with Coupler Position

Steve Lidia

E.O. Lawrence Berkeley National Laboratory


Calculation of q ext

Calculation of Qext

  • ‘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


Coupler benchmarking eigenmode

Coupler Benchmarking - Eigenmode

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


Coupler benchmarking excitation response

Coupler Benchmarking - Excitation/Response

Field probes:

Electric

Magnetic

Forward wave voltage

Cavity probe response

cosrest

t cos(rest+0)

Steve Lidia

E.O. Lawrence Berkeley National Laboratory


Coupler benchmarking analytical remarks

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

Coupler Benchmarking - Analytical Remarks

Steve Lidia

E.O. Lawrence Berkeley National Laboratory


Comparing frequency and time domain simulations

Eigenmode parameters

Comparing Frequency and Time Domain Simulations

Time domain results

Excellent agreement for single modes!

Microwave Studio simulations

Steve Lidia

E.O. Lawrence Berkeley National Laboratory


Summary

Summary

  • 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


Modifications to a tesla cavity for cw high current operations

- fin -

Steve Lidia

E.O. Lawrence Berkeley National Laboratory


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