Microwave-Based High-Gradient Structures and Linacs

Sami TantawiSLAC

04/21/23 1

Acknowledgment• The work being presented is due to the efforts of

– V. Dolgashev, L. Laurent, F. Wang, J. Wang, C. Adolphsen, D. Yeremian, J. Lewandowski, C. Nantista, J. Eichner, C. Yoneda, C. Pearson, A. Hayes, D. Martin, R. Ruth, S. Pei, Z. Li SLAC

– T. Higo and Y. Higashi, et. al., KEK – W. Wuensch et. al., CERN – R. Temkin, et. al., MIT– W. Gai, et. al, ANL– J. Norem, ANL– G. Nusinovich et. al., University of Maryland– S. Gold, NRL– Bruno Spataro, INFN Frscati

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Overview

• Review of experimental data on high gradient structure

• Efficiency and standing wave accelerator structure designs

• A 1 TeV collider – Improved efficiency– Possible parameter set

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04/21/23

Research and Development Plan

Experimental Studies

• Basic Physics Experimental Studies – Single and Multiple Cell Accelerator Structures (with major KEK and CERN contributions)

• single cell traveling-wave accelerator structures (Needs ASTA)• single-cell standing-wave accelerator structures (Performed at Klystron Test Lab)

– Waveguide structures (Needs ASTA)– Pulsed heating experiments (Performed at the Klystron Test Lab, also with major KEK and CERN

contributions)• Full Accelerator Structure Testing (Performed at NLCTA, with CERN contributions)

Can only be done at NLCTA at SLAC

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6 0 8 0 1 0 0 1 2 0 1 4 0

1 0 7

1 0 5

0 .0 0 1

0 .1

A c c e l e r a ti ng G r a d i e nt M V m

Bre

akdo

wnP

roba

bili

ty1pulse

m

600ns

150ns

Flat Top Pulse width

Breakdown Probability for a Standing Wave Accelerator Structure with a/=0.22

Typical gradient (loaded) as a function of time.

PulsedTemp Rise

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1 0 0 1 1 0 1 2 0 1 3 0 1 4 0 1 5 0 1 6 0 1 7 0

1 0 7

1 0 6

1 0 5

1 0 4

0 .0 0 1

0 .0 1

A c c e le r a ting G r a d ie nt M Vm

Bre

akdo

wn

Prob

abili

ty1pulse

m

300ns

200ns

150ns

Flat Top Pulse width

Breakdown Probability for a Standing Wave Accelerator Structure with a/=0.14

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1 0 0 1 2 0 1 4 0 1 6 0 1 8 0 2 0 0

1 0 6

1 0 5

1 0 4

0 .0 0 1

0 .0 1

0 .1

A c c e le ra tin g G ra d ie n t M V m

Bre

akdo

wnP

roba

bilit

y1pulsem

600ns

400ns

200ns

Flat Top Pulse width

Breakdown Probability for a Standing Wave Accelerator Structure with

a/=0.1

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Breakdown Probability for a Standing Wave Accelerator Structure with different

a/=0.21

8 0 1 0 0 1 2 0 1 4 0 1 6 0 1 8 0 2 0 0

1 0 5

1 0 4

0 .0 0 1

0 .0 1

0 .1

A c c e le r a ting G r a d ie nt M Vm

Bre

akdo

wn

Prob

abili

ty1pulse

m

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Breakdown Probability for a Standing Wave Accelerator Structure with different a/

4 0 5 0 6 0 7 0 8 0 9 0

0 . 0 0 1

0 . 0 1

0 . 1

1

P e a k T e m p r a t u r e R i s e C

Bre

akdo

wn

Prob

abili

ty1pulse

m

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9 0 1 0 0 1 1 0 1 2 0 1 3 0 1 4 0 1 5 0

1 0 5

1 0 4

0 .0 0 1

0 .0 1

0 .1

1

G r a d ie nt M Vm

Bre

akdo

wn

Prob

abili

ty1pulse

m

600ns

400ns

200ns

Flat Top Pulse width

Breakdown Probability for a Standing Wave Accelerator Structure with a/=0.22

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9 0 1 0 0 1 1 0 1 2 0 1 3 0 1 4 0 1 5 0

1 0 5

1 0 4

0 .0 0 1

0 .0 1

0 .1

G r a d ie nt M Vm

Bre

akdo

wn

Prob

abili

ty1pulse

m

Cu

CuZr

Hard Cu

Breakdown Probability for a Standing Wave Accelerator Structure with a/=0.22, Pulse

length=200ns

CuCr

We have not tested yet structure with small apertures and different material“Stay Tuned” This is coming very soon

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0 . 0 0 . 2 0 . 4 0 . 6 0 . 8 1 . 0 1 . 2

0 . 0

0 . 5

1 . 0

1 . 5

2 . 0

D i s t a n c e c m

Es/Ea

(Zo Es Hs)1/2/Es

Hs/Es

Field Profile over the Iris

DistanceThose two peaks are correlated for structures with different t and a/

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Peak H and Peak ExH

1 .1 1 .2 1 .3 1 .4 1 .5 1 .6

1 .2

1 .3

1 .4

1 .5

1 .6

P e a k H sE a

Peak

Zo

HsE

sE

a

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Research and Development Plan

CumulatedPhase Change

Frequency. 11.424GHz

Cells 18+input+output

Filling Time 36ns

a_in/a_out 4.06/2.66 mm

vg_in/vg_out 2.61/1.02 (%c)

S11 0.035

S21 0.8

Phase 120Deg

Average Unloaded Gradient

over the full structure55.5MW100MV/m

Full Accelerator structure testing ( the T18 structure)

5.1~__ inaccoutacc EE

120°

FieldAmplitude

•Structure designed by CERN based on all empirical laws developed experimentally through our previous work•Cells Built at KEK•Structure was bonded and processed at SLAC•Structure was also tested at SLAC

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100 150 20010

-7

10-6

10-5

10-4

RF Flat Top Pulse Width: ns

BK

D R

ate:

1/p

ulse

/m

95 100 105 110 11510

-7

10-6

10-5

10-4

Unloaded Gradient: MV/m

BK

D R

ate

: 1/p

uls

e/m

RF BKD Rate Gradient Dependence for 230ns Pulse at Different Conditioning Time

After 250hrs RF Condition

After 500hrs RF Condition

After 900hrs RF Condition

RF BKD Rate Pulse Width Dependence at Different Conditioning Time

G=108MV/m

G=108MV/m

G=110MV/m

RF Processing of the T18 Structure

After 1200hrs RF Condition

This performance may be good enough for 100MV/m structure for a warm collider, however, it does not yet contain all necessary features such as wakefield damping. Future traveling wave structure designs will also have better efficiency

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2007-09 CERN/CLIC Design Structures Tested at NLCTA

In Beamline Structure Note Performance

11/06 – 2/07 C11vg5Q16First X-band Quad

- Irises Slotted Poor: 57 MV/m, 150 ns, 2e-5 BDR – grew

whiskers on cell walls

2/08 - 4/08C11vg5Q16

Redux Refurbished

Initially good (105 MV/m, 50 ns,1e-5 BDR) but one cell degraded

4/07 – 10/07 C11vg5Q16-MoMolybdenum Version of

AbovePoor: 60 MV/m, 70 ns, 1e-6 BDR

10/08 – 12/08 TD18vg2.6_QuadNo Iris Slots but WG

DampingVery Poor: would not process above 50 MV/m,

90ns – gas spike after BD

4/08 – 7/08 T18vg2.6-DiskCells by KEK, Assembled

at SLACGood: 105 MV/m, 230 ns at LC BDR spec of

5e-7/pulse/m but hot cell developed

7/08 – 10/08 T18vg2.6-DiskPowered from

Downstream EndGood: 163 MV/m, 80 ns, 2e-5 BDR in last cell,

consistent with forward operation

12/08 – 2/09T18vg2.6-Disk

CERNCERN Built, Operate in

Vac CanVery Poor: very gassy with soft breakdowns at

60 MV/m, 70 ns

(Yellow = Quad Cell Geometry, Green = Disk Cell Geometry)

Emax/Ea

HmaxZ0/Ea

SQRT(EmaxHmaxZ0)/Ea

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SQRT(EmaxHmaxZ0)/EaHmaxZ0/Ea

Emax/Ea

SW a/=0.22

SW a/=0.14

SW a/=0.1

SW

T18 Surface Field Parameters in Comparison with SW Structures

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So, What does all this imply for a 1 TeV collider parameters?

• High gradient requires high power density/unit length, quadratic with

gradient. Hence the two beam choice for the CLIC design

• However, since one also accelerates in shorter length the total required RF

power increases linearly with gradient.

• High gradient also implies reduced efficiency;

• However, one can compensate by increasing the efficiency the accelerator

structure and RF sources.

– Increasing the efficiency of the accelerator structure would reduce the power/unit

length, and it might allow for the use of a conventional RF unit

– At the moment the best ( published ) design that respects high gradient constraints

imply an RF to beam efficiency of about 28%

– Higher efficiency RF source and accelerator efficiency imply lower RF system cost

0~ ;1

s

a

x R Ix Ex

04/21/23 22

Yet another Finite Element Code

Motivated by the desire•to design codes to perform Large Signal Analysis for microwave tubes (realistic analysis with short computational time for optimization)•study surface fields for accelerators•the need of a simple interface so that one could “play”

•A finite element code written completely in Mathematica was realized. •To my surprise, it is running much faster than SuperFish or Superlance•The code was used with a Genetic Global Optimization routine to optimize the cavity shape under surface field constraints

0 .2 0 .4 0 .6 0 .8 1 .0

0 .1

0 .2

0 .3

0 .4

0 .5

0 .6

1 2 3 4 5

0 .4

0 .6

0 .8

1 .0

Surface field penalty function

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Iris shaping for a structure with a/=0.14

Shunt Impedance 83 M/mQuality Factor 8561Peak Es/Ea 2.33Peak Z0Hs/Ea 1.23

Shunt Impedance 104 M/mQuality Factor 9778Peak Es/Ea 2.41Peak Z0Hs/Ea 1.12 •With 1 nC/bunch and bunch separation of 6 rf cycles the RF to beam efficiency~70%•The power required/m~287 MW

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Iris shaping for a structure with a/=0.1

Shunt Impedance 102 M/mQuality Factor 8645Peak Es/Ea 2.3Peak Z0Hs/Ea 1.09

Shunt Impedance 128 M/mQuality Factor 9655Peak Es/Ea 2.5Peak Z0Hs/Ea 1.04•With 0.5 nC/bunch, bunch separation of 6 rf cycles, and loaded gradient of 100 MV/m the RF to beam efficiency~53%•The power required/m~173 MW

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Feeding and Wakefield damping of a set of -mode structures

•Each cell is fed through a directional coupler. •The feeding port serve as the damping port•The system has a four-fold symmetry, only one section is shown

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Parameter Options for a 1 TeV machine, 2 1034 cm-2 s-1

Option-1 Option-2 Option-3

Frequency (GHz) 11.424

Gradient (MV/m) 100 100 80

Power/meter (MW)

287 173 126

Charge/bunch (nC) 1 0.5 0.5

Bunch separation ( RF cycles)

6

Number of bunches 300 600 600

<a/> 0.14 0.1 0.1

RF source Two beam Two Beam/Conventional

Unit

Conventional Unit

RF/Beam Efficiency(%) 73 53 60

Beam duration Pulse length (ns)

158 315 315

Repetition Rate (Hz) 60 60 6004/21/23 27

Conclusion

• With recent advances on high gradient accelerator structures, it is possible to think of an “efficient” room temperature collider

• Reduced power levels/unit length may permit designs based on conventional RF unit.

04/21/23 28