Comments on injector linac for INDUS...

Comments on injector linac for INDUS‐II

RRCAT, Indore

11 March, 2010

T. Higo, KEK

Thoughts and discussions today• 700MeV or higher / 40m in total

– Minimum requirement

– Can be based on KEKB

– Higher injection rate with multi‐bunch operation?

– Or anything other to consider

• 2.5GeV full‐energy injection– Pros on ring acceptance and injection rate and others

– Cost , Space ?

2010/3/11 2Higo at RRCAT

Existing TW linac examplesKEK‐B (S) ATF (S) KEKB (C) XFEL(C)

E GeV 8 1.3 test 8

Acc str freq. Band S S C C

Acc str length m 2 3 1 1.8

# acc str ~4*8*8 17+2 5 2*4*16

Eacc MV/m 20 26 40 35

Acc length/ klystron m 8 6 4 3.6

P_kly MW 41 60 40 50

P_kly / Lacc MW/m 5 10 10 14

These show the feasibility of S‐band linac at 30MV/m and C‐band at 40MV/m?


Rough idea for today’s discussion

• Robust and probably cheap solution– S‐band long acc structure

• Worthwhile to consider – Energy booster section with higher frequency?

– C‐band at 40MV/m level is feasible

– X‐band at 40~60MV/m is to be considered

2010/3/11 Higo at RRCAT 4

Firm base at S‐band

• Most matured technology• Possibly most cheap as for accelerator

– Take advantage of medium mass production at various high energy linac

– But optimize design to further optimization pursuing cost, efeciency, etc.

• But big power is needed to go higher gradient to shorten linac


Let us start with KEKB linac unit configuration

Klystron

Acc str (1)

Acc str (2)

Acc str (3)

Acc str (4)

SLED

Tunnel

Kly gallery


Modulator

Power source

Pulse compressor

Transport line

Distribution to multi acc sections

KEKB 8‐GeV injector linac “nominal”

• Linac: 1GeV/sector * 8 sectors– 512m acceleration

• Sector: 160MeV * 8 units– 64m acceleration

• RF Unit: 40MeV * 4 acc structures– klystron SLED 4 acc structures– 41MW klystron– 8m acceleration at 20MV/m

Starting point for design of GeV‐scale linac


Structure parameters• Pseudo CG section

– 5 types (A‐E) of 2m section for HOM cancellation to suppress BBU

• Typical parameters of type‐C– 2a = 24~20mm

– τ = 0.333– Rs = 54~62 MΩ/m– Vg/c = 1.75~0.92%


Possible optimization with CG

2m section nominal

3m section with higher power

3m section with extending smaller a

Z

Vg/c

2m smaller aperture choice


Smaller aperture higher shunt impedance.Higher power enable to contain larger aperture.

High field at S‐band is OK but clean technology is one of the cares to be taken

H. Matsumoto, KEK‐Preprint 91‐161

<vg/c>=0.4%

Reached ~90MV/m.

Stability or breakdown rate was not evaluated then.

Clean fabrication makes less dark current, but no change in beta.


40MV/m with KEKB S‐band and effectiveness of cleaning with water

No crescent shape

Nominal

Nominal and high‐pressure water rinsed

Igarashi, PAC03

Pushed up to 40MV/m.

Cleaned structure got processed faster.

Crescent did not change much on processing speed etc.


Dark current evolution in general and surface quality variation

Yamaguchi,

Processing makesless and less dark current.

smaller field enhancement factor, beta value2010/3/11 12Higo at RRCAT

Pulse compression for high peak power

To obtain higher peak power with 4 microsec and Q0=105

Beam integration in accelerator section makes acceleration voltage gain by a factor 2.

Structure output

SLED output


Well established

>700MeV design with based on half KEKB unit

• KEKB 2m*4 acc str with SLED 9.6m in total

• Half KEKB unit 2m * 2 acc str 5m• 42MW kly SLED 2 acc str• 27.5 MV/m 110MeV/unit• 7 units 770 MeV / 35m

• 2a=24.4~20.4mm , τ=0.333, vg/c=1.7~0.9%


Some natural improvement with 3m

• ATF 3m*2 acc str with SLED

• ATF unit 3m * 2 acc str 7m• 42MW kly SLED 2 acc str• 23.5 MV/m 144MeV/unit• 5 units 720 MeV / 35m

• a=12.6~9.5mm, τ=0.45, vg/c=2.2~0.9%


Possibility to increase gradient

• Smaller aperture– If beam stability is satisfied

• Check single bunch loading

• Longer structure– Mechanically not preferable

• Higher Q pulse compressor


Possibility to increase intensity

• Multi‐bunch– Energy equalization with

• SLED phase control?

• With multi‐bunch energy compensator– Different frequency?

– Phase / amplitude modulation without SLED


Possibility to reduce cost

• Take established technology and configuration

• Explore some room for further optimization on efficiency

• Take higher gradient with higher frequency with the same average power for modulator


Possibility to use C‐band• Present SuperKEKB(C)

– Double # acc str / klystron than KEKB(S)• 2 acc str / klystron

– Keep klystron peak power the same 42MW– Reduce klystron pulse width by half 2microsec– Double the acc field 40MV/m 20MV/m– Beam hole aperture 2a=14.5 ~ 12.5 ~ 10.5

• Comparing to KEK(S) 2a=24.4~20.4• How about

– Single‐bunch beam stability?– Energy spread?


Higher frequency choice for Super‐KEKB S‐band to C‐band

Ohsawa, LINAC04

Same AC energy but double the acceleration field.


Super KEKB C‐band design

Kamitani, LINAC04

Simply scaled dimensions from S‐band by a factor 2.


KEKB C‐band present configuration

Kamitani, LINAC06 & PAC07

Can be configured in tandem or in parallel.


Processing to higher field with C‐band

Kamitani, LINAC04

Easy processing up to 40MV/m in 10 days.


Problem in coupler iris

Kamitani, LINAC04Damage was speculated to be due to thin iris between coupler cell and waveguide


C‐band coupler design evolution

Sugimura, PAC05

(a)

Thin Thick Thick and round

Breakdowns at coupler were reduced.


X‐band case, 0th order estimation

• Based on 60cm‐long high‐phase advance structure– 2a=7.5~8mm– 60cm– 5π/6 mode– Tf~100ns– 50MW input ( 22MW)– 60MV/m ( 40MV/m)– 72MeV / unit ( 96MeV/unit)– 1.5m ( 3m) / unit – 35 RF units ( 26 units)– 50m ( 78m) / 2.5 GeV


X‐band possible system


50MW , ~1microsec

X2.5 in power

Transmission lossA few ~ 20%

50MW, 200ns

50MW, 200ns

60MV/m, 100ns beam

72MeV / 1.5m ~ 50MV/m in reality 50m for 2.5GeV?

60cm acc str 1 60cm acc str 2

Technical feasibility• Klystron

– SLAC 50MW XL‐4, KEK PPM exist• Pulse compression

– Delay line type or cavity type > X2.5 feasible• Transport

– TE01, etc. low‐loss mode, OK in practice• Accelerating structure

– High phase advance structure, confirmed• Load

– Various candidate exist


KEK PPM klystron


PPM klystron performance


0

20

40

60

80

100

120

0 200 400 600 800 1000 1200 1400 1600

Pulse Shotening of PPM#4 Klystron

Power [MW]

Pulse width [ns]

1/Tpulse

0.0

10.0

20.0

30.0

40.0

50.0

60.0

70.0

80.0

90.0

350.0 400.0 450.0 500.0 550.0C athode V oltage (kV )

Power (MW) / Eff.

P ow er

Efficiency

22m PC for 300ns being constructed at KEK Nextef


22m delay line

Cavity type compressor developed now by CERN


60cm accelerator structure



10-8

10-7

10-6

10-5

10-4

10-3

60 70 80 90 100 110 120

加速管放電頻度 (BD/pulse/m)BDR_KX01 60cm 400nsBDR_KX03 60cm 400nsBDR_T53VG3MC 100nsBDR_H75VG4S18 150ns BDR_T18_#2 252nsBDR_T18_#2 400nsKX03 fitKX01 fit

Eacc (MV/m)

Feasible field levelAccelerator structure breakdown rate

0.6m

0.2m

0.5m0.75m

50~65 …. 90~100 MV/m is feasiblein 0.5~1m size X‐band accelerator structure.

Possible unit configuration

KEKB (S) KEKB (C) X(1) X(2) X(3)

Freq MHz 2856 5712 11424 11424 11424

Acc str length m 3 2 0.6 0.6 1.3

# acc str / unit 4 2 2 4 1

Eacc MV/m 20 40 60 40

Acc length/ unit m 8 4 1.2 2.4 1.3

P_kly MW 42 42 50 50

PC Yes / No Y Y Y Y Y

Linac length m 150 80 50 50


Choice of even higher frequency?X‐band or higher

• X‐band (within a practical choice)– NLCTA acceleration in usual operation ~50MV/m?– LCLS bunch compressor– High gradient studies for > several thousand hours

• NLCTA (SLAC), Nextef (KEK)• >65MV/m for 0.6m section• ~100MV/m for 0.2m section

• 30GHz (beyond present‐day choice)– CTF2, CTF3 only brief test from my view point– Need power source idea


Issues to be considered other than high gradient ∵ stronger wake field

• Short‐range wake field– Single bunch emittance– Alignment of beam aperture– Bunch length– Energy spread

• Long‐range wake field– Multi‐bunch BBU HOM suppression– Multi‐bunch energy compensation– Alignment SBPM


These should be estimated, but straightforward because all calculable.

Short range transverse wake field

38

})/()/1(1{4)( 00400 ssExpss

ascZsWT −+−=

π

17.1

38.079.1

00 169.0,377L

gasZ =Ω=

402)(a

cZsWs T π

=∂∂

Initial slope;

Transverse wake field for NLC structure;

10.0 0.2 0.4 0.6 0.8 1.0

50

100

150

200

a=3mm

a=4mm

a=5mm

s [mm]

WT [V

/pC/mm/m

]

Transverse wake field for NLC.

0

0

100

200

Linear slope andstrong dependence on “a”!

2010/3/11 Higo at RRCAT

Aperture will be increased by taking higher phase advance

Medium‐damped and detuned for HOM suppression.High phase advance (150 deg/cell) for large aperture.

If this scales simply to C‐band, it becomesLs=1.2m, 2a=18 in average, 66MW 35MV/m

To realize larger aperture without reducing shunt impedance much.


Long‐range wake field for HOMDetuning by varying (a,t)

40

(a,t) distribution along structure

Dipole modedistribution in frequency and kick factor

Contour of dipole mode frequency vs (a,t)


Long‐range wake field for HOMWeak damping by manifold

12000

14000

16000

18000

20000

0 30 60 90 120 150 180

RDDS1 DispersionFd1(97-102)Fd2(97-102)Mahifold(97-102)

Phase shift / cell

2nd dipole

Manifold

1st dipole

Example of middle cells of RDDS1

Avoided crossing due to the coupling of cavity dipole mode and manifold mode.

Example cell shape

Manifold Cell


DS and DDS scheme

• Initial fast damping with detuning

• Truncation makes recurrence

• Weak damping suppress the recurrence

• Interleaving among structures helps.

42

• These design were established in X‐band but applicable to S or C.


Actual DDS example

43

Input / output waveguide

HOM damping waveguide

Detuned cells

Manifold to carry HOM to outside

This shows DDS example.DS becomes much simpler configuration,

almost the same as usual CG structure!2010/3/11 Higo at RRCAT

Alignment by Structure BPM

44

0

100

200

300

400

500

-400 -300 -200 -100 0 100 200 300 400

15 GHz Dipole Power Scan

Parabolic Fit

Measured Power

Sign

al P

ower

(au)

Beam Y Position (Microns)

Excited power

0

50

100

150

200

-400 -300 -200 -100 0 100 200 300 400

15 GHz Dipole Phase Scan

Measured Phase

Arctan(Y/27)

Phas

e (d

egre

es)

Beam Y Position (Microns)

Phase of excited fieldMeasured structure straightness

Power from manifold

Frequency filtered

Phase and amplitude

Frequency‐to‐position

Straightness measure


SBPM gives a few micron precision misalighment info w.r.t. beam

Do we choose higher frequency?• Worthwhile to consider design based on higher frequency

• It depends on beam specification is required– intensity, bunch shape, single/multi, ….

• Methods to suppress wake field can be applied also at lower frequency design

• The availability of the power source may determine the frequency choice, in addition to the cost


Frequency choice • S‐band ~20MV/m linac is already in operation

– With more power, 30~40MV/m seems available– Well matured technology

• C‐band ~40MV/m linac is realized– Technology mostly established

• X‐band ~65MV/m or higher is proved– Potential to higher field operation– Power source and components are to be improved for higher

field• Determine with estimating

– Required beam intensity and pattern– Final energy and available space– Total cost including infrastructure and needed developments

with manpower from lab.


Conclusion

• Cost effective

• Robust

• Well established at somewhere

• Then take S‐band design– With some improvement on efficiency, cost, etc.

– By choosing optimized parameters

– With cares to safely reach ~50MV/m surface field


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