CLIC crab cavity design

Praveen Ambattu

24/08/2011

Monopole x dipole mode @ 12 GHz

abs E, V/m abs H, A/m abs S, W/m2

TM010

TM110

5 mm

5 mm

Crab cavity has a E, H and S distributions different from the main linac

CLIC crab cavity numbers

• Bunch rotation:10 mrad

• Transverse space: ~100 cm

• Mode: 2/3, 11.9942 GHz

• Voltage: 2.55 MV per cavity

• Available peak power: 15 MW

• RF tolerance for 98 % luminosity:

dVrf/Vrf=2 %, drf=18 mdeg

• Peak surface field: <250 MV/m

• Peak pulsed heating: <40 K

Cell shape

• Initial optimisation was for a cylindrical cell

• 5 mm radius beampipe was chosen as a compromise among surface fields and short range wakefields

Vertical modes for 10 cell cavity

SOM band

• Vertical wakes are dominated by the SOM band which is the 1st dipole mode itself but in 90 deg plane

• For r0=35 m, SOMs needs Q<100 to meet the luminosity requirements

Kick factor x frequency SOM Q x HOM Q

• By shifting the SOM frequency with highest kick to 6.5th bunch harmonic (13 GHz), the last bunch in the train will see zero sum wakefield

• This allows relaxing the SOM damping requirement

•This can be implemented by using an asymmetric cell shape

Asymmetric cell shapes

• Achieving 1 GHz shift in dipole frequency with less structure complexity would be easy with racetrack shape.

E-field (1 J stored energy)

Single racetrack cell

H-field (1 J)

Power flow and Sc (1 J)

Sc=max(ReS+ImS/6)

ReS=sqrt(ReSx^2+ReSy^2+ReSz^2)

ImS=sqrt(ImSx^2+ImSy^2+ImSz^2)

Sc/Et2=3.87 mA/V

Transverse gradient, Et=Vt/Lcell

Vt=jVz(r)*(c/r)

Property Value

QCu 6395

Rt/Q, Ohm 54.65

Kick, MV 2.03

vgr, % -2.92

Attenuation, Nep/m 0.0056

Es/Et 3.425

Hs/Et, T(K) 0.0114, 24

Sc/Et2, mA/V 3.87

Single cell properties (1 J)

Standard coupler Waveguide coupler

Structure size mainly depends on

• power coupler type :- standard, waveguide, mode launcher etc

• feed geometry :- single or dual

• length of damping waveguides

Power coupler

Standard coupler

More space needed to include damper, also to help cell tuning

Waveguide coupler

Splitter size

Coupler comparison

Property Waveguide Standard

Longitudinal size More Less

Transverse size Less More

Wakefield More Less

End cell tuning complexity

Low High

Mechanical complexity

Low High

Dual-feed x single-feed coupler

• DFC perfectly centres the dipole mode in the end cells due to symmetric feeding and do not excite other modes

• This needs the waveguide arms of the splitter be temperature stabilised

• Assembly tolerance is also critical

• Inclusion of dampers and tuning the end cells will be difficult unless longer waveguide arms are used

• More trapped modes, hence more wakefield

• Single-feed coupler avoids all above• But the mode is not centred causing beamloading• This can be minimised by flipping the two couplers 180 deg with each other to reduce the effect of beam loading • Not needed in the prototype 1 as there is no beam

Single-feed coupler

slot_a

slot_a

slot_h

slot_h

Microwave Studio copper model

• Used a dummy waveguide, cut-off to 12 GHz

• Could be used as damping waveguide in the final cavity

• Could be avoided in the 1st prototype

• Waveguides can be on the same plane

Power flow (1W)

z

y

x

All waveguides are terminated by ports

S-parameters

Complex field (Hx) in the band

Field amplitudes at 11.9942 GHz

Hx(0)

Ey(0)

Ez(r)

on-axis

on-axis

0.5 mm off-axis

Internal reflection and phase advance at 11.9942 GHz from Hx(0)

beam pipe

Beamloading in the end cells

Ez(0) x y

Ez(0) x z

wg2 wg1

wg1

wg2

Hsurf=360 kA/m T=28 K

Coupler cell x Regular cell

Hsurf=269 kA/m T=16 K

R0.5 mm

For 13.5 MW peak power and 242 ns pulse

RF properties

Parameter Value

Total length, mm 149.979

Active length, mm 99.979

Transverse size, mm 106.424

Kick, MV 2.55

Peak power, MW 13.5

Esurf, MV/m 110

Hsurf, kA/m360 (cell) 269 (coupler)

T, K28 (cell) 16 (coupler)

Sc, W/m2 3.85*

* TD26_vg1p8_R05, Sc~5 W/m2

Cavity tuning

pin in 0 MHz/mm

pin out -0.6 MHz/mmpin in 18.2 MHz/mm

pin out -11.4 MHz/mm

pin in 27.2 MHz/mm

pin out -16.8 MHz/mm

For 1 MHz tuning, ~1.26 m in radius~50 m in pit

Simulated 0.5 mm deep pit

Bead pull• Tuning could be done by ‘non-resonant perturbation’ technique,

combined with bead-pull, identical to what has been done for the accelerating cavity

• Simulated beadpull result using a metallic disk (1.5 mm dia x 1mm thick) shown below seems to give well defined perturbation

• More accurate field measurement needs a fine cylinder made of thin surgical needle

complex S11

HFSS x MWS fields @ 1 W, 11.9942 GHz

Hx on axis

HFSS MWS

0

200

400

600

800

1000

1200

1400

1600

0 20 40 60 80 100 120 140 160

Ez off axis

RF properties MWS x HFSS

Parameter MWS HFSS

Frequency, GHz 11.9942 11.9942

|S11|, dB -50 -32

Kick, MV 2.56 2.56

Peak power, MW 13.5 9.75

Esurf, MV/m 110 110

Hsurf, MA/m 360 354

T, K 28 27

Sc, W/m2 3.85* ??

• Fields amplitudes in HFSS are higher than in MWS by a factor of 1.15

• Needs more investigation but seems OK !!

For coarse mesh inside the cavity

Discussion

• Single feed without dummy wg ?– Yes it is, as the priority is to RF test the undamped cavity

• Cooling pipes on iris or equator ?– equator

• Tuning pins 0 deg or 45 deg ?– 45 deg

• Timescales ?– Finish drawing by Dec 2011, start procurement of copper

by Jan 2012, so EuCARD money could be spent before March 2012

CLIC crab cavity final design (using CST Microwave Studio 2010)

Praveen Ambattu

26/08/2011

The design changed from what shown at the RF group meeting

• Removed extra waveguide on coupler cells

• Increased coupler slot rounding to 1 mm from 0.5 mm

• Increased waveguide corner rounding to 4 mm from 2 mm

Single cell with periodic boundary of 2/3

Property Value

Energy stored, J 1

QCu 6395

Rt/Q, Ohm 54.65

vgr, % -2.92

Esurf/Et 3.43

Hsurf/Et 0.0114

Esurf

Hsurf

Ssurf

Comparison with HFSSv13(Vasim F. Khan, CERN fellow)

Property MWS (PEC) HFSS (Cu)

Freq, GHz 11.9941 11.9959

QCu 6395 6106

Rt/Q, Ohm 54.65 53.78

Esurf/Et 3.43 3.28

Hsurf/Et 0.0114 0.0106

• MWS supports only PEC material in eigenmode simulation

• MWS used Perfect Boundary Apprx, 134,912 hexahedra per quarter (lines/lamda=40, lower mesh limit=40, mesh line ratio limit=40)

• HFSS used 8,223 tetrahedra per quarter (surface apprx= 5m, aspect ratio=5)

Mesh view

MWS HFSS

12 cell structure frequency domain simulation

• Full structure with one symmetry plane

• 1.75 m tetrahedral elements

• Calculation at 11.9942 GHz and 1 W ‘peak’ input power

Mesh view

S-parameters

Complex field (Hx) in the band

Magnetic field profile-Hx on axis at 11.9942 GHz for 1 W

Electric field profile -Ey on axis at 11.9942 GHz for 1 W

Electric field profile -Ez(y) at 11.9942 GHz for 1 W

Red: at y=0.5 mm Green: at y=0

Power flow (1 W)

Internal reflection at 11.9942 GHz

Phase advance at 11.9942 GHz

Properties of full cavity

RF properties

Mode (rad, GHz) 2/3, 11.9942

Kick (MV) 2.56

Group vel, % -2.9

Fill time, ns 11.5

Peak power (MW) 13.35

Esurf (MV/m) 103

Hsurf (kA/m) 348 (reg cell) 207 (coup slot)

T (K)26 (reg cell) 10 (coup slot)

Sc (W/m2) 3.32*

Size

Total length

(mm)149.984

Active length (mm)

99.984

Vertical size

(mm)59.354