LHC

Crab Cavities“ from virtual reality to real reality”R. Calaga, BE-RF, LHC-PW, Chamonix 2012

On behalf of the LHC-CC collaboration

The Real “ Problem”

Nominal → 4 IRs, 120(+) parasitic encountersSufficiently large crossing angle inevitable (8-12 sep)

Beam-Beam TeamCERN-ATS-2011-217

8 to 16 LR encounters

No collisions or LR

2011 MD: 36 bunches50 ns, 2 Collisions

Reducing crossing angle

2

Consequence

Φ=σ zσ x

ϕc

σ eff=√σ x2+σ z

2ϕc2

Piwinski angle

Upgrade: reduce * (by factor 2-4)Consequence → approx double the crossing angle (10 sep)

Ineffective Overlap

Note: don't forget hour-glass effect (~15% loss for */z)

Some Numbers

2011 2012 after LS1 after LS3Energy 3.5 TeV 4 TeV 7 TeV 7 TeV

* [cm] 100 60 55 15

2 [rad] 260 313 247 473

R(z =7.55cm) 0.94 0.85 0.82 0.37

R(z =10.1cm) 0.76 0.74 0.28

2 ϕ≃d.√ϵ/βipAssume:

N = 2.5 m, d=10

very inefficient

For the Upgrade

12 separation

10 separation

Nb = 2 x 1011 p/bN = 2.5 m* = 15 cm

S. White, LHC-CC11

Lpk < 7 x 1034 (12 sep), little margin for leveling

Note: don't forget synchro-betatron resonances ~2-4BBLRs might alleviate partially

Nominal

To Recover

Δ p x=qVE. sin (ϕ s+ωt )

c

“ - Bump”

RF Deflector

RF Deflector

V crab=cE tan(ϕc)

ω R12

.2sin (πQ)

cos (ϕ cc−ip−πQ)

Cavity Voltage

~6MV/ IP-side (2 cavities)

add a cavity

Why 400 MHzLHC bunches are long

RF non-linearity (longitudinal)

Higher frequency (for example 800 MHz)“ Smaller” cavitiesLess voltage (VT 1/) → Not reallyEasier phase noise control ? (see later)

800 MHz Cavity, K. Ohmi

L∝N b

2

σ2 RΦ FRF

Φ

GUINEA-PIG simulation, Y. Sun

FRF ~ 10-25%Form factor ~1 (* 10-55 cm)

=1

Pillbox Cavity

beam

Transverse Cross Section, squash

beam in/out of the plane

TM01

0

TE11

1

TM11

0

TE01

1

freq spectrum

TM01

1

TM21

0

TM11

0Y

f res∝1R

R (independent of length)

crabbing mode (HOM)

R: 400 MHz ~ 610mm 800 MHz ~ 305mm

Too big for IR regions

Lengler et al., NIM 164 (1979)Karlsruhe-CERN RF Separator

“ 1st ” SRF Deflector

Assembly into cryostat

RF separator for 10-40 GeV/c from the SPSUnknown heavy particles, baryonic states/exchange, K± & p-bar

F = 2.865 GHzVT = 2 MV/m (104 cells)

Still in use at U-70 setup at IHEP

KEK Freq: 508.9 MHzPower: 50-120 kW (Qext: 2x105, BW: 2.55 kHz)

“ 1st” e± Crab Cavity

Feb 2007

LONG R&D, but short lifetime(2007-2010)

Complex HOM Damping Scheme

THEY WORK!

The real question: will the technology beefficient/transparent for the HL-LHC operation

Real answer: you may have to wait a little while

The LHC Pillbox

Conceptually simple, but practically difficult (KEKB experience)

Main Constraints:Frequency 800 MHzDamping LOM/SOM/HOM remains a challengeComplexity of multiple frequencies in LHCOnly vertical crossing at both IPs Surface field to kick gradient ratio is poor

1-cell version, CERN, L. Ficcadenti et al.2-cell version, USLARP, L. Xiao et al.

Pillboxes → TEM Cavities

~4yr of design evolution Exciting development of new concepts(BNL, CERN, CI-DL-LU, FNAL, KEK, ODU/JLAB, SLAC)

Short History

80yrs latersimilar concepts to be applied

for LHC crab cavities

Concentric Conducting Systemshort for coax

Leading to the telephone etc..

“ Its strongly reentrant form makes the field pattern at the outer radius predominately TEM with the consequence of only moderate current flow”

E. Haebel

Freq = 100 MHzGap Voltage = 0.5 MVPbeam = 200 kW(1.6 MW @400 MHZ, NC Cavities)

/4

More History

/4 TEM ResonatorI0

V0

ab

~/4

= 1

87.4

mm

V 0

ab

~/4

~/4

gap

Z 0=V 0/ I 0

Frequency resonator lengthand “ not” the gap or radii of the concentric cylinders

194 mm194 mm

142.5 mm122 mm

BNL: I. Ben-Zvi et al.

400 MHz LHC Cavity, quasi /4

Z 0 tan(β l)=1

ωC gap

/4 Resonator, HOMs

Note, due to large aperture & residual Ez the LHC cavity will only a quasi /4 resonator

For a pure /4 resonator, next HOM is x3 the fundamental mode

Therefore, damping is a LOT more easier (for example use a high-pass filter)

56 MHz RHIC Prototype

Pedestal to cancel Ez

V0

I0

-I0

~/2

/2 TEM Resonator

Two /4 resonators → /2➔ Use HOM (TE11 like) for deflection➔ More elegant is to use two /2 resonators

Single /2Two /2

SLAC, Z. Li ODU, J. Delayen

➢ Height of the cavity is symmetric about beam pipe➢ Only compact in dimension, LHC needs both x-y compactness

~/2

Joint SLAC-ODU Effort

2010

2011

Fill these regions Full design change

SLAC, Z. LiODU, J. Delayen

/2 TEM Resonator

Symmetric Ridges

Also, Initially proposed by F. Caspers (Crab WS 2008)

/4 = 187.5 mm

Courtesy G. Burt, B. Hall4R (LU-DI-JLAB)

Four co-linear /4 resonators 500 MHz CEBAF Separator

Conical resonators for mechanical stability

Downside is that the deflecting mode is NOT the lowest order mode

4 eigenmodes, mode 2 is our crab mode

Performance Chart

Double Ridge(ODU-SLAC)

4-Rod(UK)

¼ Wave(BNL)

Cavity Radius [mm] 147.5 143/118 142/122Cavity length [mm] 597 500 380Beam Pipe [mm] 84 84 84Peak E-Field [MV/m] 33 32 47Peak B-Field [mT] 56 60.5 71RT/Q [] 287 915 318Nearest Mode [MHz] 584 371-378 575

Kick Voltage: 3 MV, 400 MHzGeo

met

rical

RF

194 mm

B1 B2

< 60 MV/m

< 100 mT

damping more complicated

Impedance Thresholds

Longitudinal impedance2.4 M total (7 TeV)

Strongest monopole mode:R/Q=200 → Qe<1x103

Damping → Qe < 100-500

Transverse

Courtesy: Burov, Shaposhnikova

HOM

HOM

HOM

HOM

Crab

Strongest dipole mode:Z < 0.6 M/m (0.58 GHz)(Qext = 500)

Longitudinal

HOM probe

Input

HOM Broadband

LOM

3-5 stage ChebyshevHigh pass filter

HOM Damping

56 MHz Prototype

(placement not fixed yet)

4 Symmetric couplers on the end caps

(notch/high-pass ?)

4 asymmetric couplers on cavity body

ODUCAV SRHW KEKCAV UKCAV QWAVER FRSCAV

Vz(x=0) [kV] 0.0 -2.1 - 2.5i -4 +1378i 0.0 0 +85.7i -0.1 -0.2i

Vx [MV] 5 5 5 5 5 5

B(2) [mTm/m] 0 0 -0.04i -32.7 - 0.1i 0.02 + 0i 25 + 0i 0 +108i

B(3) [mTm/m2 ] 1250 + 0i 229 + 0i 250 - 0i 2452 - 0.5i 464 + 0i -233 +1i

B(4) [mTm/m3] 0 0 266 - 5i 0 540 +0i -189 -14209i

RF “ Multipoles”Courtesy: A. Grudiev, R. deMaria, J. Barranco

Linear tune shifts ~ 0.0 -10-3

Non-linear effects (b3, b4) → Negligible

See slide A5 for mitigation

Cavity Tuning Thoughts

Up/down motion± 2mm → 1 kHz

Push/pull on cavity body

Scissor jack type mechanism

CEBAF TunerDouble lever(Saclay type)

SM

SM

Modified screw/nut(SOLEIL type)

SM

Low gradient (weak or moderate)

High Field (weak)

Multipacting Courtesy G. Burt, J. Delayan, Z. Li

Medium gradient (strong)beam-pipe region (similar to KEKB)

Not a serious worry, will require RF processing

RF PowerV b∝QL I b

RT

Q 0

(k Δ x)

50 kW

RF Power ~8kW (VT=3 MV)

Margin

R/Q = 300 Ib = 0.55 A

For Comparison, Main RF 300kW (V=2 MV)

RF Power Options Courtesy E. Montesinos

2.0m

2.5m

2.0m

Solid State Amplifiers190 kW, 352 MHz

Single tower < 3m

IOTs (TV Transmitter)Light Sources

Tetrode (SPS)400 MHz, ~50kW

Electrosys

2.5 m

50 kW/cavity, moderate powerSimplified (modified) LHC coupler Common platform for 3 cavities designs

Three available choicesFor SPS tests, reuse Tetrodes used in SPS tests

Crab Cryomodule

Graphic Courtesy: S. Weisz(Space in bypass extremely limited)

RF Distribution

~300m LLRF (Coupled feedback)

P. Baudrenghien

Need ~20-25 m space for amplifiers on each IP-side

Waveguides/Coax

“Preliminary thoughts”

RF Noise

Δ x IP=θck RF

δϕ

ΔV T

V T≪ 1

tan (θ/2)

σ x*

σ z

For example:c=570rad; V/V=0.4%x*=7m, x*=7.55cm

err=1.2rad

Amplitude jitter

Phase jitterFor example: = 0.0050, c=570radx

IP = 0.3m (5% of x

*)

LHC Main RF, = 0.0050 at 400 MHz (Philippe)(summing noise at all betatron bands from DC→300kHz)

Note: IOTs & SSAs are less noisy + betatron comb (0.001)

Planning Overview

M2: Compact Validation& Selection (2012-13)

M2: Beam Tests (2015-16)

Prototype Cryomodule

Final Implementation(2022-23?)

Production of Cryomodules

Detailed planning, see E. Jensen (LHC-CC11)

LS1 LS2 LS3

Cavity Testing

Sheet metal (deep drawing, spinning, hydro-forming)Multiple dies, electron-beam welding

Solid Niobium & machiningMaterial costs & leak tightness

Fabrication Options

{Total 16 cavities (2 IPs, B1 & B2)

With sheet metal (4mm thick)We need approx 500-600 kg Niobium (RRR>300)}

0 100 200 300 400 500 6000.00E+001.00E+022.00E+023.00E+024.00E+025.00E+026.00E+02

Position [cm]

Ez

[V/m

]

4R Al-Prototype Courtesy G. Burt, B. Hall

Bead-Pull

Niobium cavity to be delivered in March 2012

Nb Cavity from solid Ingot

Al-prototype for field measurements

Double Ridge FabricationCourtesy:J. Delayan, Niowave

Nov 2011 Jan 2012

NiowaveSTTR, Phase I/II

Testing April 2012

Real Reality ?

“ If it is real, we believe in it”The Church of Reality

100% 90% 80% 70%

Leveling with crossing angle

A1: Leveling, X-Angle Courtesy Beam-Beam TeamCERN-ATS-2011-217

Demonstrated in 2011 w/o affecting other IPs and emittancew/o crabs range is extremely limited

To fully exploit leveling with x-angle, an RF cavity is ideal

A2: Why SC-Cavity

Q0=GR s

Geometrical factor~ 200

Microwave resistanceCopper ~ mNiobium-SC ~ n

With ~6MV/module, NC-RF is not a viable choice

R s=1

σ δ

G=∫ E3dV

∫ H 2dA

Maximize aperture & minimize # of cavities (reduced impedance)

A choice of 2K cryogenic system optimum for crabs (LHC-CC11)

A3: SPS As a Testbed

Long. Position: 4009 m +/- 5mTotal length: 10.72 mx, y: 30.3m, 76.8m

Cavity validation with beam (field, ramping, RF controls, impedance)

Collimation, machine protection, cavity transparency

RF noise, emittance growth, non-linearities,

Instrumentation & interlocks

Present COLDEX

4 LHC Cavities in SPS (1998)

RF Power Setup (~50kW, Tetrode)

A4: SPS, BA4 Setup

Y-Chamber like, similar to present COLDEXCourtesy E. Montesinos

5 db

m/d

iv

500 kHz

500 kHz

A5: RF Noise, LHC with 1-T feedbackP. Baudrenghien

➔ Selective reduction at all frev lines (V=1.5MV, QL=60k)

➔ Using a betatron comb, we can expect ~16dB reduction at selective frequencies

Courtesy G. Burt, J. Delayan

A6: RF Non-Linearity

Voltage deviation over 5mm:Horizontal: 20% → 5%Vertical: x2 → 10%

Tuning (shaping) to suppress multipoles

A7: Other Applications

Emittance exchange x-z (P. Emma & others)

Φ=σ zσ x

ϕc

HE-LHC (16.5 TeV)= 0.6, similar to nominal(z = 6.5cm, x = 9m, c = 160mrad)

R = -12% wr.t. to head-on

Compensate offset collisions due to beam loading for LHeC (Zimmermann)May not be needed if phase modulation removes the phase-slip

Momentum cleaning: Qacc = (fcc/f0)(S. Fartoukh)For effective Qacc ~ 0.3 → 8GHz, too high freq (Y. Sun)

x

z

SRF Deflector10 MV, 366-447 MHz

3 GeV LINAC

Mode l TE113

Freq 447 MHz

R/Q 500

Epk 34 MV/m

Bpk 74 mT

Aperture 75 mm

A8: ProjectX Synergy

LHC Type Concept(s)

Courtesy M. Champion, Y. Yakovlev

A. Facco, SRF09A9: TEM Resonators

Saclay IPNO

ArgonneNew DelhiINFN LNL-MSUTRIUMF

INFN LNL

Sputtered

INFN LNL

Right here at CERN(HIE-ISOLDE) Cavity reached (ANL 72 MHz)

Ep=70 MV/m, Bp=100 mTQ0 = 1 x 109 at 4.6 K (IPAC10)