LINAC 4 RFQ DESIGN, CONSTRUCTION, COMMISSIONING, AND OPERATION

C. Rossi and the RFQ Project Team

CERN 11 September 2018

PRESENTATION OUTLINE

Design of the Linac4 RFQ

• Beam dynamics

• RF

• Mechanics

The Linac4 RFQ fabrication

Tuning and commissioning

First operation

2

ORGANIZATION STRUCTURETh

e LINA

C4

RFQ

Pro

ject

Beam Dynamics

RF Design & Tuning

Mechanical Design

RFQ

3

RFQ FABRICATION – Detailed ScheduleTh

e LINA

C4

RFQ

Pro

ject

T1 rough machining completed in July 2009Semi-finishing February 2010Finishing + Assembly March 2010 (date of the RF measurement)Brazing 1 May 2010Brazing 2 November 2010

T3 rough machining completed in June 2010Semi-finishing October 2010Finishing + Assembly December 2010Brazing 1 February 2011Brazing 2 May 2011

T2 rough machining completed in October 2010Semi-finishing April 2011Finishing + Assembly June 2011Brazing 1 July 2011Brazing 2 April 2012

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RFQ

ENGINEERING SPECIFICATION and preliminary constraints

An Engineering Specification was issued to provide a first estimate for the projectand define the boundary conditions.

A.M. Lombardi, C. Rossi, M. Vretenar - Design of an RFQ Accelerator optimized forLinac4 and SPL, CERN-AB-Note-2007-027.

The initial Linac4/SPL design was foreseeing the use of the IPHI RFQ as low energyinjector (CERN – CEA – CNRS agreement signed in 2001).

The RFQ design started with the following constraints:

• Output beam characteristics equal or compatible with the IPHI parameter set (toavoid redesigning the chopper line and/or the following accelerators);

• Mechanical and RF design compatible with the RFQ projects at the time underrealization with the participation of CERN (IPHI and TRASCO), to avoid starting acompletely new RF and mechanical design;

• Elementary modules of 1 m;

• Maximum RF Power required: 0.8 MW to allow the use of one single LEP klystron;

• 7.5% maximum RF duty cycle, to make it compatible with the SPL operation;

• Maximum beam current: 70 mA;

• Minimum extraction energy from the H-ion source: 45kV.

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PARAMETER TABLE of Reference RFQs

6Courtesy A. Pisent – PAC09, Vancouver

Design

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PARAMETER TABLE of Reference RFQs

7

Parameter Units LINAC-4 TsinghuaFrequency MHz 352.2 325Win keV 45 50Wout MeV 3 3Particle H- p-

Iout mA 80 50Pcavity+ kW 410 387Duty % 10 2.5Epeak EKilpatrick 1.84 1.8Power Coupler 1 W/G iris 1 W/G irisPave walls W/cm2 0.72 0.35Pave undercut W/cm2 19.2 3.5Length m 3.06 3.0a, min aperture radius mm 1.8 – 3.3 2.7 – 3.5ρ, vane tip radius mm 2.77 2.32 – 4.56r0, ave. bore radius mm 3.3 2.9 - 5.7ρ/r0 0.85 0.8mmax, modulation 2.36 2.07V, intervane voltage kV 78.27 59.6 -131Transmission % 93 97.2

Courtesy J. Stovall

Design

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PARAMETER TABLE

8

Linac4 RFQ Parameter Value Units

Frequency 352.20 MHz

Length 3.06 m

Vane voltage 78.27 kV

Minimum aperture a 0.18 cm

Maximum modulation 2.36

Average aperture r0 0.33 cm

r/r0 0.85

Minimum longitudinal radius 0.9 cm

Max field on pole tip 34 MV/m

Kilpatrick value 1.84

Focusing parameter 5.7

Acceptance at zero current 1.7 p mm mrad

Final synchronous phase -22 deg

Design

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PARAMETER TABLE

Parameter Specified

RFQ (2D) Units

LINAC4

RFQ

RFQ frequency 352.2 MHz 352.2

Length 3.0 m 3.06

Electrical length 3.4 l

Intervane voltage 84 kV 78

Average radius r0 3.2 mm 3.256

r/r0 ratio 0.85 0.85

Stored energy 0.433 Joules/m 0.395

Capacitance 123 pF/m

Power dissipation (Superfish) 91.8 kW/m 78.7 Quality factor 10400 6772

Shunt impedance (Superfish) 77 kW-m

Total power dissipation

(1.2*Superfish, only cavity) 331 kW 390

Energy input 0.045 MeV 0.045

Energy output 3.0 MeV 3.0

Max RF duty cycle 5 % 10

Beam peak current during pulse 70 mA 70

Beam power (70 mA) 210 kW 210 RF total peak power 541 kW 600

Minimum aperture a 0.18 cm 0.18

Max field on pole tip 35 MV/m 34

Max surface field 1.9 Kilpatrick 1.84

Focusing parameter 6.0

Acceptance at zero current 1.7 p mm mrad 1.7

Transmission(*)

93 % 95

Input transverse emittance 0.25 p mm mrad 0.25

Transverse emittance growth(*)

0 % 0

Longitudinal emittance(*)

0.14 p deg MeV 0.13

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BEAM DYNAMICS

0

0.5

1

1.5

2

2.5

0 50 100 150 200 250 300

RFQ length (cm)

a (c

m),

m

-100

-90

-80

-70

-60

-50

-40

-30

-20

-10

0

Phi

(deg

)

a

m

Phi

30

31

32

33

34

35

36

-40 10 60 110 160 210 260 310

Es (MV/m) vs length (m)

V2TERM

VSINE

VGEOM

newdesign-304cm

10

Sacrifice beam transmission withacceleration gradient;

Keep surface electric field as low aspossible;

Match to the existing MEBT;

Pole tip profile compatible withmachining by milling wheel (constantradius);

r/R0 = 0.85 (keep Kilpatrick undercontrol).

91

92

93

94

95

96

97

98

99

100

101

0

0.05

0.1

0.15

0.2

0.25

0 20 40 60 80

Tran

smis

sio

n [

%]

RM

S Em

itta

nce

[d

eg

keV

]

Beam current [mA]

Longitudinal emittance

Beam transmission

Design

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RF DESIGN

11

Maintain a constant profile for the cavity transversesection;

0.1 mm gap between 1 m modules;

Provide at least 1 Mhz separation between Q0 and D2;

End-cell tuning performed by quadrupole rods.

freq-freq2D vs tuner penetration

Dbore = Dtuner + 2mm

-10

-5

0

5

10

15

20

25

30

35

-50 -40 -30 -20 -10 0 10 20 30 40

penetration ( mm )

freq-f

req2D

( M

Hz )

Dtuner=80mm

3 Tuners / meter

Freq flush = 345.525626 MHz

Freq 2D = 346.020509 MHz

freq-freq2D vs tuner penetration

Dbore = Dtuner + 2mm

-10

-5

0

5

10

15

20

25

30

35

-50 -40 -30 -20 -10 0 10 20 30 40

penetration ( mm )

freq-f

req2D

( M

Hz )

Dtuner=80mm

3 Tuners / meter

Freq flush = 345.525626 MHz

Freq 2D = 346.020509 MHz

3 Tuners/quadrant/module.

Design

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RF DESIGN – Peak Field and RF Coupling

12Limit peak surface field at vane ends and module transitions.

Design

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RF DESIGN – Peak Field and RF Coupling

13

Power density in W/cm2, for 640 kW coupledpower.

With coupling hole diameter 12.42 mmcritical coupling is obtained for Ibeam = 70 mA

(S11 = 0.02, b = 1.59)

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RF DESIGN – Peak Field and RF Coupling

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Design

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RFQ

MECHANICAL DESIGN – Machining and Assembly Tolerances

15

Inter-vane capacitance errors can be compensated by slug tuners up to ±2.3% and±3.5% respectively for quadrupole and dipole modes.

Beam dynamics studies haveprovided the tolerances to berespected in order to avoidadditional losses by 2% andemittance increase in allplanes by 4%.

Design

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MECHANICAL DESIGN

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The mechanical design was based on the following assumptions:

• Individual modules of 1 m length;

• Pole tip machining with cutting wheel;

• Assembly with two-step brazing.

Vacuum ports assembled at second brazing,allowing excellent compensation for fieldpenetration.

Design

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RFQ

MECHANICAL DESIGN – Thermal Stabilization

17

The Thermo-mechanical Study performed at CEA showed that:

A total of 8 water channels in the cavity are enough to stabilize the RFQ operation;

The water temperature can be effectively used to fine tune the RFQ frequency.

TBody = 20 to 30 (cont.) TVane = 20 to 30 (dash.) H-tip X-shift (m) vs Q0 eigen-frequency shift (Hz)

-2.5 -2 -1.5 -1 -0.5 0 0.5 1 1.5 2

x 105

-5

0

5

10x 10

-6

LINA

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RFQ

Fabricatio

nMECHANICAL FABRICATION

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Manufacturing procedure

Rough Machining, Over thickness 3 mm

Deep drilling for the cooling channels and UScontrol

Rough Machining, Over thickness 1 mm

Heat treatment at 600 ºC

Partial Finishing, Over Thickness 0.15 mm only forthe vane tip and the brazing surfaces

Heat Treatment at 800 ºC

Finishing to nominal sizes

RF Bead-pull Measurement

First Brazing step: 4 vanes in horizontal position,vane over length 0.5 mm, brazing temperature815 ºC

RF Bead-pull Measurement

Re-machining for the end and lateral flanges

Second brazing step: flanges on the module invertical position, module over length 0.1 mm,brazing temperature 785 ºC

RF Bead-pull Measurement

Final machining: length and centering rings

LINA

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RFQ

Fabricatio

nMECHANICAL FABRICATION

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LINA

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RFQ

Fabricatio

nMECHANICAL FABRICATION – Fabrication Control by Metrology and Bead-pulls

20

+0.044

+0.013

-0.011

-0.035

+0.016

-0.038

-0.031

-0.042

+0.018

-0.016

+0.015

RF

Bea

d-p

ulls

LINA

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RFQ

Fabricatio

nMECHANICAL FABRICATION – What went wrong …

21

T3 Major Vane RF Ports on T2

Vacuum leaks on the collar

Required difficult machining …

… and new assembly technique

LINA

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RFQ

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gASSEMBLY AND TUNING

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Final Tuning Sequence

End Plates

RF Power Coupler

Piston Tuners and Dummy RF Ports

Final Tuning result

D0 D1 D2

Q0Q1

d=12.42 h=6.17

Slot = 1.54

Tun

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f the LIN

AC

4 R

FQPARAMETER TABLE

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RFQ TUNING

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LINA

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RFQ

Tun

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RF Power waveguide supporting system.

LINA

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gRF COMMISSIONING

25

RF Commissioning

28 February 2013 RF Commissioning started

(few shots already the week before);

13 March first beam accelerated.

RF Commissioning target at the 3 MeV TestStand:

480 kW, 250 msec, 1Hz;

with 1E-6 gas load in the LEBT.

LINA

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RFQ

Tun

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gRF COMMISSIONING

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LINA

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RFQ

Tun

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gRF COMMISSIONING

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First beam accelerated at the 3 MeV Test Standon 13th February 2013

Expected from simulations LINAC4 tunnel

Pictures from the commissioning after installationin the Linac4 tunnel, on 14th November 2013

RF D

esignThe RF Cavity Design

28

Relative variation of parallel capacitance of modulated vanes made us decidefor the V2TERM profile (possibility to keep a constant RFQ cavity crosssection).

COMSOL 3D simulations – Courtesy M. Desmons30

31

32

33

34

35

36

-40 10 60 110 160 210 260 310

Es (MV/m) vs length (m)

V2TERM

VSINE

VGEOM

newdesign-304cm

VSINE

V2TERM

RF D

esignEnd-of-vane enhancing factor

29

Minimize the peak surface field between adjacent vanes, separated by 0.1 mmgap

1

1.02

1.04

1.06

1.08

1.1

1.12

1.14

1.16

1.18

0 0.5 1 1.5 2 2.5 3

b ( mm )

a=1mm, 3D

a=1.5mm, 3D

a=2mm, 3D

a=2.5mm, 3D

a=3mm, 3D

a=4mm, 3D

a=1mm, 2Daxi

a=1.5mm, 2Daxi

a=2mm, 2Daxi

a=2.5mm, 2Daxi

Enhancement T1 – T2 Enhancement T2 – T3

Saturation at = 2 * Ekp

V0 = 78.27 kVEkp = 18.43 MV/m

Saturation at = 2 * Ekp

V0 = 78.27 kVEkp = 18.43 MV/m

Field Enhancement Factor vs. ellipsoid geometry

RF D

esignEnd-of-vane RFQ output

30

Max surface field at T3 output – RFQ output

Ellipse : a=4mm, b=1.8mm; Gap=2.31mm

Emax=36MV/m

RF D

esignThe RF Power Coupler

31

RF H

igh P

ow

er Op

eration

The RFQ Stability

32

The RF power sweep from 2.5 to 400 kW shows a stable RFQ at all power levels.

The quadrupole (Q) and the twodipole (S and T) components ofthe reconstructed voltage areplotted as a function of the RFpower in the RFQ cavity, at thefour pick-up cross sections.

RF H

igh P

ow

er Op

eration

The RFQ Stability vs. Temperature Variations

33

Variation of Q, S and T component relative errorswith respect to uniform distribution of vanetemperatures

Relative error of reconstructed voltage in the four RFQ quadrants.

Relative error of Q, S and T components of reconstructed voltage.