J. Bosser, P. Bourquin, M. Brouet, B. Couturier, G. Gelato, M. Giovannozzi,F. Grandclaude, J.-Y. Hémery, A.M. Lombardi, U. Mikkelsen, S. Maury, D. Möhl,

F. Pedersen, W. Pirkl, U. Raich, H.H. Umstätter and M. Vretenar.

What is a Radio Frequency Quadrupole?

Decelerator system design

Beam characteristics as seen from the users

Construction status

Conclusions/questions

The Radio Frequency Quadrupole is a linear accelerator which • focuses • bunches • accelerates a continuos beam of charged particles.

The focusing, bunching and acceleration are all performed by the electrical RF field!

RFQ HIGHLIGHTS

alternate-gradient, velocity-independent focusing

high bunching efficiency (>90% capture for accelerations)

easy-to-operate machine

cavity loaded with 4 electrodes electric field between the electrode’s tips(TE21 mode)

alternating gradient focussing structure with period length (in half RF period the particles have travelled a length /2)

longitudinal radius of curvature

beam axis

aperture modulation X aperture

)2

1(2

designing an rfq is defining aperture, modulation and phase all along the structure. Vane profile defines the beam dynamics

every rfq is a “special case” because it delivers beam that is customized to the end-user need

For accelerating rfqs there are “design” recipes (high/medium/low space charge) that have been experimentally tested

Decelerator RFQ is NOT an accelerator the other way around

longitudinal dynamics can not be reversed

longitudinal critical point is located at the end of the machine

physical emittance increases during deceleration

MAXIMUM ENERGY EXCURSION OF A PARTICLE MOVING ALONG THE SEPARATRIX

RFQ DECELERATOR DESIGN – Step one

MAXIMIZE BUCKET AREA AT LOW ENERGY END

Electrode voltage

Accelerating efficiency

Synchronous energy

phase around -160

Maximize voltage (sparking limit)

Maximize Accelerating efficiency (that’s when we run into problem)

1

1.2

1.4

1.6

1.8

2

2.2

2.4

2.6

2.8

3

0.10.25

0.4

0.55

0.7

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

A

modulation

aperture(cm)

1.1

1.3

1.5

1.7

1.9

2.1

2.3

2.5

2.7

2.9

0.10.25

0.4

0.55

0.7

0

20

40

60

80

100

120

longitudinal radius of curvature

modulation

aperture (cm)

Accelerating efficiency and longitudinal radius of curvature vs. aperture and modulation for = 3 cm (200MHz and200 keV or 100 MHz and 50 keV)

RFQ DECELERATOR DESIGN – General criteria

Highest possible modulation (compact structure)

Aperture as to keep focussing constant (to avoid transverse normalised emittance growth and to keep beam dimensions constant)

Minimise the surface electric field (sparking)

RFQ DECELERATOR DESIGN – Step two

From the high energy end

First decelerating cell :

modulation to a high value (2-3)

aperture to give the chosen focussing

phase around –160

Following cells :

Modulation of the preceding cell or as close as machining limit allows

Aperture as to keep focussing constant (tentatively)

Phase constant

decelerator rough design is generated

Optimise locally

phase advance per focussing period

max field on the vanetip

smooth abrupt changes in aperture and modulation

(this might require inserting transition cells in critical areas)

Longitudinal matching , i.e. find the “decelerating acceptance”

The separatrix at the last cell is traced backwards to the input of the RFQ. The points of the boundary are rotated counter-clockwise around the synchronous phase and synchronous energy with a cell-by-cell angular velocity given by the longitudinal phase advance.

longitudinal matching

high energy end low energy end

Need a front-end longitudinal matching section

“adiabatic” buncher system

discrete buncher system

Decelerating acceptance

BARE-BONES DECELERATING SYSTEM NEEDS RF CAVITY + RFQ

RFQ

Efficiency of deceleration Beam quality

Prepare the beam for deceleration

Decelerator

Drift

Gas target experiment : energy variability

smooth waist at the experiment, (beam collimated for the longest possible distance)

Trap experiment :mono-energetic beam : 605 keV

beam concentrated in a 1 mm radius inside the trap

Decelerate a 5.3 MeV beam to a variable energy in the range10-100 keV for gas target and trap experiments

ENERGY VARIABILITY :

Extra device (RF or electrostatic)

GOOD TRAP EFFICIENCY

minimize the output energy spreadaccurate study of the last rfq cellstransport to the trap simulation of the trap magnetic field

5.3 MeV 70 keV 30-130keV

5.3 MeV 40 keV 40-80keV

5.3 MeV 400 keV 70 keV 10-140keV,300-500keV

5.3 MeV 120 keV 80-130keV

5.3 MeV 5.3 MeV 0-100 keV

5.00 m

1

2

3

4

5

filled box = 100 MHz

RFQ

RFQ

RFQ

RFQ

RFQ

B

B

B

B

BB

B

B

B

B= rf cavity

rfq

rfq

the chosen solution

DECELERATING SET-UP:

1. LONGITUDINAL MATCHER : a coaxial TEM resonator loaded with double gap with an effective voltage of 47 keV

2. DRIFT : 615 cm , contains magnetic elements, monitors, steerers, energy corrector cavity

3. RFQ : a four-rod structure, 3.44 m long, decelerates to 60keV

4. ENERGY VARIABILITY DEVICE : the structure holding the electrodes can be raised to a potential (± 60 keV)

rfq sketch with cells, ladder and stuff

RFQD electrodes defining parameters

0

0.5

1

1.5

2

2.5

3

0 50 100 150 200 250 300 350 400

z (m)

a(cm

), m

0

1

2

3

4

5

6

W(M

eV)energy

modulation

aperture

transition cells

PRE-BUNCHER/CORRECTOR CAVITIESFrequency: 202.56 MHzEffective voltage: 50 kVAperture radius: 15 mm Length: 300 mm

RFQ Frequency: 202.56 MHzVane voltage: 167 kVMaximum electric field: 28 MV/m (1.7 Kilpatrick)Vane length: 340 cmNumber of beam dynamics cells: 75Power losses: 700 kW (rough estimate)Average radius of aperture: 0.79 cmMinimum radius of aperture: 0.4 cmVane modulation factor (max.): 2.9

BEAM RELATED PARAMETERS Transmission : 100 %Transmission in 60 5keV : 50 %Transverse design emittance: 10 mm mrad Transverse acceptance : 15 mm mrad Design energy spread: 0.5 10-3

Energy spread acceptance : 0.9 10-3

Transverse normalised-emittance growth: ~0

Computer simulation from the HO (DE1) point to the experiment

Areas to study : Transfer line to and from the RFQRFQTrap solenoid

TOOLS for the simulations

MAFIA/SFH 3d/2d : electromagnetic field PARMILA/PARMULT/PATH : multi-particles tracking

End-to-end tracking till the trap including the “real” solenoid field

Input beam

Beam at the rfq input

Beam at the rfq output

Beam at the gas target experiment (formwar window)

Beam at the trap

From HO (DE1) to the RFQ

in the RFQ

RFQ-IN5.3 MeV

RFQ-OUT60 keV

LEBT Set-up

SC solenoidRFQ2 type sol

Half length of SC solenoid: 1000 mm

Half length of magnet windings: 700 mm

227.5 mm

170 mm52.5 mm

RFQ inside wall (z=0)

Gas target experiment

60 keV

X (cm) , Xp (rad)

Y (cm) , Yp (rad)

X (cm) , Y(cm)

Gas target experiment

60 keV

energy Solenoid field Emittance Beam radius divergencekeV Gauss mm mrad mm mrad100 8300 77 1.6 4860 6500 100 2.2 4520 3750 170 3.2 6015 3250 200 3.2 6310 2660 250 3.5 70

RFQ outputExperiment

Trap

Combined field (dotted line) Nornal conductiong solenoid standalone (solid line)

Trap

Beam envelope in SC solenoid (foil at z=87 cm)(emittance 10 pi); W=63 keV then 10 keV

z=0 corresponds to RFQ inside wall

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6

z(m)

r(cm

) Before foilAfter foil

Trap

34% in 1 mm

Matching-to-the-rfq design concerns

1.AD jitter : effective overall emittance

2.Energy variation from the AD

3.Steering

4.Diagnostic and monitoring during operation

Overall emittance error distribution

RFQ acceptance

Profile (mm)of the rfq acceptance from 2 meters upstream the rfq to the rfq input plane

.

Z(from RFQ in cm) alpha Beta (m.) Radius (mm)

-200 9.81 21.90 18.-175 8.70 17.27 16-150 7.59 13.20 14-125 6.48 9.68 12-100 5.37 6.72 10-75 4.26 4.31 8-50 3.15 2.46 6-25 2.04 1.16 4.20 0.92 0.44 2.5

2 m before the RFQ

RFQ input

RFQ-dynamics design concerns

1.Alignment ladder-electrodes-beam axis

2.RF stability

3.Mechanical stability

Transport- to-the-experiment design concerns

1.Beam longitudinal length

2.Beam steering

3.Beam diagnostics during running in and during operation

Beam length

Macropulse length is determined by the AD bunching system

Decelerator system energy spread acceptance : 0.1%, i.e. 10 keV

Trap acceptance : 400 nsec for 605keV

Transmission vs.AD bunching voltage

Bunching voltage [V]

t (ns) T (keV) Transmission (DE1 to trap)

20 219 0.5 30%100 146 1.5 40%500 97.5 2.3 43%

1000 82 2.7 42%2500 62.3 3.4 39%

Sensitivity to steering

Steering is an issue:

The overall length of the system (hand-over point to the experiment) is some 10 meters

The transverse acceptance of the trap is 1 mm

Steering can be corrected.

Steering becomes a problem only when it degrades transmission and/or beam quality

RFQ dynamics is not very sensitive to steering (up to 2 mm and 5-8 mrad do not change the RFQ performance)

The overall efficiency of the system is sensitive to steeringSteerers in the LEBT

Steering –an example

remanent error 0.5 mm and 1 mrad (upstream monitors sensitivity)

beam centre position through the system (for 60 keV case)

Dx (mm) Dxp (mrad) Dy (mm) Dyp (mrad)RFQ INPUT 0.5 1 0.5 1RFQ OUT 0 -20 0 -28NC SOL IN -1.4 -20 -2.5 -28NC SOL OUT -5 5 0.6 0.9SC SOL IN -3.7 5 0.7 0.9

Conclusion : We need steerers ( and monitors) in the LEBT, can’t do everything with upstream ones

Steering strategy

Setting up phase

Operation

In-house test with an equivalent electron beam (scaling energy like mp/me=1833)

1. Test the scaling laws : use RFQ2 (90keV to 750 keV protons) for which we have extensive data of the test stand

2. Test deceleration process and our codes : use RFQ2 in reverse mode (750keV to 90 keV) and compare with particle tracking code

3. Test and validate the design of the RFQD with electrons

Test with antiprotons from the AD in a dedicated test line from sept 99

Aarhus proton beam line

1.RFQ : model, real

2.BUNCHERS

3.DIAGNOSTICS

4.FOCUSSING ELEMENTS (QUADS UPSTREAM, SOLENOID DOWNSTREAM)

5.ELECTRON TEST STAND

RFQ model

scale 1/3

bead-pull measurements : field distribution

effect of the ladder at a potential : effect on the rfq mode, flaps, ladder enclosed (increase the capacitance ladder to the ground), resistance to terminate ladder, in real RFQ absorber . Few % extra power needed.

the RF is mastered

RF contacts

between electrode and stem, between 2 parts (ladder and electrodes will be machined ½ at the time)

standard berillium-copper contacts loose elasticity at 150-200 deg (bakeout temperature)

solution : “spiral contact” stainless steel gold/silver plated

test set-up for the spiral contacts : lambda/4 resonator, mesure Q and degradation due to contacts. Tested at low power (OK), now the all system will be put under vacuum and tested at high power

RFQ construction

Cylinder, ladder, electrodes are specified.

Cylinder and electrodes machined at CERN

Ladder farmed out (Cinell, Italy)

Diagnostics

Understaffed

Upstream diagnostics : proposal will be tested in the AD beam lines

Downstream diagnostics

Electron test stand

electron gun borrowed from the LHC vacuum group

2.88 keV electron beam (equivalent to 5.3 MeV) is feasible :

Lower energies (49 eV and 408 eV) difficult : 1.Beam dynamics at low energy requires high vacuum2.We need high current at low energy 3.the RFQ2 is made of magnetic material and can’t be properly

screened

Et cetera

Bunchers : scale from LINAC3 model, farmed out to Cinell

Magnetic elements and their power supplies are in house

RF equipment : (power group) buy from OCE (check with WP)

RFQ model shows there are no difficulties in mastering the ladder at a potential

RFQ ready and tested by the end of 1999