RF DESIGN OF THE POWER COUPLER FOR THE SPIRAL2 SINGLE BUNCH SELECTOR*

F. Consoli, A. Caruso, G. Gallo, D. Rifuggiato, E. Zappalà, INFN-LNS, Catania, Italy M. Di Giacomo#, GANIL-SPIRAL2, Caen, France

Abstract

The single bunch selector of the Spiral2 driver uses high impedance travelling wave electrodes driven by fast pulse generators. The characteristic impedance of 100 Ohm has been chosen to reduce the total power, but this non standard value requires the development of custom feed-through and transitions to connect the pulse generators and the matching load to the electrodes. The paper reviews the design of these devices.

INTRODUCTION The single-bunch selector of the Spiral2 accelerator

reduces the bunch repetition rate onto the experimental target of a factor 100 to 10000. The device is constituted of a static magnetic deflector and of a pulsed RF kicker whose fields are perfectly compensated for the selected bunch. All other bunches are deviated onto a beam stop by the magnetic field. This principle, shown in Fig. 1, inverts the duty cycle required for the RF kicker and uses constant length pulses.

Figure 1: Principle of the inverted duty cycle single bunch selector.

This solution was developed in the framework of the Eurisol Design Study [1] and is justified by the high repetition rate, high voltage, and fast transient time required by high intensity ion drivers. Table 1 reports the values for the Spiral2 application.

Table 1: Single Bunch Selector Requirements

Parameter Value Units

Repetition rate 1 MHz

Rise and fall time 6 ns

Pulse total length 19 ns

Pulse max voltage 2.5 kV

Pulser power 1 kW

Table 2 gives the operating parameters for a plate

length of 546 mm and for the different kinds of ions

(identified by their mass to charge ratio A/q) of the Spiral2 driver. The difference between the beam and the plate voltages is due to the coverage factor of the meander plate, which is ~0.75.

Table 2: Voltage Requirements for the SBS

A/q Beam Voltage Pulse Voltage

1 473 V 630 V

2 792 V 1056 V

3 1182 V 1576 V

6 1773 V 2364 V

High voltage switches being too slow and not enough

powerful, the travelling wave solution was proposed in the inverted duty cycle configuration in order to reduce the involved RF power.

The electromagnetic field of the RF kicker is generated by two pulsers of opposite sign travelling along two meander-shaped microstrips (beta 0.04), each one ended by a matching load.

Characteristic impedance (Zc) of 100 Ohm has been chosen as a compromise among pulser RF power, meander loss, and meander feasibility.

Figure 2: Schematics of the meander feeding chains.

As shown in Fig. 2, the signal of each generator is transmitted via 100 Ohm cables supplied by the pulser manufacturer. From the cable, the pulse travels through the feed-through, where the vacuum window is located, and a short section of transmission line before reaching the transition to the meander strip. A similar path is followed at the output to reach the matching load.

All the feeding chain elements have been designed to minimize the insertion and transition loss to transfer the best possible pulse. CST Microwave Studio was used for RF simulation in both time and frequency domains and results of the different simulations are reported here. An

+ HV Pulser

Beam

- HV Pulser

Static B- field

z

R

RTravelling E- field

• • • • •

• • • • •

• • • • •

• • • • • • • • • •

• • • • •• • • • •

• • • • •• • • • •

1 kW – 100 Ωdummy load

100 Ω coaxial line

Connector

VACUUM CHAMBER100 Ωvacuum feedthrough

Transition between the 100 Ω line and the meander electrode

Pulse generator (~1 kW)

100 Ω coaxial line

Connector

100 Ωvacuum feedthrough

Transition between the 100 Ω line and the meander electrode

Meander line

___________________________________________

*Work supported by the European Community FP7 – Capacities –SPIRAL2 Preparatory Phase n° 212692. #digiacomo@ganil.fr

TUP277 Proceedings of 2011 Particle Accelerator Conference, New York, NY, USA

1346Cop

yrig

htc

2011

byPA

C’1

1O

C/I

EE

E—

ccC

reat

ive

Com

mon

sAtt

ribu

tion

3.0

(CC

BY

3.0)

Accelerator Technology

Tech 28: Subsystems and Technology (Other)

analysis bandthe pulse spe

Feed-ThroThe feed-t

cable are strtwo custom disk (b) vacuHN 50 Ohm

Figure 3: Simgeometry a)

Solution abandoned bwith the diskdetails. The iand the feedmachining. TMV/m, lowconsidered ointroduced bsparking riskthat the Tefldone in HV is granted byside, the diaminner conducthis value it the resulting electric fielddielectric stre

To simulaapproximatiosince it hasnfrom the datadielectric, anbe worse duAlumina, usgeometry incwhich has chand the sectiline on the vof each sectio

Figure 4: HN(left size) and

Comparativedeformation

dwidth of 150ectrum.

POWER

ugh and Cothrough and tictly related. 100 Ohm de

uum windowsstandard cabl

mulated 100 Ois almost twic

(a) has beebecause it reqk break (b), winner conduct

d-through is toThe maximum

wer than the of 3 MV/m. Cbetween the k. In fact, theon can be supstandard conny tight mechmeter ratio chctor diameter

is more diffiouter diamete

d is ~2 MVength value. ate the set oons in the mn’t been possia sheets. Only

nd probably thue to the higsed for the vcludes the secharacteristic imon before the

vacuum side, on are shown

N simulated d ending trans

e results for thare shown in

0 MHz has b

R COUPLE

onnector the connectionThree cases hvices, using c

s as shown in le and feed-thr

Ohm feed-throce smaller than

en studied quires more rwhich has beeor connection

o be brazed tom electric field

air dielectriConsequently,two conductoe disc Aluminperposed to thnectors. The anical contachanges to Zc is chosen of 1icult to braze er is 6.4 mm. V/m, well be

of standard Hmodelling had

ble to have py Teflon has b

he feed-througgher dielectriacuum windo

ction where thmpedance higshort transitiowhere Zc is ~in Fig. 4.

structure withsition to 100 O

he S parametern Fig. 5, wher

een considere

ER

n to the 100 have been stucylindrical (aFig. 3 and a srough connect

ughs. Scale on that of case

first but raroom than theen studied mon between the o avoid compd at this point ic strength v, a Teflon shors to reducena is T-shapehe ceramic, asground conne

ct. On the vac= 100 Ohm.

1.2 mm, as bethe Alumina

The corresponelow the vac

HN devices, to be introd

precise informbeen consider

gh behaviour wic constant oow. The simuhe cable is inseher than 100 Oon to the 100 ~60 Ohm. Le

h 100 Ohm cOhm (right siz

rs and for the re (c) indicate

ed for

Ohm udied: ) and set of tors.

of b).

apidly e one ore in cable licate is ~2

value, hell is e the ed so s it is ection cuum The

eyond a, and nding cuum

some duced, mation red as would of the ulated erted, Ohm, Ohm ngths

cable ze).

pulse es the

50 Othe f

FiguthrouFig.

100Be

100 equippumThe trans

Thwhilthe t

CouTo

sectithe c

F

Ohm geometryfunction:

ure 5: S paramughs of Fig. 34. Time doma

0 Ohm Coaxetween the feeOhm coaxial

pped with lmping inside th

slots do nosmission loss,

Figure 6: Slo

he ground coe the inner coip ends onto a

upler Mechao avoid the pion of line is coupler has to

Figure 7: Feed

y. The input p

meters for the and for the H

ain results for

xial Line ed-through anl line is inseongitudinal she volume coot change sias shown in F

otted coaxial l

onnection is onductor is paa pad of the m

anics presence of inused as showface the trans

d-through and

pulse is appro

(a) and (b) 10HN 50 Ohm st

the (b) and (c

nd the meanderted. The outslots for prooncerned by tignificantly iFig. 6.

ine and S para

granted by tart of the feed

meander strip.

nternal spacewn in Fig. 7. sition collar.

d 100 Ohm slo

oximated with

(1)

00 Ohm feed-tructure (c) ofc) cases.

er transition, ater coaxial isoper vacuumthe RF fields.insertion and

ameters.

tight contact,d-through and

ers, a straightConsequently

otted line.

h

-f

a s

m .

d

, d

t y

Proceedings of 2011 Particle Accelerator Conference, New York, NY, USA TUP277

Accelerator Technology

Tech 28: Subsystems and Technology (Other) 1347 Cop

yrig

htc

2011

byPA

C’1

1O

C/I

EE

E—

ccC

reat

ive

Com

mon

sAtt

ribu

tion

3.0

(CC

BY

3.0)

TRANSITBoth long

been investigline and the similar behashape.

Figure 8: geometries apulse respon

As a 1 kWthe contact talso grant thAlumina. Bwarming the 6 A of DC cu

All the prdifferent eleresults to thethe perpendic78-periods m

Different coupler of ththe transitionis the feed-thcoaxial linedeformation.through baseresults, as theof nanosecoprinciple beconclude thnegligible wi

TION TO Tgitudinal and gated for the 100 Ohm mic

aviour, with

Longitudinal and comparisses.

W pulse has to to the strip hahe cooling of

Brazing on thplate up to so

urrent. revious simulements of the pulse deformcular transitio

meander shown

CONCsolutions hav

he single buncn to the microhrough, the me having ve The coupler

ed on a disk e transient timonds only.

e achieved wat the effectith respect to t

THE MEA perpendicultransition be

crostrip. Resunegligible ef

(a) and pson of their

be transferredas to be brazef the coaxial he microstripome 45°C by d

lations show he coupler. Tmation due to ton has been usn in Fig. 9.

CLUSIONSve been studich selector meostrip. The mo

meander transitry low effedesigned withvacuum wind

mes are slowedAcceptable

with the HN t of the desthe pulse defo

NDER STRar solutions etween the coults of Fig. 8 ffect on the

perpendicularS parameters

d, we consideed. In this wa

line wire vip is possibldriving it with

the effect oTo compare the electrode ised with the w

S ied for the peander line anost critical eletion and the slect on the h the custom dow gives thed down of fewresults coulset too. We

signed couplormation due t

RIP have

oaxial show pulse

r (b) s and

er that ay we ia the e by h 5 or

of the these itself, whole

power nd for ement lotted pulse feed-

e best w tens d in

e can ler is to the

lengtof thhavechamcoupquansolut

Figumean

Thwho equareali

[1]

[2]

th, the attenuahe meander line been studiedmber, two waplers is being dntify maximution.

ure 9: Pulsnder line fed b

ACKhe authors are

reviewed thation (1) for stic simulation

G. Le Dem,Selector for tion beam", USA, 25-29 Jhttp://www.JAP. BalleyguieFremont, PImprovementSelector", LSeptember 20

ation and the ne. Manufactud too and a proater cooled edesigned at INum perform

se deformatioby the perpend

KNOWLEDe grateful to Phe design wothe input signs.

REFEREM. Di Giacthe next low PAC'07, AlbJune 2007, MACoW.org. er, M. Di GP. Bertrandt in the SLINAC2010, 010.

frequency depuring and assemototype includelectrode holdNFN-LNS. Po

mances of th

on due to thdicular transit

DGEMENTP. Balleyguier ork and sugggnal, in order

NCES como: "A Sβ continuous

buquerque, NMOPAN008, p

iacomo, M. d: "ElectroSPIRAL2 Si

Tsukuba, J

pendent delaymbling issues

ding a vacuumders and fourower tests willhe proposed

he 78-periodtions.

T (CEA/DAM)

gested to user to get more

Single Bunchs wave heavyNew Mexico,p. 158 (2007);

Michel, G.ode Designingle BunchJapan, 12-17

y s

m r l d

d

) e e

h y , ;

. n h 7

TUP277 Proceedings of 2011 Particle Accelerator Conference, New York, NY, USA

1348Cop

yrig

htc

2011

byPA

C’1

1O

C/I

EE

E—

ccC

reat

ive

Com

mon

sAtt

ribu

tion

3.0

(CC

BY

3.0)

Accelerator Technology

Tech 28: Subsystems and Technology (Other)