Imperial College London, FETS 1

RFQ development for high power beams

1. Introduction

2. Particle dynamics in the RFQ

3. Electrodynamic design of the RF resonator

4. Mechanical design & construction of the RFQ

5. Conclusions

Imperial College London, FETS 2

Introduction

The first accelerator structure is most critical because of the high space charge forces at low beam velocities

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Imperial College London, FETS 3

IntroductionRFQ :

Using a set of four electrodes to build an electrostatic focussing channel and to create longitudinal electric field components for the acceleration of the particles

by modulation of the electrodes.

Requirements : High transmission, low emittance growth, low power consumption by use of high Impedance resonator

Imperial College London, FETS 4

An RFQ has to fulfil several functions like beam matching, bunching and acceleration at once. These functions can only be provided by changing the

modulation of the RFQ electrodes along the beam path.

Particle dynamics in the RFQ

In the traditional design philosophy in different parts of the RFQ different

parameters are kept constant according to the function of this part.

Imperial College London, FETS 5

Particle dynamics in the RFQInfluence of the electrode design on the beam current limit of the RFQ :

To increase the current limit of the RFQ in modern designs all parameters of the RFQ electrodes are varied along the beam axis.

Improved design

CRYRING

Classic design

Classic design

Improved design

HERA

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Particle dynamics in the RFQInfluence of electrode voltage, injection energy and RFQ length on beam current

limit, RFQ length and power consumption and transmission

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Particle dynamics in the RFQ

Due to sparks and dark discharge, the maximum potential on the

Electrodes is limited. This limit is not only a function of the

aperture of the RFQ, but also of the RF frequency and the quality

of the electrode surfaces. The Kilpatrick factor (usually between 1.5 and 2 for RFQ’s) is the factor between the applied potential on the electrodes and

the spark limit given by the theory of Kilpatrick.

Electrode potential as a function of gap distance and RF frequency

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Electrodynamic design of the RF resonatorTo provide the electrodes with the necessary potential

different types of resonant RF structures can be used.

4 rod structure(e)

Split coaxial (d)

Double-H (c)

4 Vane structure (a & b)

Imperial College London, FETS 9

Electrodynamic design of the RF resonatorChallenges are : High shunt impedance, low resistive losses, concentration of

fields onto axis

R’

[k

m]

4-Vane

4-Rod

Split coaxial

D-H-resonator

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)4(

2

expexp

20

2

VaneQ

QRR

dVHW

N

WQ

L

NP

P

UR

SFth

Imperial College London, FETS 10

Electrodynamic design of the RF resonatorChallenges are : Field flatness is strongly influenced by endplates and

mechanical design of the resonator (tuners, couplers…)

Field flatness of 4 rod RFQ

4 Vane RFQ with large coupling windows (left) and according longitudinal potential

distibution (upper)

Imperial College London, FETS 11

Electrodynamic design of the RF resonatorChallenges are : Unwanted modes (dipole, multipole) near the working

frequency of the RFQ

Use of VCR ringsImprovement of mode structure at HERA RFQ

by the use of RLC couplers in end flanges

Imperial College London, FETS 12

Electrodynamic design of the RF resonator

Challenges are additional support : Coupling, Endplates, Tuners, Feedback systems etc.have influence on RF properties of the cavity

RLC coupler

end tuner

adjustment ringpositioner

Vane

beam axis

Imperial College London, FETS 13

Mechanical design & construction of the RFQChallenges are : Production tolerances have influence on the

particle transport (mismatch) and resonator characteristics (esp. 4 vane and dipole modes)

RIA : high tech

assembly

SNS : massive parts

Imperial College London, FETS 14

Mechanical design & construction of the RFQChallenges are : Power dissipation, resistive losses and cooling

Leada: very strong cooling !4 Rod : uncooled stems at 170oC

How much cooling is necessary ?

China Institute of Atomic Energy : No

difference between 16 and 20 channels !

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Mechanical design & construction of the RFQ

Challenges are : support for power feed troughs, pumping, mode stabilizing, active control of tolerances, etc… without

influencing the RF properties

(shunt impedance, additional modes)

SNS

Leda

Imperial College London, FETS 16

Conclusions

Frequency, RFQ length etc. are not independent from each other

But strongly coupled (“one knob machine”)

No optimum design strategy for particle dynamics known.

Choice of resonator structure strongly influences the mechanical design

Dynamic control of resonator by electro-mechanic systems (piezo) should be considered for 4-Vane structure

=> Design of an RFQ is not straight forward but a process of several iterations.

Imperial College London, FETS 17

Work to be performedYear 2 Year 3 Year 4 Year 5 Year 6Year 1

1) Decision of frequency

2) Particle dynamic design (field level, Kilpatrick) => Length of RFQ

3) Electro dynamic design of resonator and Endplates => Choice of RFQ type

4) Electro dynamic design of tuners, couplers

5) Design of models, production and tests of models

6) Mechanical design of RFQ, Endplates, Positioners

7) Mechanical design of tuner, couplers, etc

8) Production of RFQ

9) Production of tuner, couplers, and support

10) Assembly of RFQ, test and commissioning