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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
r [m
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rad]
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eV]
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**
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age kin
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In jector R FQ D TL
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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
Imperial College London, FETS 6
Particle dynamics in the RFQInfluence of electrode voltage, injection energy and RFQ length on beam current
limit, RFQ length and power consumption and transmission
Imperial College London, FETS 7
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
Imperial College London, FETS 8
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
f [MHz]
)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 !
Imperial College London, FETS 15
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