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

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

3. Introduction
RFQ :
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

4. Particle dynamics in the RFQ
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.
In the traditional design philosophy in different parts of the RFQ different parameters are kept constant according to the function of this part.

5. Particle dynamics in the RFQ
Influence 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
Improved design
Classic design
Classic design
HERA
CRYRING

6. Particle dynamics in the RFQ
Influence of electrode voltage, injection energy and RFQ length on beam current limit, RFQ length and power consumption and transmission

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

8. Electrodynamic design of the RF resonator
To provide the electrodes with the necessary potential different types of resonant RF structures can be used.
4 Vane structure (a & b)
Double-H (c)
4 rod structure(e)
Split coaxial (d)

9. Electrodynamic design of the RF resonator
Challenges are : High shunt impedance, low resistive losses, concentration of fields onto axis
R’ [kWm]
4-Vane
4-Rod
Split coaxial
D-H-resonator
f [MHz]

10. Electrodynamic design of the RF resonator
Challenges 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)

11. Electrodynamic design of the RF resonator
Challenges are : Unwanted modes (dipole, multipole) near the working frequency of the RFQ
Use of VCR rings
Improvement of mode structure at HERA RFQ by the use of RLC couplers in end flanges

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
beam axis
Vane
RLC coupler
end tuner
adjustment ring
positioner

13. Mechanical design & construction of the RFQ
Challenges are : Production tolerances have influence on the particle transport (mismatch) and resonator characteristics (esp. 4 vane and dipole modes)
SNS : massive parts
RIA : high tech
assembly

14. Mechanical design & construction of the RFQ
Challenges are : Power dissipation, resistive losses and cooling
How much cooling is necessary ?
China Institute of Atomic Energy : No difference between 16 and 20 channels !
Leada: very strong cooling !
4 Rod : uncooled stems at 170oC

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

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.

17. Work to be performed 1) Decision of frequency
Year 1
Year 2
Year 3
Year 4
Year 5
Year 6
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