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

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

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

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.

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

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

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

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)

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]

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)

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

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

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

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

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

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.

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