Work towards high performance accelerator structures

1. Work towards high performanceaccelerator structures 1. Introduction 2. Cavity & electrode design, particle dynamics 3. Critical fields, sparking, multipacting 4. Surface treatment and conditioning 5. Design of RFQ and test models 6. Conclusions

2. Work to be performed Year 1 Year 2 Year 3 Year 4 Year 5 Year 6 1) Decision of frequency  (RFQ = 324 MHz , MC = 201 MHz) 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 11) Test of production methods and surface conditioning

3. Particle dynamics in the RFQ Improved design A RFQ electrode design has to be found by numerical simulations which fulfils : • High beam transmission • low emittance growth and defines • electrode voltage • electric surface field strength Classic design HERA

4. Electrodynamic design of the RF resonator Challenges are : High shunt impedance, low resistive losses, concentration of fields onto axis 4-Vane 4-Rod Split coaxial D-H-resonator R’ [kWm] ~30 kWm For the FETS (DF~7.5 %) the average resistive power loss is app. 25 kW/m. Therefore 4 vane and 4 rod structures are possible. f [MHz]

5. Field emission of electrons from surfaces Fowler Nordheim : J: emission current density (A/cm2); B: field-independent constant [A/V2]; E: applied field (V/cm) F0: work function (eV)

6. The maximum electric field and the Kilpatrick limit 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 semi empirical theory of Kilpatrick. Electrode potential as a function of gap distance and RF frequency

7. Multipacting 1 Multipacting in a acceleration cavity (left - field plot), occurs at low and medium power levels, when primary electrons are accelerated by the electric field and generate (in resonance with the electric field) an avalanche of secondary electrons.

8. Multipacting 2 In opposite to rectangular cavities, in elliptical cavities the path of the multipacting electrons is directed towards the axis of the cavity. Therefore the avalanche is interrupted when the electrons reach the cavity iris. The magnetic self fields in the cavity do not prevent the propagation of the electrons towards the axis !

9. Multipacting 3 Range of electric field for multipacting measured for different materials. Due to the impact of the electron current on the surfaces a conditioning of the surface might occure and the multipacting levels off. * *

10. Influence of external magnetic field

11. Sparking and surface treatmentLos Alamos experiments using a 420 MHz 4 Vane RFQ in 1981

12. Surface damage due to sparks Pitting of surface

13. Conditioning by RF

14. Gas discharge and low energy ion impact on surfaces for surface conditioning by sputtering of field emitters

15. Mechanical design & construction of the RFQ-Testmodel(example TRASCO RFQ Legnaro) 20 cm technology test model Results of measurements at test model RFQ Parameters

16. Mechanical design & construction of the RFQ(example TRASCO RFQ Legnaro) While the first section of the RFQ fulfilled the requirements after production and brazing, the second one showed a deviation of 0.1 mm due to bending of the vanes and there from a deviation of the resonance frequency which could not be adjusted by the tuners. Drawing of the first section and final first section

17. Conclusions Work to be performed : 1. Numerical Design of RFQ resonator and RFQ electrodes for • High beam transmission • Low emittance growth - moderate electric field - Low power losses 2. Numerical determination of critical fields in RFQ and MICE cavity design – investigation on influence of magnetic fields on HV break down effects 3. Definition on required surface and production quality Validation of production techniques and (cold / hot) test of models 4. Experimental determination of required surface treatment and surface conditioning procedures and sparking test on models and verification of resonator design.