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Alessandra presents the latest progress on simulation studies of the RFQ3 accelerating section.
Starting from the capture region modelling results shown at the last meeting, the beam can be gently brought to final energy with 93% efficiency assuming a vane voltage of 70kV and max Efield < 30 MV/m. The approach is safe but requires a >5m minimum length of the structure.
On a different approach, more similar to the Linac4 RFQ1 design parameters, a higher maximum Efield of 35MV/m is assumed, with a Bfactor of ~6 and a minimum aperture of ~1.8mm (which determines acceptance and sensitivity to alignment)
In a first iteration, the RFQ length is kept fixed at 3m and the vane voltage is varied to achieve a transmission higher than 90% for a 70mA current beam. A design is achieved yielding 95% transmission for a small emittance beam (with 5% longitudinal losses and 6% transverse losses of particles exceeding the acceptance).
In a second iteration the final modulation is reduced and the length is varied above 3m at fixed higher Bfactor (=6) to improve transmission (with 3% longitudinal and 3% transverse final losses).
In this second redesign, the rho/r0 ratio is kept similar to Linac4 RFQ1 values not to change the capacitance too much. The higher vane voltage (85kV vs 78kV) should not induce increased sparking. The geometry of the vanes is V2term type.
Further optimization of this last design will be continued in the next month before starting the RF design of the cavity.
HP comments that the value itself of rho/R0 is not that critical, what is important is that it is kept constant along the length of the RFQ. In this way the middle of the cell would have the same geometry all over the RFQ, and an exact cross-section could be defined for the design of tuners, couplers, pumping ports etc.
A decision should be taken on the operational duty cycle of RFQ3, in order to assess the need of cooling channels.
Goal is to wrap up the beam dynami=cs and RF design of RFQ3 by the end of the year, ready for the internal review scheduled for March 2022.
The next meeting is planned for Thursday, November 4th.
Minutes by G Bellodi
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