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Hermann gives a first update on the RF design of the RFQ3 vane cross sections.
Several options have been studied and a meeting was held with the MME workshop to discuss feasibility of the geometries and requirements on the brazing/fabrication side.
A first ideal case with a rounded shaped vane was discarded as there is no allowance for rounded surfaces in brazing, unless a major change of procedure is adopted.
The Linac4 RFQ1 shape was then adapted to the new rho/r0 parameters, also relaxing the vane angle to 28 deg (it was fixed to 45deg in the past to allow incorporating in the thick vane a cooling channel for 5% duty cycle). Auxiliaries also set a requirement for an 82mm minimum diameter large intersection, which cannot always be satisfied in case of partially rounded shapes. Some hard-edge corners are also needed below the brazing surface to catch any material leak during brazing so it doesn't reach high Efield areas.
A square SNS-like vane shape with horizontally/vertically mounted auxiliaries (instead of at 45deg) was discarded because this placement excited TEM mode in the gaps and also because this cross section would require significantly smaller inner diameter to get to 352 MHz (hence higher 3D loss).
An HF-RFQ-like shape would lower by 10% the power requirement but could support only one cooling channel per quadrant (hence could only work for a low duty factor).
Four design options were approved by MME in terms of feasibility, with some cavity rounding possible using "lollipop" tools for machining.
Thermal simulations have been started for the different shape designs on the baseline assumption of 85kV vane voltage and 5% duty factor.
An RFQ length of 3.5m would correspond to 4.1lambda, hence one tank with proper dipole detuning would be sufficient.
Standard construction material and assembly procedures have been assumed for this study. However the RF design is not specific to a choice of Copper. If a material with higher field turns out to be suitable in a couple of months, not too much of this RF work will need to be redone.
If alternative materials are identified, the MME workshop will need to be contacted to confirm feasibility of production.Titanium shows a 25% higher gradient and no signs of blistering were observed. Downstream performance changes would need to be studied in detail for this higher field scenario.
Next step will be to move from a 2D to 3D cell simulation. An iteration will then be needed in the RF design for dipole-mode detuning study.
AL will need to check an inconsistency in the beam dynamics simulations between sinusoidal and 2Term vane profiles and she will summarize the final parameters in a couple of tables. A discrepancy between PATH and Parmteq simulation results is currently being investigated.
The next WP11 meeting will be organised in the second half of January 2022.
Minutes by GB, 01/12/2021.
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