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WP11 meeting
The meeting starts with an update by Hermann on the RF design of the RFQ3: he carried out simulations with the COMSOL electrostatic solver, which can produce a 3D fieldmap usable for beam tracking and to benchmark the beam dynamics results obtained with Parmteq. Simulations of individual cells with the RF solver confirm the maximum surface field values given by PARI. They also give the Q-factor and quadrupole/dipole capacitance, hence the spectrum of higher order modes. The closest modes obtained in the spectrum to the operative one are a quadrupolar mode (+2.7MHz) and a dipole mode (+2.5MHz). In the RFQ1 and RFQ2 design, dipole rods have been used to move the higher order modes and make them less dangerous but this solution should be avoided for RFQ3. Another way of detuning is by adjusting the RFQ structure length. Assuming everything else stays the same, in order to have a more symmetric dipole arrangement, the total length would either have to be reduced by 30cm (9% or 9 cells) or increased to 4.5m (+30% or +33 cells).
AL comments that an alternative to changing the length of the RFQ is to try different designs (different values of rho/r0), i.e. change capacitance to fall in easier parameter space situation.By making the length 40cm shorter one would need to move to higher fields or accept a compromised dynamics.This also makes the design closer to the present linac4 RFQ1.By changing the rho/r0 parameter, the RFQ modulation is changed and the vane profiles would need to be recalculated.
AL gives an update on the RFQ3 beam dynamics studies. Small geometry changes have been implemented since last time, after several exchanges with Hermann, but the main parameters have stayed constant (R0=0.33cm, rho=0.28cm, L=345cm and V=85kV). A cernbox folder with the reference files has been created. The guidelines design principles with respect to RFQ1 are substantially: 1) bigger aperture and easier modulation, and 2) higher vane voltage, same surface field and longer RFQ (yielding stronger transverse focusing and higher transverse acceptance). Transmission for a design input beam of 70mA current and 0.5mm mrad emittance is 93.6%. Next steps will be to generate a field map to run tracking simulations with an input beam distribution measured at the test stand. Error studies should also be carried out to assess the design tolerance to field flatness (tuning), jitter (klystron regulation), alignment between sections and machining accuracy.
AG comments that this RFQ3 design should be compared to RFQ1 by simulating performance when tracking a measured beam input distribution. Tolerance studies could be done at a later stage, though it is important to make sure that this new design is not too sensitive to certain parameters variations.
AL confirms that RFQ1 has an acceptance factor of 1.7 for a nominal beam emittance of 0.25 mm mrad, while RFQ3 has a factor > 2.
A Grudiev gives a presentation on the Efield limit of recently tested materials. In DC setup 80MV/m can be reached by the tested OFE-Cu samples with and without prior irradiation (showing there is no significant impact of blistering on the E-field limit). With Ti samples a 30% higher surface field can be reached (105MV/m in DC mode, or 45 MV/m in L4-RFQ setup): what do we gain if we increase the field wrt to Cu-only RFQ? what are the possible advantages to beam dynamics? In literature it has also been shown that cooling down Cu from 300K to 45K results in an increased E-field limit of ~45% (in this case it would mean an increase from 34 to 50 MV/m). AG proposes to study alternative RFQ3 designs for a Ti option at 45MV/m or for a cryo-Cu option with E-field limit of 50MV/m to compare with the baseline Cu performance. It is important to understand potential benefits coming from higher E-field limit.
SR comments that cryo-cooled Cu is a significant complication in terms of RFQ fabrication.
AG comments that the studies would be interesting not only to understand potential benefits for higher beam quality in Linac4 performance, but also in a broader horizon of longer term future R&D, depending on the requests for higher brightness proton beams operation (FCC?).
AG adds that in any case RF testing would be needed before building RFQ in a new material. A small prototype should be built first to judge additional complications in fabrication, and RF performance. Concerning Titanium, irradiated samples have shown absence of blistering (hence potentially more resilience to beam losses). However Ti has very low conductivity which makes assembly more complicated.
The next meeting is scheduled for February 17th.
Minutes by GB, 27/01/2022.
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