Study on modifying IPHI's RFQ for the SPL project

As part of the CERN/CEA collaboration on the IPHI (High Intensity Proton Injector) project, we are studying the possibility of modifying the Radio Frequency Quadrupole (RFQ) to meet the needs of the SPL (Super Proton Linac) project. These requirements translate into the construction of a low-energy demonstrator including a transmission line with chopper. The modular structure of IPHI's RFQ allows the insertion of this chopper line with beam re-acceleration up to 5 MeV (RFQ1 + chopper line + RFQ2). The choice of chopper line energy depends on a number of parameters, including beam rigidity, space loading (from 25 to 100 mA), structure activation, and the design of the deflected beam stop block. The output energy of IPHI's RFQ is 5 MeV. This energy is too high for a chopper operating at voltages below 1 kV, as the beam is too rigid to be effectively deflected. An energy in the 1 to 3 MeV range avoids this problem. Lower energies are not permitted, given the location of the RF inputs to this RFQ (fourth and seventh sections). The cavity is made up of 8 x 1 m sections, forming 4 x 2 m radio frequency segments. As the first three sections are not powered, the first possible cut-off is at 4 m, corresponding to an energy of around 1 MeV. As the average accelerator gradient is 1 MV/m, it is also possible to consider cuts at 5 and 6 m, i.e. at 2 and 3 MeV respectively. This study consists of a non-exhaustive inventory of the disadvantages and advantages of each of these options, after a brief review of the RFQ of the IPHI project. This inventory is based on simulations of each option for a current of 25 mA, the 100 mA case being merely a breakdown of the IPHI RFQ. The study shows that the 2 MeV case is the most favorable in terms of power distribution. If the injector is to be operated at its maximum potential, no klystron is required. In terms of rigidity and power deposited on the stop block, this case is obviously intermediate between the 1 and 3 MeV cases. This option also avoids activation of RFQ1 and the chopper line. Maintenance and availability of the injector are thus greatly facilitated, in a first phase where only these two elements would be present (no RFQ2). In a second phase, the installation of RFQ2 would enable the chopper line to be validated by re-accelerating the beam up to 5 MeV. It would then be possible to reuse the diagnostic line. In a second phase, the installation of RFQ2 would enable the chopper line to be validated by re-accelerating the beam up to 5 MeV. It would then be possible to reuse the IPHI diagnostic line designed by IPN Orsay. The main drawback of this option is the additional cost it generates by remanufacturing the ends of both cavities. This re-machining does, however, create an end section that will make it easier to adapt the beam to the chopper line. End-to-end simulations (RFQ1 + chopper line + RFQ2) for currents ranging from 25 to 100 mA would complement this study