The CEA teams in collaboration with those of ESS worked for many months on the conditioning of the RFQ delivered to ESS in 2019. On 28 July 2021, the conditioning was successfully completed with 110% of the nominal operating power and the beam passed through the RFQ for the first time on 26 November.
The conditioning of the RFQ marks the end of a multi-year process at CEA for this central component of the future ESS linear accelerator in Sweden.
The ESS RFQ ("radio frequency quadrupole") is a component of the ESS linear accelerator in Sweden in which radio frequency electric fields are used to focus, group and accelerate a flow of charged particles. The five sections of the ESS RFQ were built in three years by CEA with the companies Mécachrome and AMD, then delivered, assembled in the accelerator tunnel and tested by CEA teams in late 2019.
In order to obtain an operational RFQ, it must be "conditioned". Conditioning is the process of progressively increasing the RF ("Radio Frequency") power in the RFQ to the 730 kW necessary power to reach the nominal accelerator voltage required by the proton beam.
A small amount of power (a few kW) is applied at the beginning of the conditioning process. This power generates electronic activity in the RFQ partly due to contamination of the internal copper surfaces by the ambient air. As the RF power increases, this electronic activity gradually cleans the surfaces. The induced outgassing is then collected by the vacuum system. At the same time, the RFQ cooling system ensures that the RFQ temperature is stable in order to avoid deformations of the cavity that would lead to a change in the RFQ frequency.
The CEA teams, in collaboration with the ESS teams, started the conditioning in June 2021, and on 28 July, the conditioning of the RFQ at the ESS linear accelerator in Sweden was successfully completed. After 7 weeks, the RFQ has been conditioned up to 110% of nominal operating power and has achieved 96% availability in the final stages of stable operation.
Target: 729 kW, 3.2 ms pulse, 14 Hz frequency.
Achieved: 800 kW, 3.2 ms pulse, 14 Hz frequency.
Following the conditioning, ESS teams supported by CEA experts successfully sent and accelerated a proton beam in the RFQ for the first time on 26 November 2021. The first measurements show a transmission of over 90%.
ESS must now continue the commissioning of the RFQ, which consists of progressively increasing the beam parameters at the RFQ entrance to the nominal current corresponding to a beam power of around 250 kW.