Division of accelerators, cryogenics and magnetism

The linear accelerator in the PIP-II project consists of:

• a 2.1 MeV injector consisting of a source of 30 keV protons (H-), a low-energy line, an RF cavity operating at a frequency of 162.5 MHz and a medium-energy line for injecting into the superconducting linac ;
• a main linac that is approximately 180-meter long, comprising 5 families of superconducting cavities housed in cryomodules. These different families are designed to optimise proton acceleration in the 2.1 MeV - 833 MeV range: 
  • first, a cryomodule with half-wave cavities (HWR) at 162.5 MHz accelerating to 10.3 MeV,
  • next, a family of two types of cryomodules housing the so-called Spoke cavities at 325 MHz (SSR1, SSR2) accelerating to 185 MeV,
  • lastly, a family of two types of cryomodules housing the elliptical cavities at 650 MHz to reach the final energy.

The LB650 cryomodule, a contribution from CEA/IRFU

The LB650 cryomodules is the fourth type of cryomodule in the superconducting section of the PIP-II linear accelerator. Each type incorporates 4 superconducting β=0.61 cavities at a frequency of 650 MHz. These cavities are in-kind contributions from INFN (Italy) and VECC (India), while the frequency tuning systems and power couplers are supplied by Fermilab.
A cryomodule is a complex mechanical and cryogenic system used to accelerate the proton beam. It has two main functions:

• Cool down and keep the superconducting cavities at their operating temperature  (1.8 K or -271.35°C) while limiting the heat loads to be as low as possible (thermal efficiency); 
• Provide mechanical housing for the superconducting cavities with respect to the alignment requirements of the beam dynamics.

Mechanical and thermal functions are very often conflicting, so the design of such a system is a compromise between them.

Close collaboration between CEA/Irfu and Fermilab

Since the LB650 (Low beta) cryomodule is very similar to the HB650 (High beta) cryomodule developed by Fermilab, it was important for the CEA/Irfu to follow the HB650’s development in order to gain as much experience as possible — mainly with the strongback, which is a new way of supporting cavities compared with previous cryomodules developed by CEA/Irfu. 

• Firstly, the CEA/Irfu was involved in the international team designing the HB650 cryomodule, specifically responsible for the mechanical and thermal design of the strongback. 
• Secondly, the CEA/Irfu assembly team closely followed the assembly of the HB650 prototype cryomodule at Fermilab by carrying out a number of observation trips throughout 2022.
• Finally, the CEA/Irfu cryomodule testing team went to Fermilab in April 2023 to observe the cold RF testing of the HB650 prototype cryomodule.

Feedback from the assembly was implemented on the design of the LB650 cryomodule during the first quarter of 2023. The Final Design Review was held at the beginning of April, in which a committee of five international experts critically examined the cryomodule design and the organisation of the project at the CEA. At the end of this review, the committee considered the design of the LB650 cryomodule to be sufficiently mature. This important milestone for the CEA/IRFU ends the design phase and launches the construction phase of the pre-production cryomodule.

The procurement of the components of the cryomodules started mid-2023. The first contracts for the manufacturing will be awarded at the beginning of 2024. 
In 2022, the contract for the procurement of four 19 kW solid-state RF amplifiers was awarded to Jema France, and the contract to replace the current cold box (used to produce liquid helium) to Air Liquide.
Assembly of the pre-production cryomodule should begin at the end of 2024 or beginning of 2025. The commissioning of the PIP-II accelerator at Fermilab is currently scheduled for late 2028.
 

Contact: Nicolas Bazin