by Dr. Adnan Ghribi, ghribi@ganil.fr

Over the last 10 years, a multidisciplinary team from institutes around the world has worked to design, fabricate and assemble SPIRAL2, a new generation heavy ion superconducting linear accelerator (LINAC) at the Grand Accélérateur National d’Ions Lourds (GANIL) facility in Caen, France. A detailed introduction to the project is provided in S. Gales, “GANIL-SPIRAL2: A New Era,” Journal of Physics: Conference Series, Vol. 267, 2011.

Engineers are now commissioning the machine’s different technology blocks. The core accelerating technology of this new physics instrument is based on 26 bulk niobium superconducting RF resonating cavities. Cryogenic engineering is crucial here, not only to keep the cavities below their transition temperature but also to maintain stable operation conditions for beam acceleration at the required energies.

The refrigeration power for the cryogenics operation of the LINAC is insured by a cryoplant, extracting up to 1,300 watts at -268.8°C (4.2 K) for the cavities and 3,000 watts at -213°C (60 K) for the thermal screens.
In October 2017, the cryogenics team began the first full cooldown of the cavities, and within two weeks all the cavities were at 4.2 K. This was an important achievement that validated the mechanical system, the vacuum system, the control system and the data acquisition system. More information on the system is available in A. Ghribi et al., “Status of the SPIRAL 2 LINAC Cryogenic System,” Cryogenics, Vol. 85, 2017.

After this great success, the cryogenics team and vacuum group at GANIL worked hard to stabilize the pressure and liquid helium level control for every cryomodule. In order to be operational, each cavity needed to be plunged in a liquid helium bath and achieve a pressure stability of better than 0.004 atmospheres.

Pressure fluctuations higher than this requirement led to a slight mechanical deformation of the cavities, in turn detuning the cavities and making them unable to accelerate the beam to the required energies. Dedicated valves were used to control the pressure with stability better than 0.003 atmospheres on all the cavities during twelve consecutive hours.

During the evaluation, several pressure fluctuations appeared. In order to better understand these effects and mitigate them moving forward, two teams at GANIL and the French Alternative Energies and Atomic Energy Commission (CEA) worked on modeling the cryogenic system of SPIRAL2 in the frame of GRAAL (Grande Réfrigération pour les AccélérAteurs Linéaires), a cross institute project aimed at studying and understanding large refrigeration systems for superconducting LINACs.

The result of this work established a model-based control system design that will be tested during summer 2018. The next cooldown will also bring the accelerator closer to real operation conditions, thanks to the control of variable thermal loads on the cavities. If proved and then approved, this model and control method may open a new avenue for high reliability cost-efficient superconducting LINACs. More information will be available soon from the cryogenics team. The region of Normandy as well as the city of Caen, CNRS and CEA, has funded work on the project. I would also like to thank, on behalf of the SPIRAL2 team, all contributors from CEA-IRFU, CNRS-IPNO, CNRS-LPSC, CEA-INAC/SBT, IFJ-PAN (Krakow) and GANIL.