SRF Linac

The Superconducting Radio Frequency LINAC (SRF Linac)

The high-energy section of LIPAc accelerates the beam to 9 MeV and consists of the first of the four cryomodules of the IFMIF SRF Linac, housing eight superconducting 175 MHz half-wave resonators (HWR) and eight superconducting solenoid packages. The main cavity and magnet parameters are listed in the following table:

Frequency 175 MHz
Cavity b 0.094
Accelerating field Eacc 4.5 MV/m
Quality factor Qo 1.4 109
Max. forward Power / coupler 200 kW
Max. Tuning range ±50 kHz
Beam aperture cavity/solenoid 40/50 mm
Magnetic field Bz on axis 6 T
Field at cavity flange ≤20 mT

Two prototypical resonant cavities have been fabricated. The original design of the tuner relied on a capacitive plunger with a large membrane to allow an elastic deformation of ±1 mm. Tests at cryogenic temperature showed a low Qo, as well as a quench at low field, pointing to a suspect NbTi plunger. New tests performed removing the plunger have confirmed the validity of the cavity design. As a result, a new design based on a conventional compression tuner principle is under development which leads to a lengthening of the cryomodule to ease the integration of the mechanical tuning mechanism between the HWR tank and the solenoid package.

The RF coupler is composed of a water-cooled Cu antenna, a disk ceramic window, an external conductor made of helium cooled double wall tube and a tee transition between the input coaxial line and the coupler. Two prototype couplers have been fabricated and will be tested on a specific test bench when the 200 kW RF source will be available. The beam focussing and orbit corrections are performed by 8 sets of superconducting solenoids/steerers and beam position monitors, located before each HWR. During the cold tests at 4.2 K performed in a vertical cryostat, the theoretical critical current of the magnet was reached (at about 260 A, well above the nominal current of 210 A) with a very short training. The design of the cryomodule, including the magnetic shielding to protect the HWR from the earth magnetic field, two independent 4.5 K helium circuits to cool down the cavities and the magnets, a helium phase separator and a thermal shield cooled at 60 K by gaseous He, is almost completed.