The SPIRAL2 (Second Generation System On-Line Production of Radioactive Ions) facility will produce light and heavy exotic nuclei at high intensities by the ISOL (Isotope Separation On-Line) method.
SPIRAL2 wil continue investigations undertaken with SPIRAL1 and will provide better insight into the table of nuclides, thereby fostering the discovery of new properties of matter.
SPIRAL2 will produce deuterons (D+) beams up to 5 mA and 40 MeV, protons (H+) beams between 0.15-5 mA and 0.75 - 40 MeV and heavy ions (q/A=1/3) up to 1 mA and 14.5 MeV/nucleon. These beams will bombard different target and the resulting reactions, such as fission, transfer, fusion, ... will generate billions of new nuclei.
SPIRAL2 consists of two ECR sources (Electronic Cyclotron Resonance) to create the beam; an RF accelerating structure made of copper called radiofrequency quadrupole (RFQ) which gives the first acceleration to the beam and bunches the beam into particle buckets; the beam transport lines and the superconducting linear accelerator (linac) operating at a cryogenic temperature (as low as -269°C). The linac is composed of 26 superconducting cavities (Eacc = 6.5 MV/m @ D+) to accelerate the beam. Each cavity is coupled to a RF power amplifier via a RF power coupler. Our group is responsible for these RF couplers.
Couplers are designed at 88.05 MHz to transfer the power from the amplifiers to the cavities. This power reaches 14 kW for the D+ (5 mA for 6.5 MV/m in a gap of 0.41 m) in continuous wave (in SPIRAL2, the beam run continuously even if the linac can operate in pulsed mode too). The coupler ensure vacuum tightness between the secondary vacuum of the accelerator and the atmospheric pressure by a ceramic window. In addition, the Joule power dissipation must be carefully handled to avoid thermal overload on the superconducting cavities.
In cooperation with the mechanics service (SERM), two different types of ceramic geometry, cylindrical and disc, were investigated in terms of radio-frequency, thermal and mechanical studies. Two prototypes, one for each of these geometries, were built, controlled and tested. Tests included the RF measurement of the transmission and reflected power, the measurement of voltage and current of the coupler until 40 kW continuous wave (CW) and the RF conditioning perfomed to decrease the multipactor effect (electron emission) while maintaining non-harmful currents to the coupler. The test results for the two coupler designs were similar. Finally, the coupler based on a ceramic disc was chosen as it enables easier implentation of diagnostics, such as vacuum gauge and electron pickup antenna to monitor multipactor. To prepare the series of couplers at LPSC, a dedicated test facility was developped (40 kW CW 88.05 MHz amplifier, circulators,...) and a clean room (ISO7) was implemented as the cleanliness of the coupler surface is critical to reach high accelerating field (6.5 MV/m). The RF condtionning process was automated by the electronics and computing services. The preparation protocol was studied to optimize the coupler surface and cleanliness prior to introduction in the superconducting cavities.
The series of the 28 couplers (including spares) is ongoing and is expected to be completed in June 2014.