Our group has been carrying out Research & Development dedicated to radiofrequency (RF) activities for more than 10 years. For example, we have developped the prototype of an accelerating structure using side coupled cavities operating at room temperature within the framework of the european program CARE/HIPPI (FP6). Then within the FP7 program EUCARD, we were in charge of the network activity RFTech aiming to foster exchanges between experts in the field of RF for accelerators.
Presently, our group is carrying out R&D activities to understand parasitic effetcs which occurs in radiofrequency devices such as, accelerating cavities or power couplers, namely the “Multipactor”. The Multipactor effect is a resonant phenomenon which involves a large number of low energy electrons (from a few eV to 1 keV). It generally appears in RF fields resonators under vacuum. The apparition mechanism is the following: a primary electron is accelerated by the RF field and impacts a surface. Depending on its energy and angle, it releases one or more secondary electrons into the vacuum. Depending on their velocity and the timing, these electrons can also be accelerated and reproduce the same process. The phenomenon can grow exponentially and may lead to operational problems of the RF system such as damage of RF components or loss/distortion of the RF signal and consequently produce beam instabilities in the accelerator.
Some kinds of “recipe” exist to process the Multipactor (power coupler conditioning principle) and some codes enable to approach the mechanism for specific cases. But it is a very complex physical process which depends on various parameters : surface roughness, the material type, secondary emission coefficient. It is therefore very difficult to predict and describe it with general laws. To impove our knowledge on the subject, we are woking in the development of an experimental bench dedicated to the study of the Multipactor effect. This test bench will consist in a resonating structure which has a specific geometry to favor the apparition of multipactor. With these studies we aim at having a better understanding of this effect to improve the performances of future accelerators.