Experimental apparatus for tests with high electric fields generated by focusing high intensity THz radiation produced at ISIR (Osaka). The red laser spot indicates the region where the surface is irradiated. [courtesy of A. Irizawa – progetto MAECI]NUCLEAAR is one of the projects of the Research and Development (R&D) section of the INFN Vth Committee, which addresses the scientific and technological challenges necessary to pushing forward the next research frontiers on accelerators. The developments are mainly oriented to technological and industrial applications of accelerators, of the future colliders, and of the next generation of accelerators after LHC, which is under discussion during these months in the European Particle Physics Strategy Form.
In this context the aim of the NUCLEAAR project is the investigation of specific coatings to improve the performance of accelerating cavities. In order to design powerful and more compact accelerators, it is necessary to increase the intensity of the electromagnetic fields at which all accelerating devices operate. The present copper-based technology is limited by breakdown phenomena that damage the surface of the RF cavities operating at high electrical gradients (100-200 MV/m) for long time.
NUCEAAR is studying the properties of transition metal oxide coatings, such as MoO3 to improve the thermo-mechanical resistance properties of copper. The transition metal oxides – such as MoO3 or VO2 – are in fact characterized by a high work function, the parameter that controls the emission of electrons from the surface. It also affects the breakdown phenomena that inevitably occur within an RF cavity. Oxides are materials also characterized by a mechanical strength, certainly superior to copper, and this can in turn increase the resistance to damage of the surfaces inside a cavity.
Tests carried out on copper surfaces covered with metal oxides showed very promising results. These films exhibit a work function higher than copper, and a higher resistance to breakdowns induced by electric fields of the order of GV/m. The deposition of a thin MoO3 film on curved copper surface to assemble a cylindrical cavity is in progress. Tests will be performed to characterize the properties of this prototype of RF copper cavity.
Two images of a RF copper structure coated with an Au thin film. The structure subjected to high electric fields after an operation period shows very clear signs of damage that decrease efficiency over time. [courtesy of V. Dolgashev, SLAC]. (G. Gatti, et al., X-band accelerator structures: on going R&D at the INFN, Nuclear Instr. Meth. A 829, 2016, 206-212)
(left) Image of a copper substrate of roughness ~ 30 nm irradiated at ISIR (Osaka) with 5000 pulses of THz radiation at the angle of incidence of 30° where the damage has dimensions of ~ 400 μm; (right) Scanning Electron Microscopy image of the central region (~ 200 μm) present in the left image. Here the formation of copper oxide structures is evident after of many small discharges induced by an electric field of a few GV/m.Image obtained with a Raman microscope highlighting the damage generated on the copper surface by pulsed THz radiation. The distribution of colors shows the different concentration of copper oxide (Cu2O) on the irradiated surface.
Photograph of a curved copper element with a thin coating of MoO3 on its inner surface, ready for the irradiation tests. The possibility of assembling together these curved elements will allow testing for the first time new types of RF cavities.
A schematic representation of the work function of a metallic surface. The height (measured in eV) and the shape of the barrier (blue line) control the probability that an electron inside the material may escape from the metallic surface by “tunnel effect”.
INFN-LNF Laboratori Nazionali di Frascati
