Experimental studies of electron emission properties under magnetic field for copper samples: effect of the surface morphology
Résumé
Multipactor is a resonance effect between the RF electric field and the motion of the electrons which can decrease the performance of Radio-Frequency (RF) systems functioning under vacuum [1]. It highly depends on the electron emission properties of the RF component materials. Applications using these kind of components are telecommunication satellite [2], fusion experimental reactors with Tokamak [3] or particle accelerators [4] among others. Many experimental [5] and theoretical [6]–[9] works have been conducted to study this undesirable phenomenon. The aim of these approaches is to determine the multipactor power threshold. Above this threshold, multipactor can appears and eventually damage RF systems. In some applications concerned by the multipactor effect, RF components are submitted to DC magnetic fields. For instance, in telecommunication satellite, magnetic fields of a few tenths of Tesla produced with permanent magnets are used in circulators and isolators. In fusion reactors, rectangular copper waveguides are located under intense magnetic fields of few Tesla generated by toroidal and poloidal coils. Multipactor simulations codes are used to calculate the threshold that would trigger the electron density growth by following the electrons under the RF wave electromagnetic field [7], [8]. Multipactor modeling can also take into account an external magnetic field, which induces gyratory motions of electrons within the simulated RF structure. Studies have been made on the effect of external magnetic field on multipactor discharge [10]. However, to our knowledge, there are no multipactor simulation codes which consider the influence of DC magnetic field on the electron emission process itself. Electron emission due to an incident electron beam is a phenomenon which lies on the surface first tens of nanometer [11]. Then the Total Electron Emission Yield (TEEY) highly depends on surface condition (topography, contaminants adsorbed) and surface treatment (drying, irradiation, erosion) [11]–[15]. The presence of a uniform DC magnetic field influences the electrons trajectories by giving them cylindrical helix trajectories depending on the direction of the magnetic field as well as on the energy of the electrons. To study the effect of the magnetic field on the TEEY, a new experimental setup has been developed. A special attention to the design of the experimental setup and to the choice of the measurement methodology has been taken to circumvent the possible artefacts that are related to the high sensitivity of incoming and emitted electrons trajectories to the DC magnetic field. The new developed experimental setup and the measurement methods are described in details in [16] as well as the validation procedure of the TEEY measurement methodology based on both experiments and modelling with SPIS code [17]. In this paper TEEY measurements on copper under a DC magnetic field perpendicular to the sample surface are presented. We have studied various surface morphologies such as laminated and polished and have observed TEEY increase as well as decrease depending on the magnetic field amplitude and the surface morphology [18]. With incident electron at first cross-over energy (EC1), DC magnetic field has a greater influence on the laminate surface than the polished one (respectively TEEY decreased to 45% and 5%). Such impact of the magnetic field is discussed in regards of multipactor effect simulations codes.
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