Experimental and numerical study of the effect of dc electric field on the multipactor discharge threshold of Parallel-Plate Waveguide

International audienceThe multipactor effect is an electron avalanche like phenomenon that can occur in microwave components when electrons are accelerated by radio-frequency (RF) fields in the vacuum. This effect results from the synchronism between the RF-wave and the electron emission phenomenon that may lead to an avalanche-like increase of the electron density. Multipactor is considered as an adverse effect in many applications such as in high particle accelerator technology, fusion and in particular for space telecommunication applications. The multipactor discharge can lead to increase the system noise level and rise locally the temperature and the pressure. Local pressure rising can also appear driving to corona discharge and electrical breakdowns that can in turn, damage the RF components. The multipactor is nowadays well understood and is well modelled. A benchmark work8 comparing multipactor simulations codes and multipactor test results on a variety of microwave components has demonstrated the consistency of the multipactor threshold predictions with multipactor simulation codes when the critical gap is composed by electrical conductive materials only (such as metals), under the condition that the Total Electron Emission yield (TEEY) models are accurate enough compared to experiments. The TEEY is the number of the whole electrons emitted by the surface (including backscattered and secondary electrons) divided by the number of the incident electrons. When the critical gap contains dielectric surfaces, the situation becomes much more complex. Indeed, electrical charge build-up may occur due to extrinsic effects such as triboelectrifiaction, space radiations or due to the multipactor avalanche itself. The generated dc electric fields affect both the secondary electron emission processes into the dielectric and the trajectories of the emitted electrons into the vacuum gap of the microwave component. Dielectrics are more and more used in RF onboard satellites to reduce the dimensions and the weight of the microwave-components, even if that the effects of charge on multipactor are still not well understood. As the consequence, high RF power margin is applied for spacecraft microwave-components including dielectrics. Previous experimental works, indicated that the dc electric field applied or generated by the trapped charge into the dielectric affects the multipactor dynamics and the power threshold substantially. An applied dc electric field was already used in coaxial line as a method to induce multipactor phase instabilities and hence increase the multipactor power threshold , , .The effect of the dc bias is experimentally attested for both positive and negative polarities in coaxial geometry . Coves et al have modeled the multipactor discharge in parallel-plate dielectric loaded waveguide and demonstrated that under certain circumstances the static field generated by the multipactor turnoff the electron avalanche. More recently theoretical works , , expected that in parallel-plate geometry with small gap the multipactor voltage (or power) varies as function of the deposited charge (or applied dc electric field) an almost manner as symmetrical or close-symmetrical concave up curve with often a minimum threshold at zero deposited charge. Even if multipactor simulation and the experimental results seem to agree qualitatively, it is important to quantitatively compare modelling and experiment on the same device (geometry and TEEY) and at the same frequency. The aim of this contribution is to report results on the of both experiments and multipactor numerical calculation that have been carried out to study the effect of the dc electric field on the multipactor threshold of small gap Parallel–Plate Waveguide in Ku band