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We performed a combined secondary electron yield and x-ray photoelectron spectroscopy study on a prototypical system formed by increasing coverages of amorphous carbon (a-C) deposited on atomically clean Cu. A remarkably thin a-C layer, of about 6-8 nm, is surprisingly enough to lower below 1 the secondary emission yield of the whole system. This feature qualifies such low thickness coatings as a optimal multipacting suppressor that will not significantly affect impedance issues. The concomitant reduction of surface conductivity observed after antimultipacting coating is, in fact, a major drawback, reducing its applicability in many research fields. The consequences of this observation are discussed mainly for a-C coating applications to mitigate detrimental multipacting effects in radio-frequency devices and accelerators, but are expected to be of interest for other research fields and to hold for other conductive substrates and overlayers.

This paper presents a novel strategy to implement a CAPTCHA (Completely Automated Public Turing test to tell Computers and Humans Apart). The aim of these tests is to easily and reliably distinguish between real human users and (malicious) bots. The approach underlying FATCHA is to exploit real time capture of human actions instead of human ability to recognize visual or auditory items. The latter approach explicitly requires proposing a challenge difficult for an automatic responder but easy for a human. However, it is often the case that pursuing the first feature takes to lose the second one. Moreover the user task may be hindered by specific disabilities. According to FATCHA approach the system rather asks the user to carry out some trivial gesture, e.g., rotating or moving the head. The webcam, which is available in almost all computers or mobile devices, captures the user gesture, and the server (hosting the service to protect) matches it with the requested one. It is possible to extend the service with a second module that allows the user to authenticate himself by face recognition instead of using a password. On the contrary, FATCHA gesture challenge can be used as a liveliness test to avoid biometric spoofing. Multimodal interaction is the base for both an advanced Human Interactive Proof (HIP) test and for robust/comfortable authentication. © 2016 Springer Science+Business Media New Yor

In this paper, we propose an innovative type of CAPTCHA (Completely Automated Public Turing test to tell Computers and Humans Apart). These tests are used to allow a service to discriminate human users from (malicious) bots. With FATCHA, the user is simply asked to perform at random some trivial gesture, e.g., moving the head, which will be captured by the computer webcam and recognized by the server hosting the service. A second module in a possible composite service allows the user to authenticate by face recognition instead of using a password. In this way we significantly exploit the potentiality of multimodal interaction for both an advanced Human Interactive Proof (HIP) test and for robust/comfortable authentication.