E.~Soliman, K.~Hofmann, H.~Reeg, and M.~Schwickert, "Sensor Studies for DC Current Transformer Application", in Proc. 3rd International Beam Instrumentation Conference, Monterey, CA, USA, paper WEPF31, pp. 624–628, ISBN: 978-3-95450-141-0, \url{http://jacow.org/IBIC2014/papers/wepf31.pdf}, 2014.