In this paper, we analyzed the physical essence for producing a series of step structures or oscillating effect of dc Josephson current within one flux quantum period φ0 by placing Josephson junction, two-junction SQUID and the supercon ducting ring containing a single junction in a cavity, when one of the eigen-frequencies ωr of which is equal to Josephson frequency and the Q-Value is sufficiently high. We point out that the frequency effect of the feedback field would produce a series of induced steps for Josephson junction or two-junction SQUID, as ω = ωr, the oscillating electric-magnetic field excited by the junction current reacts on the junctions themself. Furthermore, a series of new step structures should occur within the φ0 since the amplitude of the feedback field is modulated by applied magnetic field as the modulation of the amplitude of the feedback field is coupled with that of the radiation. For the superconducting ring containing a single junction, as nω=ωr, the feedback radiation of stationary electric-magnetic wave will give rise constrained oscillation of the supereurrent with a smaller period than the φ0.
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