Dynamic operation of the in-line cryotron in bistable circuits

The dynamic operation of the in-line cryotron is tested by performing pulse measurements on individual devices and also by using the device in free-running closed-loop oscillators (similar to shift register). The gain of the device is currently insufficient to permit bit transfer from one stage of the oscillator to the next in less than 25 nsec. This propagation time is slow when compared to the 10-nsec circuit time constant L/R that was designed for each stage. The slower propagation is shown to be associated with the slow transition of resistance from the superconducting to the normal state and also from the normal to the superconducting state. Pulse measurements on the device indicate that the cryotron switching time is dependent upon the magnitude of the applied magnetic field. The times for switching resistance obtained from the pulse measurements are applied to the analysis of the dynamically operating closed-loop register. The maximum oscillating frequency of 5 Mc for a four-stage closed register was predicted by the analysis and is shown to be in good agreement with the experiment. A similar analysis, using the same cryotron limitations, shows the maximum frequency for a two-stage closed-ring register to be essentially the same as that for a four-stage register. Again, this was verified experimentally. The benefits originally projected for the biased in-line cryotron in comparison to the crossed-film devices appear to have been overestimated. The large incremental gains anticipated are not obtainable when operating the device at frequencies of about 10
⁷
cps or greater. This limitation arises from the requirement of supplying bias at 0.7H
c
and preferably 0.5H
c
to allow for a fast turnoff of the device. Consequently, the static gain is not sufficient to provide a large overdrive for a fast turn on for the device. Thermal considerations, state-of-the-art fabrication, and testing procedures are discussed along with projected cryotron improvements that could lead to flip-flop time constants of about 10 nsec.