SUPER COUPLING CURRENTS IN RUTHERFORD TYPE OF CABLES DUE TO LONGITUDINAL NON-HOMOGENEITIES OF DB/DT

In this paper it is shown that non-homogeneities in the field sweep rate dB/dt along the length of a Rutherford cable provoke a non-homogeneous current distribution during a field sweep, This process can be described by means of Super Coupling Currents (SCC's) flowing through the strands over lengths far larger than the cable pitch. These SCC's can be characterised by a characteristic length, a characteristic time, and a propagation velocity. The dependence of these three parameters on the strand resistance and the contact resistance between strands will be illustrated, Two longitudinal non-homogeneities in dB/dt are considered which are present in accelerator magnets. Firstly, an increase in dB/dt from 0 to a certain value simulating that part of the cable where the cable enters the magnet field. Secondly, a longitudinal decrease in dB/dt which occurs mainly in the heads of the magnet. It is shown that in accelerator magnets a non-homogeneous current distribution induced by the field sweep can not be avoided, However, it seems to be very difficult to estimate the amplitude of the effect.