Definition of a flux

Hello everyone,

I've a little problem with the definition of my flux, in fact I used the rectangle function to make my flux periodic but after the computing, when I draw my flux (flux in function of the time), it seems that the during of the pulse is already defined and my question was how can I choose the time of my pulse and rest ? I'm not sure I was really clear but I hope you'll can help me.

Cordially,

Samy