Calculation of two-screen emittance measurement

The two-screen measurement of emittance is a standard method often used by many accelerator laboratories in the world. In this method forming a beam waist at the location of the screen is a necessary condition. But in many cases it is very difficult to satisfy this condition. Instead, one often uses an approximate method, in which the beam spot at the screen is focused as small as possible (beam spot minimizing). Then the minimum beam spot seems as a beam waist. In this present paper, we give a new two-screen emittance measurement formula which corresponds directly to the condition “beam spot minimizing”. Thus, the approximation problem is solved completely.