Description of a Device Used to Measure Ion Beam Emittance

Abstract

The beam emittance was measured by an instrument consisting of a slowly moving sampling aperture located in front of a rapidly moving pickup wire. The position of the aperture and the difference between the position of the wire and that of the aperture were impressed upon the x and y plates, respectively, of a storage oscilloscope. Each time the pickup wire passed through the beam sampled by the aperture, a signal was produced. The width of this signal, which depended upon the convergence or divergence of the beam sampled, was used to pulse-brighten the oscilloscope. (In this way an area was plotted on the oscilloscope which was directly proportional to the beam emittance.)

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“…Beam emittance is defined as the area an ellipse in Rθ phase space containing all particles in the beam at any given point along the beam axis where R is the radius of a particle in the beam, θ is the angle dR/dz of the particle trajectory, and z is the beam axis coordinate [12]. The emittance area is invariant as the beam is transported which means that particle trajectories become more parallel as the beam diameter is made larger.…”