Quadrupole Scanning

The emittance of the electron beam can be calculated using the transverse spot size information and known Twiss parameters of the electron beam. At PBPL, the quadrupole scanning method is the preferred way of measuring emittance.

A quad scan consists of a quadrupole magnet (with focal length f in the thin lens approx.), a drift (length d), and a screen (BPM to measure the spot size). The beam size is calculated by carrying out the matrix multiplication of this arrangement (quad-drift) and is given by
.
A minimum of three measurements, for varying strengths of the quad, is used to fit a parabola and solve for the emittance, .