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It is well known that beam dispersion , along with the
Twiss parameters and emittance, contributes to the beam
spot size. So that, in general, anomalous dispersion is an
undesirable event and must be minimized by careful tuning
the machine. If not, when the spot size is used to infer
beam emittances, as it is the case of the "quadrupole
scan" method, basically employed at TTF, the unknown
dispersion can lead to overestimated values for the emittance.
This paper presents the first attempt to determine the
dispersion function at several points of the TTF Linac and
to separate its contribution to the local emittance measurement,
performed by means of the OTR imaging technique.
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