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Small, stray magnetic and RF fields in electron guns and injectors canperturb an electron beam and introduce correlations between the otherwiseorthogonal transverse trajectories. These correlations couple the x and ydynamics which increases the transverse emittance. If the correlation becomes"diluted" or randomized in the beam transport then as the correlationdisappears the 4D emittance increases. This paper discusses two importantcorrelations common to most electron injectors. The first results from thecoupling of a weak quadrupole field with beam rotation in a solenoid, and thesecond x-y coupling is generated by an asymmetric on-axis RF field due to ahigh-power RF coupler or cavity port. This paper shows that a small quadrupolefield combined with solenoidal focusing can result in significant emittancegrowth due to coupled transverse dynamics. It also shows how adding a skewedquadrupole field can exactly cancel this correlation and its emittance. Similaremittance cancellation is demonstrated for asymmetric RF fields, with thedegree of cancellation limited by the electron bunch length. Analyticexpressions are derived and compared with simulations and experiments.

Exact Cancellation of Emittance Due to Coupled Transverse Dynamics in Solenoids and RF Couplers