An Observation of a Transverse to Longitudinal Emittance Exchange at the Fermilab A0 Photoinjector

An experimental program to perform a proof of principle of transverse to longitudinal emittance exchange (ε_x,in ↔ ε_z,out and ε_z,in ↔ ε_x,out) has been developed at the Fermilab A0 Photoinjector. A new beamline, including two magnetic dogleg channels and a TM110 deflecting mode radio frequency cavity, were constructed for the emittance exchange experiment. The first priority was a measurement of the Emittance Exchange beamline transport matrix. The method of difference orbits was used to measure the transport matrix. Through varying individual beam input vector elements, such as x_in, x′_in, y_in, y′_in, z_in, or δ_in, and measuring the changes in all of the beam output vector's elements, x_out, x′_out, y_out, y′_out, z_out, δ_out, the full 6×6 transport matrix was measured. The measured emittance exchange transport matrix was in overall good agreement with our calculated transport matrix. A direct observation of an emittance exchange was performed by measuring the electron beam's characteristics before and after the emittance exchange beamline. Operating with a 14.3 MeV, 250 pC electron bunch, ε_z,in of 21.1 ± 1.5 mm·mrad was observed to be exchanged with ε_x,out of 20.8 ± 2.00 mm·mrad. Diagnostic limitations in the ε_z,out measurement did not account for an energy-time correlation, thus potentially returning values larger than the actual longitudinal emittance. The ε_x,in of 4.67 ± 0.22 mm·mrad was observed to be exchanged with ε_z,out of 7.06 ± 0.43 mm·mrad. The apparent ε_z,out growth is consistent with calculated values in which the correlation term is neglected.