An electron flat beam production mechanism in a photo RF gun linac is investigated and a flat beam emittance compensation mechanism is found. The two constants of electrons motions in the solenoid field are determined. A three-skew-quadruple channel maps the two motion constants into the emittances in the x and y planes of the decoupled beam. A theoretical model was built to understand the experiments at Fermilab A0 beamline. Particle acceleration and a non-uniform solenoidal field are included. The space charge effects are considered qualitatively and analyzed in simulations. In the magnetic field free region before the adaptor, the constants of motion and angular momentum measurement were proposed and performed in experiments. The S-shaped distribution of the flat beam, found both in experiments and simulations, is explained. And a flat beam emittance compensation mechanism is proposed to cure the S-shaped distribution and decrease the flat beam emittance.
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