Emittance Compensation in Flat Beam Production in an RF Gun Linac

Fermilab/NICADD Photoinjector Laboratory has performed a lot of experiments on flat beam production with Ya. Derbenev proposed skew quadrupole channel method [1]. A ratio of 50 of the transverse emittances in X and Y planes has been achieved. In both experiments and simulations with PARMELA [2], the resulted flat beam shows an S-shaped distribution in X-Y space. The sources of the S distribution has been investigated in this paper. A compensation method was suggested to straighten the S distribution and improve the emittance ratio. With this method, an extra solenoid located before the superconducting cavity is added to adjust the beam transverse size when the beam passes through the cavity. PARMELA simulations have shown that, with proper setup of this extra solenoid, the emittance ratio can be enhanced by a factor of about 1.6 and the S-shaped transverse distribution can also be greatly removed.