Beam focusing by decelerating RF fields in a drift tube linac

A method of beam focusing by two-electrode RF crossed lenses with decelerating fields is proposed. The lenses are arranged in accelerating gaps of a drift tube linac. The crossed lens is a set of plane electrodes with rectangular apertures such that the apertures in the neighbouring electrodes are rotated 90° each other. Different variants of focusing periods with the RF crossed lenses are considered. The βλ FD period is shown to be used for a low-energy part of the linac. The transverse phase advance for the FD period is independent on a particle phase when a synchronous harmonics of the accelerating field is absent and is analogous to a RFQ with unmodulated vanes in the case. In the main part of linac it is worth to change to the 2βλ FDOO or 3βλ FOODOO period to obtain essentially higher acceleration rate.