Accelerating field step-up transformer in wake-field accelerators

The authors present details of a scheme by which the RF power is generated in a large-diameter wake-field tube, where deflection mode generation by the intense drive beam is tolerable, and then fed into a small-diameter acceleration tube where the less-intense witness beam is accelerated by the greatly enhanced axial electric field. The witness beam generates little deflection-mode power itself, even in the small acceleration tube; thus, a final high-quality, high-energy electron beam is produced. It is pointed out that the RF gymnastics illustrated result in nominally a factor of two enhancement of the ultimate accelerating electric field as compared to the case where the drive beam is sent directly into the small-diameter structure. The principal advantage of the step-up transformer is the large reduction of deflection-mode worries and allowance of larger-diameter (weaker focused) drive beams. The acceleration parameters were examined, and transformer, matching and optimization are discussed.<>