INTRODUCTION
The improvement of high-power vacuum microwave sources plays a crucial role in the development of the new generation of high-gradient, high-frequency particle accelerators [1]. As scaling up the microwave sources to higher frequencies, a significant difficulty is the need to transport intense beams through decreasing aperture sizes, because the RF circuit dimensions decrease with the wavelength. In this regard, the use of sheet electron beams seems a promising concept, since larger amounts of current can be transported at lower current densities by increasing the width of the beam, while keeping its height of the order the RF wavelength [2]–[4]. The main drawback on the use of sheet beams in comparison to the usual round beam is that the sheet beam may be more vulnerable to some instabilities in ordinary solenoidal focusing channels [5].