A New Method to Stably Transport the SEB With a Large Aspect Ratio by PCM With a Higher Order Bias Magnetic Field

The stable transmission of sheet electron beam (SEB) with a large aspect ratio is the key to the success of high-power SEB devices. This article researches the instability of the large-aspect-ratio SEB (LAR-SEB) in the traditional period cusp magnet (PCM). The research reveals that the mismatch between the nonlinear horizontal space charge field of the LAR-SEB and the linear bias magnetic field (
${B} _{y,\text {off}}$
) in the traditional PCM is the main factor for this instability. To solve this issue, this article proposes a new method to confine the LAR-SEB using a PCM with a nonlinear higher order
${B} _{y,\text {off}}$
. The simulation confirms that this constructed magnetic field can effectively suppress the dramatic width fluctuation and thickness expansion, which contributes to the unstable transport of the LAR-SEB. In addition, to generate the required magnetic field, a new magnetic configuration, named PCM with notches (PCM-N), is proposed, in which a set of notches are added to the PCM’s pole pieces to substitute the staggering approach. This article also provides a design method to obtain the notches’ parameters by utilizing a genetic algorithm. Moreover, to verify the performance of the PCM-N, an
${X}$
-band electron optical system (EOS) is built and calculated. The results indicate that PCM-N does generate a higher order
${B} _{y,\text {off}}$
, which is highly matched to the LAR-SEB’s horizontal electric field
${E} _{x}$
, resulting in all particles passing through the long tunnel without any interception. In contrast, the transmission rate by employing the traditional PCM approach is only 68%.