  We presented a novel concept of longitudinal bunch train compression capable
of manipulating relativistic electron beam in range of hundreds of meters. This
concept has the potential to compress the electron beam with a high ratio and
raise its power to an ultrahigh level. The method utilizes the spiral motion of
electrons in a uniform magnetic field to fold hundreds-of-meters-long
trajectories into a compact set-up. The interval between bunches can be
adjusted by modulating their sprial movement. The method is explored both
analytically and numerically. Compared to set-up of similar size, such as
chicane, this method can compress bunches at distinct larger scales and higher
intensities, opening up new possibilities for generating beam with ultra-large
energy storage.
