  The undulator is a critical component to produce synchrotron radiation and
free electron laser. When a Bessel light beam carrying the orbit angular
momentum co-propagates with an electron beam, a net transverse deflection force
will be subjected to the electron beam. As a result of dephasing effect, the
deflection force will oscillate acting as an undulator. For such a laser based
undulator, the period length can reach sub-millimeter level, which will greatly
reduce the electron energy for the required X-ray production.
