the modulation section was been changed into current modulation in the drift section and increased electromagnetic radiation is then extracted in the extraction section in another device that is similar to the optical klystron deloney feinstein and pantell in 34 characteristics of the dispersive free electron laser 34 ieee quant electr vol qe 21 pp 1037 1040 july 1985 discribe a dispersive free electron laser which resembles the optical klystron in that it contains three sections that device was designed to increase the allowable energy spread of the input electron beam while still maintaining high gains in the present invention the periodic medium loaded free electron laser can also be divided up into three sections and made into an optical klystron or a dispersive free electron laser this would increase the gain these various embodiments of the optical klystron all have the three sections but uniquely differ in their methods of phase matching the electron beam velocity to the wave velocity as with the vacuum free electron laser the addition of a periodic medium into the optical klystron and the dispersive free electron laser alters the phase matching condition and permits the device to operate in the x ray region of the spectrum the phase matching condition for the present invention uniquely defines these embodiment of.