This invention relates to the field of stimulated x-ray emission and, more particularly, to the field of x-ray emission caused by a lasing type of action.
There is a recognized need for a coherent x-ray generator, but the short life of the excited state of atoms which emit x-rays has presented a problem difficult to overcome. Recently, there have been developed techniques for overcoming the effects of very short spontaneous emission lifetimes by causing a population inversion to sweep along the lasing direction at the speed of light. In the copending U.S. application Ser. No. 393,293 of W. B. McKnight et al., filed Aug. 30, 1973 and entitled "X-Ray Laser," there is disclosed a system wherein an ion beam of completely stripped nuclei, such as He.sup.++, Li.sup.+++, etc., is passed through a foil so as to effect resonant charge pickup in an excited state. The stripped nuclei are swept across the foil target at the speed of light. When the beam strikes the foil target, a population inversion in the states between which laser action occurs is established by means of beam foil excitation. The foil is located in one of the two plates which are aligned parallel to the incident direction of the beam. A voltage pulse source is connected to the end of the two plates so as to cause a voltage pulse to travel down the plates at the speed of light. The resultant electric field causes deflection of the beam onto the negative plate where the thin foil is located, so that the beam sweeps along and into the foil at the same speed as the velocity of propagation of the wavefront of stimulated emission. In this manner, a volume element with a large density of excited states on the exit side of the foil is produced due to the resonant charge exchange between the beam and the hydrogen in the foil. Since the beam sweeps downstream in coincidence with the stimulated emission wavefront, the traveling stimulated emission constitutues an x-ray laser action.
While the described technique is considered promising, there are problems associated with the production and use of the thin hydrogen-rich foil strip. The foil thickness needed to maximize the desired population inversion is so thin that such foils are difficult to make which are still thick enough so that multiple scattering will give a suitable Doppler width. In addition, the lifetime of the foils are short since they are destroyed by the impinging ion beam.
It is an object of this invention to overcome the disadvantages set forth.