Modal study of the role of excess noise in x-ray lasers

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Abstract
Current x-ray laser designs rely on amplifying spontaneous emission in a high-temperature plasma. A central problem in the study of x-ray lasers is determining and optimizing the degree of transverse spatial coherence for holographic applications. The inherent non-power-orthogonal character of the normal modes in a general amplifying medium necessarily results in excess noise and cross-correlation effects that can significantly affect the predicted coherence and intensity profiles of an x-ray laser. In particular, loosely bound discrete transverse eigenstates of the paraxial wave equation have previously been found to dominate the intensity and coherence in the low-amplification regime if the continuum eigenstates are omitted from the spectrum [London, Strauss, and Rosen, Phys. Rev. Lett. 65, 563 (1990)]. A detailed analysis of the role of excess noise on steady-state laser intensity and coherence is presented for the simple case of a transverse square gain and density profile in a finite geometry. The inclusion of the continuum portion of the spectrum is found to substantially reduce the level of excess noise and to alter the coherence predictions for low and intermediate gain-length products.