A confined electron laser is a “wiggled” electron beam confined by the ponderomotive forces inside an electromagnetic cavity. The ponderomotive forces counteract the beam expansion due to space-charge forces, thereby enabling lasing at low energies. In this Letter, we show the possibility of lasing at a wavelength of approximately 4 nm in the soft-X-ray regime. We simulate various cavity parameters and show that electron-beam confinement and, consequently, the initiation of the lasing process are independent of the cavity resonance wavelength. We also show that ponderomotive confinement can compensate for the beam divergence due to nonzero emittance, thereby enabling lasing with large emittance values for which the free-electron lasing condition is violated. Our results support the feasibility of a few femtosecond, compact, and coherent soft-X-ray source.
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