We describe theoretically the performance of a new design for quasi-phase matched harmonic generation in a gas medium. A hollow core fiber in which thick glass plates are periodically introduced allows good phase control of the fundamental light. The generated x-rays are transmitted throught small holes in the plates. An increase of the harmonic yield of up to three orders of magnitude is predicted as a result of the structure. This esign can be considered as a phase-locked travelling-wave x-ray laser.