We describe theoretically the enhanced generation and propagation of attose
cond pulses in a gas medium irradiated by sub-10-fs laser pulses. Both clas
sical and quantum calculations reveal a significant increase in the energy
of the attosecond pulse as a result of quasiphase matching in a tapered str
uctured hollow-core fiber. The increasing laser field into the fiber reduce
s the dependence of the harmonic spectrum on the dispersive phase of the la
ser pulse and ensures generation and propagation of a single attosecond pul
se in a highly ionized transient plasma. For the attosecond pulse radiation
this design acts as a phase-locked traveling-wave x-ray laser. [S1050-2947
(99)02510-X].