We present a fully ab initio quantum investigation of the frequency comb structure and coherence within each order of the high-order harmonic generation ͑HHG͒ spectrum in the high-frequency vuv-xuv regime. The HHG spectrum driven by a train of equal-spacing short laser pulses is calculated by propagating the timedependent Schrödinger equation accurately and efficiently by means of the time-dependent generalized pseudospectral method. We explore the comb structure and coherence by varying the

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... er pulse separation , the number of pulses N, and the laser intensity. We find that a nested comb structure appears within each of the harmonics, ranging from the first harmonic all the way to the cutoff harmonic, and this global pattern persists regardless of the values of and N used. The comb structure of the harmonics originates from quantum interferences among induced dipole pulses. Finally, the frequency comb structure prevails even in the presence of appreciable ionization.