  The bottleneck for an attosecond science experiment is concluded to be the
lack of a high-peak-power isolated attosecond pulse source. Therefore,
currently, generating an intense attosecond pulse would be one of the highest
priority goals. In this paper, we review a TW-class parallel three-channel
waveform synthesizer for generating a gigawatt-scale soft-x-ray isolated
attosecond pulse (IAP) using high-order harmonics generation (HHG).
Simultaneously, using several stabilization methods, namely, the
low-repetition-rate laser carrier-envelope phase stabilization, Mach-Zehnder
interferometer, balanced optical cross-correlator, and beam-pointing
stabilizer, we demonstrate a stable 50-mJ three-channel optical-waveform
synthesizer with a peak power at the multi-TW level. This optical-waveform
synthesizer is capable of creating a stable intense optical field for
generating an intense continuum harmonic beam thanks to the successful
stabilization of all the parameters. Furthermore, the precision control of
shot-to-shot reproducible synthesized waveforms is achieved. Through the HHG
process employing a loose-focusing geometry, an intense shot-to-shot stable
supercontinuum (50-70 eV) is generated in an argon gas cell. This continuum
spectrum supports an IAP with a transform-limited duration of 170 as and a
submicrojoule pulse energy, which allows the generation of a GW-scale IAP.
Another supercontinuum in the soft-x-ray region with higher photon energy of
approximately 100-130 eV is also generated in neon gas from the synthesizer.
The transform-limited pulse duration is 106 as. According to this work, the
enhancement of HHG output through optimized waveform synthesis is
experimentally proved. The high-energy multicycle pulse with 10-Hz repetition
rate is proved to have the same controllability for optimized waveform
synthesis for HHG as few- or subcycle pulses from a 1-kHz laser.
