## 1 Introduction
Currently, numerous efforts have been devoted to develop a carrier envelope phase (CEP)-stabilized MIR laser with a few-cycle duration from an optical parametric amplifier (OPA) [28; 29; 30]. To tackle the problem of the soft X-ray IAPs with low photon flux which is driven by a MIR pulse, in this work, we discuss and numerically demonstrate the efficient generation of intense soft X-ray IAPs using an MIR waveform synthesizer [32]. This demonstration realizes not only efficient cutoff-energy extension of the HHG, but also a reduction of the atto-chirp of IAPs. Our scheme is based on the idea of sub-cycle waveform shaping by synthesizing coherent multicycle MIR pulses with different wavelengths [33; 34; 35; 36]. With the development of recent laser techniques, it has become possible to precisely synthesize and control the output of multicolor coherent light sources from different laser systems, such as fiber lasers [37] and OPA [38] and optical parametric chirp pulse  
Figure 1: Transmission (**a**) and GDD (**b**) of various filters with thickness of 1000 Å. The transmission and GDD values are adopted and calculated based on the data obtained from [31]. amplifier (OPCPA) [39] systems. Indeed, a fully stabilized terawatt (TW)-scale multicolor waveform synthesizer has been demonstrated by combining a Ti:sapphire laser and OPA [40]. By synthesizing a near-IR waveform, an IAP with 2.6 GW peak power was demonstrated in the XUV region [34; 36]. In this paper, we simulate IAP generation using an MIR waveform synthesizer. In a classical calculation considering a single atom, we confirm that a longer wavelength not only helps extend the cutoff energy of HHG, but also reduces the attochirp. By mixing weak control pulses to synthesize the waveform, the cutoff can be further extended and the atto-chirp can be further reduced compared with that for a single-color pump. This result indicates that a single-cycle laser field is not an optimum waveform for generating the shortest IAP from the viewpoint of reducing the attochirp. In addition, the conversion efficiency of HHG can be increased by manipulating a waveform using a weak supplementary field. Our numerical simulations considering the propagation effect suggest that an IAP with a duration of \(\sim\)55 as can be obtained in the water window region even without atto-chirp compensation. Taking previous experimental results into account [14], therefore, we can expect to generate a GW-scale water window IAP with a pulse energy of 10 nJ order by using a 100 mJ MIR pump energy. On the basis of our simulation parameters, we present a designed experimental setup with information on how to achieve a fully stabilized TW-scale 100 mJ MIR waveform synthesizer based on a dual-chirped optical parametric amplification (DC-OPA) system. The content of this paper is organized as follows: in Section 2, we first describe the advantages of an MIR waveform synthesizer. Then, we perform numerical simulation of a water window IAP driven by an MIR waveform synthesizer. In Section 3, a design for a water window IAP light source driven by a TW-scale MIR waveform synthesizer is presented. Finally, we summarize the results and present future prospects in Section 4.