## 5 Conclusions  
TCP allows stronger yield of odd harmonics and appearance of almost similar intensities of odd and even harmonics in the case of TCP. Meanwhile, QPM allows tuning a group of enhanced harmonics in the XUV range. The practical issue here is the search for optimal experimental conditions for each particular case of laser-plasma interaction using different concepts of harmonic enhancement. TCP and QPM notably enhance the potential of laser-ablation-induced HHG spectroscopy of any solid material.  
We have demonstrated some peculiarities of quasi-phase matching in laser produced plasma, which allows us to analyze the advantages and disadvantages, as well as further amend the developed technique. Since first demonstration of this effect in plasma [1], new opportunities and schemes were developed and analyzed. Inevitably, they overlap, to some extent, with each other to explain, in some cases from the very basic rules, the advantages (and in some case disadvantages) of the QPM in plasmas. This overlap may cause some confusion; however, it would be difficult to clarify some new findings without the description of the basics and practical issues of this method.  
Apart from the novelty of the approach employed in the present work (comparison of single and two color pumps of QPM plasmas, role of number of coherent zones, competitiveness of macro-processes (QPM) and micro-processes (resonance enhancement of single harmonic), analysis of dimensional parameters of plasma jets, etc.), we have developed a new approach in the theoretical description of the process, which shows peculiarities that are not mentioned in earlier theoretical studies of QPM in plasmas.  
We have analyzed different patterns of experimentally-observed modifications of harmonic spectra at the conditions of QPM in silver, indium, and aluminum multi-jet plasmas. In particular, we have studied the role of the number of coherent zones, sizes of plasma jets and distance between them, plasma formation conditions and the characteristics of fundamental and heating radiation on the HHG efficiency at QPM conditions. We have demonstrated TCP-induced QPM at different regimes of laser-plasma interaction.  
The results of the theoretical simulations have shown good agreement with the experimental findings. The calculated spectra qualitatively coincide with the measured ones. The deeper insight into the mechanisms of the investigated phenomena can be obtained from the analysis of relationships between the characteristics of the processes like those shown in Figure 4. Figure 4a shows that the yield of H34 increases as \(I_{\text{H34}}\)-\(n^{2}\) up to number of jets (\(n\)) equal to 4. The quadratic dependence is correct in a wide range of harmonic spectra. However, further deviation of \(I_{\text{H34}}(n)\) dependence from the quadratic one can be explained by the violation of equality of the conditions for each next added jet (i.e., non-ideal conditions of propagation through jets, and/or low-order nonlinear optical properties of plasma, etc.). New possibilities for controlling QPM conditions using combined multi-jet plasmas are readily proposed. The results of our experimental and theoretical studies of the QPM conditions provide reliable interpretation of this phenomenon.  
Conceptualization, R.A.G. and A.S.A.; Software, S.Y.S.; Formal Analysis, S.Y.S. and A.V.A; Investigation, R.A.G.; Writing--Original Draft Preparation, R.A.G. and S.Y.S.; Writing--Review & Editing, A.S.A.; Supervision, A.S.A.  
This research was funded by the Russian Foundation for Basic Research, grant number 18-02-40014. The APC was funded by Materials Science and Engineering Research Institute, UAE.  
R.A.G. acknowledges the support from [PERSON].  
The authors declare no conflict of interest.