## 4 Discussion  
The practical aspects of our presented approach require answers to a few questions in order to provide a reliable pattern of the QPM in plasmas. (1) For practical purposes, one would like to know: what is the most optimal number of jets for a given harmonic? The meaning of most optimal number of jets cannot be clarified without understanding the impeding processes, which stop the quadratic growth of harmonic yield with the increase of number of jets. Our studies revealed that this deviation from the anticipated quadratic dependence occurs at \(n>5\). This quadratic rule is fulfilled only for the maximally quasi-phase matched harmonic (i.e., the one which has highest enhancement). For non-phase matched harmonic, the enhancement is not realized at all. (2) Another question is: what would be the corresponding jet characteristics in the case of \"optimal\" (i.e., most effective) QPM? The characterization of jets requires the knowledge of the ions/neutrals and free electron densities. The latter parameter defines the maximally-enhanced harmonic at a given jet length. For a given size of individual plasma jet, the QPM for maximally enhanced harmonic could be maintained at a fixed product \(H_{\rm qpm}\times N_{\rm e}\) (since \(L_{\rm coh}\approx 1\times 10^{18}(H_{\rm qpm}\times N_{\rm e})^{-1}\)[5; 6]). The electron density depends on the ablation conditions and the ionization of the target species by the heating pulse in addition to the tunneling ionization by femtosecond driving pulse. The conditions of ablation are easily adjustable and can be used for variation of \(N_{\rm e}\) and the corresponding \(H_{\rm qpm}\). (3) What would be the most optimal material for this purpose? Currently, the most optimal material is silver. The reasons for that include the extended plateau-like shape of harmonic distribution in the non-QPM regime, the relatively high conversion efficiency, and the easiness in stable Ag plasma formation, etc. Other materials, which also allow observation of this effect, are indium, manganese, gold and arsenic plasmas. (4) How much is the most attainable gain? Until now, the 20\(\times\) to 40\(\times\) enhancement factors for QPM harmonics were reported. This is the ratio of the intensities of QPM-enhanced harmonic propagated through eight to 11 plasma jets and of the same harmonic propagated through the extended imperforated (i.e., homogeneous) plasma, which has a length that is equivalent to length of all jets. (5) Can we use the QPM measurements to determine the electron density? Previously reported studies have shown this opportunity [40]. The developed plasma QPM technique allows achieving some groups of notably enhanced harmonics at fixed \(N_{\rm e}\) such that one can assume this technique could be a method to determine the value of electron density of the plasma. Determining the value of electron density of the plasma is a well-recognized problem in plasma physics especially with the case of dynamically changed \(N_{\rm e}\). Numerous techniques and theories were proposed and realized to resolve this issue. Particularly, in the case of laser-produced plasmas, the code HYADES was used to estimate the electron density of plasma at different regimes of plasma formation. The plasma QPM method allows determining the electron density from direct measurements of two other parameters from the aforementioned relation, and those parameters are the coherence length and the maximally-enhanced harmonic order. (6) What is the ultimate limit from the point of view of tuning the maximally enhanced harmonic towards the shorter wavelengths? Our aim is to demonstrate the effect of different experimental parameters on the modulation of homogeneous harmonic distribution. We did not aim to determine the limits of \(H_{\rm qpm}\) variations since this task has already been analyzed in previous studies [1; 16; 42]. Particularly, H43 of 800 nm pump was the maximally-enhanced harmonic in the case of 11-jet silver plasma structure.