## 4 Discussion
In the meantime, laser ablation of any of the above targets creates a very efficient plasma medium, which generates extreme high harmonics when the femtosecond pulse propagates 200 \(\upmu\)m above the target surface. This contradiction is explained by another model of creation of the cloud of particles, namely, plasma explosion during ablation of the targets. The dynamics of plasma front propagation during laser ablation is studied by few groups (for example [32] and references therein). The dynamics of plasma formation and spreading can be analyzed by either the time resolved ICCD images or shadowgram technique. A numerical analysis of the generation of such plasmas for the case of single-pulse interaction with the target surface was described in [33]. Previously, the dynamics of the spatial characteristics of laser plasmas generated from B and Mo targets, measured using the shadowgraphs of the plasma, was reported in [34]. For Mo, the plasma front spreads with the velocity of \(\sim\)6 \(\times\) 10\({}^{3}\) m s\({}^{-1}\). For example, the plasma front reaches 200 \(\upmu\)m distance from the target after hundred nanoseconds rather than a few thousand ns, as we can estimate if we assume the steady-state expansion of the plasma particle cloud. Obviously, the formation of \"optimal\" plasma is not restricted by appearance of the plasma front in the area of femtosecond pulse. Wemust wait until the density of the particles becomes sufficient for efficient HHG, while the free-electron concentration remains below the level when the impeding processes prevail over the harmonic generation. In our case the velocity of the \"optimal\" part of carbon nanostructures contained plasma cloud was measured to be \(1\times 10^{3}\) m s\({}^{-1}\). The appearance of continuum points out the strong heating of target surface. The strong incoherent emission of plasma also decreases the overall \"quality\" of generated XUV radiation when the former emission becomes stronger than coherent emission of harmonics. That is why the determination of \"optimal\" plasma for HHG [35] includes various factors playing an important role in amendment and suppression of harmonic yield. Briefly, moderate (10\({}^{17}\) cm\({}^{-3}\)) plasma density, a small ionization rate, and correspondingly small electron density (a few units of 10\({}^{16}\) cm\({}^{-3}\)) helped achieve the best conversion efficiencies towards the harmonics in the plateau range (10\({}^{-5}\) and higher), which was almost two orders of magnitude higher than in the case of \"overheated\" targets. The application of \"optimal\" plasma immediately allowed for the demonstration of such unique properties of HHG in ablated debris like the resonance-induced enhancement of single and group of harmonics, the resonance-induced suppression of some harmonics, the quasi-phase matching of the groups of harmonics in different ranges of XUV, the efficient application of clusters, quantum dots and relatively large nanoparticles for harmonics generation, the application of extended plasmas at the conditions when the coherence length for some harmonics does not exceed the sizes of laser-induced torches, etc. These advanced features of HHG in optimally prepared plasmas cannot be reproduced in the plasmas formed during strong ablation of target surfaces. Moreover, most of above advantages could not be repeated in the gas media commonly used of HHG. The continuum shown in the emission spectra was produced during the strong excitation of our targets. The motivation to show plasma emission spectra at these conditions is: (a) to demonstrate the similarity in emission lines of different nanostructured materials caused by the presence of carbon in each of these species and (b) to show the closeness of harmonic and plasma emission lines, which does not lead to the resonance-induced enhancement of those harmonics.