We numerically investigate the influence of plasma defocusing in high harmonic generation (HHG) by solving the first-order wave equation in an ionized medium. To quantitatively analyze of the influence of plasma defocusing, we define an enhancement factor that considers the macroscopic propagation effects including phase-matching and reabsorption. While degrading the driver pulse intensity, plasma also has a strong impact on HHG phase-matching and efficiency. With the aid of the scaling relation between the single atom efficiency and the HHG driver wavelength, our results give an estimate of HHG efficiencies with different driver wavelengths and show a limited HHG efficiency in high density media.
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