COMPARISON OF SOFT-X-RAY SPECTRA FROM OPTICAL-FIELD-IONIZED PLASMAS GENERATED WITH LINEARLY AND ELLIPTICALLY POLARIZED FEMTOSECOND LASER-PULSES

We compare x-ray spectra from optical-field-ionized plasmas generated by linearly and elliptically polarized 250-fs Ti:sapphire laser pulses . The experiment demonstrates the difference in the spectra resulting from the different electron temperatures under these two conditions. T o avoid high-density effects, such as electron heating, collisional io nization, or ionization defocusing the plasmas are generated under wel l-controlled conditions in a low-pressure gas cell, For the gases inve stigated (He, N-2, O-2, and CH4), a significantly higher x-ray intensi ty is observed for elliptically than for linearly polarized light on t he majority of lines. This finding is in accord with general theoretic al expectations and with the result of simulations. However, for linea r polarization, anomalies occur in the relative line intensities of se veral lines, Possible mechanisms responsible for this effect are discu ssed.