Analysis of spectra of high-order harmonics from relativistic singularities

High-order harmonics from relativistic singularities are emitted when a multi-terawatt femtosecond laser irradiates a gas jet target [1, 2]. The laser pulse pushes electrons producing an electron-free cavity and generates a bow wave [3]; the singularities are produced at the joining of the cavity wall and the bow wave and represent point-like coherent x-ray emitters. The emitted spectra generally consist of high-order harmonics of optical frequency. However, the spectra often exhibit additional features such as modulations with large period (up to 10s of eV) and presence of several periods in a single spectrum. Here we report on analysis of experimental spectra with the emphasis on dependence on laser and plasma parameters, and discuss possible implications.This work was supported by JSPS KAKENHI # JP 25287103, JP 25390135, and JP 26707031.JPS Spring Annual Meetin