Harmonic generation in the limit of ultra-steep density gradients is studied experimentally. Observations demonstrate that while the efficient generation of high order harmonics from relativistic surfaces requires steep plasma density scale-lengths (Lp/λ < 1) the absolute efficiency of the harmonics declines for the steepest plasma density scale-length Lp → 0, thus demonstrating that near-steplike density gradients can be achieved for interactions using high-contrast high-intensity laser pulses. Absolute photon yields are obtained using a calibrated detection system. The efficiency of harmonics reflected from the laser driven plasma surface via the Relativistic Oscillating Mirror (ROM) was estimated to be in the range of 10 −4 − 10 −6 of the laser pulse energy for photon energies ranging from 20 − 40 eV, with the best results being obtained for an intermediate density scale-length.PACS numbers: 52.59.Ye, 52.38.-r Keywords: surface high-harmonic generation, relativistic laser plasma interaction, attosecond pulse generation Ultrashort XUV pulses are a promising tool for a wide range of applications including attosecond laser physics and seeding of free-electron X-ray lasers. Typically, they are created by the nonlinear frequency up-conversion of an intense femtosecond driving laser field in a gaseous medium. Remarkable progress has been made to the present date with efficiencies reaching the level of 10 −4 at 20 nm wavelengths [1,2]. Such efficiencies are not yet available at shorter wavelengths or for attosecond pulse generation and the low intensities at which harmonic conversion takes place in gaseous media, makes harnessing the high peak power in the 0.1−1PW regime challenging. High-harmonic generation at a sharp plasma-vacuum interface via the Relativistically Oscillating Mirror (ROM) mechanism [3] is predicted to overcome these limitations and result in attosecond pulses of extreme peak power [4,5].While other mechanisms such as Coherent Wake Emission (CWE) can also emit XUV harmonics [6], the ROM mechanism is generally reported to dominate in the limit of highly relativistic intensities, where the normalized vector potential a 2 0 = Iλ 2 /(1.37 · 10 18 µm 2 W/cm 2 ) 1. The efficiency of ROM harmonics is predicted to converge to a power law for ultra-relativistic intensities [7], such that the conversion efficiency is given by η ≈ (ω/ω 0 ) −8/3 up to a threshold frequency ω t ∼ γ 3 , beyond which the spectrum decays exponentially. Here, γ is the maximum value of the Lorentz-factor associated with the reflection point of the ROM process. While these predictions correspond well with the observations made in experiments using pulse durations of the order of picoseconds in terms of highest photon energy up to keV [8,9] and the slope of the harmonic efficiency [10], no absolute efficiency measurements have been reported to date.The plasma density scale-length plays a critical role in determining the response of the plasma to the incident laser radiation. In the picosecond regime, the balance between the laser pre...

When a laser pulse hits a solid surface with relativistic intensities, XUV attosecond pulses are generated in the reflected light. We present an experimental and theoretical study of the temporal properties of attosecond pulse trains in this regime. The recorded harmonic spectra show distinct fine structures which can be explained by a varying temporal pulse spacing that can be controlled by the laser contrast. The pulse spacing is directly related to the cycle-averaged motion of the reflecting surface. Thus the harmonic spectrum contains information on the relativistic plasma dynamics.

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In this paper a method to determine spatially-resolved profiles of the electron temperature T e and density n e in an electron-cyclotron-resonance (ECR) discharge is presented. This technique is based on the observation of line emission from a neutral Li atom beam, which is injected into the plasma and excited by electron collisions. A collisional-radiative model valid for the injected Li atoms is used to predict the emission intensities as function of n e and T e for several lines theoretically. In contrast to the electron temperature regime representative for the edge of tokamak discharges (T e > 5 eV), the ECR discharge offers a T e range where selected line intensity ratios strongly depend on the electron temperature. Therefore, a comparison of the measured ratios with the calculated ones yields T e profiles for the first time. The n e measurement is performed as in tokamaks by observing the attenuation of the beam due to ionization in the plasma. We present radial profiles of T e and n e for discharges in argon and xenon under different operating conditions. These results are compared with results obtained by Thomson scattering. Our measurements give evidence for a satisfying agreement between the two methods.

Electrons have been accelerated from solid target surfaces by sub-10-fs laser pulses of 120 microJ energy which were focused to an intensity of 2x10;{16} W/cm;{2}. The electrons have a narrow angular distribution, and their observed energies exceed 150 keV. We show that these energies are not to be attributed to collective plasma effects but are mainly gained directly via repeated acceleration in the transient field pattern created by incident and reflected laser, alternating with phase-shift-generating scattering events in the solid.