We examine the interaction of intense, femtosecond laser radiation with the large ͑50-200 Å͒ clusters produced in pulsed gas jets. Both experiment and simulation show that the plasmas produced during these interactions exhibit electron temperatures far in excess of that predicted by above-threshold ionization theory for a low-density gas. Efficient heating of the clusters by the laser is followed by rapid expansion of the clusters and long-lived x-ray emission from hot, decaying, underdense plasma.

Laser pulses with a power of 10' W and a duration of 10 ' s were focused onto both gas and solid targets. Strong emission of pulsed radiation at terahertz frequencies was observed from the resulting plasmas. The most intense radiation was detected from solid density targets and was correlated with the emission of MeV x rays and electrons. Results indicate that radiative processes in such plasmas are driven by ponderomotively induced space charge fields in excess of 10 V/cm. This work constitutes the first direct observation of a laser-induced wake field.PACS numbers: 52.40.Nk, 42.65.Re, 52.25.Rv, 52.35.Mw Plasmas created by high-intensity laser pulses with subpicosecond duration have received considerable attention as novel sources of radiation. The observed emission includes coherent radiation at high harmonics of the laser frequency [1], incoherent soft x-ray bursts with subpicosecond duration [2], and the generation of hard x rays with photon energies extending beyond 1 MeV [3]. At the low frequency end of the electromagnetic spectrum, strong emission of coherent far-infrared radiation (FIR) at terahertz frequencies was recently predicted [4]. This radiation results from the space charge fields developed in such plasmas. In this Letter we report the first observation of this eff'ect.The generation of strong electric and magnetic fields in laser produced plasmas has been considered before. Electric fields on the order of 10 V/cm have been inferred in experiments involving plasma-wave accelerators [5]. In the context of high-intensity short-pulse laser interaction with plasmas, electric fields of 10 V/cm [6] and magnetic fields of up to 10 G [7,8] were predicted by several groups. Our experiments allow a comparison with this previous work by direct measurement of such fields. We note that the generation of terahertz radiation through the use of femtosecond laser pulses has been considered in a variety of schemes [9]. For example, intense pulses with energies up to 0.8 pJ were produced by illuminating a biased GaAs wafer with short laser pulses [10].In our experiment, the mechanism of FIR generation involves ponderomotive forces present at the focus of an intense laser pulse. These forces generate a large density difference between ionic and electronic charges since the laser pulse length is short enough to inertially confine the ions [6,11]. This charge separation results in a powerful electromagnetic transient [4].To estimate the magnitude of the terahertz emission we employed a hydrodynamic model for the plasma dynamics. We calculated the spatial and temporal dependence of the charge density and acceleration within the focal region and thereby determined the far-field radiation pattern. The electron fiuid can be assumed to be cold, i.e. , the thermal energy is small compared to the ponderomotive energy, U~". U~" is defined in Ref. [12]. The cold Auid approximation is justified since plasmas produced by short pulse lasers tend to have temperatures~10 eV [13], while the ponderomotive energies for our experime...

Matter with a high energy density (>10(5) joules per cm(3)) is prevalent throughout the Universe, being present in all types of stars and towards the centre of the giant planets; it is also relevant for inertial confinement fusion. Its thermodynamic and transport properties are challenging to measure, requiring the creation of sufficiently long-lived samples at homogeneous temperatures and densities. With the advent of the Linac Coherent Light Source (LCLS) X-ray laser, high-intensity radiation (>10(17) watts per cm(2), previously the domain of optical lasers) can be produced at X-ray wavelengths. The interaction of single atoms with such intense X-rays has recently been investigated. An understanding of the contrasting case of intense X-ray interaction with dense systems is important from a fundamental viewpoint and for applications. Here we report the experimental creation of a solid-density plasma at temperatures in excess of 10(6) kelvin on inertial-confinement timescales using an X-ray free-electron laser. We discuss the pertinent physics of the intense X-ray-matter interactions, and illustrate the importance of electron-ion collisions. Detailed simulations of the interaction process conducted with a radiative-collisional code show good qualitative agreement with the experimental results. We obtain insights into the evolution of the charge state distribution of the system, the electron density and temperature, and the timescales of collisional processes. Our results should inform future high-intensity X-ray experiments involving dense samples, such as X-ray diffractive imaging of biological systems, material science investigations, and the study of matter in extreme conditions.