Shaoting Gu

Experimental study of continuum lowering

We present time- and space-resolved XUV spectra of boron and carbon plasmas created by focusing 100-fs laser pulses on a solid target to an intensity of 1017 W/cm2. Emission lines originating from He-like and H-like excited states from n equals 2 to the ionization limit are observed with a spatial resolution of 100 micrometers in the direction normal to the target plane and with a temporal resolution of up to 4 ps. The position of the ionization limited is seen to depend very crucially on the plasma parameters of density and temperature, and is explained through continuum lowering effects. We observed the dynamics of the continuum lowering for plasma slices at different distances from the target, and record a maximum lowering of 40 eV in He-like carbon (10% of the ionization potential) from the disappearance of the 1snp - 1s2 line and from the position of the continuum edge.

Laser Acceleration of Protons from Thin Film Targets

A collimated beam of fast protons, witlr energies as high as 10 MeV and total number of 109, confined in a cone angle of 40°±10° has been observed when a 10 TW laser with frequencies either ω0 (corresponding to 1 μm) or 2ω0 was focused to an intensity of a few times 1018 W/cm2 on the surface of a thin film target. The protons, which originate from impurities on the front side of the target, are accelerated over a region extending into the target and exit out the backside in a direction normal to the target surface. Acceleration field gradients of ∼10 GeV/cm are inferred. The maximum proton energy for 2ω0 can be explained by the charge-separation electrostatic-field acceleration due to “vacuum heating.” In other set of experiments when a deuterated polystyrene layer was deposited on a surface of a Mylar film and a 10B sample was placed behind the target, we observed the production of ∼105 atoms of positron active isotope 11C from the nuclear reaction 10B(d,n)11C.

Near 10 MeV ion acceleration in the forward direction and isotope production with a high-intensity laser

High energy protons with the energy up to 10 MeV were accelerated in the forward direction using a tabletop laser with focusing intensity of 6 X 1018W/cm2. When a deuterated polystyrene was deposited on a front surface of a Mylar film and a boron sample was placed behind the laser target we observed the production of approximately 105 atoms of positron active isotope 11C from the reaction 10B(d,n)11C. The activation results suggest that deuterons were accelerated from the front surface of the laser target.

MeV proton beam driven by a high-intensity laser

Summary form only given. The interaction of compact high-intensity subpicosecond lasers with matter has been studied for several years, having numerous applications such as table-top electron accelerators. However, only recently an interest has developed in ion acceleration, with potential applications for the initiation of nuclear reactions on a tabletop. Critical for ion acceleration is the efficiency of laser-energy conversion into a high-energy electron component, since the latter through charge separation can produce the requisite strong electrostatic fields. Thermal expansion of a laser-driven plasma and ponderomotive electron expulsion constitute the most well-known examples of electrostatic field production. While the former mechanism has been observed for many years, the latter one has only recently been observed in experiments with gas targets. For the gas targets, when the laser pulse duration /spl tau/ is long, /spl tau/>r/sub 0//c, where r/sub 0/ is the laser focal spot radius and c is the speed of light, the radial component of the pondermotive force dominates, and ions are accelerated radially.

Electron cavitation and generation of MeV ions produced by relativistically self-guided laser pulse in He gas jet

Summary form only given. An interaction of ultra-high intensity short laser pulses with underdense plasma is of considerable interest from the standpoint of basic physics and potential application for advanced inertial confinement fusion, X-ray lasers and particle accelerators. The ultra-high electromagnetic fields in the laser focus produce an extremely high pondermotive force that expels free electrons from the laser axis and relativistically modify the electron mass, plasma frequency and the plasma refractive index so that the plasma acts as a positive lens. If the laser pulse duration is long enough, the charge separation produces strong electrostatic field /spl sim/1 GV/cm that can accelerate ions to MeV energies. All these effects lead to a relativistic self-focusing and self-guiding of the laser beam that can further increase the laser intensity and maintain it over distances much longer than a Rayleigh range.

Pondermotive acceleration of ions by relativistically self-focused high-intensity short pulse laser

We report on the observation of high energy ions with energies up to 1 MeV accelerated ponderomotively by a relativistically self-focused intense 5 TW, 400 fs laser pulse in a supersonic He gas jet on the distance of a laser spot size. Probing interferometry was used to observe on-axis electron-ion cavitation followed by the plasma expansion with a high radial velocity of /spl sim/3.8.10/sup 8/ cm/s. Using the nuclear track detector CR-39, we confirmed that the ions are preferentially accelerated in the radial direction and the total energy in high energy ions is about 1% of the laser energy. Developed kinetic modeling provides a reasonable description of a plasma channel formation and ion acceleration.

Pressure Ionization and Density Diagnostics in Subpicosecond Laser-Produced Plasmas

The atomic physics of high-density plasmas is studied extensively for its relevance to astrophysics1, inertial confinement fusion,2,3 x-ray lasers,4 and to the interaction of ultrashort lasers with solids. 5-7 Of utmost importance is the knowledge of the plasma parameters of electron density, Ne, and temperature, Te, as they govern the atomic physics in the plasma, from its ionization balance to its emission and absorption. The structure and behavior of atoms and ions, for example, can be radically affected by the presence of strong fields in high-density plasmas1, leading to such effects as extreme line broadening and pressure ionization.1,2,9 Pressure ionization and line-merging have been used in laboratory plasmas as a density diagnostic of spatially- and/or temporally-integrated spectra. 2,10–13 But in laser-produced plasmas, conditions often vary rapidly over time and space, so it is important to resolve both these dimensions for accurate diagnostics. Furthermore, several models are available to quickly extract densities from spectroscopic data but are very different and need to be carefully benchmarked in order to identify which apply for any given set of plasma parameters. Precise data for model validation is rare and usually comes from plasmas limited in density and temperature range.13 Here, we compare four models under a wide range of densities and temperatures in plasmas created with ultrafast laser pulses. These 100-fs laser pulses have the advantage over nanosecond pulses of depositing the energy of the laser impulsively, in a small target layer. Thus, the spectroscopic measurements are conducted after the laser pulse, in a freely expanding plasma, without the added complication of further energy deposition during the plasma evolution.