Experimental demonstration of particle energy, conversion efficiency and spectral shape required for ion-based fast ignition

Research on fusion fast ignition (FI) initiated by laser-driven ion beams has made substantial progress in the last years. Compared with electrons, FI based on a beam of quasi-monoenergetic ions has the advantage of a more localized energy deposition, and stiffer particle transport, bringing the required total beam energy close to the theoretical minimum. Due to short pulse laser drive, the ion beam can easily deliver the 200 TW power required to ignite the compressed D-T fuel. In integrated calculations we recently simulated ion-based FI targets with high fusion gain targets and a proof of principle experiment [1]. These simulations identify three key requirements for the success of ion-driven fast ignition (IFI): (1) the generation of a sufficiently high-energetic ion beam (approximate to 400-500 MeV for C), with (2) less than 20% energy spread at (3) more than 10% conversion efficiency of laser to beam energy. Here we present for the first time new experimental results, demonstrating all three parameters in separate experiments. Using diamond nanotargets and ultrahigh contrast laser pulses we were able to demonstrate >500 MeV carbon ions, as well as carbon pulses with Delta E/E

Laser - Ion Acceleration in the Laser Transparency Regime

Experiments on laser-induced ion acceleration from ultra-thin (nm) foil targets reveal a dramatic increase in the conversion efficiency and the acceleration of C6$+$ions in a phase stable way by the laser radiation pressure.

Enhancing solar cells with localized plasmons in nanovoids

Localized plasmon resonances of spherical nanovoid arrays strongly enhance solar cell performance by a factor of 3.5 in external quantum efficiency at plasmonic resonances, and a four-fold enhancement in overall power conversion efficiency. Large area substrates of silver nanovoids are electrochemically templated through self-assembled colloidal spheres and organic solar cells fabricated on top. Our design represents a new class of plasmonic photovoltaic enhancement: that of localized plasmon-enhanced absorption within nanovoid structures. Angularly-resolved spectra demonstrate strong localized Mie plasmon modes within the nanovoids. Theoretical modelling shows varied spatial dependence of light intensity within the void region suggesting a first possible route towards Third Generation plasmonic photovoltaics. (C) 2011 Optical Society of America

Identifying PV module mismatch faults by a thermography-based temperature distribution analysis

Photovoltaic (PV) solar power generation is proven to be effective and sustainable but is currently hampered by relatively high costs and low conversion efficiency. This paper addresses both issues by presenting a low-cost and efficient temperature distribution analysis for identifying PV module mismatch faults by thermography. Mismatch faults reduce the power output and cause potential damage to PV cells. This paper first defines three fault categories in terms of fault levels, which lead to different terminal characteristics of the PV modules. The investigation of three faults is also conducted analytically and experimentally, and maintenance suggestions are also provided for different fault types. The proposed methodology is developed to combine the electrical and thermal characteristics of PV cells subjected to different fault mechanisms through simulation and experimental tests. Furthermore, the fault diagnosis method can be incorporated into the maximum power point tracking schemes to shift the operating point of the PV string. The developed technology has improved over the existing ones in locating the faulty cell by a thermal camera, providing a remedial measure, and maximizing the power output under faulty conditions.

Design and results of a dual-gas Quasi-Phase matching (QPM) foil target

Quasi-phase matching (QPM) can be used to increase the conversion efficiency of the high harmonic generation (HHG) process. We observed QPM with an improved dual-gas foil target with a 1 kHz, 10 mJ, 30 fs laser system. Phase tuning and enhancement were possible within a spectral range from 17 nm to 30 nm. Furthermore analytical calculations and numerical simulations were carried out to distinguish QPM from other effects, such as the influence of adjacent jets on each other or the laser gas interaction. The simulations were performed with a 3 dimensional code to investigate the phase matching of the short and long trajectories individually over a large spectral range.

M-L band x-rays (3-3.5 KeV) from palladium coated targets for isochoric radiative heating of thin foil samples

We describe experiments designed to produce a bright M-L band x-ray source in the 3-3.5 keV region. Palladium targets irradiated with a 10(15) W cm(-2) laser pulse have previously been shown to convert up to similar to 2% of the laser energy into M-L band x-rays with similar pulse duration to that of the incident laser. This x-ray emission is further characterized here, including pulse duration and source size measurements, and a higher conversion efficiency than previously achieved is demonstrated (similar to 4%) using more energetic and longer duration laser pulses (200 ps). The emission near the aluminium K-edge (1.465-1.550 keV) is also reported for similar conditions, along with the successful suppression of such lower band x-rays using a CH coating on the rear side of the target. The possibility of using the source to radiatively heat a thin aluminium foil sample to uniform warm dense matter conditions is discussed.

Dual interfacial modifications of hierarchically structured iodine-doped ZnO photoanodes for high-efficiency dye-sensitized solar cells

A nanocomposite porous electrode structure consisting of hierarchical iodine-doped zinc oxide (I-ZnO) aggregates combined with the two simple solution-processed interfacial modifications i.e. a ZnO compact layer (CL) and a TiO2 protective layer (PL) has been developed in order to understand electron transport and recombination in the photoanode matrix, together with boosting the conversion efficiency of I-ZnO based dye-sensitized solar cells (DSCs). Electrochemical impedance spectra demonstrate that ZnO CL pre-treatment and TiO2 PL post-treatment synergistically reduce charge-transfer resistance and suppress electron recombination. Furthermore, the electron lifetime in two combined modifications of IZnO + CL + PL photoelectrode is the longest in comparison with the other three photoelectrodes. As a consequence, the overall conversion efficiency of I-ZnO + CL + PL DSC is significantly enhanced to 6.79%, with a 36% enhancement compared with unmodified I-ZnO DSC. Moreover, the stability of I-ZnO + CL + PL cell is improved as compared to I-ZnO one. The mechanism of electron transfer and recombination upon the introduction of ZnO CL and TiO2 PL is also proposed in this work.

K alpha yields from Ti foils irradiated with ultrashort laser pulses

We have studied the emission of Kalpha radiation from Ti foils irradiated with ultrashort (45 fs) laser pulses. We utilized the fundamental (800 nm) light from a Ti:sapphire laser on bare foils and foils coated with a thin layer of parylene E (CH). The focusing was varied widely to give a range of intensities from approximately 10(15)-10(19) W cm(-2). Our results show a conversion efficiency of laser to Kalpha energy of similar to 10(-4) at tight focus for both types of targets. In addition, the coated targets exhibited strong secondary peaks of conversion at large defocus, which we believe are due to modification of the extent of preformed plasma due to the dielectric nature of the plastic layer. This in turn affects the level of resonance absorption. A simple model of Kalpha production predicts a much higher conversion than seen experimentally and possible reasons for this are discussed.

Hot Electrons Transverse Refluxing in Ultraintense Laser-Solid Interactions

We have analyzed the coupling of ultraintense lasers (at similar to 2 X 1019 W/cm(2)) with solid foils of limited transverse extent (similar to 10 s of mu m) by monitoring the electrons and ions emitted from the target. We observe that reducing the target surface area allows electrons at the target surface to be reflected from the target edges during or shortly after the laser pulse. This transverse refluxing can maintain a hotter, denser and more homogeneous electron sheath around the target for a longer time. Consequently, when transverse refluxing takes places within the acceleration time of associated ions, we observe increased maximum proton energies (up to threefold), increased laser-to-ion conversion efficiency (up to a factor 30), and reduced divergence which bodes well for a number of applications.

Dependence of the ion energy on the parameters of the laser pulse and target in the radiation-pressure-dominated regime of acceleration

When the dominant mechanism for ion acceleration is the laser radiation pressure, the conversion efficiency of the laser energy into the energy of relativistic ions may be very high. Stability analysis of a thin plasma layer accelerated by the radiation pressure shows that Raleigh-Taylor instability may enhance plasma inhomogeneity. In the linear stage of instability, the plasma layer decays into separate bunches, which are accelerated by the radiation pressure similarly to clusters accelerated under the action of an electromagnetic wave. The energy and luminosity of an ion beam accelerated in the radiation-pressure-dominated regime are calculated.

Saturated output of a Ge XXIII x-ray laser at 19.6 nm

We report on measurements of the saturated single frequency output of a Ge XXIII x-ray laser on the J=0-->1 transition at 19.6 nm from a refraction compensating double target driven by 150 J of energy from 75-ps Nd-glass laser pulses. The 19.6-nm line completely dominated the laser output. The output energy was measured to be 0.9 mJ in a beam of 6.6x30 mrad(2) divergence, corresponding to a conversion efficiency of 6 x 10(-6).

Spectral Enhancement in the Double Pulse Regime of Laser Proton Acceleration

The use of two separate ultraintense laser pulses in laser-proton acceleration was compared to the single pulse case employing the same total laser energy. A double pulse profile, with the temporal separation of the pulses varied between 0.75-2.5 ps, was shown to result in an increased maximum proton energy and an increase in conversion efficiency to fast protons by up to a factor of 3.3. Particle-in-cell simulations indicate the existence of a two stage acceleration process. The second phase, induced by the main pulse preferentially accelerates slower protons located deeper in the plasma, in contrast to conventional target normal sheath acceleration.

Effects of front surface plasma expansion on proton acceleration in ultraintense laser irradiation of foil targets

The properties of beams of high energy protons accelerated during ultraintense, picosecond laser-irradiation of thin foil targets are investigated as a function of preplasma expansion at the target front surface. Significant enhancement in the maximum proton energy and laser-to-proton energy conversion efficiency is observed at optimum preplasma density gradients due, to self-focusing Of the incident laser pulse. For very long preplasma expansion, the propagating laser pulse is observed to filament, resulting in highly uniform proton beams, but with reduced flux and maximum energy.

Role of the plasma scale length in the harmonic generation from solid targets

We have investigated the generation of high harmonics from the interaction of 150 fsec, 790 nm, and 395 nm laser pulses with solid targets. Experiments are presented that demonstrate a strong dependence of the conversion efficiency on the temporal pulse shape and the resulting density scale length (L/lambda) of the preformed plasma. The highest conversion efficiencies are achieved for short density scale lengths (L/lambda less than or equal to 0.4), which result from high contrast ratio pulse interactions. [S1063-651X(98)50211-5].

Feasibility study of high harmonic generation from short wavelength lasers interacting with solid targets

The generation of the third and fourth harmonics from the interaction of a 1 ps, ultraviolet (UV), krypton fluoride (KrF) laser with a solid surface is investigated. The conversion efficiency is seen to increase linearly with I lambda(2), with a transition from specular harmonic emission to emission into 2 pi steradians occurring between 10(15) and 10(16) W cm(-2) mu m(2). The diffuse emission is strongly dependent on the incidence angle of the laser, with the peak in emission at around 30 degrees being consistent with measurements for resonance absorption. Finally, the conversion efficiencies are found to be in agreement with particle-in-cell (PIC) simulations including appropriate density scalelengths. (C) 1998 Elsevier Science B.V.