Particle acceleration using ultraintense, ultrashort laser pulses is one of the most attractive topics in relativistic laser-plasma research. We report proton/ion acceleration in the intensity range of 5×10 19 W/cm 2 to 3.3×10 20 W/cm 2 by irradiating linearly polarized, 30-fs, 1-PW laser pulses on 10-to 100-nm-thick polymer targets. The proton energy scaling with respect to the intensity and target thickness was examined. The experiments demonstrated, for the first time with linearly polarized light, a transition from the target normal sheath acceleration to radiation pressure acceleration and showed a maximum proton energy of 45 MeV when a 10-nm-thick target was irradiated by a laser intensity of 3.3×10 20 W/cm 2. The experimental results were further supported by two-and three-dimensional particle-in-cell simulations. Based on the deduced proton energy scaling, proton beams having an energy of ~ 200 MeV should be feasible at a laser intensity of 1.5×10 21 W/cm 2 .

Field‐emission currents from SiO2‐ or MgO‐coated carbon nanotube (CNT) emitters are larger than for uncoated equivalents. Moreover the coated emitters exhibit lower turn‐on fields and it is shown that the MgO or SiO2 films protect the CNT tips during field emission, which improves the lifetime stability of these emitters. The Figure shows an SEM image of a SiO2‐coated CNT emitter.

In the last two decades, a series of novel nanostructured carbon materials have been synthesized in the laboratory, [1] including carbon nanotubes, [2] carbon onions, [3] and carbon nanocapsules. [4,5] Intensive studies of nanostructured carbon synthesis are motivated by potential applications such as intercalation materials for Li batteries, [6] gas-storage media, cold-electron field emitters, [7] etc. Consisting of bent graphene layers (GLs), these nanomaterials possess unique electric properties due to their finite characteristic size, making them attractive for such applications. Depending on the curvature of the graphene sheets, carbon nanomaterials can demonstrate metallic or semiconducting behavior. [8] As has been shown in our recent paper, [9] alternating metallic and semiconducting regions within carbon nanomaterials produces high levels of cold-electron field emission (FE) from nanostructured carbon materials. Therefore, there is a strong need to find an industrially attractive pathway to synthesize carbon nanoparticles with this structure. It is worth noting that although a variety of methods have been developed to produce carbon nanomaterials, these methods generate only a raw product requiring either further purification (e.g., after laser ablation or arc discharge) or the removal of catalysts (after chemical vapor deposition). Herein, we report a new synthesis pathway for generating onionlike shell-shaped carbon nanoparticles (SCNPs) which is a one-step process. Our SCNPs are highly crystalline and not mixed with other types of carbon, and therefore do not need further purification. Furthermore, our method does not use any catalysts. We show that a transparent acetylene flow can produce onion-like SCNPs (which have continuous bent GLs) in bulk quantities when it is exposed to external irradiation from a continuous-wave (CW) infrared CO 2 laser. The striking feature of this process is that it is launched only above a critical threshold of laser irradiance. Below the irradiance threshold, the flame generates carbon soot. It is this critical threshold phenomenon that distinguishes our work from any other work in the field of flame-formed carbon nanoparticles. At the same time, we demonstrate that the formation of hollow SCNPs in our experiment has nothing to do with soot restructuring due to the ordering of basic structural units (BSUs), which has been recently reported in a hydrocarbon flame under pulsed irradiation from a Nd:YAG (YAG: yttrium aluminum garnet) laser.[10] The generation of SCNPs is governed by the direct growth of graphene sheets from precipitating acetylene molecules. The demonstration of the possibility of this latter process should be a significant contribution to the field of flame-formed carbon nanoparticles. We also show that our SCNPs exhibit FE performance comparable to that of carbon nanotubes. Experiments have been carried out with the setup used successfully in our previous work.[11±15] The multi-nozzle-type burner allowed us to supply gases through different annular coaxia...

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Enormously high secondary electron emission yields under electric field are observed from MgO deposited on carbon nanotubes. The yields reach a value as high as 15 000 and are strongly dependent upon the bias voltage applied to the sample. The creation of the electric field across the MgO film after bombardment of primary electrons is considered as one of key features, since positive charges are generated at the surface by departure of secondary electrons. Subsequent bombarding electrons produce other secondary electrons inside the MgO film, then the liberated secondaries are accelerated towards the surface under the strong field. Under this condition, the secondary electrons gain sufficient energy to create further electrons by impact ionization. The process continues until an equilibrium avalanche is established. To elucidate the earlier explanations, the kinetic energy spectra of secondary electrons are measured by an energy analyzer at various bias voltages in MgO/carbon nanotube samples. The analysis of spectral results with the energy band diagram gives us strong evidence for the suggested mechanism.

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We have investigated effects of electric fields on the yield of secondary electron emission ͑SEE͒ from the primary electron bombardment on magnesium oxide ͑MgO͒ covering vertically grown multiwalled carbon nanotubes ͑MWCNTs͒. We observe that the yield of SEE increases up to at least 22 000 at a special condition. The strong local field generated by the sharp tip of vertically grown MWCNTs accelerates secondary electrons generated by primary electrons. This eventually gives rise to so called Townsend avalanche effect, generating huge number of secondary electrons in a MgO film. Emission mechanism for such a high SEE will be further discussed with energy spectrum analysis.

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