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The statistical link between spontaneous fluctuations occurring in different parts of the brain can provide insights into its functional organization. Here, we used high-quality resting-state fMRI (rs-fMRI) data acquired with a test-retest paradigm to assess the reliability of several functional connectivity (FC) metrics, some of which rarely used in the current literature. After applying time/frequency methods to generate FC matrices, we restricted our focus on individual edges through different statistical measures, including but not limited to the intra-class correlation coefficient (ICC). We found that full correlation and magnitude square coherence yielded more reproducible measurements than the other metrics, as revealed by ICC values. These results have important implications when choosing metrics for quantifying FC in rs-fMRI studies, adding novel information to the current panorama of information on the test-retest reliability topic.

Changes in the structure of synaptic connections underlie various physiological and neurological processes such as the development of new synapses and neuronal circuitry related to learning and memory processes or neural plasticity after injury and recovery. Recent technological advances, including two-photon microscopy and transgenic mice overexpressing fluorescent proteins have made possible to image individual dendritic arbors and spines in cortex in living animals. The aim of this work is to assess the detectability of such fine structural changes induced by Cytotoxic necrotizing factor 1 (CNF1) also via diffusion weighted Magnetic Resonance Imaging (dMRI). In this preliminary work, classical Diffusion Tensor Imaging (DTI)-based indices were derived for two groups of mice (twelve controls and fifteen CNF1-treated) and group differences were assessed by statistical analysis. T2-based Voxel Based (VBM) and Tensor Based Morphometry (TBM) were used for benchmarking. Results highlight an increment of both Fractional Anisotropy (FA) and Axial Diffusivity (AD) and a decrement of both Mean Diffusivity (MD) and Return To Plane Probability (RTPP) mainly in the visual and hippocampal areas. Our data suggest that mouse morphoanatomical imaging is sensitive to changes in neural plasticity.1

Mg films deposited by pulsed laser ablation on a Cu substrate were tested as photocathodes in a DC diode. They showed features of robustness and a quantum efficiency of 3 × 10 - 4 at low accelerating field when illuminated by 266 nm light at normal incidence. A graphite protective layer of about 20 nm thickness deposited by laser ablation just after the Mg deposition made the cathode stable and of easy handling. The cathodes could stay for several weeks in air before being inserted in the test chamber. The preparation procedure and the tests are presented.

In diffusion MRI, numerical biomarkers are usually calculated for research and clinical purposes as Generalized Fractional Anisotropy (GFA). Recently, more eloquent indices allowing a more accurate description of tissue microstructure were derived from the SHORE model. Under certain experimental conditions, such indices express the morphological properties of the compartments where spins diffuse. Evidence of the suitability of such indices as biomarkers for stroke was provided in a previous study based on diffusion spectrum imaging (DSI) and focusing on the cortical motor loop. The goal of this work was to investigate the suitability of such indices for stratification, namely for distinguishing pathological from healthy subjects. To this end, two different paths were followed. First, the same approach used in the previous work for longitudinal analysis (statistics-based) was applied to detect inter-group variations. Then, a new approach based on the LASSO regressor was proposed. Results provided evidence of the suitability of the proposed indices for stratification purposes.

Antihydrogen production by charge exchange reaction between Positronium atoms and antiprotons requires efficient excitation of Positronium atoms up to high-n levels (Rydberg levels). A two-step optical excitation, the first from ground to n = 3 and the second from this level to a Rydberg level, is proposed and a suitable laser system is discussed. The requirements on the energy and bandwidth of the excitation laser suggest the use of optical parametric generation technology for both wavelengths. The laser system is composed by two subsystems: one for the generation of 205 nm radiation and the other for the generation of 1670 nm radiation. We have separately developed and tested the laser sources and results are here presented. (C) 2011 Elsevier B.V. All rights reserved.

An analysis of the single-spike FEL radiation in the visible range starting from noise is presented, taking into account realistic characteristics of the electron beam such as emittance and energy spread. A parametric study of the power emitted respect to emittance and energy spread is carried out and the influence of these quantity on the production of a single-spike radiation pulse is shown. Furthermore, we propose some examples of start-to-end simulations in the specific case of the SPARC apparatus from the photocathode to the end of the undulator, describing a possible experiment aimed at generating sub-picosecond light pulses and exploring the single-spike regime.

OBJECTIVE: Excessive GH secretion may lead to secondary diabetes mellitus, while prolonged GH treatment may accelerate the onset of type 2 diabetes mellitus in predisposed individuals. Turner's syndrome (TS) patients are a population at risk since they have reduced glucose tolerance (GT) spontaneously and because they are usually treated with high doses of GH. DESIGN AND METHODS: The aim of the study was to evaluate insulin sensitivity (IS) and glucose tolerance (GT) in a group of TS patients treated with GH for a period of 6 years. Forty-seven TS girls were included in the study. GH was administered at a mean weekly dosage of 0.35 mg/kg, injected subcutaneously over 6-7 days. GT was assessed according to the criteria of the Expert Committee on the Diagnosis and Classification of Diabetes Mellitus. IS was evaluated with the quantitative insulin sensitivity check index (QUICK-I). RESULTS: No significant increase of impaired GT was observed in the patients during the follow-up period, while a reduced IS was detected. IS in TS patients was already lower than in prepubertal controls (P

The photoemission behaviour of a series of diamond-based polycrystalline films irradiated by the second (2.3 eV), third (3.5 eV) and fourth (4.7 eV) harmonics produced by a Q-switched-mode-locked Nd: Yag laser has been investigated and related to the structural and compositional characteristics of the layers. The materials were polycrystalline undoped diamond films as well as Nd- and N-containing diamond films grown by CVD techniques, diamond-like and amorphous carbon layers. The morphological and structural characteristics of the films were investigated by electron microscopy, Raman spectroscopy and electron diffraction. The analysis of the photoemission curves does not evidence any improvement of the emission efficiency in the case of Nd-containing films nor for the diamond films grown in the presence of N 2. The results evidence conversely a strong correlation between the characteristics of the photoemission process at sub-band gap energies and the presence of amorphous sp 2 -C patches located at the diamond film surfaces. The photoemitting properties of our samples are discussed and rationalized by considering charge emission occurring at the sp 2 -diamond-vacuum border and the emission process governed by the ratio of amorphous sp 2 -C to crystalline sp 3 -C. The rather high values of quantum efficiency measured in the course of the present research at 3.5 and 4.7 eV suggest that a proper distribution of amorphous carbon onto a good quality diamond surface is the key factor for the preparation of efficient and stable photocathode materials.

Photoemission from polycrystalline diamond films irradiated with the harmonics of a Nd:YAG laser indicates that the amorphous or quasi-amorphous zones (patches) of sp2-C on the external surface of the diamond grains make a dominant contribution to the emission yield. This is deduced from: (a) the strong reduction of the yield as a consequence of the partial removal of amorphous carbon (a-C) patches by etching with acid and (b) the correlation between the yield and the a-C content determined by Raman spectroscopy. The junction between the a-C patches and the diamond determines a configuration of the energy bands, which allows one-photon emission with an energy of 4.7 eV (4th harmonic). We have measured samples with variable a-C content and found an efficient emission, with values of quantum efficiency on the order of 10-6. A phenomenological picture of the electron emission process leads us to locate it at the a-C–diamond–vacuum triple border. This indicates a high local efficiency and suggests that a diamond film with an appropriate distribution of a-C patches on the surface could be a very efficient material. Moreover it could work in moderate vacuum and have a high laser-radiation damage-threshold. Such a material would therefore be interesting as a robust photocathode .

Field Emission (FE) properties of vertically aligned Carbon Nanotubes (CNTs) grown in a nanoporous anodic aluminium oxide (AAO) template have been investigated. A 50-μm-thick AAO template was fabricated by electrochemical techniques. The nanotubes were synthesized in a CVD quartz hot wall furnace using C 2 H 2 /N 2 mixtures as feeding gas. I–V measurements have been performed on samples after the nanotubes growth (type I samples) and after a partial Al 2 O 3 removal (type II samples) in order to obtain segments of nanotubes protruding from the nanopores. The effects of the conditioning process and adsorbates release have been investigated. The emission curves have been analysed in the framework of the Fowler–Nordheim model. For the β factor enhancement, a different dependence on time has been evidenced for two types of investigated samples and has been tentatively correlated with materials modifications occurring under the HV polarisation (in case of type I samples) and with the damage induced by chemical etching (in case of type II samples). The values of emitted current density (up to 40 mA/cm 2 ) and the emission properties indicate that the proposed preparation methodology is suitable for the realization of robust and efficient CNT-based field emission devices and electron sources.

The photoemission from chemical vapor deposited polycrystalline diamond induced by 3.5 eV photons has been investigated and compared to that of Nd-doped diamond. The different behavior observed at low radiation intensity between undoped and doped diamond is shown to be due to the different excess carrier recombination rates in the two materials. The measured quantum efficiency of ∼3×10−6 in the two-photon regime, at relatively low accelerating field, makes this material interesting as photoemitter.

The formation of the antihydrogen beam in the AEGIS experiment through the use of inhomogeneous electric fields is discussed and simulation results including the geometry of the apparatus and realistic hypothesis about the antihydrogen initial conditions are shown. The resulting velocity distribution matches the requirements of the gravity experiment. In particular it is shown that the inhomogeneous electric fields provide radial cooling of the beam during the acceleration.

The present cesium telluride cathodes have the capability to provide, for months, trains of picosecond pulses carrying a charge equal/higher than 10 nanocoulomb, for a total charge higher than the microcoulomb. However, they are very delicate. A possible way to increase their robustness is by covering them with a protective film of nanostructured carbon. Ferroelectric ceramics, as possible new robust photocathodes, showed an interesting level of emission with 532 nm–25 ps laser pulses.

Though present photoemitting cathodes have attained quantum yields, lifetimes, dark currents, and response times as required by high-luminosity accelerators, their use is hampered by their high reactivity to residual gases even in ultra-high-vacuum conditions. Ultrathin (20–200 nm) films of nanostructured sp2 carbon, directly grown on photocathodes by supersonic cluster-beam deposition, besides providing a strongly bound, inert protective coating, are shown to maintain the photoemission efficiency of clean metallic substrate cathodes. Moreover, nanostructured carbon coating is shown to induce strong nonlinearities in photoemission at high intensities, which can be explained as due to the Auger effect.

International audience; The principle of the equivalence of gravitational and inertial mass is one of the cornerstones of general relativity. Considerable efforts have been made and are still being made to verify its validity. A quantum-mechanical formulation of gravity allows for non-Newtonian contributions to the force which might lead to a difference in the gravitational force on matter and antimatter. While it is widely expected that the gravitational interaction of matter and of antimatter should be identical, this assertion has never been tested experimentally. With the production of large amounts of cold antihydrogen at the CERN Antiproton Decelerator, such a test with neutral antimatter atoms has now become feasible. For this purpose, we have proposed to set up the AEGIS experiment at CERN/AD, whose primary goal will be the direct measurement of the Earth's gravitational acceleration on antihydrogen with a classical Moiré deflectometer.

Electron emission from a ceramic disk is due to spontaneous polarization switching induced by a high-voltage pulse applied to the surface electrodes. With a conventional front electrode, emission was erratic as a function of samples and decayed with shot number. On the contrary, when the front electrode consisted of a pattern of unconnected patches contained within a ring, emission was stable. The physics of the two configurations is investigated and a possible interpretation of the two different behaviors is given.

The emission of electrons from lead lanthanum zirconate titanate ceramics, showing ferroelectric or/and antiferroelectric properties at room temperature, after excitation by a rectangular high voltage electric pulse, is explained by the switching and relax-back of the spontaneous polarization. The reported simultaneous measurements of emitted and polarization switching pulses show clearly the expected correlation. From the shape of the switching current it is possible to foresee if the sample can emit or not.

Un-prepoled Lead Zirconate Titanate Lanthanum doped-PLZT ferroelectric cathodes have emitted intense current pulses under the action of a high voltage pulse of typically 8 kV/cm for PLZT of 8/65/35 composition and 25 kV/cm for PLZT of 4/95/5 composition. In the experiments described in this paper, the exciting electric field applied to the sample is directed from the rear surface towards the emitting surface. The resulting emission is due to an initial field emission from the metal of the grid deposited over the emitting surface with the consequent plasma formation and the switching of ferroelectric domains. These electrons may be emitted directly form the crystal or from the plasma. This emission requires the material in ferroelectric phase. In fact, PLZT cathodes of the 8/65/35 type, that is with high Titanium content, showing ferroelectric-paraelectric phase sequence, emit at room temperature, while PLZT cathodes of the 4/95/5 type, that is with low Titanium content, having antiferro-ferro-paraelectric phase sequence, emit strongly at a temperature higher than 130°C.

We show experimentally that the cavity resonances for transverse modes of a ring laser depend on the number of mirrors closing the optical cavity. Thus the dynamical behavior may change for different cavity configurations. We study the spatial and temporal evolution of the intensity in both directions of propagation of the electromagnetic field and we show that a large bore diameter tube sustains bidirectional operation at the same frequency.

The free electron laser–peniotron hybrid is studied. It is shown by computer simulations that the free electron laser–peniotron hybrid, with the synchronous condition ω−(kw+k∥)v∥−ωc≊0, is a strong interaction, and has an efficiency as high as that of the peniotron. The interaction of the free electron laser–peniotron hybrid is compared with that of the free electron laser–cyclotron hybrid.

We discuss a new design of a very high efficiency FEL amplifier in the microwave band, based on the waveguide and wiggler tapering technique. Up to 75% efficiency is reported, even in a relatively relaxed wiggler design. >

The design of a 400 MW-20 GHz multibeam FEL amplifier, aimed to be a source for a high gradient linac is presented. A 120 A, 5 MV electron beam from an electrostatic accelerator is simulated. FEL simulation confirms an efficiency as high as 70%. The space-charge effect is strongly reduced by the cavity walls.

A small model of a Cockcroft-Walton of the novel “onion” design has been built with the aim to make the measurements on the geometric shunt capacitance and its effect on the output voltage and ripple. The results are positive concerning the output voltage, negative for the ripple, which reaches some percent.

Considerable efforts have been made and are still being made to verify the validity of the principle of the equivalence of gravitational and inertial mass, one of the cornerstones of the classical theory of general relativity. Specific attempts at quantummechanical formulations of gravity allow for non-Newtonian contributions, which might lead to a difference in the gravitational force on matter and antimatter. While it is widely expected that the gravitational interaction is independent of the composition of bodies, this has only been tested for matter systems, but never yet for antimatter systems. By combining techniques from different fields, and relying on recent developments in the production of Positronium and ongoing work to laser-excite Positronium to Rydberg states, such a test with neutral antimatter has become feasible. The primary goal of the AEGIS experiment being built at the Antiproton Decelerator at CERN is to carry out the first direct measurement of the Earth‘s gravitational acceleration on antihydrogen by means of a classical Moire deflectometer.

Antihydrogen production by charge exchange reaction between Positronium atoms and antiprotons benefits from an efficient excitation of Positronium atoms to high-n levels (Rydberg levels). A two-step optical excitation, the first from ground to n = 3 and the second from this level to a Rydberg level, is proposed and a proper laser system to be developed is discussed. The requirements on the energy and bandwidth of the excitation laser suggest the use of optical parametric generation technology for both wavelengths. The laser system is composed of two subsystems: one for the generation of 205 nm radiation and the other for the generation of 1670 nm radiation. We report on the progress towards the realization of the short wavelength source. (C) 2011 Elsevier B.V.

A comparison between three possible cavity configurations, the confocal cavity, the ring cavity and the Bragg cavity is presented for a millimeter FEL set within the high voltage head of an electrostatic accelerator. A simple mechnical design to make the Bragg cavity tunable is proposed. The usual theory of the sinusoidal Bragg corrugation, in cylindrical geometry, is extended to the rectangular corrugation and, further, the theory of the Bragg mirror is extended to rectangular geometry. The features of a Bragg mirror in cylindrical and rectangular geometry are compared.

A prototype of a new kind of Cockroft-Walton accelerator is being built. The onion-wise disposal of the capacitor plates allows a high-gradient compact

The physical and technological problems relevant to a very efficient FEL are examined. It is shown that the FEL efficiency has an upper limit of 80% and that this efficiency is reachable only in the microwave region owing to the technology of the wiggler. The possible minimim value of the final electron beam energy is discussed. The results are applied to the FELTRON project.

One of the crucial issues of the new μ-wave source FELTRON is the high-voltage generator. FELTRON is a powerful electrostatic FEL providing μ-wave radiation at 20 GHz, with peak power of 200 MW, pulse length of 500 ns (derived in ten separate beams of 50 ns each) at a repetition rate of 1 kHz. This radiation power will feed the cavities of a high gradient linac for TeV colliders. The average power of the generator must be around 250 kW, at a voltage of 5 MV. A Cockroft-Walton having the “onion” configuration is presented. The features are compared with those of dynamitron and insulating core transformer generators. The operation principles and technological problems are discussed in view of pulsed FEL utilization.

A powerful, continuous-wave millimeter radiation source is presented. An electrostatic Cockcroft-Walton accelerator (2.5 MV) drives an FEL (free-electron laser) of 1 MW power. The accelerator has a new design in order to obtain a very powerful (50 MW) electron beam and very small ripple. A recovery system is used to increase RF power and efficiency. The FEL oscillator is 1.5-m long, with a permanent magnet helical wiggler. The cavity is equipped with Bragg mirrors. The FEL is set at the high voltage terminal. >

A pulsed powerful Cockcroft-Walton generator for an electrostatic accelerator is presented. The electrostatic accelerator is designed to provide a pulsed electron beam of 5.5 MeV, 100 A, and 0.5 mu s with a repetition rate of 1 kHz. The output voltage has a descending staircase waveform. This voltage provides an electron beam made up of ten sections having different energies. Two adjacent pieces have a relative energy variation Delta gamma ij/ gamma of 1%. A dispersive system spatially separates those sections into ten 50-ns-long electron beam pulses. >

Physics and simulations of a new multistage collector for the recovery of a warm electron beam are extensively discussed. The problem of the transition region between the decelerating column and the collector in an immersed flow configuration is also presented. The charge recovery of this new collector seems to be good.

The basics of a complete twin-laser system driving both the LINAC accelerator and the interaction chamber of a powerful inverse Compton source is discussed. The pourpose is the production of trains of 100 pulses, 10 ps long, with an internal frequency of 100 MHz at a repetition rate of 10 Hz. The pulse energy of the egun-laser, the one driving the accelerator photocathode, is about 10 μJ, while the Compton-laser, the one driving the interaction chamber, is 1 J. The egun-laser is conceptually based on a powerful Nd:YAG(YLF) oscillator seeded by a saturated laser pulse. The Compton-laser is based on a commercial 1 J-15 ps-10 Hz Nd:YAG(YLF) coupled to a passive enhacing cavity which can generate the pulse trains. The energy and brilliance of the two lasers are discussed in view of producing an X-ray source capable of delivering more than 10^{10} photons p

We discuss a design for a high-power pulsed electrostatic accelerator (1 GW) for a high gain microwave FEL operating in an expanded gain configuration. The resulting radiation generator, FELTRON, is tailored to power a high-gradient linac operating at 20 GHz with an accelerating gradient of 100 MV/m. The electron pulse duration is 0.5 μs and the rep rate is 1 kHz. Each electron-beam pulse is divided into ten pulses.

A 352-MHz one-cell photocathode electron gun is studied. The simulations with PARMELA are done with two values of peak accelerating fields: 21 and 10 MV/m. Three cavity profiles are compared. The results are also discussed in view of the previous analytic theory. The space-charge effect with peak current greater than 300 A covers the RF effect. The beam qualities are not sensitive to cavity design. >

A pulse generator with the following characteristics is presented: the voltage ranges in the interval 0.1–6 kV, the maximum delivered current is 150 A, the pulse length ranges within the interval 100–300 ns, the rise time and the decay times are, respectively, 10 and 25 ns on 50 Ω resistive load and the repetition rate is higher than 1 MHz. The circuit has a source capacitor of 10 nF charged at the needed voltage, the capacitor feeds the load through a parallel of two fast and high voltage solid state switches. The nanosecond rise time and the square fashion of the pulse have been accomplished arranging all the components in cylindrical symmetry. A bipolar pulse is obtained coupling two circuits with opposite polarity.

The photoemission characteristics of ceramic disks of lead zirconate titanate lanthanum doped (PLZT), have been investigated. We observe 1 nC of extracted charge under an accelerating field of 20 kV/cm in poor vacuum conditions. The emission is clearly limited by space charge effects. The extrapolated quantum efficiency results in ≈10−6. The yield of a PLZT ceramic in the ferroelectric state and its slope versus light intensity have turned out higher than those of antiferroelectric ceramic. Samples in different experimental configurations have shown different nonlinear yields.

We present experimental results relative to superluminal propagation based on a single photon traversing an optical system, called 4f-system, which acts singularly on the photon's spectral component phases. A single photon is created by a CW laser light down{conversion process. The introduction of a linear spectral phase function will lead to the shift of the photon peak far beyond the coherence length of the photon itself (an apparent superluminal propagation of the photon). Superluminal group velocity detection is done by interferometric measurement of the temporal shifted photon with its correlated untouched reference. The observed superluminal photon propagation complies with causality. The operation of the optical system allows to enlighten the origin of the apparent superluminal photon velocity. The experiment foresees a superluminal effect with single photon wavepackets., Comment: 11 pages, 2 figures

We describe in this paper a possible experiment with the existing SPARC photoinjector to test the generation of sub-picosecond high brightness electron bunches able to produce single spike radiation pulses at 500 nm in the SPARC self-amplified spontaneous emission free-electron laser (SASE-FEL). The main purpose of the experiment will be the production of short electron bunches as long as few SASE cooperation lengths and to validate scaling laws to foresee operation at shorter wavelength in the future operation with SPARX. The basic physics, the experimental parameters and 3D simulations are discussed.

Antihydrogen production by charge exchange reaction between Positronium (Ps) atoms and antiprotons requires an efficient excitation of Ps atoms up to high-n levels (Rydberg levels). In this study it is assumed that a Ps cloud is produced within a relatively strong uniform magnetic field (1 Tesla) and with a relatively high temperature (100 K). Consequently, the structure of energy levels are deeply modified by Zeeman and motional Stark effects. A two-step laser light excitation, the first one from ground to n=3 and the second from this level to a Rydberg level, is proposed and the physics of the problem is discussed. We derive a simple formula giving the absorption probability with substantially incoherent laser pulses. A 30% population deposition in high-$n$ states can be reached with feasible lasers suitably tailored in power and spectral bandwidth., Comment: 18 pages, 4 figures; changed content (with erasing of section 2), adding a new figure and new references

The formation of the antihydrogen beam in the AEGIS experiment through the use of inhomogeneous electric fields is discussed and simulation results including the geometry of the apparatus and realistic hypothesis about the antihydrogen initial conditions are shown. The resulting velocity distribution matches the requirements of the gravity experiment. In particular it is shown that the inhomogeneous electric fields provide radial cooling of the beam during the acceleration., Comment: Invited talk at Pbar08 - Workshop on Cold Antimatter Plasmas and Application to Fundamental Physics, Okinawa, Japan, 2008

Problems relevant to a continuous-wave FEL operating in the cm-mm region are investigated. The SEAFEL (small electrostatic accelerator for free electron laser) experiment, in progress at LNL (Laboratori Nazionali Legnaro), is presented. The electrostatic accelerator is of the Cockroft-Walton type and delivers a 3 A, 0.7 MeV electron beam. This current level can be reached with a beam recovery system. The “immersed-flow” configuration for the beam transport is adopted. The FEL has a 1.5 m long permanent-magnet helical undulator and a resonator with Bragg mirrors.