A. Bacci

Synaptic and intrinsic mechanisms shape synchronous oscillations in hippocampal neurons in culture

We have detected spontaneous, synchronous calcium oscillations, associated with variations in membrane potential, in hippocampal neurons maintained in primary culture. The oscillatory activity is synaptically driven, as it is blocked by tetrodotoxin, by the glutamate receptor antagonist 6‐cyano‐7‐nitroquinoxaline‐2,3‐dione (CNQX) and by toxins inhibiting neurotransmitter release from presynaptic nerve endings. Neuronal oscillations do not require for their expression the presence of a polyneuronal network and are not primarily influenced by the γ‐aminobutyric acid (GABAA) receptor antagonist picrotoxin, suggesting that they entirely rely on glutamatergic neurotransmission. Synaptic and intrinsic conductances shape the synchronized oscillations in hippocampal neurons. The concomitant activation of N‐methyl‐d‐aspartate (NMDA) receptors and voltage‐activated L‐type calcium channels allows calcium entering from the extracellular medium and sustaining the long depolarization, which shapes every single calcium wave.

Synaptogenesis in hippocampal cultures.

Hippocampal cultures offer unique advantages for the study of neuronal development and synaptogenesis. Studies performed on this model enabled dissection of the temporal sequence of events which lead to the differentiation of pre- and postsynaptic compartments.

Astrocytes are required for the oscillatory activity in cultured hippocampal neurons

Synchronous oscillations of intracellular calcium concentration ([Ca2+]i) and of membrane potential occurred in a limited population of glutamatergic hippocampal neurons grown in primary cultures. The oscillatory activity occurred in synaptically connected cells only when they were in the presence of astrocytes. Microcultures containing only one or a few neurons also displayed oscillatory activity, provided that glial cells participated in the network. The glutamate‐transporter inhibitors L‐trans‐pyrrolidine‐2,4‐dicarboxylic acid (PDC) and dihydrokainate, which produce an accumulation of glutamate in the synaptic microenvironment, impaired the oscillatory activity. Moreover, in neurons not spontaneously oscillating, though in the presence of astrocytes, oscillations were induced by exogenous l‐glutamate, but not by the stereoisomer d‐glutamate, which is not taken up by glutamate transporters. These data demonstrate that astrocytes are essential for neuronal oscillatory activity and provide evidence that removal of glutamate from the synaptic environment is one of the major mechanisms by which glial cells allow the repetitive excitation of the postsynaptic cell.

Electron Linac design to drive bright Compton back-scattering gamma-ray sources

The technological development in the field of high brightness linear accelerators and high energy/high quality lasers enables today designing high brilliance Compton-X and Gamma-photon beams suitable for a wide range of applications in the innovative field of nuclear photonics. The challenging requirements of this kind of source comprise: tunable energy (1–20 MeV), very narrow bandwidth (0.3%), and high spectral density (104 photons/s/eV). We present here a study focused on the design and the optimization of an electron Linac aimed to meet the source specifications of the European Extreme Light Infrastructure—Nuclear Physics project, currently funded and seeking for an innovative machine design in order to outperform state-of-the-art facilities. We show that the phase space density of the electron beam, at the collision point against the laser pulse, is the main quality factor characterizing the Linac.

A TTX-sensitive conductance underlying burst firing in isolated pyramidal neurons from rat neocortex

Pyramidal neurons were acutely isolated from neocortex slices of 14- to 20-day-old rats and patch-clamped under physiological conditions. Current-clamp recordings revealed firing patterns corresponding to those previously reported in slices as regular spiking (RS) and intrinsically bursting (IB), i.e., single action potentials (AP), trains of regular spikes and bursts with depolarizing after-potentials (DAP). In IB neurons, intracellular perfusion with KF blocked the high-voltage-activated Ca2+ and the Ca(2+)-dependent K+ currents, revealing APs with a 10-30 ms shoulder at -35 mV (shoulder AP), which was the supporting plateau of the intraburst spikes. The use of the A channel blocker, 4-aminopyridine, caused a three-fold reduction in the AP repolarizing rate. A study of the de- and repolarizing rates modulating the spike shape (shoulder AP, burst or single APs) suggested that the percentage of available A channels could play a crucial role in burst formation. Blockade of the residual T-type Ca2+ current by Ni2+ did not inhibit the AP shoulder, whereas it was completely and reversibly inhibited by 30 nM TTX, which did not affect AP amplitude. The AP rising rate was only halved by 100 nM TTX. The data concerning the A channel-mediated burst formation and the role of the TTX-sensitive conductance have been successfully simulated in a model cell. We suggest that bursting is an intrinsic property of the membrane of neocortex neurons, and is sustained by TTX-sensitive slowly inactivating and/or persistent Na+ conductances.

The SPARC linear accelerator based terahertz source

Ultra-short electron beams, produced through the velocity bunching compression technique, are used to drive the SPARC linear accelerator based source, which relies on the emission of coherent transition radiation in the terahertz range. This paper reports on the main features of this radiation, as terahertz source, with spectral coverage up to 5 THz and pulse duration down to 200 fs, with an energy per pulse of the order of several micro-joule, and as electron beam longitudinal diagnostics.

Laser-driven electron beamlines generated by coupling laser-plasma sources with conventional transport systems

Laser-driven electron beamlines are receiving increasing interest from the particle accelerator community. In particular, the high initial energy, low emittance, and high beam current of the plasma based electron source potentially allow generating much more compact and bright particle accelerators than what conventional accelerator technology can achieve. Using laser-generated particles as injectors for generating beamlines could significantly reduce the size and cost of accelerator facilities. Unfortunately, several features of laser-based particle beams need still to be improved before considering them for particle beamlines and thus enable the use of plasma-driven accelerators for the multiple applications of traditional accelerators. Besides working on the plasma source itself, a promising approach to shape the laser-generated beams is coupling them with conventional accelerator elements in order to benefit from both a versatile electron source and a controllable beam. In this paper, we perform start-to-end simulations to generate laser-driven beamlines using conventional accelerator codes and methodologies. Starting with laser-generated electrons that can be obtained with established multi-hundred TW laser systems, we compare different options to capture and transport the beams. This is performed with the aim of providing beamlines suitable for potential applications, such as free electron lasers. In our approach, we have analyzed which parameters are critical at the source and from there evaluated different ways to overcome these issues using conventional accelerator elements and methods. We show that electron driven beamlines are potentially feasible, but exploiting their full potential requires extensive improvement of the source parameters or innovative technological devices for their transport and capture.

Linear and Nonlinear Thomson Scattering for Advanced X-ray Sources in PLASMONX

Thomson scattering of laser pulses onto ultrarelativistic e-bunches is becoming an advanced source of tunable, quasi-monochromatic, and ultrashort X/gamma radiation. Sources aimed at reaching a high flux of scattered photons need to be driven by high-brightness e-beams, whereas extremely short (femtosecond scale or less) sources need to make femtosecond-long e-beams that collide with the laser pulses. In this paper, we explore the performance of the PLASMONX TS source in several operating regimes, including preliminary results on a source based on e-bunches produced by laser wakefield acceleration and controlled injection via density down ramp.

Large-bandwidth two-color free-electron laser driven by a comb-like electron beam

We discuss a two-color SASE free-electron laser (FEL) amplifier where the time and energy separation of two separated radiation pulses are controlled by manipulation of the electron beam phase space. Two electron beamlets with adjustable time and energy spacing are generated in an RF photo-injector illuminating the cathode with a comb-like laser pulse followed by RF compression in the linear accelerator. We review the electron beam manipulation technique to generate bunches with time and energy properties suitable for driving two-color FEL radiation. Experimental measurements at the SPARC-LAB facility illustrate the flexibility of the scheme for the generation of two-color FEL spectra.

The Project Plasmonx for Plasma Acceleration Experiments and A Thomson X-Ray Source at SPARC

We present the status of the project PLASMONX, recently approved by INFN. This project, based on a collaboration between INFN and CNR-IPCF, aims at a long term upgrade of the SPARC system with the goal to develop at LNF an integrated facility for advanced beam-laser-plasma research in the field of advanced acceleration techniques and ultra-bright X-ray radiation sources and related applications. The project, in its first phase, foresees the development at LNF of a High Intensity Laser Laboratory (HILL) whose main component is a 100 TW-class Ti: Sa laser system synchronized to the SPARC photo-injector. Experiments of self-injection and acceleration of electrons into laser driven plasma waves will be conducted at HILL-LNF, early in this first project phase. Eventually an additional beam line will be built in the SPARC bunker in order to transport the SPARC electron beam at an interaction point, where a final focus system will allow to conduct experiments either of laser-beam co-propagation in plasma waves for high gradient acceleration, or experiments of laser-beam head-on collisions to develop a Thomson source of bright ultra-short X-ray radiation pulses, with X-ray energies tunable in the range 20 to 1000 keV and pulse duration from 30 fs to 20 ps. Preliminary simulations of plasma acceleration with self-injection are illustrated, as well as external injection of the SPARC electron beam.