Mordechai Botton

The role of pre-plasma in the efficient generation of protons by intense laser-matter interaction from structured targets is investigated. Optimal energy coupling between laser and plasma is found by varying the fluence and arrival time of an independently controllable ultrashort pre-pulse with respect to the main interaction pulse. The coupling is evaluated based on the energy of the accelerated protons. The accelerated proton energy is maximized at optimal pre-pulse delay and fluence conditions. Plasma emission spectrum and Particle-in-Cell simulations provide a possible explanation of the obtained experiment results.

Interaction between high-intensity lasers with solid targets is the key process in a wide range of novel laser-based particle accelerator schemes, as well as electromagnetic radiation sources. Common to all the processes is the generation of femtosecond pulses of relativistic electrons emitted from the targets as forerunners of the later-time principal products of the interaction scheme. In this paper, some diagnostics employed in laser–solid matter interaction experiments related to electrons, protons, ions, electromagnetic pulses (EMPs) and X-rays are reviewed. Then, we present our experimental study regarding fast electrons and EMPs utilizing a femtosecond-resolution detector previously adopted only in accelerator facilities.

Laser–plasma interactions have been studied in detail over the past twenty years, as they show great potential for the next generation of particle accelerators. The interaction between an ultra-intense laser and a solid-state target produces a huge amount of particles: electrons and photons (X-rays and -rays) at early stages of the process, with protons and ions following them. At SPARC_LAB Test Facility we have set up two diagnostic lines to perform simultaneous temporally resolved measurements on both electrons and protons.

Consolidating multiple high-energy femtosecond scale lasers is expected to enable implementation of cutting edge research areas varying from wakefield particle accelerators to ultra-high intensity laser pulses for basic fresearch. The ability to guide while augmenting a short-pulse laser is crucial in future laser based TeV particle accelerators where the laser energy depletion is the major setback. We propose, analyze and experimentally demonstrate consolidating multiple femtosecond pulse lasers in coupled curved capillaries. We demonstrate a proof of principle scheme of coupled curved capillaries where two femtosecond laser pulses are combined. We found that the details of the coupling region and injection scheme are crucial to the pulse consolidations. Furthermore, our simulations show that high-intensity short pulse laser can be guided in a small curvature radius capillary. Incorporating these finding in a curved capillary laser coupler will be a significant step towards realization of meters long TeV laser based particle accelerators.

Nowadays, plasma wakefield acceleration is the most promising acceleration technique for compact and cheap accelerators, needed in several fields, e.g., novel compact light sources for industrial and medical applications. Indeed, the high electric field available in plasma structures (>100 GV/m) allows for accelerating electrons at the GeV energy scale in a few centimeters. Nevertheless, this approach still suffers from shot-to-shot instabilities, mostly related to experimental parameter fluctuations, e.g., laser intensity and plasma density. Therefore, single shot diagnostics are crucial in order to properly understand the acceleration mechanism. In this regard, at the SPARC_LAB Test Facility, we have developed two diagnostic tools to investigate properties of electrons coming from high intensity laser–matter interaction: one relying on Electro Optical Sampling (EOS) for the measurement of the temporal profile of the electric field carried by fast electrons generated by a high intensity laser hitting a solid target, the other one based on Optical Transition Radiation (OTR) for single shot measurements of the transverse emittance. In this work, the basic principles of both diagnostics will be presented as well as the experimental results achieved by means of the SPARC high brightness photo-injector and the high power laser FLAME.

Enhanced proton acceleration to high energy by relatively modest ultrashort laser pulses and structured dynamic plasma snow targets was demonstrated experimentally. High proton yield emitted to narrow solid angle with energies of up 25 MeV were detected from interaction of a 5 TW laser with snow targets. The high yield was attributed to a carefully planned prepulse and microstructured snow targets. We studied experimentally the minimal energy requirements for the adequate prepulse and we are using PIC simulations to study the dynamics of acceleration process. Based on our simulations, we predict that using the proposed scheme protons can be accelerated to energies above 150 MeV by 100 TW laser systems.

Summary form only given. We present a simple non-intrusive experimental method allowing a complete single shot time resolved measurement of the relaxation of laser induced plasma filaments. The method is based on filament interaction with low intensity microwave radiation in a rectangular single mode waveguide. The suggested diagnostics allow a complete single shot temporal analysis of filament plasma decay with temporal resolution better than 0.3 ns and high spatial resolution along the filament. This experimental method does not require prior knowledge of filament dimensions or control over its exact spatial location. The experimental results are compared to numerical simulations of air plasma chemistry. Results reveal the role of various decay mechanisms including the importance of O₄⁺ molecular levels. Microwave diagnostics was used to study plasma lifetime prolongation by properly timed irradiation of the filament with a relatively love intensity nanosecond laser pulse. Lifetime prolongation of high density plasma filament by more than an order of magnitude has been demonstrated. The experimental results are in good agreement with our theoretical model that follows the evolution of the temperature and density of various molecules, atoms and ion species. The results point to the possibility of generating extremely long time duration, stable high density plasma filaments in air.

A modified complex-envelope (CE) alternating-direction-implicit (ADI) finite-difference time-domain solver for electromagnetic (EM) fields is presented. Like the conventional ADI scheme, the modified scheme is based on splitting the time step into two halves. In each substep, Maxwell's equations are solved implicitly in one direction and explicitly in the perpendicular direction. The new scheme is a combination of the conventional CE-ADI scheme and the leapfrog ADI scheme that was recently reported. The main features are a more compact form of the tridiagonal equations and greater flexibility in the application of boundary conditions. The boundary conditions on the six faces of the solution box are general and can be specified either as perfect conductor, symmetry boundary conditions, cavity modes, or arbitrary external specification. The new compact CE-ADI scheme is unconditionally stable; thus, the time step can be larger than the one imposed by the Courant-Friedrichs-Lewy (CFL) condition. Fast stable EM simulations of a planar slow-wave structure are demonstrated, with time steps exceeding the CFL limit by two orders of magnitude.

Summary form only given. We describe recent progress in the development of the NEPTUNE code (nonlinear evolution of particle trajectories under non-stationary excitation) for large-signal simulation of sheet-beam amplifiers. The NEPTUNE model uses a new formulation of the ADI-FDTD method to represent three- dimensional electromagnetic fields in the beam tunnel of a sheet-beam structure. Recently, two new features have been introduced. First, a model for particle trajectory integration has been implemented to represent a spatially distributed beam. This is based on a related scheme used previously in the successful TESLA code for round-beam amplifier simulation. Second, a number of additional boundary conditions have been implemented to allow for coupling to either external resonators or waveguides for incoming and outgoing waves. We describe the models used and verify simulation results for a number of examples. Finally we discuss ongoing efforts to optimize the algorithms and improve the performance of the code.