Nelson Lopes

AWAKE, The Advanced Proton Driven Plasma Wakefield Acceleration Experiment at CERN

The Advanced Proton Driven Plasma Wakefield Acceleration Experiment (AWAKE) aims at studying plasma wakefield generation and electron acceleration driven by proton bunches. It is a proof-of-principle R&D experiment at CERN and the world׳s first proton driven plasma wakefield acceleration experiment. The AWAKE experiment will be installed in the former CNGS facility and uses the 400 GeV/c proton beam bunches from the SPS. The first experiments will focus on the self-modulation instability of the long (rms ~12 cm) proton bunch in the plasma. These experiments are planned for the end of 2016. Later, in 2017/2018, low energy (~15 MeV) electrons will be externally injected into the sample wakefields and be accelerated beyond 1 GeV. The main goals of the experiment will be summarized. A summary of the AWAKE design and construction status will be presented.

Laser-wakefield accelerators as hard x-ray sources for 3D medical imaging of human bone

A bright μm-sized source of hard synchrotron x-rays (critical energy Ecrit > 30 keV) based on the betatron oscillations of laser wakefield accelerated electrons has been developed. The potential of this source for medical imaging was demonstrated by performing micro-computed tomography of a human femoral trabecular bone sample, allowing full 3D reconstruction to a resolution below 50 μm. The use of a 1 cm long wakefield accelerator means that the length of the beamline (excluding the laser) is dominated by the x-ray imaging distances rather than the electron acceleration distances. The source possesses high peak brightness, which allows each image to be recorded with a single exposure and reduces the time required for a full tomographic scan. These properties make this an interesting laboratory source for many tomographic imaging applications.

Plasma Channels for Electron Accelerators Using Discharges in Structured Gas Cells

We present a new scheme to produce preformed plasma waveguides to extend the acceleration length of laser-plasma accelerators. The plasma is produced in a hydrogen background by a high-voltage discharge between two hollow conic electrodes through a sequence of thin dielectric plates with apertures that fix the initial plasma diameter and position. The thickness of the dielectric plates is close to their aperture diameter and negligible when compared with the distance between them. This results in a fully ionized plasma cylinder that expands almost freely, evolving into a radial parabolic density profile with a minimum on axis as required for high-intensity guiding. In this paper, besides the description of the guiding device and its operation, we report the plasma density measurements of the first prototype with 16.6-mm length obtained by Mach-Zehnder interferometry as well as laser guiding results.

A Plasma Lens for High Intensity Laser Focusing

A plasma lens based on a short hydrogen‐filled alumina capillary discharge is experimentally characterized. For a plasma length of about 5mm, the focal length, measured from the plasma entrance, was ∼ 11 to 8mm for on axis densities of ∼ 2.5 to 5 × 1018cm−3, respectively. The plasma temperature away from the walls of the 1/2mm diameter capillary was estimated to be ∼ 8eV indicating that the plasma is fully ionized. Such a lens should thus be suitable for focusing very high intensity pulses. Comparisons of the measured focusing strength to that predicted by a first‐order fluid model [N. A. Bobrova, et al., Phys. Rev. E 65, 016407 (2002)] shows reasonable agreement given that some of the observed plasma parameters are not predicted by this model.

Radiological Issues in Laser‐Matter Interactions at Under 200 PW and High Repetition Rates

The Extreme Light Infrastructure (ELI) project aims to provide laser beams up to 200 PW, with intensities in the 1023–1025 W⋅cm−3 range. This enables ultra‐relativistic laser‐matter experiments, such as the development of compact particle accelerators. PIC simulations have shown that under certain conditions a directed proton beam can emerge from a thin foil at energies of several GeV per particle, with laser‐to‐particle energy conversion efficiencies better than 50%. At the same time electrons with energies in the 1 GeV range are scattered in arbitrary directions, including backwards and sideways. This omnidirectional source term, along with the main beam, need to be properly contained for safety and operational reasons. For that purpose the primary particle distribution given by the PIC simulations is post‐processed with Geant4 and the secondary radiation and consequent material activation is determined. The relatively high repetition rates expected for ELI, up to 1 shot per minute at ∼100 PW and 1 kHz ...

Photon acceleration as the laser wakefield diagnostic for future plasma accelerators

In the near future, laser-plasma particle accelerators will be able to sustain electric fields in excess of 100 GeV/m, along plasma channels several Rayleigh lengths long. For these extreme conditions, present day laser wakefield diagnostics such as Frequency Domain Interferometry will not be able to resolve the wakefield structure and determine the magnitude of the electric field. In this paper, we present a detailed comparison between frequency-domain interferometry and a photon acceleration based wakefield diagnostic. We determine the experimental parameters for which photon acceleration becomes the only viable diagnostic technique. Dispersion effects on the probe beam and the implications of an arbitrary phase velocity of the plasma wave are discussed for both diagnostic techniques. We also propose an experimental set-up for a photon acceleration diagnostic allowing for the simultaneous measurement of the electric field structure and the laser wakefield phase velocity. Comparison with results from photon acceleration experiments by ionization fronts are also presented.

Formation of ionization fronts by intense short laser pulses propagating in a neutral gas

Propagation of short laser pulses in gases resulting in relativistic ionization fronts is studied numerically using a kinetic formulation based in the photon number phase-space distribution function. With this approach we are able to follow the dynamics of the laser pulse both in time and spectral content. The advance of the photon number is obtained by solving a Klimontovich type equation. The properties of the emergent laser pulse, responsible for the ionization front, such as duration, chirp and spectrum are continuously monitored by adequate diagnostics of the photon number phase-space distribution. In particular, a detailed analysis of the evolution of the laser pulse velocity is presented.

Experimental observation of photon acceleration of femtosecond laser pulses in relativistic under-dense ionization fronts

Frequency up-shifts from photon acceleration ands flash ionization of ultrashort laser pulses counter-propagating and co-propagating at a given angle with respect to a laser produced relativistic under-dense ionization are studied.