8 research outputs found
Current channel evolution in ideal Z pinch for general velocity profiles
Recent diagnostic advances in gas-puff Z pinches at the Weizmann Institute
for the first time allow the reconstruction of the current flow as a function
of time and radius. These experiments show an unexpected radially-outward
motion of the current channel, as the plasma moves radially-inward [C.
Stollberg, Ph.D thesis, Weizmann Institute, 2019]. In this paper, a mechanism
that could explain this current evolution is described. We examine the impact
of advection on the distribution of current in a cylindrically symmetric
plasma. In the case of metric compression, with |v_r| proportional to r, the
current enclosed between each plasma fluid element and the axis is conserved,
and so the current profile maintains its shape. We show that for more general
velocity profiles, this simple behavior quickly breaks down, allowing for
non-conservation of current in a compressing conductor, rapid redistribution of
the current density, and even for the formation of reverse currents. In
particular, a specific inward radial velocity profile is shown to result in
radially-outward motion of the current channel, recovering the surprising
current evolution discovered at the Weizmann Institute.Comment: 12 pages, 6 figure
On the Stark Effect of the O I 777-nm Triplet in Plasma and Laser Fields
The O I 777-nm triplet transition is often used for plasma density diagnostics. It is also employed in nonlinear optics setups for producing quasi-comb structures when pumped by a near-resonant laser field. Here, we apply computer simulations to situations of the radiating atom subjected to the plasma microfields, laser fields, and both perturbations together. Our results, in particular, resolve a controversy related to the spectral line anomalously broadened in some laser-produced plasmas. The importance of using time-dependent density matrix is discussed
Direct observation of relativistic broken plasma waves
International audiencePlasma waves contribute to many fundamental phenomena, including astrophysics, thermonuclear fusion and particle acceleration. Such waves can develop in numerous ways, from classic Langmuir oscillations carried by electron thermal motion, to the waves excited by an external force and travelling with a driver. In plasma-based particle accelerators, a strong laser or relativistic particle beam launches plasma waves with field amplitude that follows the driver strength up to the wavebreaking limit, which is the maximum wave amplitude that a plasma can sustain. In this limit, plasma electrons gain sufficient energy from the wave to outrun it and to get trapped inside the wave bucket. Theory and numerical simulations predict multi-dimensional wavebreaking, which is crucial in the electron self-injection process that determines the accelerator performances. Here we present a real-time experimental visualization of the laser-driven nonlinear relativistic plasma waves by probing them with a femtosecond high-energy electron bunch from another laser-plasma accelerator coupled to the same laser system. This single-shot electron deflectometry allows us to characterize nonlinear plasma wakefield with femtosecond temporal and micrometre spatial resolutions revealing features of the plasma waves at the breaking point
Low divergence proton beams from a laser-plasma accelerator at kHz repetition rate
Proton beams with up to 100 pC bunch charge, 0.48 MeV cut-off energy and
divergence as low as a were generated from solid targets at kHz
repetition rate by a few-mJ femtosecond laser under controlled plasma
conditions. The beam spatial profile was measured using a small aperture
scanning time-of-flight detector. Detailed parametric studies were performed by
varying the surface plasma scale length from 8 to 80 nm and the laser pulse
duration from 4 fs to 1.5 ps. Numerical simulations are in good agreement with
observations and, together with an in-depth theoretical analysis of the
acceleration mechanism, indicate that high repetition rate femtosecond laser
technology could be used to produce few-MeV protons beams for applications.Comment: 6 pages, 4 figures (main text). 7 pages, 6 figures (supplemental
material
Low divergence proton beams from a laser-plasma accelerator at kHz repetition rate
International audienc
Simultaneous generation and detection of energetic particle and radiation beams from relativistic plasma mirrors driven at kHz repetition rate
We report on the first simultaneous measurement of high-order harmonics, relativistic electrons and low divergence proton beams generated from plasma mirrors driven at kHz repetition rate by relativistic-intensity milliJoule-energy femtosecond laser pulses. This setup enables detailed parametric studies of the particle and radiation spatio-spectral beam properties for a wide range of controlled interaction conditions, such as pulse duration and plasma density scale length. This versatile setup should aid in further understanding the collective laser absorption mechanisms at play during the laser-plasma interaction and in optimizing the secondary beam properties for potential applications
Characterization of spatiotemporal couplings with far-field beamlet cross-correlation
International audienceAbstract We present a novel, straightforward method for the characterization of spatiotemporal couplings in ultra-short laser pulses. The method employs far-field interferometry and inverse Fourier transform spectroscopy, built on the theoretical basis derived in this paper. It stands out in its simplicity: it requires few non-standard optical elements and simple analysis algorithms. This method was used to measure the space-time intensity of our 100 TW class laser and to test the efficacy of a refractive doublet as a suppressor of pulse front curvature (PFC). The measured low-order spatiotemporal couplings agreed with ray-tracing simulations. In addition, we demonstrate a one-shot measurement technique, derived from our central method, which allows for quick and precise alignment of the compressor by pulse front tilt (PFT) minimization and for optimal refractive doublet positioning for the suppression of PFC