Evidence of magnetic reconnection (MR) events driven by relativistic electrons is observed between two high-intensity laser/plasma interaction sites. The two laser foci were on average 20um FWHM containing 50TW of power each, delivered with a split f/3 paraboloid onto copper foil targets at a focused intensity of 1019 W/cm2 with the HERCULES laser system. Cu K-alpha emissions from the interactions were imaged with a spherically bent Quartz crystal, and by motorizing one half of the paraboloid vertically the focal separation was varied between 0- 400um.
Splitting the beam halves revealed an enhanced region between the foci with the highest a maximized K-alpha signal intensity at one inter-beam separation, evidencing inflow from relativistic electron driven MR. A filtered LANEX screen was imaged to search for outflow/jet electrons along the plane of the target surface and normal to the axis defined by the two spots, to calculate the electron temperature and to search for spatial profile nonuniformities potentially directly originating from reconnection events. Ongoing 2D and 3D PIC simulations are being conducted to better understand and model the measured electron outflow dynamics.
An ultra-relativistic electron beam passing through a thick, high-Z solid target triggers an electromagnetic cascade, whereby a large number of high energy photons and electron-positron pairs are produced. By exploiting this physical process, we present here the first experimental evidence of the generation of ultra-short, highly collimated and ultra-relativistic positron beams following the interaction of a laser-wakefield accelerated electron beam with high-Z solid targets. Clear evidence has also been obtained of the generation of GeV electron-positron jets with variable composition depending on the solid target material and thickness. The percentage of positrons in the overall leptonic beam has been observed to vary from a few per cent up to almost fifty per cent, implying a quasi-neutral electron-positron beam. We anticipate that these beams will be of direct relevance to the laboratory study of astrophysical leptonic jets and their interaction with the interstellar medium.
Experiments at the HERCULES laser facility, originally reported by C. Zulick, et al in Applied Physics Letters (2013), have produced neutron beams with energies up to 16:8(±0:3) MeV using 73Li(d,n)84Be reactions. These efficient deuteron reactions required the selective acceleration of deuterons through the introduction of a deuterated plastic or cryogenically frozen D2O layer on the surface of a thin film target. It was shown that a optimized frozen D2O layer, formed in situ, yielded the highest efficiency deuteron acceleration with deuterons constituting over 99% of the accelerated light ions. The deuteron signal was optimized with respect to the delay between the heavy water deposition and laser pulse arrival, as well as the temperature of the target. A total conversion efficiency of laser energy to neutron energy of 1(±0:5) × 10−5 was obtained. The simulated neutron signal was found to be in reasonable agreement with the experimental spectra. The scattering of neutrons through shielding and target materials was investigated with MCNPX and determined to have a small effect on the observed neutron energies.
KEYWORDS: Electrons, X-rays, Plasma, Optical filters, Photons, Pulsed laser operation, Electron beams, X-ray sources, Simulation of CCA and DLA aggregates, Synchrotrons
The dynamics of relativistic electrons in a laser driven plasma cavity are studied via measurements of their
radiation. For ultrashort laser pulses at comparatively low focused laser intensities (3 < a0 < 10), low density
and long f-number of 10, electrons are predominantly accelerated in the wakefield leading to quasi-monoenergetic
collimated electron beams and well collimated (< 12 mrad) beams of comparatively soft x-rays (1-10 keV) with
unprecedented small source size (2-3 μm). For laser pulses with increasing laser intensity (10 < a0 < 30),
density and short f-number (< 5), electrons are accelerated directly by the laser, leading to divergent quasimaxwellian
electron beams and divergent (50-95°) beams of hard x-rays (20-50 keV) with relatively large source
size (> 100 μm). In both cases, the measured x-rays are well described in the synchrotron asymptotic limit of
electrons oscillating in a plasma channel. At low laser intensity transverse oscillations are small as the electrons
are predominantly accelerated axially by the laser generated wakefield. At high laser intensity, electrons are
directly accelerated by the laser. A betatron resonance leads to a tenfold increase in transverse oscillation
amplitude and electrons enter a highly radiative regime with up to 5% of their energy converted into x-rays.
We report on multi-MeV ion beam generation from the interaction of a 10 TW, 400 fs, 1.053 μm laser focused onto thin foil targets at intensities ranging from 1017 to 1019 W/cm2. Ion beam characteristics were studied by changing laser intensity, the preformed plasma scale-length and target material initial conductivity. We manipulated the proton beam divergence by using shaped targets. We observed nuclear transformation induced by high-energy protons and deuterons. A fully relativistic two-dimensional particle-in-cell simulation modeled energetic ion generation. These simulations identify the mechanism for the hot electron generation at the laser-plasma interface. Comparison with experiments sheds light on the dependence of ion-energy on preplasma scale length and solid density plasma thickness as well as relates ion energies for multi-species plasma.
Generation of relativistic electrons from the interaction of a laser pulse with a high density plasma foil, accompanied by an underdense preplasma in front of it, has been studied with 2D particle-in-cell (PIC) simulations for pulse duration comparable to a single-cycle and for single-wavelength spot size. The primary mechanism responsible for electron acceleration is identified. Simulations show that the energy of the accelerated electrons has a maximum versus the pulse-duration for relativistic laser intensities. The most effective electron acceleration takes place when the preplasma scale length is comparable to the pulse-duration. Electron distribution functions have been found from PIC simulations. Their tails are well approximated by Maxwellian distributions with a hot temperature in the MeV range.
High energy protons with the energy up to 10 MeV were accelerated in the forward direction using a tabletop laser with focusing intensity of 6 X 1018W/cm2. When a deuterated polystyrene was deposited on a front surface of a Mylar film and a boron sample was placed behind the laser target we observed the production of approximately 105 atoms of positron active isotope 11C from the reaction 10B(d,n)11C. The activation results suggest that deuterons were accelerated from the front surface of the laser target.
The protons with the energy up to 10 MeV accelerated in the forward direction from the thin Mylar film by relativistically intense 10 TW, 400 fs laser pulse have been observed. When a deuterated polystyrene was deposited on the front surface of the film and a boron sample was placed behind the target the production of 105 atoms of positron active isotope 11C from the reaction 10B(d,n)11C have been measured. The activation results suggest that ions (protons and deuterons) were accelerated from the front surface of the target.
We present time- and space-resolved XUV spectra of boron and carbon plasmas created by focusing 100-fs laser pulses on a solid target to an intensity of 1017 W/cm2. Emission lines originating from He-like and H-like excited states from n equals 2 to the ionization limit are observed with a spatial resolution of 100 micrometers in the direction normal to the target plane and with a temporal resolution of up to 4 ps. The position of the ionization limited is seen to depend very crucially on the plasma parameters of density and temperature, and is explained through continuum lowering effects. We observed the dynamics of the continuum lowering for plasma slices at different distances from the target, and record a maximum lowering of 40 eV in He-like carbon (10% of the ionization potential) from the disappearance of the 1snp - 1s2 line and from the position of the continuum edge.
A novel subpicosecond x-ray streak camera (called PX1) was developed by the INRS group for subpicosecond time resolved spectroscopy in x rays and X-UV range. Using the PX1 camera, we have measured keV x-ray pulses with a 950 fs FWHM and a 850 fs rise time. The camera has also been coupled to ultrafast photoconductive switches and tested in jitter-free mode as a signal averaging detector. This instrument allows to analyze ultrafast changes in short wavelength signals with an unlimited dynamic range.
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