697 research outputs found
Introducing the Fission-Fusion Reaction Process: Using a Laser-Accelerated Th Beam to produce Neutron-Rich Nuclei towards the N=126 Waiting Point of the r Process
We propose to produce neutron-rich nuclei in the range of the astrophysical
r-process around the waiting point N=126 by fissioning a dense
laser-accelerated thorium ion bunch in a thorium target (covered by a CH2
layer), where the light fission fragments of the beam fuse with the light
fission fragments of the target. Via the 'hole-boring' mode of laser Radiation
Pressure Acceleration using a high-intensity, short pulse laser, very
efficiently bunches of 232Th with solid-state density can be generated from a
Th layer, placed beneath a deuterated polyethylene foil, both forming the
production target. Th ions laser-accelerated to about 7 MeV/u will pass through
a thin CH2 layer placed in front of a thicker second Th foil closely behind the
production target and disintegrate into light and heavy fission fragments. In
addition, light ions (d,C) from the CD2 production target will be accelerated
as well to about 7 MeV/u, inducing the fission process of 232Th also in the
second Th layer. The laser-accelerated ion bunches with solid-state density,
which are about 10^14 times more dense than classically accelerated ion
bunches, allow for a high probability that generated fission products can fuse
again. In contrast to classical radioactive beam facilities, where intense but
low-density radioactive beams are merged with stable targets, the novel
fission-fusion process draws on the fusion between neutron-rich, short-lived,
light fission fragments both from beam and target. The high ion beam density
may lead to a strong collective modification of the stopping power in the
target, leading to significant range enhancement. Using a high-intensity laser
as envisaged for the ELI-Nuclear Physics project in Bucharest (ELI-NP),
estimates promise a fusion yield of about 10^3 ions per laser pulse in the mass
range of A=180-190, thus enabling to approach the r-process waiting point at
N=126.Comment: 13 pages, 6 figure
Paramagnetic reentrant effect in high purity mesoscopic AgNb proximity structures
We discuss the magnetic response of clean Ag coated Nb proximity cylinders in
the temperature range 150 \mu K < T < 9 K. In the mesoscopic temperature
regime, the normal metal-superconductor system shows the yet unexplained
paramagnetic reentrant effect, discovered some years ago [P. Visani, A. C.
Mota, and A. Pollini, Phys. Rev. Lett. 65, 1514 (1990)], superimposing on full
Meissner screening. The logarithmic slope of the reentrant paramagnetic
susceptibility chi_para(T) \propto \exp(-L/\xi_N) is limited by the condition
\xi_N=n L, with \xi_N=\hbar v_F/2 \pi k_B T, the thermal coherence length and
n=1,2,4. In wires with perimeters L=72 \mu m and L=130 \mu m, we observe
integer multiples n=1,2,4. At the lowest temperatures, \chi_para compensates
the diamagnetic susceptibility of the \textit{whole} AgNb structure.Comment: 4 pages, 4 figures (color
Dynamics of laser-driven proton acceleration exhibited by measured laser absorptivity and reflectivity
Proton acceleration from nanometer thin foils with intense laser pulses is investigated experimentally. We analyzed the laser absorptivity by parallel monitoring of laser transmissivity and reflectivity with different laser intensities when moving the targets along the laser axis. A direct correlation between laser absorptivity and maximum proton energy is observed. Experimental results are interpreted in analytical estimation, exhibiting a coexistence of plasma expansion and light-sail form of radiation pressure acceleration (RPA-LS) mechanisms during the entire proton acceleration process based on the measured laser absorptivity and reflectivity
Ultrasmall divergence of laser-driven ion beams from nanometer thick foils
We report on experimental studies of divergence of proton beams from
nanometer thick diamond-like carbon (DLC) foils irradiated by an intense laser
with high contrast. Proton beams with extremely small divergence (half angle)
of 2 degree are observed in addition with a remarkably well-collimated feature
over the whole energy range, showing one order of magnitude reduction of the
divergence angle in comparison to the results from micrometer thick targets. We
demonstrate that this reduction arises from a steep longitudinal electron
density gradient and an exponentially decaying transverse profile at the rear
side of the ultrathin foils. Agreements are found both in an analytical model
and in particle-in-cell simulations. Those novel features make nm foils an
attractive alternative for high flux experiments relevant for fundamental
research in nuclear and warm dense matter physics.Comment: 11 pages, 5 figure
Temporally Resolved Intensity Contouring (TRIC) for characterization of the absolute spatio-temporal intensity distribution of a relativistic, femtosecond laser pulse
Today's high-power laser systems are capable of reaching photon intensities up to 10(22)W cm(-2), generating plasmas when interacting with material. The high intensity and ultrashort laser pulse duration (fs) make direct observation of plasma dynamics a challenging task. In the field of laser-plasma physics and especially for the acceleration of ions, the spatio-temporal intensity distribution is one of the most critical aspects. We describe a novel method based on a single-shot (i.e. single laser pulse) chirped probing scheme, taking nine sequential frames at frame rates up to THz. This technique, to which we refer as temporally resolved intensity contouring (TRIC) enables single-shot measurement of laser-plasma dynamics. Using TRIC, we demonstrate the reconstruction of the complete spatio-temporal intensity distribution of a high-power laser pulse in the focal plane at full pulse energy with sub-picosecond resolution
Proton acceleration by irradiation of isolated spheres with an intense laser pulse
We report on experiments irradiating isolated plastic spheres with a peak laser intensity of 2-3 x 10(20) W cm(-2). With a laser focal spot size of 10 mu m full width half maximum (FWHM) the sphere diameter was varied between 520 nm and 19.3 mu m. Maximum proton energies of similar to 25 MeV are achieved for targets matching the focal spot size of 10 mu m in diameter or being slightly smaller. For smaller spheres the kinetic energy distributions of protons become nonmonotonic, indicating a change in the accelerating mechanism from ambipolar expansion towards a regime dominated by effects caused by Coulomb repulsion of ions. The energy conversion efficiency from laser energy to proton kinetic energy is optimized when the target diameter matches the laser focal spot size with efficiencies reaching the percent level. The change of proton acceleration efficiency with target size can be attributed to the reduced cross-sectional overlap of subfocus targets with the laser. Reported experimental observations are in line with 3D3V particle in cell simulations. They make use of well-defined targets and point out pathways for future applications and experiments.DFG via the Cluster of Excellence Munich-Centre for Advanced Photonics (MAP) Transregio SFB TR18NNSA DE-NA0002008Super-MUC pr48meIvo CermakCGC Instruments in design and realization of the Paul trap systemIMPRS-APSLMUexcellent Junior Research FundDAAD|ToIFEEuropean Union's Horizon research and innovation programme 633053Physic
Rough Surface Effect on Meissner Diamagnetism in Normal-layer of N-S Proximity-Contact System
Rough surface effect on the Meissner diamagnetic current in the normal layer
of proximity contact N-S bi-layer is investigated in the clean limit. The
diamagnetic current and the screening length are calculated by use of
quasi-classical Green's function. We show that the surface roughness has a
sizable effect, even when a normal layer width is large compared with the
coherence length . The effect is as large as that
of the impurity scattering and also as that of the finite reflection at the N-S
interface.Comment: 12 pages, 3 figures. To be published in J. Phys. Soc. Jpn. Vol.71-
Diamagnetic response of cylindrical normal metal - superconductor proximity structures with low concentration of scattering centers
We have investigated the diamagnetic response of composite NS proximity
wires, consisting of a clean silver or copper coating, in good electrical
contact to a superconducting niobium or tantalum core. The samples show strong
induced diamagnetism in the normal layer, resulting in a nearly complete
Meissner screening at low temperatures. The temperature dependence of the
linear diamagnetic susceptibility data is successfully described by the
quasiclassical Eilenberger theory including elastic scattering characterised by
a mean free path l. Using the mean free path as the only fit parameter we found
values of l in the range 0.1-1 of the normal metal layer thickness d_N, which
are in rough agreement with the ones obtained from residual resistivity
measurements. The fits are satisfactory over the whole temperature range
between 5 mK and 7 K for values of d_N varying between 1.6 my m and 30 my m.
Although a finite mean free path is necessary to correctly describe the
temperature dependence of the linear response diamagnetic susceptibility, the
measured breakdown fields in the nonlinear regime follow the temperature and
thickness dependence given by the clean limit theory. However, there is a
discrepancy in the absolute values. We argue that in order to reach
quantitative agreement one needs to take into account the mean free path from
the fits of the linear response. [PACS numbers: 74.50.+r, 74.80.-g]Comment: 10 pages, 9 figure
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