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Characterizing the energy distribution of laser-generated relativistic electrons in cone-wire targets
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Image of FIG. 1.
FIG. 1.

(a) TITAN cone-wire experimental setup and diagnostic layout. (b) Measured Cu Kα emission from 1.5 mm long Cu wire.

Image of FIG. 2.
FIG. 2.

Transversely integrated lineouts along the Cu wire from the Kα imager for various prepulse energies of 17, 100, 500, and 1000 mJ. The lineouts were normalized to their peak.

Image of FIG. 3.
FIG. 3.

Contours of fast electron density (nfast), radial electric field (ER) and azimuthal magnetic field (Bθ) simulated at 1.0, 4.5, and 7.5 ps. The electrons using the two-temperature distribution are injected at z = 0 with 40° Gaussian spread.

Image of FIG. 4.
FIG. 4.

Fast electron particle plots at 5.0 ps for three energy groups: (a) less than 50 keV, (b) between 0.5 and 2.5 MeV and (c) >3.5 MeV. The energy of electrons surfing along the wire is ∼1 MeV as shown in (b). The electron bunch between 500 and 1500 μm corresponds to the time-of-flight of electrons at speed of light for 0.7 ps duration. The vertical edge of the bunch is due to aspect ratio.

Image of FIG. 5.
FIG. 5.

Energy partition of fast electron transport simulation with Tcold = 800 keV and Thot = 6 MeV for the intrinsic prepulse case. The total injected energy (blue) is distributed to the fast electron energy (pink), thermal energy (red), field energy associated with the electrons (light blue), ionization (black) and energy into radiation loss (green). The energy carried by electrons leaving from the simulation boundaries is shown in dotted-blue. The energy error of the simulation is plotted in dotted-red.

Image of FIG. 6.
FIG. 6.

Comparisons of the measured wire profile for the 17 mJ to simulations using single temperature: (a) T = 500, 800, and 1200 keV and (b) T = 4, 8, and 12 MeV. (c) The simulated escaped electron spectra for 1.2, 4, 6, 8, and 12 MeV. They are compared with the measured electron spectrum for the intrinsic prepulse case (shot 17s5). (d) Comparison of the measurement and model with two-temperature distribution (Tcold = 700 keV, Thot = 6 MeV, Ehot/Etotal = 70%).

Image of FIG. 7.
FIG. 7.

Result of least square fitting for the measurement at 17 mJ shot. The input electron distribution was varied by changing Tcold and energy fraction in the cold component while Thot = 6 MeV and the total injected current were fixed. The contour is plotted in log scale of the least square values normalized by the minimum. The dotted line represents twice the minimum of lease squares (log10(2) = 0.3).

Image of FIG. 8.
FIG. 8.

Comparisons of the measured Kα wire profile for the 17 mJ case with the simulated for different initial angular momentum. The spread of the electrons was modeled with a Gaussian function (exp[ − (θ/dθ)2]) and dθ = 5, 40, and 80°.

Image of FIG. 9.
FIG. 9.

Fits of LSP simulations of the Kα wire profiles to the measurements for the prepulse energies of 17(open circle), 100 (open square), 500 (asterisk), and 1000 mJ (open diamond). In the fitting, Tcold and total injected energies were varied with fixed Thot of 6 MeV. As the prepulse energy increases, the slope of the fall-off decreases from 700 keV to 400 keV while the energy percentage in the hot component increases from 70.0% to 96.6%. The total electron energy entering the wire decreases from 5.2 J to 2.2 J.

Image of FIG. 10.
FIG. 10.

(a) The coupling efficiencies inferred from the fits using exponential and relativistic-Maxwell distributions as a function of the prepulse energy. The coupling efficiencies were calculated for the coupling from the laser to all electrons into the wire and from the laser to 1-3 MeV electrons. (b) The electron energy distributions using the inferred parameters from the fits to the measurements for 17 mJ (solid) and 1000 mJ (dotted) cases. The distributions were reconstructed with exponential (red) and relativistic Maxwellian (black) distributions.

Image of FIG. 11.
FIG. 11.

(a) The measured Kα spectrum of the cone-wire shot for 17 mJ was compared with modeled Cu Kα spectra with FLYCHK for 10, 50, and 100 eV temperature. The modeled spectra were convolved with the instrument function. (b) Calculated total Kα signals within the crystal bandwidth as a function of electron temperature. The signals were normalized by that for 20 eV.


Generic image for table
Table I.

Summary of inferred electron spectrum parameters using exponential energy distribution for the prepulse energies considered.


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Characterizing the energy distribution of laser-generated relativistic electrons in cone-wire targets