Abstract
The fraction of laser energy converted into hot electrons by the twoplasmondecay instability is found to have different overlapped intensity thresholds for various configurations on the Omega Laser Facility [T. R. Boehly et al., Opt. Commun. 133, 495 (1997); J. H. Kelly et al., J. Phys. IV 133, 75 (2006)]. A factorof2 difference in the overlapped intensity threshold is observed between two and fourbeam configurations. The overlapped intensity threshold increases by a factor of 2 between the 4 and 18beam configurations and by a factor of 3 between the 4 and 60beam configurations. This is explained by a linear commonwave model where multiple laser beams drive a common electronplasma wave in a wavevector region that bisects the laser beams (resonant commonwave region in kspace). These experimental results indicate that the hotelectron threshold depends on the hydrodynamic parameters at the quartercritical density surface, the configuration of the laser beams, and the sum of the intensity of the beams that share the same angle with the commonwave vector.
This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DEFC5208NA28302, the University of Rochester, and the New York State Energy Research and Development Authority. The support of DOE does not constitute an endorsement by DOE of the views expressed in this article.
I. INTRODUCTION
II. EXPERIMENTAL SETUP
A. Laser setup
1. OMEGA EP planar geometry
2. OMEGA planar geometry
3. OMEGA spherical geometry
B. Targets
1. Planar target
2. Spherical target
C. Diagnostics
1. XRay spectrometer
2. Hardxray detector
III. EXPERIMENTAL RESULTS
IV. COMMONWAVE MODELING
A. Multiple linearly polarized beams
B. Multiple beams with polarization smoothing
V. INTERPRETATION OF EXPERIMENTAL RESULTS
A. Beam geometry and polarization
B. Number of contributing beams
C. Plasma parameters
VI. CONCLUSION
Key Topics
 Plasma waves
 28.0
 Laser beams
 27.0
 Hot carriers
 14.0
 Polarization
 14.0
 Polarized particle beams
 13.0
Figures
Schematic of the laserbeam configurations on (a) OMEGA EP, (b) OMEGA planar, and (c) OMEGA spherical. On OMEGA EP experiments, the polarizations of the beams are from vertical [inset (a)], and on OMEGA experiments the beams used polarization smoothing.
Schematic of the laserbeam configurations on (a) OMEGA EP, (b) OMEGA planar, and (c) OMEGA spherical. On OMEGA EP experiments, the polarizations of the beams are from vertical [inset (a)], and on OMEGA experiments the beams used polarization smoothing.
Hotelectron fraction ( ) as a function of vacuum overlapped laser intensity. Single, two, and four correspond to OMEGA EP planar experiments where the beams are linearly polarized; 18 (60) corresponds to OMEGA planar (spherical) experiments where the beams have polarization smoothing. For each configuration, the overlapped intensity is given by the vacuum intensity of the laser beams on the target surface. The dashed lines are drawn to guide the eye. In each case, the overlapped intensity at quartercritical density is about half the vacuum overlapped intensity. For the eighteen beam configuration, at an overlapped intensity of , the signal was lower than the diagnostic detection threshold (red arrow).
Hotelectron fraction ( ) as a function of vacuum overlapped laser intensity. Single, two, and four correspond to OMEGA EP planar experiments where the beams are linearly polarized; 18 (60) corresponds to OMEGA planar (spherical) experiments where the beams have polarization smoothing. For each configuration, the overlapped intensity is given by the vacuum intensity of the laser beams on the target surface. The dashed lines are drawn to guide the eye. In each case, the overlapped intensity at quartercritical density is about half the vacuum overlapped intensity. For the eighteen beam configuration, at an overlapped intensity of , the signal was lower than the diagnostic detection threshold (red arrow).
(a) The commonwave region for two beams is given by a plane that bisects the wave vectors ( ) of the laser beams (red plane). (b) A common EPW can be driven only by multiple laser beams that share the same angle to the commonwave vector ( ) in order to satisfy the dispersion relation for each daughter EPW ( ).
(a) The commonwave region for two beams is given by a plane that bisects the wave vectors ( ) of the laser beams (red plane). (b) A common EPW can be driven only by multiple laser beams that share the same angle to the commonwave vector ( ) in order to satisfy the dispersion relation for each daughter EPW ( ).
(a) 3D representation of the maximum growth rate for a single beam with polarization smoothing (gray hyperboloids). Multiple beams with polarization smoothing can couple through the common wave along the commonwave line ( , red dashed line) at an angle . (b) Normalized singlebeam with polarization smoothing growth rate in the plane ( ). The Landau cutoff ( , where k is the maximum value between and ) for is represented with a black dashed line. The normalized multiplebeam growth rate is equal to the singlebeam growth rate along . (c) Normalized multiplebeam growth rate calculated along the commonwave line for (solid red line), (dashed red line), and (dotted red line). The cutoff for small and large corresponds to the Landau cutoff calculated for .
(a) 3D representation of the maximum growth rate for a single beam with polarization smoothing (gray hyperboloids). Multiple beams with polarization smoothing can couple through the common wave along the commonwave line ( , red dashed line) at an angle . (b) Normalized singlebeam with polarization smoothing growth rate in the plane ( ). The Landau cutoff ( , where k is the maximum value between and ) for is represented with a black dashed line. The normalized multiplebeam growth rate is equal to the singlebeam growth rate along . (c) Normalized multiplebeam growth rate calculated along the commonwave line for (solid red line), (dashed red line), and (dotted red line). The cutoff for small and large corresponds to the Landau cutoff calculated for .
The hotelectron fraction is plotted as a function of the commonwave gain for each experimental configuration tested. The errors bars shown are a result of the uncertainties in the laser beam intensities, and the temporal variation in the hydrodynamic conditions after the plasma has reached steady state.
The hotelectron fraction is plotted as a function of the commonwave gain for each experimental configuration tested. The errors bars shown are a result of the uncertainties in the laser beam intensities, and the temporal variation in the hydrodynamic conditions after the plasma has reached steady state.
The projection of the commonwave vector in the plane ( ) for a single beam with polarization smoothing (PS).
The projection of the commonwave vector in the plane ( ) for a single beam with polarization smoothing (PS).
Tables
List of parameters defining the commonwave gain that were varied during the experiments. is the number of beams that share an equivalent angle with the common electronplasma wave [see Eq. (3) ] with the largest growth rate, is in units of and is in units of .
List of parameters defining the commonwave gain that were varied during the experiments. is the number of beams that share an equivalent angle with the common electronplasma wave [see Eq. (3) ] with the largest growth rate, is in units of and is in units of .
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