Thesis (Ph. D.)--University of Rochester. Department of Physics and Astronomy, 2018.
A laser-driven magnetized liner inertial fusion (MagLIF) experiment was designed
for the OMEGA laser system by scaling down the original point design for the
Z machine. 1-D hydrocode modeling was used to select pulse length, thickness
of the cylindrical shell, and initial gas pressure that maximizes the neutron yield
within the constraints of convergence ratio and implosion velocity. An implosion
velocity of twice that of Z machine implosions was chosen, and other factors in the
point design matched the Z design within a factor of 2. 2-D modeling of the point
design provides estimates for end losses, total neutron yields, and better estimates
of implosion velocity and convergence ratio. Scaling of the OMEGA point design to
MJ-class lasers was considered.
Focused experiments to establish laser preheat conditions and implosion uniformity
for laser driven cylinders were performed in preparation to integrate each piece
in the first ever MagLIF experiments. Implosion uniformity and velocity was determined
using x-ray self-emission of the cylinders in
ight. A laser balance of 83 %
of peak energy for oblique beams overlapped in the center and peak energy of the
normal beams irradiating the ends of the cylinder gives the most uniform cylindrical
implosion. Implosion velocities were determined for 4 different shell thicknesses.
Azimuthal uniformity was corrected prior to discovering a natural Legendre mode
5 imposed from the default laser pointing, and the effect of the correction will be
measured in future experiments.
The laser transmission, backscatter, and sidescatter from laser entrance hole
(LEH) windows was measured to understand LEH burn through, which is important
in determining preheat timing and quantifying the amount of laser energy that
reaches the gas. Backscatter and sidescatter was negligible in both the window only
experiments and full cylinder preheat experiments. Analysis of soft x-ray emission
from Ne doped deuterium gas filled cylinders indicates that the minimum preheat
temperature of 100 eV was achieved in the implosion region. Furthermore, comparison
of the x-ray spectra from experiments with simulations indicate that hydrocode
predictions of laser preheating are accurate enough to be considered predictive. According
to hydrocode predictions, the entirety of the gas region is preheated to an
electron and ion temperature of 200 eV by the end of the laser pulse, and the timing
is consistent with the 100 eV minimum preheat measured from the implosion region.
The summary of neutron yields obtained from the first integrated MagLIF experiments
demonstrate yield enhancement from magnetized cylindrical implosions.
Complications of the preheat beam and the Hall parameter estimations show there
is no benefit to neutron yield or ion temperature from preheating. Secondary D-T
fusion yield was used to infer final fiR, and the ratio of initial to final fiR is used
to estimate convergence ratio. Increasing convergence ratio was shown to decrease
yield by the relation YDD Cô€€€0:818. Yield decreases due to high convergence causes
1-D results to diverge from experimental measurements. Hall parameter estimations
using the neutron-averaged ion temperature as an approximate measurement of the
electron and ion temperatures and assuming a fixed number of magnetic
ux loss show that integrated shots are weakly magnetized compared to magnetized cylinxiv
drical compression. Future integrated MagLIF experiments must include integrated
shots with higher magnetic field to demonstrate the benefits of preheat. Optimal
preheat timing was determined to be 1.0 ns before the start of the laser pulse, but
more measurements are required to confirm this result.
