Volume VII–System Description
OMEGA EP Operations Manual
Page of 35
The conventional laser beams on OMEGA and the NIF produce relatively low peak power and
thus suffer several limitations for radiography applications. The primary limitations that are ameliorated
using high-power, short-pulse, high-energy backlighter laser beams such as those available from
OMEGA EP are
(a)
The x-ray backlighter power is low. This results in an inability to make the backlighter
brighter than some of the HED physics targets that are at high temperatures themselves
and therefore emitting copious amounts of x rays.
(b)
The x-ray photon energies are below ~15 keV, limiting the target thickness or atomic
number that can be radiographed.
(c)
The x rays are emitted for the relatively long duration of the laser pulse. This limits the
time resolution
D
t
and, thus, the image quality that can be achieved. It also impacts the
spatial resolution
D
x
because of motional smearing (given by
D
x
=
v
D
t
for material that
is moving at velocity
v
).
(d)
Few high-energy protons are produced, limiting the possibility of measuring material
properties other than the x-ray opacity. The ionization state and electromagnetic fields
could be measured with energetic protons.
OMEGA EP will overcome the limitations of backlighting with conventional OMEGA and
NIF beams. It will allow dedicated backlighting beams in both the OMEGA and OMEGA EP target
chambers. The short-pulse capability of illuminating targets at 10
20
W/cm
2
and above allows new regimes
of backlighting and the use of dedicated backlighting beams avoids compromising the symmetry of
implosion experiments on OMEGA.
OMEGA EP will have essentially the same high energy petawatt (HEPW) beam configuration
envisioned for implementation on the NIF advanced radiographic capability (ARC) machine. This will
allow scientists to develop backlighting techniques for use on the NIF by exploiting OMEGA EP’s higher
shot rate and lower shot cost.
1.1.2 HED Backlighting Experiments
OMEGA EP’s enhanced backlighting capabilities will be used for many different experiments.
Examples are described in the following subsections.
1.1.2.1 Cryogenic Implosion Fuel Conditions
The HEPW beams from OMEGA EP backlight OMEGA cryogenic target implosions. The
backlighter must provide sufficient brightness to overcome the implosion self-emission and produce an
image that can be used to infer the density distribution of the hot core with a high signal-to-noise ratio.
Using both of the OMEGA EP short-pulse-capable beamlines, it is possible to irradiate a 50-
n
m-radius
Si backlighting target at an intensity of 5
#
10
17
W/cm
2
using 20-ps pulses. Assuming an efficiency of
2
#
10
–5
, this provides ~16 mJ/eV into 4
r
, which should be sufficient to produce a useful image.
Summary of Contents for Volume VII-System Description
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