S-AD-M-005
Chapter 1: System Overview
January 2006—Page 5 of 35
1.1.2.2 High-Temperature Opacity Measurements
A major application of HED facilities is to measure the opacity of high-temperature materials.
The backlighter brightness needs to exceed that of the high-temperature target. The backlighter spectrum
should also be sufficiently flat to remove any ambiguities between backlighter and target spectral
features. On OMEGA EP, radiation temperatures significantly higher than those achieved on OMEGA
would be expected from a 2-kJ, 20-ps beam focused into a small hohlraum. This would be adequate for
developing opacity experiments and the techniques needed for NIF backlighting, as well as performing
opacity measurements on OMEGA.
1.1.3 Fast Ignition
1.1.3.1 Background
Indirect-drive, hot-spot ignition is the baseline approach to achieving ignition and gain on the
NIF, and direct-drive, hot-spot ignition is the main alternative. Fast ignition coupled to a direct-drive or
indirect-drive implosion is a third approach of significant current interest since it can potentially increase
the target gain or reduce the ignition laser energy requirements. In the fast-ignition concept, the high-
energy driver is used only to compress the fuel without creating a central hot spot. A burning hot spot
is then formed by the rapid deposition of energy into the main fuel. Separation of the formation of the
hot spot from the compression of the main fuel could, if there are no unexpected physics issues, reduce
the energy requirement of the driver.
Fast ignition would make high-gain applications with drivers that have less energy than the full
NIF (but more than OMEGA) possible and may relax requirements on efficiency and drive symmetry. Fast
ignition can also be used with drivers such as ion-beam or
Z
-pinch radiation sources that can compress
thermonuclear fuel to a high density. The science of fast ignition is more complex and less mature than
central hot-spot ignition, so experimental tests under plasma conditions close to fast-ignition conditions
are crucial. OMEGA EP will be the best-suited facility to perform the most important fast-ignition
experiments because of OMEGA’s unique ability to compress cryogenic targets.
For ignition, the energy
E
required to be deposited by a fast-ignition beam is
E
= 140 (100/
t
)
1.8
kJ,
where
t
is the fuel density in g/cm
3
. Consequently, fast ignition is unlikely to be achieved with OMEGA
EP since the current estimate is that ~100 kJ in 10 ps is required in the high-intensity beams. The main
uncertainty in this estimate is the coupling of absorbed laser energy to the compressed core. The hot-
electron temperature (the average particle energy) generated by the HEPW beam is readily estimated to
be ~1 MeV for the electrons to be stopped efficiently in an areal density of a few hundred mg/cm
2
, as
required for hot-spot formation. This areal density is approximately equal to the range of alpha particles
in the hot spot.
Hydrodynamic simulations have been used with considerable success to model “traditional” ICF
implosions. Existing software codes, however, are inadequate for fast-ignition design, and more complex
models, including physical phenomena that are, at present, poorly understood, need to be developed. It is
currently believed that 3-D hybrid codes with particles for the fast electrons and fluids for the background
are required and that magnetic-field generation and neutralizing reverse current will be important.
Summary of Contents for Volume VII-System Description
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