Pulsed Power Fusion Program update Page: 3 of 9
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has learned more about the coupling of driver energy into the capsule ablator and the capsule
response to this energy deposition.
At the same time as the driver requirements were changing, the driver options for pulsed
power were evolving as well. Early driver technologies included intense electron beams and z
pinches. In the late 70's after the development of very intense electron beam diodes, a shift
from electrons to ions was dictated by the more favorable deposition characteristics of the
more massive ions. Z-pinch experiments continued to improve in radiated power and energy
and were applied to other applications but funding from the ICF program was suspended in
favor of ions. While advances in understanding in the physics of intense light ion beam were
substantial, the record ion beam intensities that have been achieved (-5 TW/cm2 - protons, -3
TW/cm2 - lithium) remain far below the requirements of ICF (-100 TW/cm2).
By 1994 on the Saturn accelerator, z-pinch x-ray power and energy output had been
increased to the level (-20 TW and -400 kJ) that important ICF experiments could begin. The
success of these experiments and the recent breakthroughs in z-pinch performance (- 290 TW
and 2MJ of x rays, >150 eV in a hohlraum) have led to another major shift in emphasis in
pulsed power fusion. Beginning in 1999, the entire pulsed power fusion effort at Sandia will
be directed toward developing z-pinch x-ray sources for ICF. This paper will describe the
present status of the pulsed power fusion program at Sandia and the plans to further develop
this promising path to fusion.
The long term goal of ICF research is the production of high yield. High yield in this
context means thermonuclear yield of 200-1000 MJ. In 1994, glass laser technology was
chosen to demonstrate ignition in the laboratory. The National Ignition Facility will be a large
laser capable of delivering 1.8 MJ of 3o (350 nm) light to a target. The 1.8 MJ of laser energy
will result in - 100 - 150 kJ of x-ray energy absorbed in the capsule. The predicted
thermonuclear yield with this absorbed x-ray energy is 2 - 20 MJ. Ignition demonstration on
the NIF would be a major step forward toward obtaining high yield in the laboratory.
However, a high-yield target is expected to require approximately 1-2 MJ of x-rays absorbed
in the capsule ablator with radiation symmetry on the surface of the capsule of better than 1%.
In order to achieve this radiation symmetry, a large case-to-capsule radius ratio is needed.
Consequently, a driver will be required with approximately 10 MJ of x-ray energy,
prohibitively expensive using laser technology. In addition, the x-ray power pulse incident on
the capsule must have an appropriate time variation (shaped pulse) to keep the DT fuel on a
low adiabat during compression. Furthermore, any energy application of ICF will require a
repetitive pulse capability (4 Hz). Pulsed-power-driven ICF offers an attractive alternative
with affordable, high energy, high efficiency drivers and the potential for repetitive pulse
Progress in Z-pinch Development
Recent progress in x-ray generation using z pinches has been remarkable and has prompted
a major shift in Sandia's ICF program. Z pinches are now seen to provide the fastest route to
demonstration of high yield in the laboratory. Pulsed power accelerators have been used for
many years to drive magnetic implosions (z-pinches). The load in these implosions has varied
from cylindrical arrays of wires arranged at constant radius, to gas puffs and low density
foams. These loads have historically coupled extremely well to pulsed power accelerators with
resulting high electrical-to-kinetic energy efficiency. In the application as an x-ray source, the
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Quintenz, J.P.; Adams, R.G. & Allshouse, G.O. Pulsed Power Fusion Program update, article, June 1, 1998; Albuquerque, New Mexico. (digital.library.unt.edu/ark:/67531/metadc704068/m1/3/: accessed October 15, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.