A Staged Z-Pinch is projected to
achieve
breakeven fusion in a compact laboratory device (Phys.Rev.Lett.74, p. 7141(1995).
Initially
a pulsed, electrical
generator drives current through an annular-plasma
shell of several cm radius
(staged pinch).
Coaxial with the shell is a target plasma and embedded magnetic field. As the shell
implodes eddy currents are induced in the target, via flux compression.
The result is a target-current risetime that is decreased by orders
of magnitude and a target current that is amplified many-fold
relative to the shell current. The large value of compressed magnetic
field confines alpha particles and provides transient shear to
improve stability against MHD instabilities as the target heats
adiabatically and to a lesser degree, Ohmically.(example of a field stabilized pinch)
Theoretical analysis of the Staged Z-Pinch has considered
parameters that will achieve break-even yield using single-step
staging in a small facility. In zero-D with a D-T
target our code
predicts: 5 MA target-current, t ~ 0.1 ns confinement time, 6x10
25
cm-3 density, 10 keV ion temperature, nt ~ 3x1014
cm-3-s Lawson criterion, and a 3-fold energy gain (Eneutron
s/Ebank
). Using the LLNL TRAC-2 hydro code, including ideal equation
of state, magnetic diffusion, radiation losses and thermal transport
we have preliminary one-D outputs for a krypton z pinch and DT target
that give similar results (one-D
results).
These calculations were benchmarked against
a Livermore radiation hydrodynamics code, run in 1-D for a "
non-optimized" configuration
and confirm features of current transfer, current amplification,
and rapid target heating. The LLNL code predictions are for 10% energy gain.
Recent calculations have been in 2-D and confirm our predictions for a stable implosion
(two-D results).
Moreover, the experimentally measured neutron yield of 1010 neutrons/shot
from a deuterium target plasma
agrees with the calculations.It is important to note this configuration has not yet
been fully optimized and that a deuterium-tritium pinch would have provided
approximately a hundredfold increase in yield, that is 1012 neutrons/shot
fusion cross sections.
Our experimental facility is called ZOT
and utilizes a low-voltage, capacitor-bank,
photo of the electrical driver.
At full charge the characteristics are: Wstored = 62.5
kJ, V
charge = 50 kV, Imax ~ 2 MA, t1/4
~ 1.8 usec. Two capacitor banks (each 25 uFd, 60 kV), switched
by railgaps, symmetrically feed a mylar-insulated, plate-transmission line:
6.4-mm
thick, 2-m wide, and 2.5-m long
(schematic view of the facility).
The gas injector is a high pressure (700 psi)
double valve system with independent control for the outer shell and target.
The coaxial injection nozzles are mounted on the cathode electrode.
The anode-cathode gap is 1.5 cm, the annular gas puff is 5-cm diameter,
and the target gas 2-cm diameter;
the total system inductance is 32 nH.
In the staged pinch the target plasma
could be formed
from a fiber injected between the discharge electrodes and pre-p
ulsed
prior to initiation of the outer pinch. The fiber would be extruded-cryogen
ic
deuterium (a form of solid hydrogen). Cryogenic fibers of H2,
D2 and Ne have been
extruded on a test stand (cryogenic
extruder)
and positioned with mm accuracy using an external-guiding system.
On another test stand
we are developing techniques
to pre-explode the fiber. To date we have studied 50-micron diameter,
exploded-copper wires subjected to a prepulse of 1010 A/sec for
1.5 microsec. The diagnostics include a nitrogen laser and an
x-ray backlighter to capture time-resolved images of the process.
Shadowgraphs and
interferograms of
the exploded fiber are displayed at various times after current
initiation. The images characterize the symmetry of the wire explosion
along it's axis. Radial jets are observed to be uniformly distributed
along the wire length with a scalelength of the order of several
hundred microns. The short wavelength of these jets is not a serious
concern for pinch instabilities, as wavelengths longer than several
mm are usually the most hazardous in pinch disruptions. Backlighter
images will be available in coming months.
Progress on the Staged Z-Pinch will be periodically
updated on this Web page.(See also the conferences papers
directory for additional
information). If confirmed experimentally the Staged
Z-Pinch would dramatically advance the understanding of
means to obtain net fusion
energy from a laboratory controlled reaction. This project is
not funded.
