Improved Confinement and Collective Behavior in Inertial
Electrostatic Confinement Fusion Devices
Improved
Confinement and Collective Behavior in Inertial Electrostatic
Confinement Fusion Devices
PI:
Dr. Raymond Sedwick, Grad RAs: Carl
Dietrich, Thomas McGuire
Nuclear fusion power represents the
future for clean and safe terrestrial power.
It also has the potential to be the power plant
of choice for space travel, but unfortunately not with
the designs favored by current terrestrial fusion research.
The inertial electrostatic confinement (IEC)
concept is ideal for space travel since it sheds massive
magnets in favor of lightweight spherical metal grids
which confine the fusing ions with purely electric fields. Also, the concept is ideally suited for direct
electric conversion for power at very high efficiencies,
eliminating much of the massive radiator required by
nuclear fission and standard fusion schemes.
MIT’s
contribution to this concept lies in the drastic improvement
of the ion confinement via the introduction of multiple
grids to create a more ordered system.
Also, operating at lower background pressure
with particle injection via guns allows for much monger
ion lifetimes. A
2-D Monte-carlo collision, particle-in-a-cell model is used to simulate
the external and internal fields and the dynamics for
hundreds of thousands of simulated particles.
These investigations have verified the improved
confinement characteristics of a multiple grid system.
The code is also used as a predictive tool for
examining grid configurations in support of an experimental
verification effort in the lab.
Also,
a curious synchronizing collective behavior is observed
in simulation. Particles injected uniformly in 3 separate beam
paths ‘clump’ and form pulses.
As the simulation progresses, these pulses are
observed to synchronize between the beam channels.
The steady-state behavior under constant injection
is then observed to be a global pulse with the majority
of the confined ion arrived near the center of the device
at the same time. Two
snapshots from a steady-state plasma are shown below.
 
The
complete 3-D electrostatic potential structure of the
system is difficult to model numerically using conventional
finite element or finite difference techniques because
of the large open space between the relatively fine
grid wires, so a semi-analytic formulation was developed
based on a spherical harmonic representation of the
potential. This
technique allows more rapid solution of the vacuum potential. Once the vacuum potential is solved, a method
of evaluating the effectiveness of ion confinement and
fusion output was developed based on iterative integration
of the paraxial ray equation at successively higher
beam densities. Once
the maximum confined beam density is estimated, the
potential fusion output of the system is calculated
and that figure of merit is used to compare different
designs. This
entire framework has been automated and inputted into
a Simulated Annealing (SA) design code developed by
Professor Olivier de Weck
and modified by Carl Dietrich. The use of SA design gives confidence that the
final solutions arrived at by the code, although not
guaranteed to be optimal, are likely to be near the
global-optimal design as opposed to a locally-optimal
design which could be arrived at by inputting an initial
design guess into a design code that works on a gradient
search technique. The output of the SA design code is then in putted
into the ‘fmincon’ gradient
search code in MATLAB to get as near as possible to
the global optimum solution given the imposed practical
constraints on the design.
The final solutions will be compared with 2D
particle-in-cell (PIC) simulations as a cross-check
to the predicted density limits.
Once this optimal design is determined, the experiment
will be built. The initial device will have only 3 grids and
an ion source. The
confinement time of ions in the system will be measured
by monitoring the current of ions flowing to the grid
wires while modulating the input current from the ion
gun. The device will then be constructed with multiple
grids in the optimum configuration, and the confinement
time will be measured again.
This experiment should conclusively show if the
multi-grid design can actually improve ion confinement
in IEC fusion devices over the conventional 2-grid approach.
The
experimental apparatus shown here features a large,
80 cm diameter vacuum chamber. The completely dry pumping provided by a turbopump backed by a scroll pump is capable vacuums on the
order of 10^-8 torr, enabling
the investigation of the very low-pressure regimes.
High voltage power supplies and other electronics
are shown mounted to the rear of the tank.
Initial experiments will be conducted at voltages
up to 10 kV and use glow-discharge, electron-bombardment
ionization and then a lab-produced ion gun as the device
pressure is dropped.
The experimental effort is ongoing.
|
