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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 inputted 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.