In the most basic form of IEC, pure electrostatic confinement, a large voltage difference between two concentric grids focuses charged particles into the center. The outer grid is grounded while the inner grid is held at a negative potential of tens of kV. The inner grid usually has a geometric transparency of greater than 90%, thus the majority of ions, which are accelerated toward the center, pass through the grid. As the ions converge at the center, some may have sufficient energy to fuse with another accelerated ion, or a background atom.
The simplest method of obtaining ions for acceleration is through a glow discharge; a diagram for such a system is shown in Fig. 1. In a small chamber, an acceleration voltage of 30 kV at a current of approximately 10 mA requires a chamber pressure of about 10 mTorr. Though the glow discharge method is very simple to implement, it has several disadvantages. First, for a given voltage and current the system can only operate at one pressure. At the chamber pressures required, the mean free path of the ions is short and many never make it to the center. Also, even though ions are produced throughout the chamber volume, those not produced near the outer grid never fall through enough of a potential difference to fuse.
Using an external ion source can reduce many of the problems with the glow discharge method. This can be accomplished with ion guns, or through the production of electrons that then ionize the background gas. A diagram of the latter approach is is shown in Fig. 2. In this method a hot filament suspended near the outer grid produces thermionic electrons. A third grid is placed just inside of this filament, and biased several hundred volts above the outer grid. Electrons produced by the filament accelerate towards this positively biased grid and can ionize background gas as they travel. As electrons pass through the grid they are repelled by the high negative potential of the inner grid, and so are trapped in a region around the middle ionization grid. This design allows operation at chamber pressures several orders of magnitude lower than the simple glow discharge design.
My current design uses a spherical geometry and the stainless steel vacuum chamber's case serves as the outer grid. Inner grids are constructed from spot-welded wire. At low power levels simple stainless steel wire is fine; as power levels increase, heating from ion bombardment increases, and the inner grid should then be constructed from a refractory metal.
The system is operated by exhausting the chamber with a vacuum pump, and then flowing deuterium gas through the chamber while the pump is left running. In glow discharge mode, adjustment of chamber pressure sets the voltage and current at which the system will operate. Pressure levels can be adjusted by varying both a valve, which throttles the vacuum pump, and a leak valve, which supplies the deuterium gas. With the triple grid design, adjustment of the ionization grid voltage serves to control the ion current. Also, in the triple grid, implementation voltage and current are almost independent parameters, unlike operation in the glow discharge regime. An external high voltage DC supply supplies voltage to the inner grid. The connection between the grid and supply is isolated from the chamber wall by a high voltage feedthrough. Reaction rates are measured via a neutron detector placed just outside the chamber.
| Fig. 1. Simple two grid setup. |
| Fig. 2. Triple grid setup. |