Negative ions in inertial electrostatic confinement devices

The UW-Inertial Electrostatic Confinement (IEC) device is comprised of concentric, nearly transparent, spherical metallic grids within a cylindrical vacuum vessel. The central grid, which can be held at high negative potentials (~ -100 to -200 kV) is the device cathode, while the outer grid, held at ground potential, is the device anode. This configuration accelerates ions, created near the anode, toward the center of the device from a weakly ionized cold plasma which exists outside the anode. The fill gas for this device is typically deuterium at 1-4 mTorr, thus leading to D-D fusion rates on the order of 10
⁸
fusions/s. The contribution by fast neutrals from charge exchange reactions in IEC devices has been investigated extensively by Emmert and Santarius [2009] at the University of Wisconsin-Madison as well as Khachan, Moore, and Bosi at the University of Sidney [2003]. Important phenomena that have not been considered to date are electron attachment and charge transfer reactions that create deuterium anions within IEC devices. Electron attachment involves the attachment of a thermal electron to a neutral deuterium gas molecule, in an excited vibrational state, thus producing a negatively charged deuterium molecule. Charge transfer in hydrogen occurs at higher energies than electron attachment, and involves a transfer of two electrons from background neutral gas to a positive ion moving through the neutral gas at high energy (~100 to ~1 times 10
⁴
eV). Negative ions were observed in the UW-IEC device using a Faraday trap diagnostic, consisting of a current collection plate with a secondary electron suppression grid, situated so as to collect the divergent negative ion flux from the spherical potential well. The charge to mass ratio and velocity of the divergent deuterium anions were measured using a magnetic deflection energy analyzer.