Plasma Direct Energy Converter for Thermal Ions Using a Slanted Cusp Magnetic Field and Two-Stage Deceleration

Abstract

A direct energy converter (DEC) designed for thermal ions escaping from a fusion reactor consists of a cusp magnetic field and one-or two-stage decelerating electrodes. In this CUSPDEC, magnetized electrons are deflected along the field lines of the cusp magnetic field to the line cusp region and collected by an electron collector, while weakly magnetized ions can traverse the separatrix and enter into the point cusp region. Thus, ions are separated from electrons, and flow into an ion collector to produce DC power. A normal cusp magnetic field enables us to separate electrons and ions for low energy electrons from a test plasma source, but not for electrons with much higher energies from the tandem mirror GAMMA10. The reason for this is found that the high energy electrons do not follow the field lines due to a high potential applied to the ion collector for ion deceleration. Use of a slanted cusp field has resolved the difficulty resulting in good separation. The efficiency of energy conversion of separated ions with wide spread in energy is ~55 % for a one-stage decelerating electrode. An additional lateral electrode, together with the existing collector, constitutes a two-stage ion collector that provides distributed ion-decelerating fields. The system has revealed improvement in efficiency. From the measured voltage-current characteristics, the efficiency of this two-stage collector is estimated to have a value of 65-70 % at an optimum condition.