Studies on Plasma Direct Energy Converters for Thermal and Fusion-Produced Ions Using Slanted Cusp Magnetic and Distributed Electric Fields

Two types of direct energy converters, cusp direct energy converter (CUSPDEC) and traveling wave (TW) DEC, used to produce electricity from thermal ions and fusion products in an advanced fueled fusion, are investigated using small-scale devices. In 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. By using a normal cusp magnetic field, the particle separation is achieved for low energy electrons, but not for electrons with much higher energies. This difficulty is overcome by applying a slanted cusp magnetic field, which has the capability of separating electrons with energies as high as 8 keV. Efficiencies of energy conversion of separated ions with large thermal spread of energy are measured to be ~55 %. An additional lateral electrode, together with the existing collector, constitutes a two-stage ion collector that provides distributed ion-decelerating fields. From the measured voltage-current characteristics, the efficiency of this collector is estimated to be improved to 65-70 %, which is consistent with the calculation. Fusionproduced fast ions enter into TWDEC and are velocity-modulated by RF fields, bunched, and then decelerated by RF traveling-wave fields on the decelerator to produce RF power. The TWDEC device has shown that the energies of ions of 3-6 keV can be decreased by 10-15 % for a one-wavelength decelerator. This would give a total efficiency of 60-70% for a full-length decelerator. A novel system is being investigated for further improvement, in which the incoming ions are deflected transversely, according to each energy, to form a fanshaped beam and a distributed electrode array for modulation and deceleration generates traveling waves appropriate to each ion path depending on the energy.