REDUCTIONOFELECTRONFLOW CURRENTAND LOCALIZEDANODE ENERGY DEPOSITIONIN TRANSITIONSFROM COAXIALFEEDSTOADISK

Many conceptual designs forfusion energyinvolve combining energy fromanumber offeed lines toacentrally located load. Onesuchdesign with70coaxial lines (each delivering la 11\MAatV - 7MV)merging into acenter disk hasbeenshowntoyield large (le - 10-40 MA)electron flow currents. Themagnetic nulls inherent inthis typeofcurrent adder geometry causeanextreme amountofenergy tobe deposited inlocalized areas oftheanode. Arevised design is proposed using 10coaxial lines delivering la 7M\AatV - 9 MV that provides better magnetic insulation andtherefore lowers theelectron flowcurrents. Computer simulations were usedtoexamine these designs. The3-Delectromagnetic, particle-in-cell (PIC)codeQUICKSILVER' developed at Sandia National Laboratories wasusedforthenumerical simulations. The capabilities of QUICKSILVERare especially well-suited tothisproblem since ithasmany features that weredeveloped tomodelavariety ofpower flowissues intheZandZRaccelerators. Thesimulation results showthat switching toa10-feed line structure (versus a 70-feed linestructure) yields a vast improvement in(lowering of)theelectron flowcurrent (Je 1-2MA). Unfortunately, thisreduction inelectron flow current onlytranslates into amodestimprovement inthe energy deposition ontheanodeinthetransition anddisk regions duetotheabrupt coax-disk transition andthe necessarily small A-Kgapinthedisk region. Theseissues wereaddressed using twomethods toattempt tolower this energy deposition. Smoothing thegeometrical transition from thefeedcoaxial lines tothediskprovides approximately a factor of3reduction inthepeaklocal energy deposition. However, lengthening theA-Kgapsinthelocalized areas whereanode energy deposition isproblematic provides more thanafactor of16reduction inthepeakenergy deposition andthecombination ofthetwomethods yields better thana 22fold reduction inthepeakenergy deposition.