of the inner electrode 120 a the mounting surface 120 ab further includes first second and third smooth unthreaded blind holes configured to receive alignment pins 530 aa 530 ab and 530 ac respectively or 530 a collectively details shown in fig5 g radially aligned at a radius between 6 0 and 6 1 preferably between 6 02 and 6 03 inches from the center of the inner electrode 120 a u201c radially aligned u201d means the distances to the center are equal the holes 530 a have a diameter between 0 11 and 0 12 inch a depth of at least 0 1 inch a 45 u00b0 chamfer of about 0 02 inch wide on an entrance edge and a fillet with a radius of at most 0 02 inch on a bottom corner the first hole 530 aa is offset by about 10 u00b0 clockwise azimuthally from one of the unthreaded blind holes 540 a the second hole 530 ab is offset by about 92 5 u00b0 counterclockwise azimuthally from the first hole 530 aa the third hole 530 ac is offset by about 190 u00b0 counterclockwise azimuthally from the first hole 530 aa referring to fig1 a the inner electrode 120 a is clamped to the backing plate 140 a by a clamp ring 150 a engaging the outer step 534 a on the lower face and a plurality of bolts 160 a engaging the threaded blind holes 520 a in the mounting surface 120 ab the clamp ring 150 a includes a series of holes which receive fasteners such as bolts screws threaded into threaded openings in an underside of the backing plate 140 a to avoid contact of the clamp ring 150.