A corona igniter (20) comprises a central electrode (22) surrounded by an insulator (24), which is surrounded by a metal shell (26). A ceramic combustion seal (30) is disposed along the gap (32) between a shell lower end shell (52) and the insulator nose region (48) to provide a hermetic seal therebetween. The ceramic combustion seal (30) is typically a bushing, cylinder, or ring formed of sintered alumina. A glass material or glass/ceramic mixture (60) typically adheres the ceramic combustion seal (30) to the shell (26) and the insulator (24). Alternatively, the ceramic combustion seal (30) is brazed to the shell (26), and the glass material or glass/ceramic mixture (60) adheres the ceramic combustion seal (30) to the insulator (24). Said shell (26) includes a shell inner surface (56) facing said insulator (24) and presenting a shell inner diameter (D s ) extending across and perpendicular to said center axis (A); said shell inner diameter (D i ) increases adjacent said shell lower end (52); and said ceramic combustion seal (30) is disposed in said gap (32) along said increased shell inner diameter (D s ) adjacent said shell lower end (52).