A remote plasma source is disclosed that includes a core element and a first plasma block including one or more surfaces at least partially enclosing an annular-shaped plasma generating region that is disposed around a first portion of the core element. The remote plasma source further comprises one or more coils disposed around respective second portions of the core element. The remote plasma source further includes an RF power source configured to drive a RF power signal onto the one or more coils that is based on a determined impedance of the plasma generating region. Energy from the RF power signal is coupled with the plasma generating region via the one or more coils and the core element.