A method and apparatus for producing ions wherein an ion generator produces periodic pulses of electric energy of a single polarity for some applications and alternating positive and negative polarities for other applications. A plurality of the electric pulses are limited in amplitude by a substantially sinusoidal half-wave envelope and the positive and negative pulses are preferably of different amplitudes. The ground electrode is arranged about and upstream and downstream of the ionizing electrode to provide a highly effective electric field substantially normal to the gas flow. A heater heats the gas to increase ionization. An ultrasonic sound wave generator pulses the gas with sound waves prior to, during or after ionization of the gas to group the ions of a like charge in distinct pressure wave fronts or distinct areas so as to reduce recombination of ions thereby making more ions available per volume and also increasing the total energy of the ions produced. Multiple sound wave generators increase the energy of the base frequency or selected harmonics. An inlet passage to the generator of a selected length increases the energy. A discharge passage of a selected length reinforces and/or eliminates selected harmonics. A discharge nozzle with angularly inclined and outwardly enlarged venturis cool the heated ionized gas.