A Long Helium Plasma Column Generated At 13.56 Mhz Up To Atmospheric Pressure

A cold uniform plasma column is obtained inside a 1 mm radius quartz tube using linear plane parallel (silver conductive paste) electrodes located outside the tube. At a driving frequency of 13.56 MHz a pure helium plasma can be sustained from 10 to 760 Torr with or without any gas flow. The measured power density lies between 1 and 10 W/cm
³
. At 760 Torr, the plasma is limited to the electrode length (arbitrarily chosen at 25 cm) and behaves like the RF-a mode. However, the plasma easily expands outside the electrode area when the pressure is reduced below 100 Torr. In fact, from 15 to 50 Torr the plasma expands significantly outside the electrode area, a feature which is related to the leak of the electromagnetic field outside the electrode zone. In addition, the OES recorded inside and outside the electrode area are very different, suggesting different electron temperatures and densities. The expansion of the plasma outside of the electrode length reaches its maximum at 20 Torr.