A three phase rotating field microwave plasma design for a low-flow helium plasma generation

A new 2.45 GHz frequency powered helium plasma source with rotating microwave field generated within the three electrodes has been developed. A stable symmetrical low-flow helium discharge is formed at atmospheric pressure with a minimum field strength at the axis. Thus, a triangle shaped ring plasma is obtained with a channel at the center through which the sample is introduced using a laminar dual-flow injector. The helium plasma operates at a microwave input power of 30–150 W and total gas flow of 200–1500 ml min−1 and it is self-igniting. The preliminary investigation of the plasma fundamental parameters provides a He excitation temperature up to 4000 K, rotational temperature (OH) up to 3100 K and electron number density up to 7.5 × 1014 cm−3. To demonstrate the plasma robustness, both the tolerance for water loading and the excitation and ionisation capability for hydride-forming elements as well as for ionic chlorine are presented. The detection limits for As, Sb and Hg obtained by continuous hydride generation without hydrogen separation were 12, 10 and 9 ng ml−1, respectively.