The Blue Core Paradigm

The blue core mode in magnetized rf plasmas has been the topic of recurring debates since it was first observed in 1968 in an argon helicon wave dominated discharge. Blue cores are typically seen with plasma densities in excess of 1012 cm−3. Because high degrees of ionization are desirable in a variety of applications, the study of the blue core mode has been a dynamic research topic. Observing a blue core is often taken as a sign of the existence of electron-heating powered by helicon waves. However, many clues indicate that a blue core is neither a sufficient nor a necessary condition of wave heated regimes. Direct measurements of helicon waves at the correct phase velocities to energise electrons have been made in discharges without the observation of blue cores, while studies report "helicon blue core mode" without direct proof of helicon waves contributing to the ionization.

Two new experiments have looked at the generation of blue cores without any evidence of helicon waves existing in the discharge. Both experiments share the same physical dimensions and magnetic field topologies but differ in the type of antenna employed and the driving frequency; a double-saddle driven at 13.56 MHz on one-hand, a single loop at 27.12 MHz on the other. In each, a Helmholtz pair of solenoids can be progressively moved away from the antenna to observe the effect on the discharge. Strong ArII emission and densities ≥ 1012 cm−3 are obtained for Prf ≥ 200 W and applied magnetic fields ≥ 600 G, when the Helmholtz pair is placed 30 cm away from the antenna. EEPFs measured along the magnetic field lines crossing the skin-depth region under the loop antenna are bi-Maxwellians with hot electrons in the inelastic energy range up to 50 cm away from the antenna. Plasma potential mappings show a strong ion radial confinement and an axial trapping of the electrons, matching with the location of maximum plasma density and of the blue core.