M. Umair Siddiqui

Double layer-like structures in the core of an argon helicon plasma source with uniform magnetic fields

A hot (Te ≈ 10 eV) electron population is observed in the core of a 3 mTorr argon helicon plasma source at 500 W RF power and 900 G uniform axial magnetic field strength, 12 cm from the edge of the helicon antenna. A double layer-like structure consisting of a localized axial electric field of approximately 8 V/cm over 1–2 cm is observed adjacent to the hot electron population. The potential step generated by the electric field is shown to be large enough to trap the hot electrons. To our knowledge this is the first observation of these structures in the core of a helicon discharge.

Electron heating and density production in microwave-assisted helicon plasmas

Microwaves are injected into argon and helium helicon plasmas at 6 to 20 mTorr neutral pressure, 1.2 kW pulsed microwave power, up to 500 W continuous RF power, and up to 1 kG magnetic fields, with the objective of heating the tail of the electron energy distribution function (EEDF) and populating ion metastable states. Langmuir probes are used to measure the EEDF and optical emission spectroscopy is used to monitor ion emission. The injection of microwave power in argon helicon plasmas is shown to heat the high energy tail of the EEDF without increasing the plasma density. Argon ion emission is shown to increase by a factor of 4. Injection of microwaves into a helium helicon plasma is shown to cool the bulk of the of the EEDF and increase the plasma density. Previously absent helium ion emission lines are observed with the injection of microwaves. All the microwave results are shown to be independent of RF power within the limits of the system.

Presheath and double layer-like structures in the core of an argon helicon plasma source with uniform magnetic fields

Summary form only given. An argon helicon plasma source is operated at ~3 mTorr neutral pressure, 500 W RF power, and 900 G axial magnetic field strength generating plasma densities of ~1012 cm-3. A grounded plate is placed two antenna lengths down stream from the edge of the m = -1 helicon antenna. A double layer-like structure consisting of a localized axial electric field of approximately 8 V/cm over 1-2 cm is observed upstream from the grounded plate. The high potential side is shown to contain a population of relatively hot (Te ~ 10 eV) electrons. The magnitude of the potential step is shown to be enough to trap the hot electrons. The interaction between the double layer-like structure and the plates presheath is investigated.

Measurement of magnetized presheaths using laser-induced fluorescence in argon plasmas

Summary form only given. The classic Bohms Criterion is valid only for unmagnetized, weakly-collisional plasmas. For magnetized plasmas where the magnetic field is obliquely incident to the boundary, Chodura and later Riemann, claimed that the presheath, accelerates ions to the sound speed along the magnetic field lines. After this an additional “magnetic presheath” must exist, which scales with the ion gyro radius, and accelerates the ions from the sound speed along the magnetic lines of force, to the sound speed perpendicular to the boundary. Riemann and Franklin claimed that both the presheath and magnetic presheath had a single structure. Previous experiments attempted to verify the presheath scale lengths by measuring plasma potential structures, however none actually measured ion flow velocity, and as such their presheath lengths were not properly defined. Kim et. al. found that in magnetized and collisional plasmas, the presheaths had two distinct potential structures, one that scaled with the ion collision length and on that scaled with the ion gyro radius, contrary to Riemann and Franklins claims. In this work the authors investigate magnetic and collisional presheath scale lengths in an inductive Argon plasma, using laser-induced fluorescence to measure ion flow speed thus and define the presheath boundaries. The scale lengths and presheath potential structures are compared to theory presented by Chodura, Riemann and Franklin.