Wave propagation and noncollisional heating in neutral loop and helicon discharges

Abstract

Heating mechanisms in two types of magnetized low pressure rf ͑13.56 MHz͒ discharges are investigated: a helicon discharge and a neutral loop discharge. Radial B-dot probe measurements demonstrate that the neutral loop discharge is sustained by helicon waves as well. Axial B-dot probe measurements reveal standing wave and beat patterns depending on the dc magnetic field strength and plasma density. In modes showing a strong wave damping, the plasma refractive index attains values around 100, leading to electron-wave interactions. In strongly damped modes, the radial plasma density profiles are mainly determined by power absorption of the propagating helicon wave, whereas in weakly damped modes, inductive coupling dominates. Furthermore, an azimuthal diamagnetic drift is identified. Measurements of the helicon wave phase demonstrate that initial plane wave fronts are bent during their axial propagation due to the inhomogeneous density profile. A developed analytical standing wave model including Landau damping reproduces very well the damping of the axial helicon wave field. This comparison underlines the theory whereupon Landau damping of electrons traveling along the field lines at speeds close to the helicon phase velocity is the main damping mechanism in both discharges.

Collisionless electron heating in periodic arrays of inductively coupled plasmas

“…19 The theoretical predictions of the anomalous skin effect have been well confirmed experimentally. [20][21][22][23] In recent years, various antenna coil geometries have been reported, e.g., solenoid, planar, or dome shapes. [24][25][26] Further, small single loop elements oriented perpendicular to the wall have been proposed for being arranged in large arrays in order to overcome the spatial size limits set by single coils.…”

Electric field measurements on the INCA discharge

“…To obtain in situ information about the amplitude and the phase of the induced electric field as well as information about the strength of the capacitive coupling compared to the inductive coupling into the plasma an optical diagnostic called radio frequency modulation spectroscopy (short: RFMOS) is used [13][14][15] . For this diagnostic the temporal modulation of the emission from a chosen line (from now on called modulation) is measured using an ICCD camera.…”

“…To obtain a relation between the intensity modulation ηnω and the electric field the following approach is applied. The rate equation for the excited state is combined with the assumption of a time independent, isotropic near Maxwellian distribution of the electrons that is displaced by a small, time dependent oscillation velocity [13][14][15] . From this, the measured modulation η(1,2)ω can be expressed as follows:…”

Characterization of neutral loop discharges in the VINETA device

“…This process is called partial electron cyclotron resonance (PECR). Finally, a collisionless wave damping mechanism in the complex magnetic field of the NL setup has been proposed [9]. In that setup, a helicon wave field is observed that is modified by the presence of a magnetic NL configuration.…”

“…Due to the well controlled density profile, these discharges can be used for precise plasma etching via the direct control of the NL radius [4]. Several concurrent plasma production mechanisms have been proposed to account for the increased plasma density near the NL [1,[5][6][7][8][9].…”