Time-dependent measurements of the temperature and density of the Ar * ( 3 P 0 ) metastable atoms in two high-density pulsed helicon discharges, deduced from the absorption profile of the 772.42 nm argon line, are presented. A single-mode tuneable diode laser is used for recording these profiles, and temperatures up to 1000 K are obtained from their Doppler width. As in high-density plasmas the metastable and ground-state atoms are strongly coupled by electron impact collisions, the temperature of the metastable atoms reflects the gas temperature. From the time variation of the Ar * ( 3 P 0 ) density during the discharge pulse we were able to deduce the density of the ground-state argon atoms and found for the helicon regime that the neutral atoms can be depleted by more than a factor of 10. Over a wide range of plasma parameters, we examined the axial asymmetry that is characteristic for helicon discharges with helical antenna coupling. In particular, we analysed the plasma-induced emission of the Ar (750 nm) and Ar + (461 nm) lines as well as the electron density in the beginning of the plasma pulse to better understand the evolution of the plasma and the transition to the helicon regime. The measurements of the gas temperature and the metastable density reveal the asymmetry to become pronounced when the discharge changes from inductively coupled plasma to the helicon discharge. In the helicon regime, the argon atom depletion of up to 90% and ionization degrees up to 65% were achieved in the antenna zone. In the afterglow, all excited states of argon, including the Ar * ( 3 P 0 ) state, are fed by electron-Ar + ion recombination. The measured very high temperature of Ar * ( 3 P 0 ) atoms is partially related to the temperature of the Ar + ions. Production of Ar + in its excited states is also observed during several hundreds of microseconds in the afterglow. It is related to the electron-Ar 2+ ion recombination and indicates the presence of a significant amount of doubly charged argon ions in the present helicon plasmas.

Pulsed helicon discharges produced through m = 1 or m = 2 helical antennas are investigated. Special attention is paid to the axial asymmetry, which is characteristic for helicon sources with helical antenna coupling. The axial profiles of the RF wave fields as well as the energy deposition profiles reveal that the RF power is predominantly absorbed via helicon modes with positive azimuthal mode numbers m (m = +1, +2) travelling in positive magnetic field direction. This can be attributed to the fact that the helicon modes with negative m are strongly damped or even evanescent if the radial plasma density gradient is steep enough. In particular, we examined the RF power absorption in the m = 1 helicon discharge, that cannot be understood in terms of collisions. Various non-collisional absorption mechanisms as well as absorption via excitation of Trivelpiece-Gould waves are discussed. We also present first measurements carried out on the large-volume plasma injected from the m = 1 helicon source into a diffusion chamber.

The evolution of a high-density helicon discharge in argon and argon–helium mixtures was studied by spectroscopic methods as well as standard diagnostics. Applying optical emission spectroscopy, the electron temperature was deduced from the ratio of different emission lines. The spectral data were evaluated using collisional-radiative (CR) models taking account of secondary processes like the transitions between excited levels and excitation from metastable niveaus. In addition, time-resolved laser absorption spectroscopy was applied to determine the temperature and the density of the Ar*(3P0) metastable atoms from the absorption profile of the 772.42 nm argon line. Due to strong coupling by electron collisions, the temperature of the metastable atoms up to 1000 K reflects the gas temperature of the ground state atoms. The gas temperature and the gas density, which is readily obtained from the temperature, are needed as input parameters for the CR models. The spectroscopic results were compared with Langmuir probe measurements which partially utilized passive compensation. The main issue of this study is electron heating due to helicon wave absorption. Applying a double radio-frequency (rf) pulse technique with variable rf power in the second pulse, the growth of the electron temperature was measured as a function of rf power. To clarify whether or not fast electrons are generated in the helicon wave due to resonant electron heating, phase-sensitive emission measurements of the Ar II (480 nm) line with a short lifetime (τ ≈ 7 ns) were performed on the rf time-scale (period 73.7 ns). However, no modulation of the Ar II emission synchronous with the helicon phase was observed. Rather, the temporal growth of the electron temperature reveals the electrons to be predominantly heated in the bulk of the energy distribution function.

A quasi-optical interferometer operating at the frequency f = 274.2 GHz has been employed for measuring electron densities of pulsed plasmas in a wide range. Line-integrated densities down to nearly 10 14 m −2 can be measured with temporal and spatial resolutions of 1 µs and less than 10 mm, respectively. A wavelength of about 1 mm requires a quasi-optical set-up making use of grid and dielectric beam splitters as well as elliptic mirrors for focussing the probing and reference beams. Due to the Michelson-like set-up, the probing beam traverses the plasma twice thus doubling the sensitivity as compared with the Mach-Zehnder configuration. The large difference between the probing and reference paths enables one to shift the phase by varying the frequency in a narrow band. When the detector output signal of the interferometer is recorded thereby shifting the phase stepwise, one obtains a cosine function for each sample time from which the phase shift by the plasma and, thus, the electron density, is deduced. The performance of the interferometer is demonstrated on a pulsed high-density helicon discharge as well as the large-volume plasma in a diffusion chamber connected with the helicon source.

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