Joerg Winter

Dust: A new challenge in nuclear fusion research?

Small particles (dust) exist in magnetic confinement fusion devices. Their origin is due to plasma–surface interactions. Dust particles may contain significant amounts of hydrogen isotopes, 50% of which will be tritium in future devices. It is important to assess and understand the processes by which dust is formed and by which it interacts with the fusion device and its plasma. Dust may be a safety hazard due to its high chemical reactivity and due to the mobile tritium inventory. Dust may influence the plasma performance and the operation of fusion devices. The radioactive decay of tritium incorporated into carbonaceous dust may lead to its charging and to the formation of a nuclear induced plasma associated with levitation and transport of dust inside the vacuum vessel. Some important aspects of dust in fusion devices will be discussed, making use of information from the area of plasma–surface interactions in fusion devices and from the field of dusty plasma research.

A Candidate Analog for Carbonaceous Interstellar Dust: Formation by Reactive Plasma Polymerization

Carbonaceous compounds are a significant component of interstellar dust, and the composition and structure of such materials is therefore of key importance. We present 1.5‐15 � m spectra of a plasma-polymerized carbonaceous material produced in radio-frequency discharge under low pressure, using C2H2 as a precursor component. The infrared spectra of the resulting spheroidal carbonaceous nanoparticles reveal a strong aliphatic band (3.4 � m feature), weak OH and carbonyl bands, and traces of aromatic compounds, all characteristics identified with dust in the diffuse interstellar medium of our Galaxy. The plasma polymerization process described here provides a convenient way to make carbonaceous interstellar dust analogs under controlled conditions and to compare their characteristics with astronomical observations. Here we focus on a comparison with the IR spectra of interstellar dust. The IR spectrum of carbonaceous dust in the diffuse interstellar medium is characterized by a strong 3.4 � m

Size dependence of optical properties and internal structure of plasma grown carbonaceous nanoparticles studied by in situ Rayleigh-Mie scattering ellipsometry

We have analyzed the growth process of a-C:H nanoparticles in Ar–C2H2 and Ar–CH4 plasmas by means of in situ Rayleigh-Mie scattering ellipsometry. Complex refractive index (m=n−ki), mean particle radius (r), and particle size distribution (σ) of a-C:H nanoparticles are carefully determined and compared with those of a-C:H thin films deposited at similar conditions. It is found that, in both plasmas, small particles (r<25nm) have graphitelike properties whereas large particles have polymerlike characteristics. These results indicate that the particles have internal structures of a hard core and soft mantles on it. The size distribution of the nanoparticles in both Ar–CH4 and Ar–C2H2 plasmas is essentially monodisperse with a narrow Gaussian size distribution. A systematic comparison between the growth and atomic structure models for a-C:H materials and criteria for interstellar dust (ISD) grains known from astrophysical observations is made. Significant agreement is found in a wide range of radii, supporti...

Anomalous behaviour of the electron density in a pulsed complex plasma

The electron density in a pulsed complex plasma is measured by means of time resolved microwave interferometry. Depending on the discharge conditions the temporal evolution of the electron density exhibits an anomalous behaviour. The electron density starts to decrease when the rf-power is switched on and it starts to increase when the rf-power is switched off. This anomalous behaviour can be attributed to the charging and decharging of nanoparticles.

Dust particle formation in low pressure Ar/CH4 and Ar/C2H2 discharges used for thin film deposition

Abstract This paper deals with the study of the temporal and spatial evolution of the dust formation in two types of capacitively coupled discharges in Ar/C 2 H 2 and Ar/CH 4 gas mixtures used for thin film deposition. To initiate the particle growth in the Ar/CH 4 discharge it is necessary either to apply transiently high power to the discharge or to inject transiently a pulse of C 2 H 2 . In the Ar/C 2 H 2 discharge, however, the particles are formed spontaneously at constant low power. Due to the different initiation process the further temporal evolution of the dust formation is significantly different for both kind of gas mixtures. In the case of argon/acetylene the formation of dust particles shows a periodical behavior, which is not observed in the argon/methane mixture. The dust particles are detected by means of laser light scattering and by measuring the extinction of the laser after passing the discharge. The chemical nature of the particles was studied in situ by means of a multi pass FTIR-spectrometer. The thin film deposition was measured with an in situ ellipsometer.

The response of a capacitively coupled discharge to the formation of dust particles: Experiments and modeling

The influence of dust particles on the properties of a capacitively coupled Ar–C2H2 discharge is studied both experimentally and theoretically. The results of measurements of the intensity and spatial distribution of the emitted light, the line width of the fast component of Hα line and of the electron density during the particle growth are presented. To analyze the experimental results a one-dimensional discharge model is developed. Using the model the effects of dust grains on the power absorption (taking into account stochastic and Ohmic heating in the plasma sheaths), the optical emission intensity profile, the sheath size, the rf electric field and on the energy of positive ions bombarding the electrodes are investigated. In particular, it is shown that the decrease of the power absorption in the sheaths of complex plasmas is due to the dependence of the stochastic and Ohmic heating in the plasma sheaths on the electron temperature and the current flowing across the discharge plates. The results of t...

Terahertz time-domain spectroscopy as a new tool for the characterization of dust forming plasmas

We report the application of terahertz time-domain spectroscopy as a new tool for plasma diagnostics. The short broadband THz pulses were radiated from a low temperature grown GaAs emitter by free charge carriers which were generated by focusing a 20 femtosecond TiSa-laser pulse onto the emitter. For sensitive signal recording a coherent detection scheme was applied. This allowed the measurement of the amplitude and sign of the electromagnetic field of the THz pulse after passing the plasma chamber. Fourier transformation allowed us to obtain the full spectrum in the frequency domain.We compared the transmitted THz intensities of a pure argon (Ar) and an acetylene (C2H2)/argon plasma. The presence of the ethynyl–radical (CCH) and cyclopropenylidene (c-C3H2) in the (C2H2)/argon plasma could be confirmed by the observations of rotational transitions in the region from 8 to 16 cm−1 corresponding to 0.3–0.5 THz.

Ionization wave propagation on a micro cavity plasma array

The simulation was performed using the computer modeling platform nonPDPSIM, described in detail in Refs. 10–12 and briefly discussed here. Poisson’s equation for the electrostatic potential is self-consistently coupled with driftdiffusion equations for the transport of charged species and the surface charge balance equation. The set of equations is simultaneously integrated in time using an implicit Newton iteration technique. This integration step is followed by an implicit update of the electron temperature by solving the electron energy equation. To capture the non-Maxwellian behavior of the electrons, the electron transport coefficients and rate coefficients are obtained by solving the zerodimensional Boltzmann’s equation for the electron energy distribution. A Monte Carlo simulation is used to track the trajectories of sheath accelerated secondary electrons. The transport of photons is treated by means of a Green’s function propagator. The discharge is sustained in argon at atmospheric pressure. The species in the model are electrons, Ar(3s), Ar(4s), Ar(4p), Ar þ ,A r �, and Ar þ . The photon transport we tracked in the model is dimer radiation from Ar � .I n

Investigation on interactions of fluid gallium with capacitively coupled rf-plasmas

This paper deals with the study of erosion and surface phenomena of liquid Ga exposed to a capacitively coupled rf-plasma. We have observed a very high erosion yield in Ar plasmas which cannot be explained by sputtering alone. Time resolved observations of the plasma–gallium interaction indicate that the formation of gas cavities inside the Ga and their explosive outgassing might be one of the important erosion mechanisms in our experiments.