Emissive probes offer the possibility for a direct determination of the plasma potential since for sufficient electron emission its floating potential is ideally equal to Φ pl . By means of two such probes the true electric field can be derived. Emissive probes also deliver reliable results if there are electron drifts and beams in the plasma. A conventional emissive probe consists of an electrically heated loop of refractory wire. We have also developed emissive pin probes of graphite or LaB6, heated by a focused infrared laser beam. In this contribution we first discuss the question to what extent emissive probes perturb the plasma and thereby might falsify the measurement. Furthermore, we present recent measurements with an array of emissive probes in a small tokamak to detect plasma potential fluctuations and electric fields and the related radial plasma transport. Eventually, we present results of a laser-heated emissive probe in a helicon discharge device.
A new fireball configuration has been developed which produces vircator-like instabilities. Electrons are injected through a transparent anode into a spherical plasma volume. Strong high-frequency oscillations with period corresponding to the electron transit time through the sphere are observed. The frequency is below the electron plasma frequency, hence does not involve plasma eigenmodes. The sphere does not support electromagnetic eigenmodes at the instability frequency. However, the rf oscillations on the gridded anode create electron bunches which reinforce the grid oscillation after one transit time or rf period, which leads to an absolute instability. Various properties of the instability are demonstrated and differences to the sheath-plasma instability are pointed out, one of which is a relatively high conversion efficiency from dc to rf power. Nonlinear effects are described in a companion paper ͓R. L. Stenzel et al., Phys. Plasmas 18, 012105 ͑2011͔͒.
We derive two equations describing the coupling between electromagnetic and electrostatic oscillations in one-dimensional geometry in a magnetized cold and non-relativistic plasma. The nonlinear interaction between the wave modes is studied numerically. The effects of the external magnetic field strength and the initial electromagnetic polarization are of particular interest here. New results can, thus, be identified. V C 2014 AIP Publishing LLC.
Due to their ability to suppress a large part of the electron current and thus measuring directly the plasma potential, ion sensitive probes have begun to be widely tested and used in fusion devices. For these probes to work, almost perfect alignment with the total magnetic field is necessary. This condition cannot always be fulfilled due to the curvature of magnetic fields, complex magnetic structure, or magnetic field reconnection. In this perspective, we have developed a plasma potential probe (named Bunker probe) based on the principle of the ion sensitive probe but almost insensitive to its orientation with the total magnetic field. Therefore it can be used to measure the plasma potential inside fusion devices, especially in regions with complex magnetic field topology. Experimental results are presented and compared with Ball-Pen probe measurements taken under identical conditions. We have observed that the floating potential of the Bunker probe is indeed little affected by its orientation with the magnetic field for angles ranging from 90° to 30°, in contrast to the Ball-Pen probe whose floating potential decreases towards that of a Langmuir probe if not properly aligned with the magnetic field.
As discovered in recent work, plasma fireballs have the ability to exert considerable force onto ions and neutrals and, hence, induce macroscopic gas flows. This property makes them interesting objects for fundamental scientific research. Furthermore, there are also the possibilities for applications in the field space propulsion. As there is a lack of fundamental understanding of these plasma phenomena, this article aims to enhance the physical knowledge of fireballs by presenting a mathematical model for the calculation of the force that can be provided by them. It will be shown that all the main plasma parameters such as the plasma potential and the electron density can be derived completely with the knowledge of the potential of the electrode and the radial electron temperature profile. The calculations show very good agreement with the experimental data if two species of electrons (i.e., fast and slow) are considered. Both electron populations have different temperature profiles as is shown with measurements. Furthermore, it will be demonstrated that the potential drop throughout the fireball is much larger than previously thought and that this larger potential drop can considerably contribute to the acceleration of ions in the double layer. This mechanism makes it more likely that the force exerted by the fireball is rather caused by heating of the neutrals via collisions with those accelerated ions and the high energetic ions themselves than by collisions between fast electrons and neutrals.
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