R. Friedl

Controllable evaporation of cesium from a dispenser oven

This instrument allows controlled evaporation of the alkali metal cesium over a wide range of evaporation rates. The oven has three unique features. The first is an alkali metal reservoir that uses a dispenser as a cesium source. The heating current of the dispenser controls the evaporation rate allowing generation of an adjustable and stable flow of pure cesium. The second is a blocking valve, which is fully metallic as is the body of the oven. This construction both reduces contamination of the dispenser and enables the oven to be operated up to 300 °C, with only small temperature variations (<5 °C). By minimizing the temperature variation, the built up of the alkali metal at a cold spot is significantly hindered. The last feature is an integral surface ionization detector for measuring and controlling the evaporation rate. The dispenser oven can be easily transferred to the other alkali-metals.

Temperature dependence of the work function of caesiated materials under ion source conditions

The key parameter for the performance of negative hydrogen ion sources based on surface conversion is the work function of the converter surface. In order to enhance the negative ion yield caesium is introduced into the source which lowers the converter work function upon adsorption. However, the Cs layer is subject to background pressures of 10−7–10−6 mbar and to H2 or D2 low-temperature plasmas which can have a major impact on the resultant work function. In order to determine the work function of caesiated surfaces under ion source relevant conditions and to identify influencing parameters, systematic investigations are performed at the laboratory experiment ACCesS. It is confirmed, that the work function of a sample surface in a vacuum of 10−6 mbar is decreased upon the evaporation and adsorption of caesium. However, the adsorbed layer is determined by the formation of Cs compounds with residual gases from the background pressure leading to an increased work function by about 0.6 eV compared to pure C...

Influence of cesium on the plasma parameters in front of the plasma grid in sources for negative hydrogen ions

The cesium dynamics in ion sources for negative hydrogen ions based on surface conversion mainly determine the performance of the neutral beam injection (NBI) heating system for fusion. To investigate the role of the evaporated cesium in the hydrogen plasma, which e.g. causes a decrease of the co-extracted electron current, experiments at a flexible laboratory setup (planar inductively coupled plasma) have been performed aiming at the direct influence of cesium on the plasma parameters in the plasma volume. In order to distinguish between effects resulting from the high atomic mass and those from plasma and surface chemistry, xenon admixtures (similar mass to Cs) were also investigated. A comprehensive set of diagnostics is applied simultaneously: optical emission and white light absorption spectroscopy, Langmuir probe measurements and residual gas analysis. As expected admixtures of xenon lead to a reduction of Te and an increase of ne. For cesium, however, the major effect is a remarkably reduction of n...

Fundamental studies on the Cs dynamics under ion source conditions

The performance of surface conversion based negative hydrogen ion sources is mainly determined by the caesium dynamics. Therefore, fundamental investigations in vacuum and plasma are performed at a flexible laboratory setup with ion source parameters. Studies on the influence of Cs on the plasma parameters of H2 and D2 plasmas showed that ne and Te in the bulk plasma are not affected by relevant amounts of Cs and no isotopic differences could be observed. The coating of the vessel surfaces with Cs, however, leads to a considerable gettering of hydrogen atoms from the plasma volume and to the decrease of ne close to a sample surface due to the formation of negative ions.

Generation of an atmospheric plasmoid from a water discharge: An analysis of the dissipated energy

A plasmoid in air at atmospheric pressure of about 20 cm in diameter and up to 500 ms duration is generated from a water discharge which is powered for a short time period by a capacitor bank. The analysis of the electrical circuit and the comparison with experimental values show that the energy dissipated into the system is given by the conventional equation for discharging capacitors. The resistance of the system is governed by the resistances of the water reservoir, the plasma, and the plasma-water transition, which are represented as one time-averaged resistance in the equation. Thus, the dissipated energy can be influenced by the energy available (capacitance and voltage), the voltage-on time, the conductivity of the water, the electrode gap and the size of the container (plate electrode) within the experimental boundaries. An estimation of the energy channels for a discharge at standard conditions revealed that the dominant part of the energy is dissipated into the water reservoir. About 25% of the ...

Influence of H2 and D2 plasmas on the work function of caesiated materials

Caesium-covered surfaces are used in negative hydrogen ion sources as a low work function converter for H–/D– surface production. The work function χ of the converter surface is one of the key parameters determining the performance of the ion source. Under idealized conditions, pure bulk Cs has 2.14 eV. However, residual gases at ion source background pressures of 10−7–10−6 mbar and the plasma surface interaction with the hydrogen discharge in front of the caesiated surface dynamically affect the actual surface work function. Necessary fundamental investigations on the resulting χ are performed at a dedicated laboratory experiment. Under the vacuum conditions of ion sources, the incorporation of impurities into the Cs layer leads to very stable Cs compounds. The result is a minimal work function of χvac ≈ 2.75 eV for Cs evaporation rates of up to 10 mg/h independent of substrate material and surface temperature (up to 260 °C). Moreover, a distinct degradation behavior can be observed in the absence of a C...

Workshop on performance variations in H− ion sources 2012: PV H−12

This paper briefly summarizes a workshop held in Jyvaskyla the day after NIBS’12. The half-day workshop aimed at globally capturing the issue of performance variations in H− sources. There was a focus on production facilities and facilities that work under production-like conditions, because there are often high expectations to be met.

Correlation of size, velocity, and autonomous phase of a plasmoid in atmosphere with the dissipated energy

The visual properties of a large plasmoid rising from a water container into the air for up to 450 ms are brought into correlation with the total energy dissipated into the system, and, in particular, with the energy used for plasma generation. The latter parameters are deduced from the time-resolved discharge current and voltage of the capacitor bank which is used as energy supply. By varying the experimental parameters, the energy dissipated to the system varies between 5 kJ and 30 kJ from which 10% to 30% is transferred to the plasma. Clear correlations are obtained for the size of the plasmoid changing from 15 cm to 35 cm in width, the ascent velocity ranging from 1 m/s to 2 m/s, and the rising height for which up to 85 cm is measured. For the relation of the autonomous phase with the energy transferred to the plasma, two trends are observed: 450 ms duration is achieved in maximum with the present setup being almost independent on the electrode gap, the voltage-on time, the water conductivity, or the ...

Initial Phase of a Large Atmospheric Plasmoid Generated Above a Water Surface

A water plasmoid in atmospheric pressure air with an autonomous phase of ~500 ms is formed from a discharge above a water surface, which is powered only for 150 ms. The plasma ball has a diameter of ~20 cm and is often referred to as the ball lightning phenomena. High speed cameras and optical spectra are used for diagnostic purposes. An image of the initial phase reveals the formation of the plasma ball from the inner electrode with streamers above the water surface.