The transport of barium atoms and ions in the vicinity of the hot spot in fluorescent lamps operating at 25 kHz is investigated by a combined experimental and theoretical approach. By laser-induced fluorescence, the particle densities of barium atoms and ions were measured time-resolved at different distances from the spot centre. In addition, the time-dependent cathode fall voltage was measured using an improved band method. The model combines a kinetic part for the electrons with a fluid part for the barium atoms and ions. Both parts are spatially resolved in spherically symmetric geometry. The space-dependent electron Boltzmann equation yields the electron density and the ionization rate coefficient of barium as functions of the cathode fall voltage. These results are used to solve the time-dependent particle balance equations of barium atoms and ions which include the ionization of barium as gain and loss terms, respectively. Good agreement between the measured and calculated particle densities of barium atoms is obtained. A sensitive dependence of the ionization frequency and of the barium particle densities on the cathode fall voltage was found.

The method of laser induced fluorescence (LIF) is applied to fluorescent lamps (FL) in order to investigate processes of electrode erosion in the vicinity of the electrodes. The life time of FLs which are ignited by instant start is mainly limited by sputtering of the coil electrodes and in final breaking. This sputtering of tungsten mainly occurs during the ignition in the glow discharge phase. Therefore, the density of W atoms is measured in the electrode region during ignition. Temporal and spatial resolved profiles were measured by LIF which has been combined with fast imaging. The life time of FLs which are started with preheated coils is also caused mainly by electrode failures. But the reason differs from the instant start case because here the loss is caused mainly by evaporation. End-of-lamp life is reached if the emitter material which is deposited at the coil to reduce the work function of the coil is lost completely. LIF is used to measure the density of the eroded emitter material, namely Barium atoms. First result of phase resolved absolute Ba atoms densities are presented.

Investigations of fluorescent lamps (FL) are often focused on the electrodes, since the lifetime of the lamps is typically limited by the electrode lifetime and durability. During steady state operation, the work function lowering emitter material, in particular, barium, is lost. Greater barium losses occur under dimming conditions, in which reduced discharge currents lead to increased cathode falls, the result of the otherwise diminished heating of the electrode by the bombarding plasma ions. In this work the barium density near the electrodes of (FL), operating in high frequency dimming mode is investigated using the high-sensitivity method of laser-induced fluorescence. From these measurements we infer barium loss for a range of discharge currents and auxiliary coil heating currents. We show that the Ba loss can very easily be reduced by moderate auxiliary coil heating.

The high energy ion bombardment during instant start of a fluorescent lamp (FL) leads to intense sputtering of the electrode material including tungsten and emitter. Thus, a cold started FL often suffers from early failures due to coil fracture. The main goal of this paper is to investigate tungsten erosion. We have employed the ultra-sensitive method of laser-induced fluorescence. This technique is particularly well-suited to determining absolute population densities of neutral and singly ionized atoms of liberated electrode material. In addition to FL, our investigations have been performed also on hollow cathode lamps (HCLs). These are useful because they provide a variable source of sputtered tungsten atoms and can serve as tuning tools for precise adjustment of the laser radiation.We will present absolute atomic tungsten population densities in a commercial FL and in an HCL. Furthermore, the results of a theoretical investigation of the argon plasma and the tungsten density in the HCL are represented.

The erosion of the graphite divertor plates in the ASDEX Upgrade tokamak is measured spectroscopically. Spatial profiles of the D0 and C+ influxes across the outer target plate are determined from measured absolute line intensities. Plasma parameters (ne, Te) at the target, which are required to determine the appropriate photon emission efficiencies for these lines, are obtained from an in-vessel reciprocating Langmuir probe above the target plate. Yields for the erosion of the graphite by the incident D+ flux are determined from the ratio of the measured C+ to D0 fluxes. Over a range of moderate densities the measured yields of ⩽4% are explicable in terms of physical sputtering alone. Chemical sputtering by low energy Franck-Condon neutrals probably contributes, however, to the total erosion. At higher densities detachment of the plasma from the targets occurs owing to formation of a MARFE near the X point. Under these conditions localized physical sputtering of the targets ceases. The impurity level (Zeff) is, however, maintained following detachment, indicating a corresponding maintenance of carbon influx, perhaps due to chemical erosion of the total graphite surface and/or an improvement in particle confinement in the detached state

Several experiments were conducted in ASDEX Upgrade to prove the suitability of tungsten as a divertor target material under the conditions of a high density and low temperature divertor. The observed fluxes from a tungsten tile into the plasma are low, in keeping with the extremely low sputtering yields. In addition, the very favourable effect of 'prompt redeposition' (redeposition during the first gyration) could be confirmed by the experiments. Cooling of the edge region by neon injection seems permissible, i.e. neon impurity sputtering did not increase the eroded fluxes of tungsten. The transport and accumulation behaviour were investigated by means of the laser blow-off technique. No accumulation effects could be observed in ohmic discharges. In discharges with NBI heating but without ICRH, strong accumulation can occur. High heat flux tests were performed on graphite tiles coated with plasma sprayed tungsten, which withstood a thermal load of 15 MW/m(2) lasting 2 s as well as 1000 cycles of 10 MW/m(2) for 2 s without disabling damage. Owing to the encouraging results, an experiment using a tungsten divertor is planned in ASDEX Upgrade.

The possible use of tungsten as divertor target material is discussed. In addition to its extremely low sputtering yields, another very favourable aspect of this material is the high probability of prompt redeposition. First experimental observations are encouraging. The erosion fluxes from a tungsten target tile are low and the ionization lengths as short as expected. Additional cooling of the edge region by adding neon seems permissible. Accumulation of tungsten is not observed as long as there is sufficient sawtooth activity in the plasma centre. (author). 8 refs, 3 figs.

The erosion, plasma penetration and redeposition of divertor plate material such as C, Si, V, Mo and W have been studied in the ASDEX-Upgrade divertor. Long-term erosion and redeposition have been studied using evaporated markers, while time resolved measurements and plasma penetration were studied spectroscopically on W tiles. The expected reduction of erosion due to plasma ion sputtering could be confirmed. This reduced erosion is accompanied by largely enhanced deposition for high-Z ions, when the gyroradius exceeds the ionisation length.

Passive spectroscopy has been used for in situ studies of the erosion of various divertor target materials in ASDEX-Upgrade. Yields measured for the erosion of graphite over a range of moderate densities can be explained by physical sputtering alone, although there may also be small contributions from chemical processes. At high density, detachment of the divertor plasma occurs and sputtering of the target ceases. Under such high-density conditions high-Z materials, e.g. tungsten, offer a promising alternative to graphite. First studies of the erosion of these materials confirm the expected low sputtering rates and the short ionisation lengths of sputtered neutrals. The latter implies efficient prompt redeposition which should suppress production of higher ionisation stages and associated problems with self-sputtering.

Due to the open field lines, the scrape-off layer and divertor region of tokamak plasmas is a complex, two-dimensional system, involving transport parallel and perpendicular to the magnetic field, as well as interaction of the plasma with surfaces and with the neutral gas. Therefore sophisticated two-dimensional codes are required to model the divertor and edge physics. In this paper, the B2-EIRENE code package is used to simulate the ASDEX-Upgrade scrape-off layer plasma and the neutral gas dynamics in a fully self-consistent way. Specific ASDEX-Upgrade discharges are modelled using the actual magnetic configuration and in-vessel components. Single fluid as well as multifluid calculations including self-consistent target and wall erosion of carbon are described. At given input power and bulk plasma radiation, typical divertor plasma profiles from Langmuir probes are fitted by varying the separatrix density and the transport coefficients. On the basis of such multifluid fits, spectroscopic divertor diagnostics are numerically modelled and compared with measured profiles, and reasonable agreement is found.

Feedback-controlled puffing of neon and deuterium has been applied to control the edge-localized-mode behavior and the target plate power deposition during high-power $H$-mode discharges in ASDEX Upgrade. A regime has been found in which more than 90% of the heating power is lost through radiation and divertor detachment occurs, without deterioration of the energy confinement. The plasma remains in the $H$ mode, exhibiting small-amplitude, high-frequency ELM's, which do not penetrate to the target plates in the strike zone region.

The completely detached high confinement (CDH) regime established recently in high power ASDEX-Upgrade single null divertor discharges is described. The standard CDH mode scenario with feedback controlled external deuterium and neon puffing is analysed in detail with respect to its divertor detachment and confinement properties. Impurity transport and pumping effects and the density operation windows of the CDH mode are outlined. First results with other impurity radiators (N,Ar) are given. Finally the CDH mode operation space is discussed in terms of main chamber and scrape-off layer plus divertor radiation and possible consequences for future fusion experiments are outlined.

The introduction presents a historical review of the role of molecules in tokamak research starting from the first installations at the Kurchatov Institute. Molecular impurities were mostly considered as a transient conditioning problem, but with the use of carbon for wall and limiter elements, it was perpetuated. New results about the elementary processes involved in hydrogenic carbon erosion are reported and the existing data base is briefly discussed. Results from mass spectrometry are presented as well. as data from optical spectroscopy including determination of CD4 and CD fluxes from molecular band intensities. A typical yield of about 5% for hydrogenic chemical erosion is obtained. In combination, all these results show the impact of hydrogenic carbon erosion. They strongly suggest that with boronized walls it remains as the dominating process for the carbon fluxes in the SOL and even dominates the carbon concentration in the central plasma in spite of a high SOL screening action for hydrocarbons.