B. Schweer

Recent developments for plasma edge diagnostics using atomic beams

Abstract Li-atom beams with velocities of 1.5 × 105 cm/s and 1× 106 cm/s have been used to measure ne profiles in the density range 1011–1013 cm−3 from the Li line emission with a spatial resolution of about 1 mm. Injecting in addition a different type of atoms, the ionization rate of which shows a strong dependance on the electron temperature in the interesting range of 5–100 eV (e.g. carbon and helium), also radial Te profiles are obtained. A high intensity 30 keV Li beam is employed to measure radial profiles of impurity ion concentrations (e.g. He, C, 0) by charge exchange recombination spectroscopy. For the different purposes several types of injectors have been developed: thermal beams using ovens, suprathermal beams using laser ablation techniques, and a high energy beam using a plasma ion source in combination with a charge exchange cell. They are located at different poloidal and toroidal positions. The spatial line emission profiles of the beams are recorded either by Si diode array cameras or by a photomultiplier in combination with a scanning mirror so that a spatial resolution better than 2 mm is achieved. The combination of all these systems delivers information about the structure of the TEXTOR boundary layer in poloidal and toroidal direction.

Molybdenum test limiter experiments in TEXTOR

Limiter experiments with a Mo test limiter have been carried out in TEXTOR under various conditions with ohmic and NBI heating. Maximum power loads reached about 20 MW/m2 resulting in surface temperatures up to the melting point. A maximum fraction of 8% of the total convective energy in the plasma was deposited onto the Mo limiter. Molybdenum impurities are mainly produced by physical sputtering due to the impact of C and O impurities. Under ohmic heating conditions the Mo impurity radiation increased with increasing plasma density, though the relative source strength of the Mo release decreased. Approaching a critical density of about 3*1013 cm-3 Mo accumulated in the plasma centre leading to strong central radiation with hollow temperature profiles, which resulted in a minor disruption. Under NBI heated plasma conditions (co-NBI, 1.3 MW) Mo radiation decreased with increasing plasma density as did the relative Mo source at the limiter; at high plasma densities the influence of the Mo limiter on plasma impurities and plasma performance became negligible. No instability was observed under these conditions. The different behaviour between ohmic and NBI heating conditions is explained by the different relative importance of neoclassical and anomalous transport under ohmic and NBI heating, respectively. The observed instability in high density ohmic plasmas is caused by Mo accumulation in the plasma centre and is attributed to neoclassical transport processes

Plasma edge physics in the textor tokamak with poloidal and toroidal limiters

Abstract Investigations of the plasma edge in TEXTOR are presented on the one hand by comparing results obtained with poloidal and toroidal limiters and on the other hand by discussing general problems of plasma edge physics which are independent of the limiter configuration. The characteristic properties of plasma flow to the different limiters are analyzed and show e.g. that the fraction of total ion flow to the limiter is much larger in the case of a toroidal limiter (80%). Density and heat flux profiles are presented which demonstrate that for both types of limiters a significant steepening of the scrape-off layer (SOL) occurs close to the limiter, leading to a small heat load e-folding length of 5–8 mm. The velocity distribution of recycled neutral hydrogen at a main limiter has been determined from the Doppler broadening of the H α line. The data clearly show that a large fraction of particles (30–50%) is reflected at the limiter surface having energies of about the sheath potential. Significant isotopic effects (H/D) concerning the plasma edge properties and the plasma core are presented and their relation to enhanced particle and energy transport in hydrogen compared to deuterium is discussed. A decrease of the cross field diffusion coefficient with increasing density can be deduced from density profile measurements in the SOL and a comparison with density fluctuations is given. The role of oxygen for impurity release is demonstrated. A new type of wall conditioning —boronization — is described, with two major improvements for quasi stationary conditions: reduction of oxygen and better density control. Best results with ICRH have been obtained under these conditions.

Investigation of light-ion sputtering of titanium using laser-induced fluorescence

Abstract Relative sputtering yields and velocity distributions for Ti sputtered by light ions have been measured, using He + , D + and H + -ion bombardment at energies between 40 eV and 12 keV at normal incidence. The sputtered neutral Ti atoms were excited from the ground state with an excimer pumped dye laser. A photomultiplier with an interference filter in front of it was used to observe the fluorescence light, which was proportional to the density of sputtered Ti-atoms. From the fluorescence signal and from the mean velocity the relative sputtering yields were determined as a function of projectile type, energy, and irradiation fluence. Measured relative sputtering yields show reasonable agreement with yields obtained by other experimental methods. A strong dependence of the velocity distribution on the projectile type and energy was found.

First results from TEXTOR

The paper summarizes the main results obtained during the first six months of experimental work on TEXTOR with ohmically heated plasmas. The machine(R = 175 cm; a = 45-50 cm) was operated at wall temperatures up to 303 oC with hydrogen gas, a toroidal field of 2 T and a maximum plasma current of 500 kA, yielding central electron temperatures of 1.2 keV, maximum line-averaged densities of 4 x 1013 cm-3 and discharge durations of up to 2.7 sec (for Ip≤340 kA). Besides providing long-lasting reproducible plasmas for forthcoming plasma wall interaction research, the following tasks were accomplished: (1) control of low Z impurities by effective wall cleaning and conditioning; (2) accurate plasma positioning for extended pulse durations; (3) determination of the current density distribution by way of Faraday rotation measurements; (4) investigation of plasma limiter interaction including spectroscopic determination of released neutral particle fluxes and measurement of the power density distribution in the scrape-off layer by infrared-thermography.

Measurement of the velocity distribution of metal atoms sputtered by light and heavy particles

Abstract The velocity distribution of neutral metal atoms in controlled fusion devices as well as their density and flux density can be determined by means of fluorescence spectroscopy. The method permits high time and space resolution and does not require any additional measurement. Using a pulsed dye laser we have applied fluorescence spectroscopy to the measurement of the velocity distribution of sputtered iron atoms. Sputtered particles have been generated in an uhf-chamber by bombarding an iron or stainless steel target at normal incidence with a 10 keV He-, D- or A-beam of 7 cm diameter. Velocity distributions have been measured in the direction parallel and perpendicular to the target plate. The potential and the limitations of the method are discussed.

Radio frequency ion source for plasma diagnostics in magnetic fusion experiments

Low-divergent quasistationary neutral beams are often applied in modern magnetic fusion devices as a diagnostic tool providing unique information about plasma parameters. The most important requirements of these beams are sufficiently large current and energy of the particles, so that the beam can penetrate to the plasma core. Also the duration of the beams must be long enough, i.e., close to that of a plasma discharge, amounting to at least a few seconds for large fusion devices. We developed a neutral beam injector for plasma diagnostics in the tokamak TEXTOR-94 which is capable of meeting these requirements. The maximum beam energy is 50 keV and the source operated in hydrogen delivers an ion current of up to 2 A with a pulse duration of up to 4 s. The low divergent beam (∼0.5°– 0.6°) is geometrically focused 4 m downstream from the source having a 1/e width of ∼ 70 mm at the focal point. The beam can be modulated with a frequency variable up to 500 Hz. The ion source plasma is produced by a radio freq...

Measurement of the density and velocity distribution of neutral Fe in ISX-B by laser fluorescence spectroscopy

Abstract The density and velocity distribution of neutral Fe atoms in the plasma-edge of the ISX-B tokamak have been measured by laser fluorescence spectroscopy. Fe neutrals were excited at 302 nm using a flashlamp-pumped dye laser. The fluorescence light was detected at 382 nm with an interference filter and photomultiplier system. The frequency-doubled laser output (700 W) and pulse length (500 ns) allowed saturation pumping of the Fe transition thereby enabling absolute measurement of densities. Insertion of an additional Fabry-Perot etalon into the laser cavity narrowed the laser bandwidth to 0.6 pm corresponding to a velocity resolution of 600 m/s (0.1 eV for Fe). Neutral densities were measured for two locations in the plasma-edge as a function of discharge time. Results show a detection sensitivity of 1012 atoms/m3 with a neutral Fe density of a few 1012 atoms/m3 in the first 20 ms of the discharge and 1014 atoms/m3 30 ms after the discharge. The velocity distribution of the neutral Fe at the end of the discharge is thermal (400–1000 K) while the smaller peak at the beginning of the discharge shows evidence of higher velocities.

High confinement and high density with stationary plasma energy and strong edge radiation cooling in the upgraded Torus experiment for technology oriented research (TEXTOR-94)

An overview of the results obtained so far for the radiative I-mode regime on the upgraded Torus Experiment for Technology Oriented Research (TEXTOR-94) [Proceedings of the 16th IEEE Symposium on Fusion Engineering (Institute of Electrical and Electronics Engineers, Piscataway, NJ, 1995), Vol. 1, p. 470] is given. This regime is obtained under quasistationary conditions with edge neon seeding in a pumped limiter tokamak with circular cross section. It combines high confinement and high β (up to a normalized beta, βn=2) with low edge q values (down to qa=2.8) and high density even above the Greenwald limit together with dominant edge radiative heat exhaust, and therefore shows promise for the future of fusion research. Bulk and edge properties of these discharges are described, and a detailed account is given of the energy and particle confinement and their scaling. Energy confinement scales linearly with density as for the nonsaturated Ohmic Neo-Alcator scaling, but the usual degradation with total power remains. No deleterious effects of the neon seeding on fusion reactivity and plasma stability have been observed.An overview of the results obtained so far for the radiative I-mode regime on the upgraded Torus Experiment for Technology Oriented Research (TEXTOR-94) [Proceedings of the 16th IEEE Symposium on Fusion Engineering (Institute of Electrical and Electronics Engineers, Piscataway, NJ, 1995), Vol. 1, p. 470] is given. This regime is obtained under quasistationary conditions with edge neon seeding in a pumped limiter tokamak with circular cross section. It combines high confinement and high β (up to a normalized beta, βn=2) with low edge q values (down to qa=2.8) and high density even above the Greenwald limit together with dominant edge radiative heat exhaust, and therefore shows promise for the future of fusion research. Bulk and edge properties of these discharges are described, and a detailed account is given of the energy and particle confinement and their scaling. Energy confinement scales linearly with density as for the nonsaturated Ohmic Neo-Alcator scaling, but the usual degradation with total power ...

Development and testing of a bulk tungsten tile for the JET divertor

The ITER-like wall project has been launched to design, manufacture and test all the necessary components in view of their installation in a dedicated shutdown 2008?09. One of the RD (ii) W lamellae design, which is a mechanical assembly of several stacks of W blades. Both concepts were realized as test tiles and tested under high heat flux loading. The lamellae test tile showed a high performance fulfilling the requirements. Finally, the lamellae concept was chosen to be the primary solution and comprehensive numerical analyses were performed for the further optimization of the concept.