B. Unterberg

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

Particle transfer in edge transport barrier with stochastic magnetic field

Charged particle losses at the plasma edge affected by resonant magnetic perturbations (RMP) are considered by taking into account the electron and ion flows both parallel and perpendicular to perturbed field lines. Calculations are done for H-mode plasmas of low collisionality, i.e., under conditions where significant pump out of particles has been observed in experiments on the DIII-D tokamak [J. Luxon, Nucl. Fusion 42, 614 (2002)] with RMP from the I-coils. It is demonstrated that the perpendicular ion flux, arising by magnetic field stochastization due to the deviation of poloidal rotation from the neoclassical one, is of more importance than the parallel ion flow. With both loss contributions included, computations provide a pump out level in agreement with observations if the screening of RMP by the plasma rotation is taken into account. The impact of possible enhancement in the perpendicular electron transport due to fluctuations observed with RMP in the edge transport barrier is assessed.

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 ...

Materials for DEMO and reactor applications-boundary conditions and new concepts

DEMO is the name for the first stage prototype fusion reactor considered to be the next step after ITER towards realizing fusion. For the realization of fusion energy especially, materials questions pose a significant challenge already today. Heat, particle and neutron loads are a significant problem to material lifetime when extrapolating to DEMO. For many of the issues faced, advanced materials solutions are under discussion or already under development. In particular, components such as the first wall and the divertor of the reactor can benefit from introducing new approaches such as composites or new alloys into the discussion. Cracking, oxidation as well as fuel management are driving issues when deciding for new materials. Here composites as well as strengthened CuCrZr components together with oxidation resilient tungsten alloys allow the step towards a fusion reactor. In addition, neutron induced effects such as transmutation, embrittlement and after-heat and activation are essential. Therefore, when designing a component an approach taking into account all aspects is required.

Effects of impurities released from high Z test limiter on plasma performance in TEXTOR

To investigate the feasibility of high Z metals as plasma facing materials, a Mo test limiter was inserted into the TEXTOR edge plasma and the Mo content in the core plasma and its flux from the limiter have been observed. In OH plasma the additional radiation due to Mo impurities increased with line averaged density n e . Beyond n e of about 3 × 10 19 m −3 , the central radiation increased strongly with time and a hollow temperature profile appeared. In contrast, Mo impurity radiation decreased with n e in NBI (co-injection) plasma and no clear Mo accumulation in the plasma center was observed. However, when the edge plasma was cooled by neon injection in NBI plasma, the Mo concentration in the plasma center substantially increased. The Mo flux from the limiter showed no remarkable change during the OH and NBI density scan or with neon cooling. The possible reasons for the Mo behavior in the core plasma are discussed

On description of magnetic stochasticity in poloidal divertor tokamaks

A generic approach to study the stochastic field lines formed near the magnetic separatrix of poloidal divertor tokamaks due to nonaxisymmetric magnetic perturbations is proposed. The method is based on the determination of the so-called Poincare integral [S. S. Abdullaev, Phys. Rev. E 70, 046202 (2004)] defined as an integral over the vector potential of the perturbation field taken along the closed field lines orbit. This integral allows us to obtain the analytical estimations for the characteristics of chaotic field lines near the magnetic separatrix, like the Chirikov parameter, the widths of the stochastic layer and magnetic footprints, also the statistical characteristics of chaotic field lines, the quasilinear field line diffusion coefficients, and the Kolmogorov lengths. These estimations are in good agreement with the direct numerical calculations of corresponding quantities. A field line convection coefficient is introduced to describe the preferential outward drift of open chaotic field lines n...

Investigation of radiation enhanced sublimation of graphite test-limiters in TEXTOR

Abstract Graphite test-limiters have been used as single main limiters in TEXTOR and exposed to convective heat loads of typically 10–20 MW/m 2 . Local carbon release has been studied by means of local spectroscopy of CI-and CII-emission lines. Surface temperature distributions were determined by CCD camera measurements in the IR spectral region. It was found that the increase of the local carbon flux was below 15% when the surface temperature increased locally up to about 1900°C demonstrating that radiation enhanced carbon emission (RES) is negligible under these conditions. The suppression of RES-emission is discussed in view of present RES models.

Spectroscopic measurements of the ion temperature profile in front of a limiter in TEXTOR-94

Ion temperatures have been measured in front of a limiter in TEXTOR-94 under various plasma conditions by means of the Doppler broadening of the C5+ line (λ = 529 nm), which is excited by charge exchange processes with hydrogen (deuterium) atoms recycled at the limiter surface. The ion temperatures have been compared with electron temperatures measured by atomic beam techniques. It has been found that the ion temperatures are considerably larger than those of the electrons. In the scrape-off layer (SOL) the ratio between the ion and electron temperatures (Ti/Te) can reach values of up to three at low plasma densities, but this ratio decreases with increasing density. The SOL e-folding length of the ion temperature is much larger than that of the electron temperature. The influence of the test limiter position rL on the ion temperature measurements has been investigated but found to be of no significant importance.

Long-distance correlation and zonal flow structures induced by mean E×B shear flows in the biasing H-mode at TEXTOR

Long-distance toroidal correlations of potential and density fluctuations have been investigated at the TEXTOR tokamak [H. Soltwisch et al., Plasma Phys. Controlled Fusion 26, 23 (1984)] in edge electrode-biasing experiments. During the biasing-induced H-mode, the dc E×B shear flow triggers a zonal flow structure and hence long-distance correlation in potential fluctuations, whereas for density fluctuations there is nearly no correlation. These results indicate an intimate interaction between the mean and zonal flows, and the significance of long range correlations in improved-confinement regimes.

Predictive modelling of L and H confinement modes and edge pedestal characteristics

The results of predictive self-consistent modelling of plasma parameters in low (L) and high (H) confinement modes by the one-dimensional transport code RITM, with particular emphasis on the properties of the edge transport barrier, are presented and discussed. The same transport model is used under both L- and H-mode conditions and includes contributions from ion temperature gradient (ITG), trapped electron, drift Alfven (DA) and drift resistive ballooning instabilities described in the fluid approximation. The computations predict the formation of the edge transport barrier at a high enough heating power due to stabilization of ITG and DA modes, dominating the edge transport in the L-mode, through the effects of the density gradient and the pressure gradient at low collisionality, respectively. The calculated radial profiles and scalings for pedestal and confinement characteristics are compared with measurements on JET, DIII-D and JT-60U tokamaks.