A key to improved ion core confinement in the JET tokamak: ion stiffness mitigation due to combined plasma rotation and low magnetic shear.

New transport experiments on JET indicate that ion stiffness mitigation in the core of a rotating plasma, as described by Mantica et al. [Phys. Rev. Lett. 102, 175002 (2009)] results from the combined effect of high rotational shear and low magnetic shear. The observations have important implications for the understanding of improved ion core confinement in advanced tokamak scenarios. Simulations using quasilinear fluid and gyrofluid models show features of stiffness mitigation, while nonlinear gyrokinetic simulations do not. The JET experiments indicate that advanced tokamak scenarios in future devices will require sufficient rotational shear and the capability of q profile manipulation.

Experimental evidence of momentum transport induced by an up-down asymmetric magnetic equilibrium in toroidal plasmas.

The first experimental evidence of parallel momentum transport generated by the up-down asymmetry of a toroidal plasma is reported. The experiments, conducted in the Tokamak à Configuration Variable, were motivated by the recent theoretical discovery of ion-scale turbulent momentum transport induced by an up-down asymmetry in the magnetic equilibrium. The toroidal rotation gradient is observed to depend on the asymmetry in the outer part of the plasma leading to a variation of the central rotation by a factor of 1.5-2. The direction of the effect and its magnitude are in agreement with theoretical predictions for the eight possible combinations of plasma asymmetry, current, and magnetic field.

Experimental study of the ion critical-gradient length and stiffness level and the impact of rotation in the JET tokamak.

Experiments were carried out in the JET tokamak to determine the critical ion temperature inverse gradient length (R/LTi=R|nablaTi|/Ti) for the onset of ion temperature gradient modes and the stiffness of Ti profiles with respect to deviations from the critical value. Threshold and stiffness have been compared with linear and nonlinear predictions of the gyrokinetic code GS2. Plasmas with higher values of toroidal rotation show a significant increase in R/LTi, which is found to be mainly due to a decrease of the stiffness level. This finding has implications on the extrapolation to future machines of present day results on the role of rotation on confinement.

Transport of parallel momentum induced by current-symmetry breaking in toroidal plasmas.

The symmetry of a physical system strongly impacts on its properties. In toroidal plasmas, the symmetry along a magnetic field line usually constrains the radial flux of parallel momentum to zero in the absence of background flows. By breaking the up-down symmetry of the toroidal currents, this constraint can be relaxed. The parallel asymmetry in the magnetic configuration then leads to an incomplete cancellation of the turbulent momentum flux across a flux surface. The magnitude of the subsequent toroidal rotation increases with the up-down asymmetry and its sign depends on the direction of the toroidal magnetic field and plasma current. Such a mechanism offers new insights in the interpretation and control of the intrinsic toroidal rotation in present day experiments.

Evidence of inward toroidal momentum convection in the JET tokamak.

Experiments have been carried out on the Joint European Torus tokamak to determine the diffusive and convective momentum transport. Torque, injected by neutral beams, was modulated to create a periodic perturbation in the toroidal rotation velocity. Novel transport analysis shows the magnitude and profile shape of the momentum diffusivity are similar to those of the ion heat diffusivity. A significant inward momentum pinch, up to 20 m/s, has been found. Both results are consistent with gyrokinetic simulations. This evidence is complemented in plasmas with internal transport barriers.

Toroidal momentum pinch velocity due to the coriolis drift effect on small scale instabilities in a toroidal plasma.

In this Letter, the influence of the "Coriolis drift" on small scale instabilities in toroidal plasmas is shown to generate a toroidal momentum pinch velocity. Such a pinch results because the Coriolis drift generates a coupling between the density and temperature perturbations on the one hand and the perturbed parallel flow velocity on the other. A simple fluid model is used to highlight the physics mechanism and gyro-kinetic calculations are performed to accurately assess the magnitude of the pinch. The derived pinch velocity leads to a radial gradient of the toroidal velocity profile even in the absence of a torque on the plasma and is predicted to generate a peaking of the toroidal velocity profile similar to the peaking of the density profile. Finally, the pinch also affects the interpretation of current experiments.

Direction of impurity pinch and auxiliary heating in tokamak plasmas.

A mechanism of particle pinch for trace impurities in tokamak plasmas, arising from the effect of parallel velocity fluctuations in the presence of a turbulent electrostatic potential, is identified analytically by means of a reduced fluid model and verified numerically with a gyrokinetic code for the first time. The direction of such a pinch reverses as a function of the direction of rotation of the turbulence in agreement with the impurity pinch reversal observed in some experiments when moving from dominant auxiliary ion heating to dominant auxiliary electron heating.

Experimental study of trapped-electron-mode properties in tokamaks: threshold and stabilization by collisions.

Trapped electron modes are one of the candidates to explain turbulence driven electron heat transport observed in tokamaks. This instability has two characteristics: a threshold in normalized gradient and stabilization by collisions. Experiments using modulated electron cyclotron heating in the ASDEX Upgrade tokamak demonstrate explicitly the existence of the threshold. The stabilization with increasing collisionality is evidenced by a strong decrease of the propagation of heat pulses, explained by a transition to ion temperature gradient driven transport. These results are supported by linear gyrokinetic calculations.

Role of kinetic effects on the polarization current around a magnetic island.

The polarization current due to a magnetic island rotating in a tokamak plasma is believed to play a central role in the initial evolution of the neoclassical tearing mode. Monte Carlo delta f simulations are performed which also cover a parameter range that is not amenable to analytic treatment but is relevant to experiments. It is shown that the polarization current can change sign when the island rotation frequency is of the order of the ion parallel streaming around the island. Moreover, the current is reduced when the island width is of the order of the ion banana width. Finally, the transition to the enhanced high-collisionality regime is shown to occur for collision frequencies higher than those typical of today's experiments.

Density peaking, anomalous pinch, and collisionality in tokamak plasmas.

The existence of an anomalous particle pinch in magnetized tokamak plasmas is still questioned. Contradictory observations have been collected so far in tokamaks. Clear experimental evidence that density peaking in tokamak plasmas drops with increasing collisionality is provided for the first time. This phenomenon is explained by means of existing theoretical models based on the fluid description of drift wave instabilities, provided that such models include the dissipative effects introduced by collisions on the mentioned instabilities. These results reconcile the apparent contradictions found so far in the experiments.

Reduction of the ion drive and rho*(theta) scaling of the neoclassical tearing mode.

In the theoretical description of the neoclassical tearing mode it is usually assumed that the ion banana width w(b) is much smaller than the island width W. This assumption is questionable at least for the island size at the mode onset. We show that a significant fraction of the (ion) bootstrap current survives inside the island when w(b) is comparable to W. This effect also leads to a linear scaling of beta(onset)theta with the normalized ion poloidal gyroradius rho*(theta), in agreement with the experimental results of ASDEX Upgrade.

Reaching high poloidal beta at Greenwald density with internal transport barrier close to full noninductive current drive.

In the ASDEX Upgrade tokamak, high poloidal beta up to beta(pol) = 3 at the Greenwald density with H-mode confinement has been reached. Because of the high beta, the plasma current is driven almost fully noninductively, consisting of 51% bootstrap and 43% neutral beam driven current. To reach these conditions the discharge is operated at low plasma current ( I(P) = 400 kA) and high neutral beam heating power ( P(NBI) = 10 MW). The discharge combines an edge (H mode) and internal transport barrier at high densities without confinement-limiting MHD activities. The extrapolation to higher plasma currents may offer a promising way for an advanced scenario based fusion reactor.

[Pollinosis in Canton Ticino. A prospective study in Locarno]. / Pollinose im Kanton Tessin. Eine prospektive Studie in Locarno 1990-1993.

A study was carried out in the southern part of Switzerland (Canton Ticino) in 1990-1993 to determine the spectrum of cutaneous sensitivity to a large amount of pollens and several perennial allergens (50), using skin prick tests in a sample of 503 consecutive patients suffering from hay fever. The Canton Ticino is a very specific geographical and botanical area which includes several plants of the alpine and Mediterranean flora as well as representatives of the tropical flora, forming a unique pattern from a botanical and allergological point of view. The results of the study indicate that in this region the patients have symptoms of hay fever almost 10 months a year (from December until October) with a peak in May and June (88% and 73% of the patients respectively have symptoms in these months). The symptoms are mainly localized in the upper respiratory tract and eyes. Asthmatic symptoms were found in about 23% of the patients. At the top of the allergen list were grass-pollens (72% of the patients had sensitivity); rye (69%); olive tree (54%); birch (46%); chestnut (37%); ash tree (36%); alder tree (33%); ragweed (17%); parietaria (18%). Of real interest in this study area are, besides the classical allergy-inducing pollens, those of chestnut, parietaria, olive tree, ash tree and cupressaceae (for example cypress). Chestnut pollens represent about 30% of the airborne pollens in this region. 30% of the patients had sensitivity against dust mites (Dermatophagoides pteronissimus und D. farinae), and 20% against cats. Sensitivity against 8 mould spores was 2-9%.

[Hyposensitization in pollinosis. Results of a 3-year controlled study with 2 depot-allergoid grass pollen extracts: aluminum hydroxide-adsorbed allergoid and tyrosine-adsorbed allergoid]. / Zur Hyposensibilisierung der Pollinose. Ergebnisse einer kontrollierten Studie über drei Jahre mit zwei Depotallergoid-Graspollenextrakten: Aluminiumhydroxid-adsorbiertes Allergoid (AGD) und Tyrosin-adsorbiertes Allergoid (TA).

For controlled hyposensitization treatment over a period of three years 36 patients with confirmed grass pollen sensitization had been selected in 1986 and randomly distributed to receive preseasonal injection therapy: 23 patients were treated with an average of seven AGD (aluminium-adsorbed allergoid) injections, and 13 patients had received six TA (tyrosine-adsorbed allergoid) injections. Evaluation of the trial data collected during three years of preseasonal treatment showed the following results of tolerance and efficacy: Systemic side-reactions registered during therapy were only mild and transient and occurred in the average after 3% of the AGD injections and after 10% of the TA injections. Local reactions over 5 cm diameter were registered after 7% in the AGD group and after 9% in the TA group. Before therapy there was no significant difference (p greater than 0.05) between the groups; after three years of therapy the AGD injections had resulted in a mean net rise of specific IgG of 220% (significant, p = 0.001); during the same time, TA injections had resulted in a final net increase of 10% (not significant, p greater than 0.05). Both treatment forms did not lead to any statistically relevant changes of specific IgE values. After three years of hyposensitization treatment, patients of both groups had improved; but an advantage was documented for patients treated with AGD on the basis of scores for objective assessment as well as for registered symptom and medication scores.