Henric Bergsåker

Initial results from the rebuilt EXTRAP T2R RFP device

The EXTRAP T2R thin shell reversed-field pinch (RFP) device has recently resumed operation after a major rebuild including the replacement of the graphite armour with molybdenum limiters, a fourfol ...

Diagnostics of mobile dust in scrape-off layer plasmas

Dust production and accumulation pose serious safety and operational implications for the next generation fusion devices. Mobile dust particles can result in core plasma contamination with impuriti ...

Migration of tungsten dust in tokamaks: role of dust-wall collisions

The modelling of a controlled tungsten dust injection experiment in TEXTOR by the dust dynamics code MIGRAINe is reported. The code, in addition to the standard dust–plasma interaction processes, also encompasses major mechanical aspects of dust–surface collisions. The use of analytical expressions for the restitution coefficients as functions of the dust radius and impact velocity allows us to account for the sticking and rebound phenomena that define which parts of the dust size distribution can migrate efficiently. The experiment provided unambiguous evidence of long-distance dust migration; artificially introduced tungsten dust particles were collected 120° toroidally away from the injection point, but also a selectivity in the permissible size of transported grains was observed. The main experimental results are reproduced by modelling.

Capture by aerogel—characterization of mobile dust in tokamak scrape-off layer plasmas

The aim of this letter is to demonstrate the feasibility and potential of the novel in situ dust diagnostic method-capture by aerogel targets. Aerogel, a highly porous material with a density of a ...

Properties of the edge plasma in the rebuilt Extrap-T2R reversed field pinch experiment

The edge region of the rebuilt Extrap-T2R reversed field pinch experiment has been investigated using Langmuir probes. Radial profiles of main plasma parameters are obtained and compared with those of the previous device Extrap-T2. The spontaneous setting up of a double shear layer of E×B toroidal velocity is confirmed. The particle flux induced by electrostatic fluctuations is calculated and the resulting effective diffusion coefficient is consistent with the Bohm estimate. A close relationship between electrostatic fluctuations at the edge and non-linear coupling of MHD modes in the core is found.

Correlation between internal tearing modes and edge electrostatic fluctuations in a reversed‐field pinch

Edge electrostatic fluctuations, in the Extrap T1 reversed‐field pinch [Nucl. Fusion 34, 427 (1994)], are observed to be correlated to internal tearing mode activity. Bispectral analysis of the edge electrostatic fluctuations shows the occurrence of nonlinear coupling between the low frequency internal tearing‐mode‐related activity and the high frequency, external, electrostatic fluctuations. In addition, the fluctuation levels of both the edge electrostatic fluctuations and the internal tearing modes have comparable scaling with plasma current. These results suggest that suppression of the internal tearing mode activity may decrease the edge electrostatic fluctuations and the related particle loss in the reversed‐field pinch configuration.

Effects of E×B velocity shear on electrostatic structures

Bursts of electrostatic turbulence measured in the edge region of two reversed field pinch experiments (RFX and T2R) are observed to be associated with electrostatic structures. The corresponding E×B velocity drifts have been reconstructed showing features reminiscent of monopolar vortices. These vortices have been grouped in two classes depending on their rotation direction. It is observed that the prevalent rotation direction depends on the shear of the surrounding mean E×B drift velocity, showing a clear analogy with ordinary fluids.

Deep deuterium retention and Be/W mixing at tungsten coated surfaces in the JET divertor

Surface samples from a full poloidal set of divertor tiles exposed in JET through operations 2010-2012 with ITER-like wall have been investigated using SEM, SIMS, ICP-AES analysis and micro beam nu ...

Anomalous particle transport and flow shear in the edge region of RFP's

The effect of the E × B velocity shear on the particle flux carried by electrostatic fluctuations at the edge is discussed comparing the results of the experiments T2R (R = 1.2 m, a = 0.18 m) and RFX (R = 2 m, a = 0.5 m). In both experiments electrostatic turbulence accounts for most of the particle flux. The shear of the E x B velocity is close or larger than the value required for turbulence suppression or reduction. Particle fluxes exhibit bursts which carry a large fraction of the total particle flux, and which tend to cluster during magnetic relaxation phase. On the average a Bohm estimate of the experimental plasma diffusivity accounts for the experimental data. Finally a discussion on the methods for turbulence control is given.

Reynolds and Maxwell stress measurements in the reversed field pinch experiment Extrap-T2R

The complete Reynolds stress (RS) has been measured in the edge region of the Extrap-T2R reversed field pinch experiment. The RS exhibits a strong gradient in the region where a high E × B shear takes place. Experimental results show this gradient to be almost entirely due to the electrostatic contribution. This has been interpreted as experimental evidence of flow generation via turbulence mechanism. The scales involved in flow generation are deduced from the frequency decomposition of RS tensor. They are found related to magnetohydrodynamic activity but are different with respect to the scales responsible for turbulent transport.