Peter Peleman

Dynamics of turbulent transport in the scrape-off layer of the CASTOR tokamak

In this paper, the propagation of turbulent events along radial and poloidal probe arrays in the scrape-off layer (SOL) of the tokamak CASTOR [M. Hron et al., Czech. J. Phys. 49, 181 (1999)] is observed using a novel correlation technique. The typical turbulent structures appear as dipoles on the floating potential signals and as positive bursts on the density. It is found that both dipoles and density bursts propagate over long radial distances in the SOL while propagating poloidally. A comparison of the measured velocities of the potential and density fluctuations shows that the density structures move radially faster than the potential ones. This allows us to understand how the density is convected by the turbulence. The density moves radially along the potential valleys created by the poloidal dipoles without modifying them. In the framework of an interchange turbulence, this property indicates that charge exchange collisions with neutrals in the SOL of CASTOR lead to a high viscosity that damps the z...

Advanced probes for edge plasma diagnostics on the CASTOR tokamak

Understanding of underlying physics in the edge plasma of tokamaks requires knowledge of the plasma density, potential, electron and ion temperature, ion flows and their fluctuations with a high spatial and temporal resolution. A family of electric probes, which have been designed and tested for this purpose in the CASTOR tokamak, is reviewed and examples of their performance are given. In particular, we focus on description of the 1D and 2D arrays of Langmuir probes for spatially resolved measurements of the edge turbulence, the Ball pen and emissive probes for direct measurements of the plasma potential, the optimized Gundestrup probe for measurements of parallel and perpendicular ion flow, and the tunnel probe for fast measurement of electron and ion temperatures. Additional information on individual diagnostics is available in the listed references. PACS 52.70.Ds

Investigation of the reliability of a 1-D fluid model for Mach probe measurements

In this paper we show how a two dimensional fluid model can be used to interpret data obtained from an inclined Mach-probe or a Gundestrup probe. We use an analytical approximation of the solution of the differential equations describing the relation between the plasma flow and the measured ion saturation currents at the probes surface. The parameters of this analytical solution are determined by comparison with the exact numerical solution of the equations. In this way we are able to measure the parallel as well as the perpendicular Mach numbers over the whole parameter range with a minimum accuracy of 90%.

Multi-machine studies of the role of turbulence and electric fields in the establishment of improved confinement in tokamak plasmas

An extensive (INTAS) research programme started in 2002 to investigate the correlations between, on the one hand, the occurrence of transport barriers and improved confinement in the medium-size tokamaks TEXTOR and T-10 and on the smaller tokamaks FT-2, TUMAN-3M and CASTOR, and on the other hand, electric fields, modified magnetic shear and electrostatic and magnetic turbulence using advanced diagnostics with high spatial and temporal resolution, and various active means to externally control plasma transport. It also requires one to characterize fluctuations of various important plasma parameters inside and outside transport barriers (TBs) and pedestal regions with high spatial and temporal resolution using advanced diagnostics, and to elucidate the role of turbulence driving and damping mechanisms, including the role of the plasma edge properties. Furthermore, one needs to determine the cross-field transport from the measurements and compare this with available theoretical models. This has been done in a strongly coordinated way, exploiting the complementarity of TEXTOR and T-10 and the backup potential of the three other tokamaks, which together have all the relevant experimental tools and theoretical expertise. Physical mechanisms of several TBs have been studied: electron internal transport barriers in T-10 and TEXTOR, ergodization-induced TB in TEXTOR, TB in ohmic discharges in TUMAN-3M, periodic bias-induced TBs in CASTOR. Geodesic acoustic modes (GAM) have been investigated in T-10, TEXTOR and TUMAN-3M. Core turbulence has been characterized in T-10, and small-scale turbulence has been revealed in FT-2.

Novel advanced Gundestrup-like probe for the measurements of flows and edge plasma parameters in TEXTOR

A novel advanced Gundestrup-like probe head for local measurements of equilibrium and fluctuating plasma parameters in the plasma edge of TEXTOR is described. This probe assembly enables us to simultaneously determine the toroidal and poloidal plasma flows, the ion saturation current, density, electron temperature, floating potential, as well as their fluctuating properties. An improved analytical probe model is used to correctly relate the ratio of the ion saturation currents measured at the upstream and downstream collecting surfaces to the plasma flow. The probe is mounted on a fast reciprocating manipulator resulting in a high radial resolution of the profiles. A unique feature of the fast probe is the electrical linear motor drive which allows predefining any wave form of the radial position. The high speed of the probe drive reduces the exposure time which enables us to measure several radial profiles within a single discharge deep inside the last closed flux surface. We describe the first experimen...

Characterization of intermittent bursts at the edge of the CASTOR tokamak

Temporal characteristics of density bursts in the scrape-off layer of the CASTOR tokamak were investigated. Intermittent positive bursts of density are observed by means of a radial array of Langmuir probes. Globally (comparing the closest and furthermost radial positions with respect to the center of the discharge chamber) we observe a radial decrease of average burst rate together with a radial increase of the average burst duration. Recently, we observed a monotonic radial decrease of average burst rate together with an increase of the average burst duration in the Tore Supra tokamak [1, 2], but the decay length is significantly shorter than in CASTOR. This is most probably due to radially elongated turbulent events (streamers), which govern the radial transport at the edge of the CASTOR tokamak [3] and are responsible for the appearance of density bursts.

Joint Experiments on the Tokamaks CASTOR and T-10

Small tokamaks may significantly contribute to the better understanding of phenomena in a wide range of fields such as plasma confiement and energy transport; plasma stability in different magnetic configurations; plasma turbulence and its impact on local and global plasma parameters; processes at the plasma edge and plasma‐wall interaction; scenarios of additional heating and non‐inductive current drive; new methods of plasma profile and parameter control; development of novel plasma diagnostics; benchmarking of new numerical codes and so on. Furthermore, due to the compactness, flexibility, low operation costs and high skill of their personnel small tokamaks are very convenient to develop and test new materials and technologies. Small tokamaks are suitable and important for broad international cooperation, providing the necessary environment and manpower to conduct dedicated joint research programmes. In addition, the experimental work on small tokamaks is very appropriate for the education of students,...