R. Gomes

Enhancement of the ISTTOK plasma confinement and stability by negative limiter biasing

Experimental results concerning the plasma response to the biasing of the tokamak ISTTOK localized limiters, on a strong flat-top plasma current reference discharge, are reported. Modifications of central beta as well as of energy confinement time are determined through time-resolved measurements of the line-averaged plasma density, electron density profile, electron temperature and ohmic power. Gross particle confinement variations are confirmed by the associated changes of the ratio between the line-averaged electron density and the radiation level. Plasma stability modifications are analysed by measurements of the plasma column transverse displacement, plasma poloidal rotation frequency and sliding fast Fourier transform spectra of both the magnetic and the electron density fluctuations. The evolution of the amplitude as well as the frequency of the most important tearing modes is determined. Negative bias leads to better particle and energy confinement, and improved stability. Positive bias reduces both confinement and stability, causing a significant transitory vertical displacement of the plasma column as well as of its current axis.

Analysis of the ISTTOK plasma density profile evolution in sawtooth discharges by heavy-ion beam probing

This paper describes the temporal evolution of the major internal plasma parameters during a complete sawtooth cycle of the tokamak ISTTOK discharges, with emphasis on the time-resolved determination of the plasma density profile by heavy-ion beam probing.

Fast Camera Images of Surviving Gallium Droplets During Plasma–Liquid Gallium Jet Interaction Experiments in ISTTOK

Liquid metals have received a renewed interest in fusion research due to the advantages that these materials could offer to overcome the erosion problem of plasma facing components presently used in large-size fusion devices. In ISTTOK tokamak, the plasma-liquid gallium jet interaction is currently being studied. One of the main technical problems faced during the implementation of the experimental setup was to minimize the amount of gallium that remains in the vacuum chamber. Despite the large effort put in fulfilling this task, liquid-metal droplets could not be completely eradicated from the plasma. Here, pictures of one of those remaining droplets traveling through the plasma are presented and discussed.

The International Joint Experiment ISTTOK

The 3rd International Joint Experiment has been carried out on ISTTOK following the last RUSFD (technical meeting on Research Using Small Fusion Devices) conference, held at Lisbon, in the scope of the IAEA Coordinated Research Project. This program has allowed a great knowledge interchange among the “small” fusion devices community and, in particular, in the development of common work-programs. This communication gives an overview of the impact of such activities. The JE have proved to be a very useful applied forum to share knowledge and to initiate young scientists in some fields of tokamak operation and diagnostics. Many of the ISTTOK 2008 relevant results were obtained under this scope.

Effects of hydrogen permeation on W, Mo and Cu Langmuir probes at ISTTOK

The microstructures of tungsten, molybdenum and copper wires used as Langmuir probes at ISTTOK edge plasma have been investigated. The probes cross-sections evidenced extensive grain growth, intergranular bubbles and increased hardness at the plasma exposed regions. Internal surfaces of large bubbles exhibited slip bands resulting from plastic deformation induced by high H2 pressure. Elastic recoil detection analysis was used to measure H concentration profiles. The present results suggest that H2 bubble formation in first wall components under long-term high thermal loads should be closely monitored in nuclear fusion devices. Strategies for H damage mitigation are proposed and discussed.