ISTTOK

Figure 1: Association EURATOM - IST / Centro de Fusão
Nuclear, Lisbon (Portugal)

History ot IST
ISTTOK Parameters
ISTTOK Milestones
IISTTOK Objectives and Achievements

History of Instituto Superior Técnico (IST)

A research and development programme in the field of controlled fusion has been carried out in Portugal, within the framework of the European Union Fusion Programme since 1987. To fulfill one of its objectives (the creation of a home laboratory) a small tokamak ISTTOK was installed at "Instituto Superior Técnico" (Lisbon).

The construction of ISTTOK started in mid 1990, at "Centro de Fusão Nuclear", reusing some parts of the former tokamak TORTUR, which included the support structure, vacuum vessel, copper shell, toroidal magnetic field coils, transformer, capacitor banks, radiofrequency-generator for the gas pre-ionization, and the discharge cleaning system.

Other components, such as the vacuum systems, the poloidal windings, and the power supply for the toroidal field coils, as well as its diagnostics (Ha monitor, microwave interferometer and reflectometer, Thomson scattering, Visible spectrometer, X-rays analyser, heavy ion beam deflection analyser), and the control and data acquisition system were locally designed, constructed and commissioned. ISTTOK has been in operation since 1993.

ISTTOK is a large aspect ratio circular cross-section limiter tokamak with the following main parameters:

ISTTOK Parameters

Major radius	46 cm
Limiter radius	8.5 cm
Vessel radius	10 cm
Copper shell radius	11 cm
Toroidal magnetic field	up to ~ 2.8 T
Transformer flux swing	0.22 Vs
Plasma current	~ 7-10 kA
Discharge duration	~ 45 ms, (200 ms in AC discharges)
Electron density	~ 5-8 x 1018m-3
Electron temperature	~ 100-200 eV
Ion temperature	~ 150 eV
Energy confinement time	~ 0.8 ms
Safety factor	q(0) ~ 1, q(a) ~ 5

Figure 2:ISTTOK Machine

ISTTOK Milestones

1990	Beginning of the tokamak reassembly.
1991	Vacuum system in operation.
1992	Electrical, magnetic and optical diagnostics installed. Control and data acquisition system fully operational.
1993	Thomson scattering diagnostic and Heavy ion beam deflection analyser installed.
1994	First consistent plasma physics studies performed.
1996	Design and construction of an alternating square wave power supply for the primary of the tokamak transformer, based on a condenser bank and a fast polarity switch circuit, using a bridge of IGBTs (Insulated Gate Bipolar Transistor).

ISTTOK Objectives and Achievements

The ISTTOK project has had, since the beginning of the tokamak operation, four main objectives:

• to attract plasma physics students,
• to provide basic experimental programme on fusion oriented physics and technology,
• to develop new state of the art diagnostics and
• to carry out a scientific programme, mainly based on the influence of externally applied electric and magnetic perturbations on plasma confinement and stability. Another important area of research was the attainment of long duration discharges based on an alternating loop voltage

The main achievements of ISTTOK were:

• 1996 - Analysis of the plasma density profile evolution in sawtooth discharges by heavy ion beam probing.
• 1997 - Operation of the tokamak in a multicycle alternating flat-top plasma current regime.
• 1998 - Enhancement of the plasma confinement and stability by negative limiter biasing.
• 1999 - Evolution of the plasma temperature and internal poloidal magnetic field profiles by heavy ion beam probing.
• 2000 - Edge plasma turbulence and transport studies, related to both limiter and electrode biasing schemes, based on the analysis of the signals of a set of Langmuir probes.
• 2001 - Application of emissive probes to determine the plasma potential radial profile in the scrape-off layer.