Centro de Fusão Nuclear

ISTTOK is a small tokamak with a circular cross-section, a poloidal graphite limiter and an iron core transformer.

This magnetic confinement experiment has been built from some components of the ex-tokamak TORTUR (supporting structure, vacuum chamber, copper shell, transformer, toroidal magnetic field coils, radio-frequency generator, and condenser banks) which was de-commissioned by the Association EURATOM/FOM in 1988.

The other components of the tokamak, as well as its diagnostics and control and data acquisition system, were designed and constructed by physicists, engineers and technicians of the Association EURATOM/IST.

The main geometrical parameters of ISTTOK are indicated in Table 1.

Parameters	Values
Major radius	46 cm
Minor radius	8.5 cm
Maximum toroidal magnetic field	2.8 Tesla
Transformer flux swing	0.25 Vs

Table 1 – Geometrical parameters of ISTTOK

The ISTTOK construction started, officially, on January 1st, 1990, date on which the contract of association EURATOM/IST enter into force. The operation of this experiment in an inductive regime started on February 1991.

Typical ISTTOK discharges are characterized by the parameters referred to in Table 2.

Parameters		Values
Plasma current		~ 7 kA
Discharge duration		~ 45 ms
Plasma density at r=0		~5´10¹⁸ m^-3
Electron temperature at r=0		~120 eV
C^III ion temperature at r=0		~100 eV
Energy confinement time		~0.8 ms
Beta at r=0)		~0.6%
Safety factor	q(0)	~1
	q(a)	~5

Table 2 – Internal parameters of the ISTTOK plasma

The objectives of the Project Tokamak ISTTOK are:

Creation of an experimental pole for fusion plasmas in an academic environment;

Formation and training of personnel in physics, engineering and technologies associated with nuclear fusion;

Development of new diagnostic techniques and test of digital instrumentation dedicated to control and data acquisition;

Realization of a programme of plasma physics studies based on the operation of a tokamak in an alternate plasma current regime and on the influence of electric and magnetic external signals on plasma confinement and stability.

Tokamak Description

Vacuum chamber

Copper shell

Vacuum system

Iron core transformer

Toroidoal magnetic field solenoid

Vertical and horizontal magnetic field solenoid

Condenser banks

Gas injection system

Vaccum chamber conditioning system

Vacuum chamber. This component consists in two similar half-torus connected by insulating material, each one composed by six rigid sections in inox connected by thin (0.15 mm) inconnel bellows.

Copper shell. This component envolves the toroidal chamber, has a thickness of 1.5 cm and is internally covered by an 1.5 mm insulating layer (up to 12 kV). Its functions are: (a) – to support the vacuum chamber and (b) – to suppress the plasma column position fluctuations (T<2ms).

Vacuum system. This system is composed by a magnetic levitation turbo molecular pump (500 l/s) supported by double stage rotary pumps. Both ionisation and capacitance manometers measure the residual (~ 5*10 -9 torr) as well as the work (2 * 10 -4 torr) pressure.

Iron core transformer. This component has a primary winding of 2x20 spires in the central post and an auxiliary winding with 20+10 spires in the external post, allowing the variation of the transformer ratio between 10:1 and 70:1. The flux swing is 0.22 Vs.

Toroidol magnetic field solenoid. The toroidal magnetic field, up to 2.8 T, is obtained by a set of 24 water cooled coils. Due to a power limitation (1 MW) ISTTOK is operated at low magnetic field (0.5 T).

Vertical and horizontal magnetic field solenoid. The vertical and horizontal magnetic fields are produced by toroidal windings with 2x6 (horizontal field) and 4x8 (vertical field) spires inserted between the main toroidal coils and the copper shell.

Condenser banks. The discharges are made by using two condenser banks: a fast and high voltage bank (1 mF, 5kV) for the pre-discharge and a slow and high energy bank (0.5 F, 500 V) to assure that the discharge may continue until the saturation of the transformer iron core.

Gas injection system. It is constituted by several electromagnetic and pneumatic valves that allow the filling of the vacuum chamber and by a piezoelectric valve for additional gas injection in a pulsed regime.

Vacuum chamber conditioning system. This system conditions the vacuum chamber by converting impurities (Oxigen, Carbon, etc) in pumpable c omponents (H2O, CH4) through tokamak discharges. Luminiscent discharges are made by radio-frequency power (3 MHz, 200 W) delivered to the tokamak limiters. Inductive discharges are made by feeding the primary of the transformer by an alternating voltage (50 Hz) during small periods, under a pulsed regime

ISTTOK Tokamak Photos [+]

Tokamak Physics Programme

Recently, the ISTTOK scientific programme has been based on: (i) study of the influence of external biasing on the plasma confinement and stability; (ii) operation on alternating plasma current regimes; (iii) testing of the liquid metal limiter concept; (iv) study of the fluctuation induced transport and their driving mechanisms; (v) development and upgrade of diagnostics.

Study of the influence of external biasing on the plasma confinement and stability

Improvement of the plasma confinement and stability by electrode biasing;
Comparison between negative and positive electrode bias;
Control of the edge turbulent transport by electrode biasing;
Study of the correlation between the ExB flow shear and confinement improvement;
Development of emissive electrodes for the biasing experiments.

Tokamak operation in alternating current regimes

Operation of the tokamak ISTTOK in a multi-cycle alternating plasma current regime, to obtain long duration discharges;
Implementation of a real-time plasma control system and study of the control of long-time AC discharges in ISTTOK;

Testing of the liquid metal limiter concept

Verify the feasibility of the ISTTOK operation with a LML;
Study of the influence of the LML on the ISTTOK plasma performance;
Experimental measurements of the Gallium jet power removal capability;
Study the dynamic behaviour of the liquid metal jets in magnetic fields.

Study of the fluctuation induced transport and their driving mechanisms

Detailed study of the fluctuation induced turbulent transport using different types of electrical probes;
Study the importance of temperature fluctuations in the estimation of the turbulent particle flux.

Development and upgrade of diagnostics

Upgrade of the time of flight energy analyser of the heavy ion beam diagnostic to increase of the signal to noise ratio;
Development of a large-area robust emissive electrode for biasing experiments;
Development of a Laser induced fluorescence diagnostic for absolute Ga density determination;
Further studies on the ISTTOK plasma emissivity reconstruction using analytical methods, aiming at the best choice for real-time implementation in ISTTOK;
Development of sensors for plasma force measurements