Tokamak ISTTOk

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
CIII 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

 




ISTTOK general view

Control room and data acquisition rack

ISTTOK birds view with the heavy
ion beam diagnostic on the front

 

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.

  1. 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
  2. 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;
  3. 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.
  4. 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.
  5. 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

Diagnostics

Tokamak ISTTOK Diagram

Magnetic diagnostics:

  • Rogowski coil: There are two coils of this type, with 500 spires each and 3.25 mm in diameter, measuring both the internal (plasma) and the external (total) toroidal currents.
  • Sin and Cos coils: Each of these coils, 3.2 mm in diameter, has 300 spires. Together they are used for the determination of the position of the plasma current axis.
  • Mirnov coils: There are twelve Mirnov coils distributed along the poloidal direction, in a single transverse plane. They measure the MHD activity and may be used for the determination of the position of the plasma column.
  • Toroidal loop: This diagnostic is constituted by a single loop and measures the loop voltage.
  •  

    Electric probes: Four different electric probe systems are routinely used: (i) a radial array of probes (rake probe); (ii) a turbulent transport probe; (iii) a Gundestrup probe; and (iv) an array of emissive probe.

  • Rake probe: The rake probe consists of a boron-nitride head carrying nine tungsten tips with a spatial resolution down to 4 mm, given information of the edge floating potential and ion saturation current radial profiles with high temporal and spatial resolution.
  • Gundestrup probe: Information on the plasma flow patterns in the edge plasma is obtained using a Gunderstrup probe, which consists of eight conducting segments mounted around an insulating cylindrical housing in order to measure the polar diagram of the ion saturation current.
  • Turbulent transport probe: The turbulent transport probe consists in radially and poloidally separated pins measuring the floating potential and the ion saturation current and allows the determination of the turbulent particle flux.
  • Emissive probes: The array of emissive probes has been specifically designed to investigate the difference between emissive and Langmuir probes for turbulence studies;
  •  

    Interferometer: A single channel interferometer, at 100 GHz, with heterodynic detection used to determine the line-averaged density;

    Bolometer tomography: The bolometer tomographic diagnostic is based on 3 linear 10-pixel detectors. The reconstruction of the plasma emissivity profile is performed using overdetermined analytical methods (Cormack based);

    Heavy-ion beam diagnostic: A 10 μA, 22 keV, Cesium primary beam is used to probe the plasma. Secondary ions are collected by a surface array detector with a maximum of 16 x 4 copper cells, sampling cylindrical plasma volumes with the primary beam cross section and a length of about 10 mm. This diagnostic allows the determination of the density, temperature and poloidal magnetic field radial profiles. The heavy-ion beam diagnostic has been recently upgraded to allow the determination of the plasma potential profile using the time-of-flight technique;

    Line radiation monitors: Photodiodes with interference filters of narrow bandwidth centered in the Hα line or in impurities lines (e.g. CIII);

    Visible and VUV spectroscopy: Visible and UV radiation emission comes mainly from impurity lines. A high resolution spectrometer is used for ion temperature and ion rotation measurements (Doppler spectroscopy). Another diagnostic based on a spectrometer, a multi-channel (x7) fiber to collect the light and a CCD camera for radiation detection is used to measure radial density profile of different impurity lines at the plasma edge;

    Residual gas analyzer: The residual vacuum of ISTTOK is analyzed by means of a quadrupolar RGA (Spectramass DATAQUAD);

    Emissive electrode: A movable emissive electrode has been developed for the biasing experiments in ISTTOK. The emissive electrode consists of a LaB6 (Lanthanum Hexaboride) disk with a diameter of 16 mm and covered by a Tantalum cylinder, which is protected by Boron Nitride cup as insulating material to be exposed to the plasma. When heated the electrode emits up to 30 A of steady state current,

    Data Acquisition

    The Control and Data Acquisition System (CODAS) of ISTTOK has served not only to support and operate the tokamak but also to test new ideas and developments specific of fusion devices. From the beginning of the ISTTOK project, state of the art equipment has been used in this area. ISTTOK has served as a test bed for new hardware and software developments that later ware exported to other fusion devices. As a consequence of this approach, the ISTTOK CODAS has evolved continuously, starting from the early VME buses, through PCI and nowadays ATCA. Recently, a strong afford has been placed in real time control and remote participation. As an example, an ISTTOK 'virtual control room' has been developed and it is available at the moment over the Internet.

    Remote Data Access

    A common software layer between end-users and laboratories has been developed at CFN in order to allow an easy and unified data access to any of the participating associations data. Detailed information on the access to the ISTTOK database may be found here (Link).

    Education and Training

    ISTTOK has been used to support the experimental part of post-graduation programmes on plasma physics and engineering. During the past eight years about twenty Master and five PhD thesis have been written based on work performed on this tokamak. Visits to our laboratories are often organized for high school and undergraduate students. Furthermore, in the past 3 years a one week summer course has been organized for typically 10 high school students. This programme of education and training has allowed CFN to create a research team with the adequate number of qualified professionals.

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