ISTTOK - Wiki
IST Tokamak - ISTTOK
The ISTTOK Tokamak ("Instituto Superior Técnico TOKamak") is a research fusion device located at Instituto Superior Técnico.
It has a circular cross-section due to a poloidal graphite limiter and an iron core transformer. This magnetic confinement experiment has been built from some components (supporting structure, vacuum chamber, copper shell, transformer, toroidal magnetic field coils, radio-frequency generator, and capacitor banks) of the precessor TORTUR tokamak which was de-commissioned by the Association EURATOM/FOM in 1988.
Goals and research objectives
The scientific programme has been based on:
- Edge plasma turbulence studies;
- Operation and control on alternating plasma current regimes;
- Testing of liquid metal limiter concept;
- Development and upgrade of diagnostics;
- Test bed facility for new data acquisition concepts and hardware.
Understanding turbulent transport is important to optimize the plasma confinement and for predicting the performance of future devices. Research on the comprehensive investigation of the multi-scale physics aspects of the edge turbulence, from its origin to the impact on transport including plasma-wall interaction. ISTTOK is ideal for edge physics studies due to its flexibility, low operation costs, short time scale for diagnostics implementation and compatibility with electrostatic probes. The universality of several edge physics phenomena was verified in different devices, confirming the relevance of these studies in small devices. Present activities include:
- Contribute to a better understanding of spontaneously generated large-scale sheared flows and its role in regulating plasma transport.
- Characterize and control the SOL intermittent transport.
- Develop different electrical probes systems for edge studies.
- Provide synergies between theory and experimental groups working on edge/SOL physics.
- Develop analysis tools required to process the massive experimental data obtained .
Further studies on the ISTTOK plasma emissivity reconstruction using analytical methods, aiming at the best choice for real-time implementation in ISTTOK. Upgrade existing tomographic cameras to 3 x 16 channels in order to improve spatial resolution for enhanced emissivity reconstruction. Developing and building a retarding electrostatic field cylindrical energy analyzer (CEA) to measure plasma potentials in the core and their fluctuations.
Due to iron core saturation, operation of ISTTOK in single pulse mode is limited to 35 ms plasma duration. ISTTOK has suffer a major update of its control systems and power supplies to operate in AC mode increasing that parameter to more than 1000 ms. Within this line of research present activities include:
- 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 duration AC discharges in ISTTOK.
- Development of advanced gas feed control to improve reproducible and reliable plasmas with high performance.
Testing of liquid metal PFC
Liquid metals as PFCs
One of the most challenging issues with regards to the operation of a future fusion reactor is related to plasma-wall interaction. Solid materials exposed to fusion plasmas are known to suffer significant damage that reduces wall lifetime. Liquid metals have proven to be of relevance for PFC protection. Due to ISTTOK characteristics its programme has also focused on testing liquid metals PFC´s and their properties:
- Develop a novel Liquid Metal Limiter based on micro-channels structures.
- Build and test the new LML concept using gallium as liquid metal.
- Expose this PFC to ISTTOK plasmas.
- Study the behavior of the limiter surface temperature increase with liquid metal re-circulation.
Data acquisition and control
ISTTOK provides more than 100 acquisition channels, two spectrometers imaging systems and several I/O control channels, supported by a robust database with easy methods for retrieving information (supporting Matlab, Python, Mathematica ,C++, JAVA, etc).
By using MARTe and this infrastructure it was implemented a control system based on an optimized state-space model for real time control of the plasma current magnitude and centroid position retrieved by a twelve mirnov coils poloidal set.
ISTTOK uses as well numeric-integrated digital channels to determine the local magnetic field and by such approach calculate the better current profile in real-time.
Nominal plasma parameters
Tortur was a tokamak designed in Grenoble with an initial primary to secondary winding ratio of 1:1. It starts to operate in 1974 at the FOM institute at Nieuwegein on the Neederlands.
For the primary was used the stabilizing cooper-shell structure. A very high current had to be feed in this primary and short discharges were obtained (up to 8 ms). The vessel was a glass-quartz type. It's name origin was due to the acronym TORus for TURbulent heating. The underlying idea was to generate a suficient large current through the plasma to the limit were micro-instabilites will be excited enhancing the plasma resistivity and allowing further ohmic-heating.
Tortur was not capable to diagnose properly the heated plasma and the high primary voltage needed posed severe insulation problems. In 1978 it undergoes a major upgrade. A new stainless steel 625 vessel was installed being divided in two C-sections and equiped with 33 ports of 35mm, constructed by Avica Equipment, England. Basically the vessel were made based on port structures welded together by a thin inconnel bellow of 0.15mm with an inner diameter of 200mm. Such thin metal structure allowed for a suficient fast penetration of the electric fiels used for the turbulent heating.
By 1981 Tortur undergo again a vessel upgrade of the former one by the same manufacturer, assuming the actual design as in ISTTOK . A higher toroidal magnetic field was comissioned to improve confinement (up to 5T) in paralell with a structural reinforcement. Besides that it was equipped with a iron-core transformer yoke switchable in a 20:1 or 40:1 ratio, allowing to extend plasma discharges up to 50 ms. This pulse extension was possible as well by the instalation of a quadrupole for creating the necessary vertical and horizontal magnetic fields for plasma stability, centering the plasma in the chamber.
By then the copper-shell was used sollely for the fast turbulent heating by firing a 25uF and up to 20kV for a steep-rising heating pulse at any presetable time duplicating the current(30->60kA) for ~10us. This thecnic allowed up to 1keV plasmas.
Prior to the main discharges Tortur was "cleaned" by creating a glow discharge with the RF generator, heating the vessel up to 180ºC and producing small discharges with a low toroidal field (~50 mT) to outgas the vessel from impurities.
It is worth to mentioned that all aquisition systems was based on CAMAC boards (5 crates) controlled by a PDP 11/23 Digital Inc. mainframe computer (-1988) and all data from the transient recorders being backuped on tape.
After the commissioning in Portugal, Tortur became the IST TOKamak, ISTTOK. During XX century is was operated with two capacitors banks as it's predecessor and only with forward current. The main systems have been updated, namely the vacuum systems, power supplies, the plant control systems and data acquisition. Discharges were started based on a cold plasma generated by the TORTUR RF generator (1.7MHz) but due to the high noise have been soon replaced by an electron beam source (80 eV, tungsten filament) situated on the high field side close to the SOL. The hot plasma was started by the high voltage bank (1-5kV, 1mF, 2-3ms) and then an electrolytic bank (0.4F, 350V, 40-50ms) took over. With this electron gun to pre-ionize the gas it was not necessary anymore to use the high-voltage capacitor to disrupt the gas.
ISTTOK were since the beginning equipped with a tunnable toroidal field in view of adjusting the cyclotronic frequency to a desired value. This was quite uncommon in such class of tokamaks were the toroidal field was basically imposed either by a fixed transformer ratio from the grid or by using a capacitor bank. Conversely, due to the lack off available power in campus, toroidal field had to be limited to ~0.5T.
After a first exploitation phase with very similar Tortur parameters except for the toroidal field, ISTTOK@XXI became the first tokamak equipped with a switched IGBT primary circuit with a considerable larger capacitor bank (3.8F), allowing multiple AC discharges.
ISTTOK is one of the few fusion devices equipped with an heavy ion beam diagnostic.
List of main components
This section lists exhaustively ISTTOK main components which are referred in more detail on article ISTTOK main components
- Toroidal field
- Current supply
- Poloidal fields
- Power amplifiers
- Support structure
- Vacuum system
- Primary vacuum
- High vacuum
- Gas injection
- Background pressure
- Gas puffing
- Residual gas analyzer
- Broadband spectrometer
The diagnostics systems in tokamaks usually serve two proposes: either to allow a proper control of the machine or being themselves the research object. The former constitutes the "technical diagnostics" allowing (i) a correct machine's operation, (ii) estimation of the discharge performance, (iii) the usage in the real time control loop and (iv) serve some interlock security services. The "scientific diagnostics" constitute the research object normally are not yet ready to be used without a previous data validation from the involved team. Those ones can be as well oriented to study just a particular plasma behavior or academic activity, without the future objective to serve as a regular diagnostic. Some of the diagnostics can be inoperative for a given time according to data acquisition limitations or maintenance.
List of main diagnostics
Please refer to a more detail article on ISTTOK Diagnostics.
- Halpha monitors
- Electron Cyclotron Emission
- Soft X-rays
- Multifilter electron temperature diagnostic
- Broad band spectroscopy
- Residual gas analyzer
- Fast camera
- Vacuum gauges
- Heavy ion beam
- Langmuir Probes
- Thomson Scattering (Presently in operation at TCA-BR)
- Helium injection
- Reverse field analyzer
- Reflectometry (Presently in operation at TCA-BR)
- Heavy Ion Beam Probe