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Remote tokamak operation Jan Stockel Institute of Plasma Physics, Prague, [email protected] & Vojtech Svoboda, Ondrej Grover – GOLEM operators Faculty of Nuclear Physics and Physical Engineering, CTU in Prague with a local help of Milos Vlainic (Ghent University) Aims of this lecture • • Prepare participants for the workshop on remote operation of GOLEM this afternoon (necessary technical details) Explain very basics of tokamak physics Fusion Days@NS, Novi Sad 2016 Motivation of tokamak research Tokamak is an experimental facility, which allows to heat the plasma to temperatures 200 milion Kelvins (20 x higher temperature then in the core of the Sun) and confine it by the magnetic field for a sufficient time. The main goal of the tokamak reseach is to achieve the thermonuclear fusion for production of electricity Existing tokamaks since 1959 => ITER (under construction) 2025 DEMO – prototype of fusion reactor (design phase) 2045 Commercial Fusion power plant 2050-2070 However, a lot of research (tokamak physics + technologies) has to be still performed!! Tokamak basics Tokamak is composed of three basic components • • • Large transformer with primary winding Plasma ring as secondary winding Coils for confinement of plasma ring by magnetic field (toroidal solenoid) Electric current I generated in the plasma ring by the transformer • delivers the ohmic power Pohmic = I2Rplasma to plasma (heating) • generates the poloidal magnetic field in the plasma ring Bpoloidal ~I/2pa Tokamaks ( ~ 175 facilities since 1960) EURATOM Germany France GB Italy Switzerland Portugal Czech JET – the largest tokamak ASDEX U, TEXTOR 94 TORE – SUPRA MAST FT-U TCV ISTTOK COMPASS, GOLEM USA Japan Russia Canada China South Korea India Brazil Iran Pakistan D IIID, ALCATOR C-mod, NSTX JT- 60, ….. T-10, TUMAN 3, FT-2, GLOBUS (spherical), T11-M STORM-1M EAST, HT-7, J-TEXT, HL-2A, …. KSTAR Aditia, SINP, (SST-1 under construction) ETE, TCABR IR-T1 GLAS Currently ~ 30 operational tokamaks, which differ in: Major radius 0.4 - 4 m, Magnetic field 0.5 - 4 T, Plasma current 0.01 - 4 MA Main source: Conventional Tokamaks Pulse length 0.01 – ~300 sec from http://tokamak.info/ The GOLEM tokamak in Prague The GOLEM tokamak is a small and also the "simplest" tokamak Located in Prague at the Faculty of Nuclear Physics and Physical Engineering (Czech Technical University) Unique feature: Can be operated remotely via Internet Parameters Toroidal magnetic field Plasma current Plasma density Electron temperature <0.5 T <10 kA <10^19 m^-3 <100 The oldest tokamak still operational! Front view of GOLEM (schematically) Iron core of the transformer Toroidal magnetic field coils (28) Primary winding of the transformer (24 turns) Diagnostic ports (18) Any vertical magnetic field is required for equilibrium position of the plasma column in the horizontal direction in the basic mode of operation (Iron core, copper shell, low plasma pressure, short pulse,….) Toroidal magnetic field 28 TF coils - charged by the capacitor bank C = 24.3 mF Tyristor switch (always 5 ms after start of DAS) Signal of the magnetic sensor Is numerically integrated Typical temporal evolution of The toroidal magnetic field Toroidal electric field • Toroidal electric field E tor in the "empty" vessel is required for plasma breakdown • E tor is generated by by the time varying current in the primary winding of the tokamak transformer Etor M dI prim * 2πR dt M –mutual inductance between primary winding and the plasma ring R – major radius of the torus Primary winding is charged by a condenser bank Toroidal electric field at the breakdown should be as low as possible!!!! Toroidal current Toroidal current is measured inductively by means of Rogowski coil (a toroidal solenoid surrounding the tokamak vessel form outside) Again, the output signal of the RC has to numerically integrated! Note: The RC measures the total toroidal current Itotal = Ivessel + Iplasma The vessel current must be subtracted to get plasma current Ivessel = Uloop/Rvessel Rvessel = 10 mOhm Evolution of a vacuum shot – no plasma No working gas in the tokamak vessel Voltage Vloop is induced along the torus The GOLEM vessel is the stainlesssteel torus without any insulating break => the loop voltage induces the current in the tokamak vessel Time delay between Btor and Etor Time delay can be selected A proper time delay is selected to optimize breakdown of the working gas Poloidal Limiter & pre-ionization A circular diaphragm is installed inside to reduce interaction of plasma with the inner surface a = 85 mm Some free electrons have to exist inside the vessel – pre-ionization of the working gas by an electron gun (hot filament) is used But background cosmic radiation can be also exploited Pumping and gas handling system b = 0.1 m R0 =0.4 m Pressure of working gas is adjusted according your request! GOLEM Basic engineering scheme Available at the remote control room of GOLEM http://golem.fjfi.cvut.cz/current/index.html http://golem.fjfi.cvut.cz/NoviSad To summarize the remote operation We need only five “buttons” to operate the GOLEM tokamak We have to select: • Pressure of the working gas Recommended range 5 – 30 mPa • Preionization of the working gas – ON (Top electron gun) or OFF • Charging voltage of the condenser bank of the toroidal magnetic field Recommended range 100 – 1400 V • Charging voltage of the condenser bank to generate the toroidal electric field (current in primary winding) Recommended range 300 – 1000 V • Time delay between triggers switching the toroidal magnetic field and the toroidal electric field (the loop voltage) – Recommended range 0 – 10 000 ms TOP view of the tokamak Electron Fully ionized are plasma fills Density of charged accelerated the vessel (in in 0.1-10 toroidal ms – Free electron(s) particles increases direction depending ionize the size theof appear and inon the vessel exponentially in time working tokamak)gas Conditioning of the tokamak vessel • Inductive heating of the vessel to 150 - 250o C (after opening the vessel to atmospheric pressure • Glow discharge cleaning Diagnostics available for the remote workshop 1. Toroidal magnetic field in Tesla 2. Total toroidal current (vessel & plasma) in Amps 3. Plasma current in Amps 4. Loop voltage in Volts 5. Intensity of HXR radiation measured by a proportional detector 6. Interferometer 7. Rake of 16 Langmuir probes 8. Mirnov coils 9. Visible camera Organization of the workshop Participants will convene in the computer room (here) Participants are divided in five experimental groups Each group is supervised by: 1. Ana Kostić 2. Jordan Cavalier 3. Ondřej Kudláček 4. Branka Vanovac 5. Ondřej Ficker NOTE: Even though two mentors are Serbian, the official language of the Workshop is English – thus students will be pursued to communicate in English ;) Requirements from participants • Each student is asked to occupy one PC with internet connection (hopefully there will be enough of them) • Each student is asked to use an available software to process and plot experimental data from GOLEM database (EXCEL, ORIGIN, MatLab, Python…) Active participation is highly required!!!!! • AFTER THE WORKSHOP – each student will write a report (A rough template will be given, while details what to write in report will be given by your mentors. Tip: Do not make tables of data!!!) – PRICE: 2 best will go to COMASS training next summer!!! GOLEM tokamak – long story The GOLEM tokamak is the oldest tokamak in the word which is still operational with a long term history!! Built by the godfather of tokamak research, Lev Artsimovich in Kurchatov Institute, Moscow around 1960 as TM-1 The first real tokamak plasma was achieved (simultaneously with the T-2 tokamak!) Moved to Institute of Plasma Physics Prague and baptised as CASTOR in 1977 Replaced by COMPASS tokamak 2007 Moved to Czech Technical University and baptised again as GOLEM Prague GOLEM The word golem is used in the Bible to refer to an embryonic or incomplete substance Similarly, golem is often used today in metaphor as an entity serving man under controlled conditions but hostile to him in others. Prague GOLEM was created by Rabbi Loew, who is buried at Jewish cemetery, Just 50 meters from the Faculty of Nuclear Physics and Physical Engineering THE END (there are some additional slides with simple tasks if somebody wants to play more) Task 1: Produce the highest Te Measured loop voltage has resistive and inductive component U loop I p R p L p dI p dt The inductance of the plasma ring on GOLEM is L p 1 μH The inductive component can be neglected, if the loop voltage is measured, when the plasma current is maximum => U Rp I loop max p The (Maxwellian) plasma resistivity is linked to the electron temperature. A rough estimate for GOLEM I max p Te 32.5 U loop 2/3 eV, kA, V Task 2: Produce the lowest Ubreak Breakdown voltage Ubreak ~ 6.5 V Power supply of the primary winding has a relatively high resistance Resistance of the secondary winding without plasma is the vessel – 10 Ohm Plasma column represents an additional resistance in parallel to the vessel resistance => the total resistance of the secondary winding is reduced Task 3: Produce the highest q(a) Extremely important quantity, which characterize stability of the plasma ring It denotes the number of turns a magnetic field line goes around a torus toroidally to finalize a single poloidal turn a Btor q(a) R B pol Poloidal magnetic field is a function of the plasma current m0 I p B pol (a) 2pa 2 => In practical units Btor q(a) 90.3 Ip [T, kA] 2pa 2 Btor q(a) m0 R I p Task 4: Investigate the role of glow discharge Plasma should be as clean as possible!!! Where ‘clean’ means that there is no atoms different than the working gas. Z eff j njZ j 2 njZ j j njZ j 2 ne Zeff donates the effective ion charge of the plasma. Perfectly clean hydrogen, deuterium or tritium plasma would have Zeff=1. Artsimovich at 1st IAEA Fusion Energy Conference in Salzburg (1961): Braams and Stott ''Nuclear Fusion: Half a Century of Magnetic Confinement Fusion Research" (2002), p. 164 Task 5: Measurements with Langmuir Probes Theory: see lecture of Jordan Cavalier :D Experiments: Jordan has many ideas . . . - floating potential - profiles of electic field - H vs He (old shots) - correlations (very advanced) Is somebody interested looking into the old data? #16312 #16346 Runaway electrons (lecture by Žana Popović) A fraction of electrons in tokamak plasma is accelerated by the toroidal electric field to very high velocities. These electrons are usually called runaway electrons. If the runaway electron hits a material surface (the limiter, the vacuum vessel), the high-energy photon(s) is generated. High energy photons are registered by a proportional detector Task 6: Find the most energetic HXR photon A signature of runaway electrons Individual spikes are HXR photons resulting from impact of a high energy on the limiter Amplitudes of spikes are proportional to energy of HXR photons (calibrated by radiosotop Cs) Energy [keV] = Amplitude [V]* 492