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Control system and breakdown studies on a small spherical tokamak Gutta.
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Control system and breakdown studies on a small spherical tokamak Gutta.

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  1. Control system and breakdown studies on a small spherical tokamak Gutta. G.M. Vorobyov, D.A. Ovsyannikov, A.D. Ovsyannikov, E.V. Suhov, E. I. Veremey, A. P. Zhabko St. Petersburg State University Zubov Institute of Computational Mathematics and Control Processes, Faculty of Applied Mathematics and Control Processes Acknowledgements This work was partly funded by the IAEA CRP “Joint Research Using Small Tokamaks” This work is carrying out in the framework of Saint-Petersburg State University project “Innovation educational environment in a classical university G Vorobjev, GUTTA, Chengdu

  2. History and main parameters of Gutta Main diagnostics and data acquisition Plasma position control systems Main experimental results ECR breakdown studies b/d using reversed current Iron core Horizontal position control studies Conclusions and future plans OUTLINE G Vorobjev, GUTTA, Chengdu

  3. GUTTA, IOFFE, USSR (1980-1986) GUTTA was one of the first attempts to built a spherical tokamak, G.M. Vorobyev et al, Ioffe Institute, 1980-86 Main parameters: major radius R, cm 16 minor radius a, cm 8 aspect ratio A 2 vessel elongation k 2 toroidal field, T 1.5 plasma current Ip, ka100 GUTTA at Ioffe Institute, 1984 GUTTA is now fully operational at St. Petersburg State University, Russia G Vorobjev, GUTTA, Chengdu

  4. MAIN DIAGNOSTICS • Magnetics: 2 Rogowski coils for Ip, Rogowski coils for PF and TF currents, 2 flux loops at midplane; • Z and R position control, shape control: array of 24 pick-up coils (2 components at one toroidal position), 6 Mirnov coils - toroidal array at midplane; • Photomultiplier • 94 GHz interferometer • Spectrometer/monochromator with CMOS camera • RF power detector at 900 in toroidal direction at midplane G Vorobjev, GUTTA, Chengdu

  5. DATA ACQUISITION AND PROCESSING Control and diagnostics complex ADC boards Measurement channels number96 Input voltage range, В ±1,25 Input resistance, Ом 100 Sampling interval, μs 2,4,6,8,10,12,14,16 Input signals sampling5461 digital capacity 11bit + sign G Vorobjev, GUTTA, Chengdu

  6. Spectroscopic diagnostics Spectroscopic diagnostics block-scheme G Vorobjev, GUTTA, Chengdu

  7. Optical diagnostics Spectrograph SpectraProSP-2358: Specifications (1200g/mm Grating): Focal length: 300mm Aperture Ratio: f/4 Optical Design: Imaging Czerny-Turnerwith original polishedaspheric mirrors Optical Paths: 90° standard, 180° andmulti-port optional Scan Range: 0 to 1400nm mechanical range Operating Range: 185nm to the far infraredwith available gratings andaccessories Resolution: 0.1nm at 435.8nm Dispersion: 2.7nm/mm (nominal) Accuracy: ±0.2nm Repeatability: ±0.05nm Drive Step Size: 0.0025nm (nominal) Focal Plane Size: 27mm wide x 14mm high Spectrograph SpectraProSP-2358 pco.1200 hs CMOS detector G Vorobjev, GUTTA, Chengdu

  8. Plasma control systems on Gutta consists of: Vertical and horizontal position feedback control systems. Horizontal plasma position pre-programmed control. Horizontal control system was build, tested and commissioned Testing and tuning of vertical control system are in progress. Plasma control systems G Vorobjev, GUTTA, Chengdu

  9. Horizontal feedback control system Main parameters of horizontal feedback control system: Power switch Voltage: 500V Current: 400A (1,2 kA in pulse) Frequency: 100 kHz Capacitor bank: Voltage: 450V Current: 39600 µF Charge and voltage control system Capacitor bank Start pulse Diagnostics Control signal Displacemet signal Power switch Integrator Comparator Magnetic flux changing Current Diagnostic coils Vertical field coil Vertival magetic field Magnetic flux Plasma column G Vorobjev, GUTTA, Chengdu

  10. Horizontal program control Main parameters of horizontal pre-program control system: Power switch: Voltage: 500V Current: 400A (1,2 kA in pulse) Frequency: 100 kHz Capacitor bank: Voltage: 450V Current: 39600 µF Digital controller: PIC 16F876 Communications: UART Charge and voltage control system Start pulse Capacitor bank Control signal Digital controller Power switch Settings Vertical field coil PC Plasma column G Vorobjev, GUTTA, Chengdu

  11. Vertical feedback control system Main parameters of vertical control system: Power switch: Voltage: 1000V Current: 200A (400 A in pulse) Frequency: 100 kHz Capacitor bank: Voltage: 1000V Current: 19800 µF Charge and voltage control system Capacitor bank Start pulse Diagnostics Control signal Summation unit Comparator Power switch Integrator Magnetic flux changing Displacemet signal Current Diagnostic coils Vertical field coil Vertival magetic field Magnetic flux Plasma column G Vorobjev, GUTTA, Chengdu

  12. Horizontal control system Green- Magnetic flux through midplane Yellow- Control pulses Red-magnetic flux zero level White-control system threshold value Control feedback system OFF Green- Magnetic flux through midplane Yellow- Control pulses Red-magnetic flux zero level White-control system threshold value Control feedback system ON G Vorobjev, GUTTA, Chengdu

  13. ECR discharge, experiment set-up. MICROVAWE POWER WAVE LENGTH 30mm FUNDAMENTAL RESONANCE FOR B0=0.15T G Vorobjev, GUTTA, Chengdu

  14. ECR breakdown in pure Toroidal field • breakdown delay increases at low pressure • no dependence of b/d delay on RF power at 5 - 20 kW • Ha intensity reduces with RF power • very similar dependence of Ha intensity on pressure to what observed on START G Vorobjev, GUTTA, Chengdu

  15. Comparison of ECR b/d on START and GUTTA START: 2.45GHz ~1.0 kW, 3.5ms TF < 0.2 T, O- and X-mode launch GUTTA: 9.4 GHz, 5 - 20 kW, 0.4 msTF ~ 0.15 T, O-mode launch • Ha intensity reduces with RF power • very similar dependence of Ha intensity on pressure to what observed on START • no pronounced maximum of Ha dependence at 5 kW G Vorobjev, GUTTA, Chengdu

  16. ECR Discharge. Top, green – visible light; bottom, yellow – RF power at 900 in toroidal angle Gas pressure 1.75*10-4 torr Microwave power 20kW Gas pressure 1.75*10-4 torr Microwave power 20kW During ECR discharge with constant microwave power and some specific conditions (such as middle gas pressure, high microwave power, not very good conditioned wall) regular self-oscillations of visible light emission appear G Vorobjev, GUTTA, Chengdu

  17. ECR Discharge. Top, green – visible light; bottom, yellow – RF power at 900 in toroidal angle Gas pressure 3.75*10-5 torr Microwave power 20kW Gas pressure 2.5*10-5 torr Microwave power 20kW At even lower filling pressure breakdown delay increases G Vorobjev, GUTTA, Chengdu

  18. ECR Discharge. UV lamp assisted b/d Top, green – visible light; bottom, yellow – RF power at 900 in toroidal angle Gas pressure 2*10-5 torr Microwave power 4 kW Ultra-violet on – clear b/d Gas pressure 2*10-5 torr Microwave power 4 kW Ultra-violet off – no b/d Ultra-violet lamp assists breakdown at low pressure G Vorobjev, GUTTA, Chengdu

  19. Self-oscillations of light emission – old results Light emission during ECR discharge in tokamak Light emission during electrode discharge in linear device B.N. Shustrov, A I. Anisimov, N. Blashenkov. G.Y. Lavrentyev. G.G. Petrov, “Self-organizing in gas discharge”, Preprint Ioffe Institute, Leningrad,1988 The same processes observed in another devices and even in electrode discharges G Vorobjev, GUTTA, Chengdu

  20. Why there is a breakdown delay? Common view is that after microwave power is ON, electron density rises to threshold value, after breakdown occurrence. Delay may depend on gas pressure, microwave power and poloidal fields. 1 ms 1 ms 5 ms Top, yellow – visible light; bottom, green – microwave power G Vorobjev, GUTTA, Chengdu

  21. Reverse current preionization Top, yellow – visible light; bottom, green – Loop voltage • Reverse current preionization experiments were carried out. • Preionization using plasma current reversal is as effective as ECR preionisation (same light emission level) G Vorobjev, GUTTA, Chengdu

  22. ECR preionization Breakdown does not occur without microwave power. Top, yellow – visible light; bottom, green – microwave power, red-loop voltage Standard breakdown order ECR breakdown not happens, however ohmic field breakdown occurs. 1 ms 4 ms Delay between ECR and ohmic field breakdown is increasing up to 4ms. Delay between ECR and ohmic field breakdown is increasing up to 1ms. G Vorobjev, GUTTA, Chengdu

  23. ECR preionization Top, yellow – visible light; bottom, green – microwave power, red-current in TF coils 15 ms 8 ms Delay between ECR and ohmic field breakdown is increasing up to 15ms. Toroidal field between breakdowns is absent. Delay between ECR and ohmic field breakdown is increasing up to 8ms. 50 ms 30 ms Delay between ECR and ohmic field breakdown is increasing up to 50ms. Toroidal field between breakdowns is absent. Delay between ECR and ohmic field breakdown is increasing up to 30ms. Toroidal field between breakdowns is absent. G Vorobjev, GUTTA, Chengdu

  24. ECR preionization experiments • Delay in light oscillations at constant microwave power during ECR discharge, ECR and Ohmic field breakdown depends not only on processes in vacuum chamber, but on vacuum vessel wall conditions • Preliminary cleaning methods, ultraviolet radiation before breakdown, ECR preionization (even without breakdown) affects these conditions. • Consequence of such influence stay for a long time, which is typical not for charged particles lifetime, but for chemical processes on vacuum vessel walls. G Vorobjev, GUTTA, Chengdu

  25. Plasma Formation in CTF • No central solenoid in CTF concept design requires alternative formation schemes Ferrite steel shielding of the central post and ferrite central rod can provide enough flux for breakdown and initial current formation for use of ferrite steel in JTF-2M see: M Sato, et al., Fusion Eng. Des., 51-52 2000 1073 Fe pin radius = 0.18m gives 100 mVsec which is enough to ramp Ipl to 300kA. CTF, Culham design with iron pin G Vorobjev, GUTTA, Chengdu

  26. Plasma Formation in CTF Inspired by Culham’s new CTF design with the use of Ferritic steel central rod, 1:5 (scale) model of the CTF central post has been installed in GUTTA We plan to use GUTTA tokamak for proof-of-principle demonstration G Vorobjev, GUTTA, Chengdu

  27. z measuring coils Plasma Formation in CTF: GUTTA 1:5 model Soft iron rod and Al imitation of TF coil (not shown in photo) Induction coils: 50Hz, 4A x 1000turns plasma measured flux structure • Flux measurements have been done with and without TF coil G Vorobjev, GUTTA, Chengdu

  28. V z, cm Plasma Formation in CTF: GUTTA 1:5 model • How much flux at midplane can be produced? • flux loss by factor of 5 due to iron saturation, some of it can still be used during ramp-up • solid TF coil requires radial cuts for flux penetration Coil signal (flux) vs distance from induction coil: red – without TF coil; black – with TF coil G Vorobjev, GUTTA, Chengdu

  29. Future plans • Developing and verification of plasma mathematical models and control methods. • Studies of plasma vertical instability dynamics. • Optical measurements to determine plasma temperature. G Vorobjev, GUTTA, Chengdu