IAEA – 2 nd RCM – 2006

1. IAEA – 2nd RCM – 2006 Plasma studies in the ETE spherical tokamak G.O. Ludwig, E. Del Bosco, L.A. Berni,J.G. Ferreira, R.M. Oliveira, M.C.R. Andrade, J.J. Barroso, P.J. Castro, C.S. Shibata. Laboratório Associado de Plasma Instituto Nacional de Pesquisas Espaciais 12227-010 São José dos Campos, SP, Brazil CRP – Joint research using small tokamaks 23-26 October, 2006 - Beijing, China Instituto Nacional de Pesquisas Espaciais - Laboratório Associado de Plasma

2. ETE Spherical Torus General view of the Spherical Tokamak Experiment – October 2006. Instituto Nacional de Pesquisas Espaciais - Laboratório Associado de Plasma

3. ETE main parameters and objectives • Explore the physics of low aspect ratio. • Investigate plasma edge conditions. • Undertake diagnostics development. • Follow worldwide spherical tokamak advancements. Instituto Nacional de Pesquisas Espaciais - Laboratório Associado de Plasma

4. Comments on ETE operation and power circuit • Maintenance work in the TF circuit (6 months in 2005) after two failures. • About 100 contacts thoroughly inspected, tightened and measured (~μ). • Two damaged contacts fixed – reduction of ~0.5m in overall resistance of ~3m. • Routine tokamak operation resumed. • Design of a double swing OH circuit. • Control at the secondary of transformers used as variable inductors. • Low cost • Dissipates 10-15% of capacitor banks energy in control resistor. • Simulation shows 15 and 30ms pulses with pre-programmed control. • Feedback control possible. Instituto Nacional de Pesquisas Espaciais - Laboratório Associado de Plasma

5. Topic 2. Edge physics, plasma surfaceinteraction and technology • Subtopic 2.8. Fuelling, recycling, wall conditioning • Proposed activity: Improvement of ETE vacuum conditioning by increasing the baking temperature and optimizing glow discharge operation accompanied by a systematic study of the effects produced on plasma temperature and impurity lines. • Progress report: The procedures for vacuum conditioning by baking and glow discharge cleaning adopted in ETE are being revised and improved in preparation for the assembly of a complete set of graphite limiters and magnetic sensors inside the vessel (cf. subtopic 5.3. – magnetic diagnostics, equilibrium reconstruction). • Shielding of thermocouple cabling reduces spurious signals that deactivate baking. • Two new temperature sensors – temperature at central column lower than outer wall. • May need direct heating (AC) of inner wall when full graphite limiters are installed. • New baking routine – 8 hours once a weak, glow discharge in the morning followed by normal tokamak operation. Instituto Nacional de Pesquisas Espaciais - Laboratório Associado de Plasma

6. Vacuum conditioning Evolution of residual gas composition with the new baking routine. Instituto Nacional de Pesquisas Espaciais - Laboratório Associado de Plasma

7. Topic 3. Heating, current drive and plasma formation • Subtopic 3.1. Heating power source development • Proposed activity: Work will be carried out on the design and construction of a high-power 6.7GHz monotron, and on the study of advanced concepts of corrugated waveguides and cavity resonators, looking for future plasma heating applications. • Progress report: Tests of the electron gun for the monotron are almost completed. It is expected to complete the assembly and initiate tests of the full monotron in the next few months. If successful, this development will lead to a microwave source for preionizing the plasma in ETE. • Thermal testing of a 10kV/5A electron gun for the 6.7GHz monotron completed. • Innovative 10kV/3×8A power supply – grounded solid-state switch in series with the cathodes of 3 tetrodes in parallel – tested with 10kV/(10~100μs)/100Hz – upgradeable to ~500μs pulse for preionization (long pulse operation depends on bank energy). • Monotron test bench assembled and cold-tested – 0.5dB power loss. Instituto Nacional de Pesquisas Espaciais - Laboratório Associado de Plasma

8. Heating power source development Monotron power supply (left) and test bench (right). Instituto Nacional de Pesquisas Espaciais - Laboratório Associado de Plasma

9. Topic 3. Heating, current drive and plasma formation • Subtopic 3.5. Startup • Proposed activity: Theoretical work will be undertaken in the area of plasma startup simulations applied to the ETE conditions. • Progress report: To simulate the plasma startup conditions, a model including all the external flux sources and eddy current effects in ETE (central column and vacuum vessel) has been completed. • The total electromotive force calculated with this model is in very good agreement with the voltage measured in loop voltage sensors placed along the poloidal perimeter of the vacuum vessel of ETE. • This effort completes the first step in simulating the startup conditions in ETE. • Next step: Insert plasma in the model and initiate start-up simulations together with the development of equilibrium reconstruction tools (cf. subtopic 5.3 – magnetic diagnostics, equilibrium reconstruction). Instituto Nacional de Pesquisas Espaciais - Laboratório Associado de Plasma

10. Eddy currents model verification • Vacuum vessel and location of loop voltage sensors (left). • Comparison of signals measured with sensors and calculated with eddy current model (right). Instituto Nacional de Pesquisas Espaciais - Laboratório Associado de Plasma

11. Current drive studies (ECCD) • Additional activity: Simple dynamical models are needed for examining the efficiency and time dependence of electron cyclotron current drive (ECCD) in different tokamak operating conditions. With this application in view, a fully relativistic fluid model for the energetic beam of current-carrying electrons is being developed. Equilibrium solutions for TCV with toroidal launching angles 15o (high-energy beam on the tail of background plasma) and 25o (loss-cone distribution). The whistler mode propagating in the background becomes unstable in the latter case (Weibel instability). This indicates a loss mechanism of beam particles, mainly by pitch angle scattering, that may reduce ECCD efficiency. Instituto Nacional de Pesquisas Espaciais - Laboratório Associado de Plasma

12. Topic 4. MHD and control • Subtopic 4.8. MHD instabilities • Proposed activity: Theoretical work will be undertaken in the formulation of edge plasma models, particularly looking into effects of plasma viscosity at the boundary. • Progress report: Work has been carried out showing the influence of edge viscosity in the behavior of natural as well as laboratory plasmas. • For natural plasmas the turbulence associated with the Rayleigh-Taylor instability, which is driven during the contracting stage of the decaying return stroke of a lightning discharge, creates anomalous viscosity that defines the structure of bead lightning. • For laboratory plasmas studies have been carried out of the kink mode instabilities in circular cylindrical plasmas taking into account the effect of viscosity at the boundary. The m = 1 kink mode is barely changed, but the higher order m 2 modes are significantly damped if the collisionality is high. This indicates that in a tokamak the ballooning modes could be strongly affected by edge viscosity. This topic will be resumed in a future investigation. Instituto Nacional de Pesquisas Espaciais - Laboratório Associado de Plasma

13. Bead lightning (example of viscosity effects) Beaded structure in a triggered lightning discharge, shown at 1ms time intervals. The calculated e-folding time (~0.5ms) and wavelength (~44cm) at the final stages of the sausage and kink instabilities are consistent with the observations, provided the anomalous viscosity is enhanced significantly above the classical level. Instituto Nacional de Pesquisas Espaciais - Laboratório Associado de Plasma

14. MHD equilibrium and neoclassical effects • Additional activity: Theoretical work on the modeling of neoclassical effects in the equilibrium of low aspect ratio tokamaks has been carried out. A detailed study of the bootstrap current dependence upon plasma profile parameters and their effect on equilibrium quantities has lead to a new scaling law for the bootstrap current fraction. Comparison of scaling laws with equilibrium calculations: present work (black), Hoang’s scaling (green) and histogram of errors for the ETE theoretical data set. Instituto Nacional de Pesquisas Espaciais - Laboratório Associado de Plasma

15. Topic 5. Diagnostics improvement and development • Subtopic 5.1. Core diagnostics • Proposed activity: The single-channel Thomson scattering (TS) diagnostic will be upgraded to 10 channels applying a time-delay technique. • Progress report: Upgrade of the TS to 10 channels has been completed and tested. The time-delay technique uses several optical fibers of different lengths to transmit the light signals to the same polychromator. Because the effective area of the detector is limited it is not possible to introduce more than 10 fibers inside the polychromator. • One micro-lens is placed in front of each fiber to enlarge the observation region inside the plasma to 4mm. This unpublished TS system result will become an essential diagnostic tool for ETE. • For the next year improvements will be introduced concerning the S/N ratio, dump system and analysis of signals with on-line presentation of profiles after each shot. Instituto Nacional de Pesquisas Espaciais - Laboratório Associado de Plasma

16. Multipoint Thomson scattering system Light collection optics of the 10-channel TS diagnostic (left) and stack of fiber optics reels forming the delay-line (right). Instituto Nacional de Pesquisas Espaciais - Laboratório Associado de Plasma

17. Test of the multipoint Thomson scattering system Temperature profiles for two different times using the 10-channel TS. The inner and outer limiters are placed at 10cm and 55cm, respectively. Instituto Nacional de Pesquisas Espaciais - Laboratório Associado de Plasma

18. Topic 5. Diagnostics improvement and development • Subtopic 5.2. Edge diagnostics • Proposed activity: The single-channel optical detection system of the fast neutral lithium beam (FNLB) probe will be upgraded to 8 channels using a multi-channel photomultiplier. • Progress report: The upgrade of the FNLB probe was successfully performed with the substitution of the old single-channel optical detection system by an array of 8 channels for plasma monitoring. The high spatial (1cm) and temporal (tens of nanoseconds) resolutions previously obtained were retained • The 64 cells photomultiplier was coupled to an objective lens via fiber optics and interference filters that allow the processing of up to 64 signals. • An array of 88 cells was implemented to probe 8 radial points of the discharge, allowing gain amplification by a factor of eight, in comparison with the case of using just one cell for each observed radial position. • Additional tests will be performed to initiate measurements of the plasma density and its fluctuations along a 15cm radial extent from the plasma edge in ETE. Instituto Nacional de Pesquisas Espaciais - Laboratório Associado de Plasma

19. Lithium beam probe development 10 keV neutral lithium beam probe for measurements of the edge plasma density and its fluctuations (left) and 8 channels optical detection system of lithium beam emission (right). Instituto Nacional de Pesquisas Espaciais - Laboratório Associado de Plasma

20. Test of the multi-channel lithium beam probe H2 calibration curve for channels 1 to 4 (top) and raw signals corresponding to gas puffing and pump down (right). Channel 1 corresponds to the outer edge and channel 4 to a point 15cm inward. Channels 2 and 3 are at intermediate positions. Instituto Nacional de Pesquisas Espaciais - Laboratório Associado de Plasma

21. Topic 5. Diagnostics improvement and development • Subtopic 5.2. Magnetic diagnostics, equilibrium reconstruction • Proposed activity: Magnetic measurements coupled to equilibrium modeling will be used to define the plasma edge location. • Progress report: A new set of magnetic sensors (pick-up coils, Rogowski coil, total flux coils) is being developed for magnetic measurements and equilibrium reconstruction studies, as well as a two-foil soft X-ray spectrometer for the determination of the electron temperature with high time resolution. • The set of magnetic sensors will consist of: 39 two-components probes; 2 total Rogowski coils (one inside and the other outside the vacuum vessel); 4 partial Rogowski coils; 8 total flux loops inside the vessel and 6 outside; and 6 partial flux loops (saddle coils) in the central limiter. • Also, 12 Langmuir probes will be installed in the central limiter. • The magnetic diagnostics will make extensive use of the new 64-channels data acquisition system based on the PCI standard that was recently purchased (CFN). Instituto Nacional de Pesquisas Espaciais - Laboratório Associado de Plasma

22. Magnetic diagnostics development Cross-section of ETE at azimuthal angles of 44o and 76o showing the positions of magnetic sensors (left) and upper arrangement of magnetic sensors inside the vacuum vessel of ETE. Instituto Nacional de Pesquisas Espaciais - Laboratório Associado de Plasma

23. Topic 8. Expertise exchange • Proposed activities: Visits to other laboratories and collaboration of visiting scientist in ETE are planned, mostly involving the institutions participating in the CRP/JRUST. • Progress report: The IAEA CRP on Joint Research Using Small Tokamaks has motivated a mutual cooperation between LAP/INPE and CFN/IST. A data acquisition system was integrated by CFN and will be soon installed in ETE. A member of the ETE group assists the CFN in transferring the TS from ISTTOK to the TCA-BR tokamak, in further cooperation between CRP participants. • A Nuclear Fusion Network is being established in Brazil with the participation of several Brazilian institutions and universities. • Agreements in the area of fusion should be formalized between the Brazilian Ministry of Science and Technology and EURATOM countries, involving the participation of all groups that take part of the Network. Instituto Nacional de Pesquisas Espaciais - Laboratório Associado de Plasma