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Edge plasma diagnostics in t okamaks (advanced electrical probes)

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  1. Edge plasma diagnostics intokamaks(advanced electrical probes) Jan Stöckel + CASTOR team Institute of Plasma Physics, Association EURATOM/IPP.CR Prague, Czech Republic • In close collaboration with • G. Van Oost, Ghent University • J. Gunn, M. Kocan, P. Devynck, CEA Cadarache, France • E. Martines,Consorzio RFX, Padova, Italy • R. Schrittwieser, C. Ionita, P. Balan, Innsbruck University, Austria • Decisive role of the edge plasma in global plasma confinement in tokamaks (what to be measured?) • Fluctuation and flow measurements byelectric probes (how to measure?) IAEA TCM RUSFD, Lisabon, October24-26, 2007

  2. Built in Kurchatov Inst. Moscow1958 Operational in IPP Prague since1977 Reconstructed (new vessel)1985 Associated to EURATOM 1999 Last shot 26 Oct 2007 CASTOR -Czech Academy of Sciences TORus Will be replaced soon by COMPASS tokamak (UKAEA): see posters R. Panek and M Hron Major radius 0.4 m Minor radius 0.1 m Toroidal magnetic field <1.3 T Plasma current <15 kA Pulse length 50 ms Plasma temperature 0.20keV Plasma density ~ 1*1019 m-3 Confinement time < 1 ms Edge density ~2*1018 m-3 Edge temperature 10-40 eV • Research topics • Edge plasma physics • Plasma turbulence • Diagnostics development • LHCD Physics at the plasma edge is almost independent on machine size

  3. PlasmaCore x Btor Edge of a tokamak (schematically) Scrape off layer (open magnetic field lines – connected to a material surface (divertor or limiter) Last Closed Flux Surface (separatrix) Edge plasma - a „layer“ insulates the hot core from the cold wall its importance for plasma confinement recognized in 80s – transport barriers • Heat and particles are transported from the core to wall through the edge plasma • Transport coefficients  and D must be as low as possible • However, the opposite is true -  and D are 100-1000x larger than predictions because of plasma turbulence • Decisive role of plasma turbulence was recognized ~30 years ago, but its nature is not yet fully understood

  4. WALL What is the edge turbulence(result of fluid modelling of plasma edge in CASTOR) Central part of plasma column is not modelled BRIGHT COLORS Density is higher than the average DARK COLORS Density is lower than theaverage LFS HFS 20 cm Flute-like structure(s) of density or potential), which follow the magnetic field lines and propagates poloidally

  5. What diagnostics are required? • It is evident that understanding of underlying physics in the edge plasma requires simultaneous measurement: • Plasma parameters like density, temperature, potential with a good spatial and temporal resolution (turbulence) • Plasma flows in poloidal (and toroidal) direction • A practical solution for small scale experiments => electric (Langmuir) probes • Alternatively in large experiments (more sophisticated diagnostics): • Fast cameras are used to visualize the density fluctuations ( ~100 k USD) • Beam emission spectroscopy • Microwave scattering • ……

  6. Single Langmuir probe Ion saturation current Floating potential Iprobe=Iionsat{1 - exp [- e(Vfloat-Vprobe)/kTe]}

  7. Design of Langmuir Probe Array (radial profiles on a small-scale tokamak) Rake probe on CASTOR floating potential • 16 small tips (diam.=0,7 mm, l=2 mm) • Distance of tips 2.5 mm • Total extension 35 mm LCFS LCFS Limiter Wall Isat ~ density • All tips are biased simultaneously • Probe currents are measured • I-V characteristics are fitted • Radial profiles averaged over six • identical discharges El. temperature Radius [mm]

  8. Design of Langmuir Probes on Large Tokamaks has to be quite different!! Fast-scanning probeon Tore Supra Probes Probe construction must be sufficiently robust to survive extreme heat loads Probe head must reciprocate during discharge. Typically 200 ms - inward motion 200 ms - outward motion fast radial motion Graphite shield

  9. Reciprocating probe head on TORE-SUPRA (5 insertions of the probe head will be seen on movie) Might not be enough!

  10. Single Langmuir probefor fluctuation measurement Time required to measure a single I-V characteristic is typically >1 ms Example of a simple circuit (just Isat or Vfloat) used on CASTOR for > 100 probes simultaneously Typical power spectrum • Mean value of the signal • Level of fluctuations • Frequency spectrum

  11. d Double Langmuir probefor fluctuation measurement t Cross-correlation function is calculated and Mean propagation velocityof fluctuations v = d/ t can be determined from the delay t

  12. But what about temperature fluctuations?A solution - SegmentedTunnelProbe on CASTOR Advanced Langmuir probe measuring simultaneously ne,Te,T//,iJust the ion saturation currents from individual electrodes are measuredTemporal resolution is determined only by data acquisition system. Low-cost and robust - only three DC signals of ion saturation current are measuredNo expensive electronics. Immediate access to fluctuating quantities Diameter of the tunnel ~ several Larmor radii (CASTOR d=5 mm) ne~ I1+ I2+Ib Te~ (I1+ I2) / Ib T//,i~ I1/ I2

  13. Two examples of time resolved Measurements with STP However, massive PIC simulations are needed in order to absolutely calibrate the probe for Te and T//,i measurements, for particular probe design, tokamak configuration and the range of plasma parameters to interpret measured signals for J//,i = 1 ÷ 3 kAm-2 for J//,i = 3 ÷ 6 kAm-2 Electron temperature Ion temperature Jsat (~ density) Floating potential

  14. Space/time resolved measurements of plasma turbulence Poloidal array of 124 probes (for a better insight to the physics of the edge turbulence) • Array surrounds the whole poloidal circumference of the tokamak • Poloidal resolution  = 2.9 deg (3 mm) • Metallic support represents the poloidal limiter • 64 fast data acquisition channels available (1 s sampling) B

  15. Poloidal structure of edge turbulence

  16. Cross-Correlation in the Poloidal Direction Poloidal periodicity is more evident from cross-correlation analysis Poloidal mode number can be easily determined The reference probe is located at the top of the torus Poloidal direction Time lag [ms]

  17. 2D Array of Langmuir Probes 2Dmatrix of 64 tips Poloidal resolution ~ 6 mm Radial resolution ~ 4.5 mm As shown before by numerical modelling, the character of the edge turbulence is rather complex. To know as much as possible about turbulent structures (characteristic dimensions, life time, wavelength, …) 2D arrays of the Langmuir probes should be used!

  18. 2D Structure of Edge Turbulence on the CASTOR Tokamak 2D matrix of 64 Langmuir probes limiter separatrix radial position [mm] poloidally: 42 mm A snapshot of potential structures

  19. 2D Structure of Edge Turbulenceas measured by thematrix of Langmuir probes 42 mm in the poloidal direction Movies:1000 frames by 1 s Total duration = 1 ms POTENTIAL HILL POTENTIAL VALLEY 22 mm in radial direction

  20. Measurement of ion flow velocityby Planar (Mach) Probe B Mach number in the direction parallel to the magnetic field lines is calculated from ion saturation currents measured by the upstream and downstream collectors using a simple formula vII Isat(downstream) Isat(upstream)

  21. Alternative approach - Gundestrup Probe Polar diagram of Ion saturation current The Gundestrup cauldron B Top View Bt, Ip Parallel & perpendicularMach numbes are derived with a high temporal resolution from the shape of the polar diagram Several (eight) segments with a different orientation with respect to magnetic field lines Used on ISTTOK as well

  22. Tools for edge plasma diagnostic and fluctuation measurements on CASTOR (Summary) • Classical Langmuir probes – IV characteristics, local Te, ne, Ufl at the plasma edge, routine measurements • Radial & Poloidal arrays of Langmuir probes for spatially-temporally resolved measurements of plasma fluctuations • Oriented probes - Gundestrup and Mach probe for flow measurements during biasing experiments • Advanced probes – Segmented tunnel probe - a quite novel concept for fast Te and Ti measurements • Ball pen probe & Emissive probe – Direct measurement of plasma potential (not discussed here)

  23. Conclusions • Edge plasma is an important region in tokamaks – confinement, transport barriers • Edge plasma diagnostics with a good spatial and temporal resolution are required to understand the underlying physics • Electric probes (arrays) are extremely useful tools for that purpose • Small tokamaks (flexibility, routine operation) are suitable for: • Testing of novel diagnostics • Investigation of relevant edge physics (in particular the plasma (turbulence) • Training J Stöckel,et al, Advanced probes for edge plasma diagnostics on the CASTOR tokamak, Journal of Physics, Conference Series, 63 (2007), 012001 http://www.iop.org/EJ/journal/conf