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Scrape-off Layer Power Flux Measurements in the Tore Supra Tokamak J. P. Gunn 1

Scrape-off Layer Power Flux Measurements in the Tore Supra Tokamak J. P. Gunn 1 R. Dejarnac 2 , P. Devynck 1 , N. Fedorczak 3 , V. Fuchs 2 , C. Gil 1 , M. Kočan 4 , M. Komm 2 , M. Kubič 1 , T. Lunt 4 , P. Monier-Garbet 1 , J. -Y. Pascal 1 , F. Saint-Laurent 1

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Scrape-off Layer Power Flux Measurements in the Tore Supra Tokamak J. P. Gunn 1

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  1. Scrape-off Layer Power Flux Measurements in the Tore Supra Tokamak J. P. Gunn1 R. Dejarnac2, P. Devynck1, N. Fedorczak3, V. Fuchs2, C. Gil1, M. Kočan4, M. Komm2, M. Kubič1, T. Lunt4, P. Monier-Garbet1, J. -Y. Pascal1, F. Saint-Laurent1 1CEA, IRFM, F-13108 Saint-Paul-lez-Durance, France. 2Association EURATOM-IPP.CR, Za Slovankou 3, 18200 Prague 8, Czech Republic. 3Center for Momentum Transport and Flow Organization, USCD, San Diego, California 92093, USA. 4Max-Planck-Institut für Plasmaphysik, EURATOM Association, Boltzmannstr. 2, 85748 Garching, Germany.

  2. OUTLINE Improving accuracy: New integral method of SOL power measurement using probes Closing the power balance: Role of "paradoxical quantum mechanical backscattering" of low energy electrons Application: Power decay length in inboard-limited ITER start-up plasmas TS44052 : inboard-limited discharge for ITER start-up studies

  3. probe ½PSOL main source of heat + part. fluxes LFS blobby transp. ½PSOL MODEL UPSTREAM SIDE OF PROBE COLLECTS HALF OF PSOL assume probe does not perturb plasma any more than a limiter does radiation inside LCFS (~75% from bolometry) power density to probe = power density to wall tunnel probe retarding field analyzer model neglects possible role of poloidal drifts

  4. New method : (1) INTEGRAL OF RFA COLLECTOR CHARACTERISTIC GIVES q//,i DIRECTLY Advantages of integral method : Less sensitive to fluctuations. No knowledge needed about sheath potential, distribution functions, collisionality, surface properties.

  5. New method : (2) INTEGRAL OF TUNNEL PROBE CHARACTERISTIC GIVES q//,e DIRECTLY 2D PIC calculations Electron component of power flux Integral of electron current measured by tunnel probe -automatically includes effect of non-ambipolar parallel currents (must correct for secondary electron emission) N.B. Tunnel probe is a concave Langmuir probe It is immune to sheath expansion - precise J//,i measurements

  6. NOVEL METHOD TO DIRECTLY MEASURE S.E.E. USING TUNNEL PROBE STRONG S.E.E. ALWAYS OBSERVED Magnetic field misaligned w.r.t. tunnel axis. Apparent backplate electron current reduced (s.e.e.) Some electrons strike tunnel, but no net s.e.e. due to glancing angle (Larmor gyration). J. P. Gunn, PPCF (2012), accepted paper.

  7. NEARLY CONSTANT TOTAL EMISSION YIELD AT LOW Te UBIQUITUOUS PHENOMENON In the past it was always assumed that S.E.E. should be negligible at low energies. Enhanced backscattering of low energy electrons compensates decrease of true secondary emission due to energetic electron impact. Postulated by Pitts and Matthews, JNM 176-177, 877 (1990). consequences for plasma-wall interactions -decreased sheath potential, increased power flux to surfaces (sheath transmission coeffecient γ>>7), SOL cooling

  8. E electron z vacuum solid work function + Fermi energy "PARADOXICAL QUANTUM MECHANICAL BACKSCATTERING" RECENTLY RECOGNIZED IN MANY FIELDS OF TECHNOLOGY DeBroglie wavelength of low energy electron >> interatomic spacing quantum mechanical reflection of wave packet from negative potential step R. Cimino, et al, PRL 93, 014801 (2004). Garrido, et al, Am. J. Phys. 79, 1218 (2011). J. Cazaux, J. Appl. Phys. 111, 064903 (2012). - problems in particle accelerators caused by electron clouds - spacecraft charging - limitations of Hall thruster performance

  9. SECONDARY ELECTRON EMISSION DOMINATES THE PHYSICS OF POWER FLOW TO THE SURFACE New integral method finds low PSOL if S.E.E. neglected. Force 100% power balance to deduce S.E.E. coefficient. Resulting coefficients agree with independent tunnel probe measurement.

  10. assuming δSEE=0 SIMILAR RESULTS OBTAINED FOR ALL DISCHARGES Compilation of results for which both RFA and tunnel probe measurements were made in identical plasmas Probes can measure power correctly

  11. ITER start-up experiments in Tore Supra IN 2009-2010, 3 SESSIONS TO STUDY SOL POWER SCALING Head load specifications given by ITER physics basis [NF 39, 2391 (1999)] (derived from measurements in L-mode plasmas in X-point divertor tokamaks) TS λq does not follow ITER scaling law. Better fit :

  12. SCATTER OF SCALING LAW PREDICTION IS NOT RANDOM. Measurements in Tore Supra follow systematic trends that are different than the predictions of the ITER scaling law. Example : for fixed PSOL and safety factor qa, ITER scaling law predicts λq increase with density. We observe the opposite!

  13. Scrape-off layer power flux measurements in the Tore Supra tokamak CONCLUSIONS Development of combined RFA / tunnel probe method has led to significant improvement in accuracy of power measurements. Strong secondary electron emission seems to be the rule -indirect evidence : s.e.e. needed to close power balance -direct measurements : tunnel probe, RFA sheath potential Paradoxical quantum backscattering -very low energy electrons reflected with nearly 100% probability PSOL can be correctly measured by probes upstream of the strike point. How does the power map to the wall? -need to compare with IR cameras. Old measurements in Tore Supra and recent experiments in JET-ILW suggest that cross field transport near limiters can deflect power radially

  14. WHAT ABOUT POWER BALANCE USING SMALL PINS? Previously published "good agreement" might be illusionary! J//,ioverestimated by 200-400% -due to sheath expansion Sheath heat transmission underestimated by similar factor -due to neglect of secondary emission The two large errors can roughly cancel each other

  15. O-26 Tilmann Lunt, Thursday PROBES DON'T PERTURB THE SOL? C'est cela, oué

  16. EXCELLENT AGREEMENT WITH INTERFEROMETRY TUNNEL PROBE = ACCURATE PARTICLE FLUX MEASURMENTS Dedicated experiment : plasma shape was adjusted so that an interferometer cord passed through the SOL (20° toroidally away from tunnel probe) Using magnetic equilibrium calculations, probe data was integrated to reconstruct interferometer measurement.

  17. extra slides

  18. POINT-BY-POINT PROFILE ANALYSIS TO UNDERSTAND PSOL SCATTER PROBE PSOL DOES NOT TRACK MACROSCOPIC MEASUREMENTS Despite a large change in radiated power, the probe measurements barely budge!

  19. POWER BALANCE IS UNACCEPTABLY POOR Tunnel probe measurements from a single experimental session Despite good J//,i measurements, PSOL still has enormous scatter! IF WE CANNOT GET GOOD POWER BALANCE, HOW CAN WE BELIEVE λq MEASUREMENTS??? Seems to be a systematic behaviour depending on discharge.

  20. ITER start-up experiments in Tore Supra – POWER BALANCE IS THE SHEATH TRANSMISSION FACTOR IS THE CULPRIT? A better estimate of γ can be obtained from RFA measurements of Ti in the same discharge...

  21. WHICH IS THE HIDDEN PARAMETER MUCKING THINGS UP? ION TEMPERATURE IS NOT THE CULPRIT Special session to measure Ti using retarding field analyzer Including RFA Ti measurements in the calculation of sheath transmission coefficient (γ) is not enough We can only get variations of 10-30%, but we need up to 300%!

  22. SIMILAR s.e.e. COEFFICIENTS OBTAINED FOR ALL DISCHARGES Combined RFA - tunnel probe analysis STRONG S.E.E. RESOLVES THE OBSERVED DISCREPANCIES!

  23. ITER start-up experiments in Tore Supra – RESULTS λqNEARLY TWICE SMALLER WITH NEW METHOD Similar scaling of power decay length is obtained using new analysis, but values are usually lower.

  24. STANDARD PROBE MEASUREMENTS CAN BE (SORT OF) CORRECTED Power balance from tunnel probe data This graph shows the repartition of ion and electron power based on Langmuir probe measurement of JSAT and Te using standard sheath theory, as a function of secondary electron emission coefficient. As S.E.E. increases, the sheath potential decreases. Secondary electron emission can be included in standard sheath theory (but we still need to guess Ti ). Approximate s.e.e. coefficient can be deduced by imposing 100% power balance.

  25. ITER start-up experiments in Tore Supra IN 2009-2010, 3 SESSIONS TO STUDY SOL POWER SCALING Head load specifications given by ITER physics basis [NF 39, 2391 (1999)] (derived from measurements in L-mode plasmas in X-point divertor tokamaks) Trend OK but too much scatter! q// [ MW/m2 ] PSOL qa / a λq TS λq does not follow ITER scaling law. Better fit : λq~PΩ-0.7

  26. ITER start-up experiments in Tore Supra NOVEL FEEDBACK CONTROL TO MAXIMIZE PARAMETER RANGE EXPERIMENTAL PROCEDURE Feedback control of plasma current on pre-programmed ramp of safety factor Feedback control of gas injection rate on pre-programmed constant radiated power fraction. Shot-by-shot increase of frad to increase density Repeat series for different B, and for HFS and LFS contact

  27. ACCURACY OF J//,i DEPENDS ON PROBE MAGNETIZATION SMALL CYLINDRICAL PINS DO NOT MEASURE J//,i CORRECTLY J//,i = Isat / AEFF ;AEFF>>AGEO TS turbulence probe Small pin (~1 mm diameter) is not in strongly magnetized regime - overestimates J//,i by 200% - 400% AEFF depends on density, temperature, and flow Consistent with 3D SCEPTIC simulations (spherical probe) [Patacchini, Hutchinson, PPCF 53 (2011) 025005.]

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