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Overview

Scenario and operational issues for high current L. Zanotto , R. Cavazzana, S. Dal Bello, F. Milani. Overview. Lessons learned from 1.5MA pulses and first pulses at I PLASMA > 1.5MA during 2008 High current issues Horizontal equilibrium Error field correction at the gaps Start-up

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Overview

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  1. Scenario and operational issues for high currentL. Zanotto,R. Cavazzana, S. Dal Bello, F. Milani

  2. Overview • Lessons learned from 1.5MA pulses and first pulses at IPLASMA > 1.5MA during 2008 • High current issues • Horizontal equilibrium • Error field correction at the gaps • Start-up • Density control • Scenarios towards 2MA • Roadmap RFX-mod Programme workshop – 20-22 january 2009

  3. Energy transfer units (PT): early operations @ high current were difficult due to failures in the VCB. PT upgraded in 2008 Glow Discharge Cleaning (GDC): essential to recover density control and to condition the wall MHD control: both PR power supplies and control algorithms must work reliably Diagnostics: Interferometer, Thomson Scattering, Pellet Some key plant components have proven to be essential for 1.5MA campaigns RFX-mod Programme workshop – 20-22 january 2009

  4. Energy transfer units have been upgraded during 2008 • Motivation: vacuum breakers not able to interrupt 50kA at high I2t • Modification and test of one unit (new breaker in parallel): • beginning of 2008 • Upgrade extended to other units: • Summer 2008 • Final commissioning @ 50kA: • October 2008 • Increased interruption capabilities • Regimes @ Ip > 1.5MA (Imag up to 50kA) enabled RFX-mod Programme workshop – 20-22 january 2009

  5. Glow Discharge Cleaning • GDC is old and aging can be critical • Density control techniques shall be improved to limit as much as possible the use of GDC • Some interesting stats: • Total GDC duration in 2008 (excl. baking, boron.)  80h • Total GDC duration during high current session  10h • Amount of shift time used for GDC  30h (6.25% over the 59 high current shifts) RFX-mod Programme workshop – 20-22 january 2009

  6. Dedicated runs to understand wall behavior have proven to be a useful investment Density control techniques have improved a lot in 2008 ... but as current increases the operative window narrows! M.E. Puiatti, PoP R. Cavazzana, SciOp meeting nG [1020 m-3] 3 0.5 1.0 Ip [MA] 1.5 2.0 RFX-mod Programme workshop – 20-22 january 2009

  7. Vessel temperature RFX-mod Programme workshop – 20-22 january 2009

  8. 350° RUN 1401 04/06/2008 280° 96 °C 210° Toroidal angle 140° 70° 0 Time [h:m:s] RFX-mod Programme workshop – 20-22 january 2009

  9. About 370 shots (over about 60 runs) @1.41.6MA On best 2008 sessions, up to 10-13 useful shots were performed (1.5 2 pulses per hour) A considerable amount of shots at 1.5MA was obtained during 2008 Daily pulse rate was limited mainly by • GDC during sessions (due to excessive wall loading) • Wall temperature evolution during the day • FPSD due to PT, not optimized equilibrium control RFX-mod Programme workshop – 20-22 january 2009

  10. Lessons learned • It is worthwhile to spend as much time as possible for optimisation • Optimisation and refinement of density control techniques increase operation efficiency • Understanding the wall behaviour and further develop density control techniques is important • Operative density window is narrowing as the current increases • Without GDC it is impossible to operate at high current • Pulse rate at high current is 10-13 pulses/day (if the machine works perfectly) RFX-mod Programme workshop – 20-22 january 2009

  11. Some tentative plasmas @ Ip > 1.5MA have already been performed RFX-mod Programme workshop – 20-22 january 2009

  12. Reversal at low plasma current Fast ramping Good control of the density (minimization of particle loss before reversal)  Equilibrium control more critical  Higher energy fed to MHD modes (higher dynamo required)  Error field correction at the gaps more demanding  Lower achievable plasma current with same mag. current  A start-up scenario with low bias toroidal field was used for 2008 high current campaigns RFX-mod Programme workshop – 20-22 january 2009

  13. Horizontal equilibrium control optimization • The equilibrium control loop showed instabilities leading to unbalance in the PVAT subunits (Shut Down of the Discharge) • Noise reduction and filtering of control signals FPSD @ 125ms • Optimise control loop regulator to reduce shift oscillations during flat-top • Feedback start delay to avoid error accumulation RFX-mod Programme workshop – 20-22 january 2009

  14. Horizontal equilibrium control optimization (2) • A pulsed bias vertical field is applied in high current discharges... • ...but the vertical field penetration is not uniform (gaps) • Additional error fields at start-up arise • Limited benefit on the equilibrium control as the pulse amplitude is increased •  optimise radial field correction at start-up •  find a different start-up to enhance uniform penetration • Assess plasma shape See proposal #99 by R. Cavazzana et al. RFX-mod Programme workshop – 20-22 january 2009

  15. The start-up needs optimization • Fast or slow? • Fast • Higher transfer resistance (limited by max. PT voltage) • Higher volt-sec efficiency (higher peak current before PCAT on) • Worse control of error fields and equilibrium • Slow • Lower transfer resistance • Lower volt-sec efficiency (lower peak current before PCAT on) • Better control of error fields and equilibrium 25091 : Rt 0.584 Ω 25326: Rt 0.467 Ω 25329: Rt 0.420 Ω RFX-mod Programme workshop – 20-22 january 2009

  16. Flux consumption & rise time (#23800-#25672) Rt = 0.584 Rt = 0.420  Rt = 0.467  Rt = 1.011  RFX-mod Programme workshop – 20-22 january 2009

  17. If the “classic” start-up scenario is pursued... PCAT See proposal #52 by R. Cavazzana et al. ... 50kA seems not enough to reach 2MA  PCAT needed to fill the gap, or more efficient start-ups (it means faster... ) RFX-mod Programme workshop – 20-22 january 2009

  18. Can a different start-up be imagined? • Assessing the flux consumption and efficiency is important (see R. Piovan’s talk) • ... but the slower we ramp-up, the less efficient we are! • PCAT capability can be enhanced (2xTFAT+2xPMAT) • Total flux cons. theoretical capability 15Vs (12Vs @ 2MA)+PCAT RFX-mod Programme workshop – 20-22 january 2009

  19. Roadmap to 2MA • Mandatory: assess the capability of the machine to withstand a pulse duration > 0.5s and high current scenarios • Power supplies (TC, PR, PVAT) • First wall, vessel • Define some reference start-up scenarios • Begin from the experience gained in 2008 • Low bias toroidal field start-up (early reversal) • Mandatory: optimise equilibrium and error field control • Further optimise density control • Develop new scenarios • More efficient? • Very slow ramp-up? • Take advantage of PCAT enhanced capability RFX-mod Programme workshop – 20-22 january 2009

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