RFX-mod programme workshop, January 2009 Scenario and operational issues for high current L. Zanotto, R. Cavazzana, S. Dal Bello, F. Milani

RFX-mod Programme workshop – january 2009 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  Density control Scenarios towards 2MA  Roadmap

RFX-mod Programme workshop – january 2009 Some key plant components have proven to be essential for 1.5MA campaigns Energy transfer units (PT): early 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

RFX-mod Programme workshop – january 2009 Energy transfer units have been upgraded during 2008 Motivation: vacuum breakers not able to interrupt 50kA at high I 2 t Modification and test of one unit (new breaker in parallel):  beginning of 2008 Upgrade extended to other units:  Summer 2008 Final 50kA:  October 2008 Increased interruption capabilities Ip > 1.5MA (Imag up to 50kA) enabled

RFX-mod Programme workshop – january 2009 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 – january 2009 Density control techniques have improved a lot in 2008 Dedicated runs to understand wall behavior have proven to be a useful investment n G [10 20 m -3 ] Ip [MA] but as current increases the operative window narrows! M.E. Puiatti, PoP R. Cavazzana, SciOp meeting

RFX-mod Programme workshop – january 2009 Vessel temperature

RFX-mod Programme workshop – january 2009 Time [h:m:s] Toroidal angle 96 °C 70° RUN /06/ ° 210° 280° 350° 0

RFX-mod Programme workshop – january 2009 A considerable amount of shots at 1.5MA was obtained during 2008 About 370 shots (over about 60  1.6MA On best 2008 sessions, up to useful shots were performed (1.5  2 pulses per hour) 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 – january 2009 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 pulses/day (if the machine works perfectly)

RFX-mod Programme workshop – january 2009 Some tentative Ip > 1.5MA have already been performed

RFX-mod Programme workshop – january 2009 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 – january 2009 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 125ms Optimise control loop regulator to reduce shift oscillations during flat-top Feedback start delay to avoid error accumulation

RFX-mod Programme workshop – january 2009 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 – january 2009 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 : Rt Ω 25326: Rt Ω 25329: Rt Ω

RFX-mod Programme workshop – january 2009 Flux consumption & rise time (#23800-#25672) Rt =  Rt =  Rt =  Rt = 

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

RFX-mod Programme workshop – january 2009 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 2MA)+PCAT

RFX-mod Programme workshop – january 2009 Roadmap to 2MA 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 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