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Progress on ITER Rampdown Modeling with TSC. C. Kessel Princeton Plasma Physics Laboratory ITPA-IOS Meeting, 3/31-4/3/2009, Naka, Japan. ITER Rampdown Modeling Requirements. Rampdown in ITER is complicated by several constraints and the desire to avoid disruptions Rampdown Ip
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Progress on ITER Rampdown Modeling with TSC C. Kessel Princeton Plasma Physics Laboratory ITPA-IOS Meeting, 3/31-4/3/2009, Naka, Japan
ITER Rampdown Modeling Requirements • Rampdown in ITER is complicated by several constraints and the desire to avoid disruptions • Rampdown Ip • Rampdown elongation to avoid vertical instability • Terminate burn • Avoid additional flux consumption after EOB, avoid increase in CS1 coil current • Maintain divertor strike points on allowed target ranges to handle power and particle control • Maintain diverted configuration down to very low Ip (1.5 MA)
Several Possible Strategies • Ip rampdown rate • Transport regime • H-mode • Heated L-mode • Ohmic • H-mode/L-mode/ohmic • Density trajectory during rampdown • Auxiliary power trajectory and H-mode threshold • (Some) Uncertain features • Density drop during H to L, or H to L to H • Response to drops in auxiliary power • H-mode threshold, H to L back-transition threshold
Power and H-mode Thresholds • Typical flattop ELMy H-mode power to compare to Pth • 80 MW alpha power • 40 MW injected power • Giving 120 MW of input power • 47 MW of radiated power (Pbrem = 21 MW, Pcycl = 8 MW, and Pline = 18 MW) • Giving 73 MW of input power • If we use rule of thumb for power radiated inside the pedestal (Pbrem + Pcycl + Pline/3) • Giving 85 MW of input power • ---> role of radiated power in estimating H-mode sustainment???
Density Evolution in Rampdown • From the threshold formulas, it is clear density will play an important role in the transport regime we sustain in rampdown, and the auxiliary power requirements for it • Fastest density drop is assumed to be P* = 5 x E, roughly based on pumped divertor experiments • Slowest density drop is at same rate as Ip to keep n/nGr ≤ 0.85 • Otherwise density drop could be anywhere in between • Strike points locations not optimal • Divertor physics regime dependences
Auxiliary Power into the Plasma • There is a total of 73 MW available from the day-1 H/CD mix • 33 MW NNBI • 20 MW ICRF • 20 MW EC • NNBI has a density permissible, nL > 5 x 1019 /m3, so in rampdown this source is not available for the entire phase • ICRF and EC would prefer plasma that “sees” midplane port substantially, so vertical height of plasma is constrained • If we are injecting power we must have strike points in allowed locations of divertor slots to handle the power
Classification of rampdowns • Non-heated L-mode rampdown • Heated L-mode rampdown • H-mode rampdown • Slow, medium and fast density rampdowns
Focused on Plasma Midplane Coupling, But we Need to be Coupled to Divertor Also J-toroidal
Results • The flux state does not advance for any of the cases examined • The plasma shifts by about 5 cm inward with the complete shutdown of power and transition to L-mode at EOB • Simulations with more abrupt density drops showed this got up to 7-8 cm, but still no FW contact • The CS1 coil rise at the beginning of the rampdown is lower when the plasma stored energy stays higher, either by H-mode or heating with L-mode • Need to check whether forcing ICS1 to remain flat when it reaches it maximum current is OK • Other pre-programming/feedback tricks may be sufficient to avoid the ICS1 limit from creating a problem (disruption)
Result, cont’d • The li rises more slowly in the H-mode than in the L-mode in the first half of the rampdown, but due to higher edge temperatures it rises faster in H-mode later • Use experimental guidance on how H-mode temperature pedestal behaves as power is brought down • Vertical stability of plasmas as li rises • Uncertain features and complicated behavior • The ratio n/nGr can easily exceed 1.0 when Ip is dropping, unless n and Ip drop together • Injected power to stay in H-mode P(MW) > 86.2 ne200.73, given large incremental drops from NB’s • Tped must stay sufficiently low in H-modes to avoid high values, drives negative edge current, drives higher li than L-mode • Modeling precise transition features is difficult but possible with experimental prescription, what are they?