H-mode performance and pedestal studies with enhanced particle control on Alcator C-Mod J.W. Hughes, B. LaBombard, M. Greenwald, A. Hubbard, B. Lipschultz, K. Marr, R. McDermott, M. Reinke, J.L. Terry MIT Plasma Science and Fusion Center American Physical Society 49th Annual Meeting of the Division of Plasma Physics Orlando, FL 14 November 2007
Goal: Understand influences of particle control on H-mode behavior Density control desirable in H-mode - Mitigate unwanted density rises that deteriorate confinement - Extend operation to lower collisionality for studies of ELMs and core density peaking - Low-density targets for lower hybrid current drive, advanced scenario development Fueling control in C-Mod discharges has typically been limited to gas puffing... did this limit attainable pedestal parameters? Addition of an upper chamber cryopump to C-Mod provides new tool for particle control Cryopump Pumping slots Design, operation of cryopump, Ohmic density control: B. LaBombard NP8.83 Slide x 2 of 9
H-mode edge density profiles resilient to modification from fueling Varying plasma current I P systematically varies pedestal plasma transport, in particular regulating n e,ped. Typically, n e,ped ~ 0.4 x n G (QI P ) D 2 puffing experiments in steady enhanced D α (EDA) H-modes show clamped dn, dp Suggestive of transport regulation via critical gradient physics Decreasing plasma current Base case Puffed with ~15 torr-l Question. What are the limitations to particle control? Is the H-mode edge as resilient to pumping as it is to puffing? Ohmic edge density gradient also stiff! LaBombard BI1.1 Slide x 3 of 9
Edge pressure gradient also regulated by plasma transport H-modes display a ballooning-like scaling for dp, despite being ideal MHD stable Over a wide range in operational space, appropriately normalized dp appears regulated by edge collisionality 50 EDA ELM-free 2 1/ 2 pe ~I P PSOL Δ p e gradient (MPa/m) Mid-pedestal α MHD 10 2 α MHD q 2 P R B 2 1 10 70 Mid-pedestal ν* Question. Can we use the cryopump as a tool to push to lower-collisionality H-mode regimes? Does machine performance improve? Similar relationships seen observed in SOL of Ohmic plasmas: LaBombard BI1.1 Slide x 4 of 9
H-mode pedestal density response to pumping evaluated at varied I P Normal B T (unfavorable BxdB) USN discharges evaluated with both B T directions Dashed lines signify unpumped n e,ped Typ. τ E,H 1.0MA 0.8MA 0.6MA 0.4MA Reversed B T (favorable BxdB) In both cases, n e,ped scales with I P Normal field: BxdB directed away from X-point - Lower starting n e,ped than for favorable db drift - Unvaforable drift direction -> radiative ELM-free H-modes, typically of short duration [cf. Hubbard, APS06] - Ohmic core pump-out times ~ 100-200ms Reversing field gives favorable db drift - Promotes longer, EDA target plasmas for comparison - Somewhat reduced pumping capacity due to lower recycling at the pumping slot (reversal of in-out divertor asymmetry) Pedestal density slightly lower on average when pumping, most prominently at higher I P (q<4) Time following L-H transition (s) Slide x 5 of 9
ELM-free H-mode n e,ped reduced by 15-20% kev kev MW MW 10 20 m -3 Normal field + USN (unfavorable BxdB direction), q 95 =3.7 Line-averaged n e Pumped / Unpumped P ICRF P rad T e,0 T e,95 H ITER89 time (s) Here pumping lowers edge collisionality promptly by lowering edge n e and roughly fixed p e Transient improvement of confinement in pumped case: consequence of boosted T e,ped 10 20 m -3 kev kpa Profiles are from 2nd and 3rd H-modes of each discharges ne Te pe Base case: Transient ELM-free H-modes, collapsing due to radiation R - R LCFS (mm) Thomson scattering profiles Slide x 6 of 9
"Pump-out" of higher q EDA H-modes Reversed field + USN (favorable BxdB direction), q 95 ~6 Examples at left have varied L-mode fueling 10 20 m -3 Typ. τ E,H Line-averaged n e Pumped H-modes reach identical equilibrium values of both global and pedestal n e Steady state <n e >, n e,ped independent of the kev MW MW T e,95 P ICRF P rad initial H-mode starting condition, and lower than in nominal unpumped case Less evidence of prompt density depression than in ELM-free cases Core plasma sheds particle inventory on 100-200ms time scale Edge T e rises in response to density drop, so n e,ped that initial edge pressure is approximately 10 20 m -3 time (s) maintained Resembles a time-reversal of H- mode D2 puffing experiments! (Hughes, APS05, IAEA06) Slide x 7 of 9
Observed increases in confinement linked to edge pedestal modification Many H-mode regimes (moderate density, q 95 ) exhibit very little density drop when pumped Stiff density gradients at fixed pedestal width a possible explanation const. p curves ~ Ip 2 unpumped pumped 1.0MA 0.8MA 0.6MA USN, normal field w/ Ptot = 3.8 +/- 0.2 MW Even in cases with clamped density pedestals, pumping provides a significant reduction in edge collisionality, via T e,ped temperature increase Likely a modification of SOL n e, T e is lowering the separatrix collisionality Boost in edge dp is obtained by reducing ν*, consistent with prior observations Provided radiation is kept low, pumping enhances H-mode performance through core profile stiffness Slide x 8 of 9
Summary and Future Work Upper divertor cryopump providing an additional tool for particle control on Alcator C-Mod Density control in H-modes can be more challenging than in their Ohmic counterparts - Prompt density pedestal formation at the L-H transition not suppressed by reduction in neutral fueling - Obtained n e,ped can be reduced by pumping, though still tied mainly to I P - n e,ped in pumped discharges modified by magnetic topology and db drift direction, which affect (1) the level of recycling at the pump and (2) pedestal transport Pumping generally lowers edge collisionality of H-mode discharges - Starting H-mode density reduced in H-modes with q 95 <4 - Long-term (>>τ E ) density pump-out in higher-q steady EDA H-modes - Even cases with marginal n e,ped reduction demonstrate edge ν* reduction, along with a boost in edge dp, global confinement (nearly a time-reversal of H-mode gas puffing experiments) Need to better understand transport physics clamping edge pedestal gradients, identify more suitable discharges for particle control Further pursuing physics studies at reduced collisionality - Pedestal structure and ELMs, high performance operation, core density peaking Slide x 9 of 9