Plasma-wall interactions during high density operation in LHD 18th PSI (May 26, 2008, Toledo, Spain) Edge plasma physics/ Plasma-wall interactions during high density operation in LHD S. Masuzaki for A. Komori and the LHD Experimental Groupp

OUTLINE Introduction Edge plasma control for high density operation in LHD Superdense core plasma with Internal Diffusion Barrier (IDB) Characteristics of the edge plasma in SDC-IDB discharges Edge magnetic field structure and plasma profiles Divertor plasma properties Impurity behavior Summary

Introduction One of the characteristics of heliotron/stellarator plasma is rather large tolerative for high density operation, and the central density of over 11021m-3 was achieved Density limit in LHD is observed as nc = 0.25 {(Ptot – dWp/dt)B/(a2R)}0.5 nc: critical edge density When the edge density reaches the nc, detachment occur. If the edge density continues to increase, plasma is collapsed by radiation. Edge plasma control is a key issue for high density operation.  Divertor Time evolutions of plasma parameters with and without radiation collapse.

Divertors in LHD Helical Divertor (HD) Local Island Divertor (LID) Intrinsic double-null divertor in the heliotron-type magnetic configuration. Ergodic boundary Now, it is ‘OPEN’ type. Closure plan is under way. Utilize m/n=1/1 island gerenated by perturbation coils LID is a ‘CLOSED’ divertor High pumping efficiency(> 50 %) has been achieved. P2-02 M. Shoji Demonstrate a high performance of a reactor-relavant helical plasma

Finding an Internal Diffusion Barrier (IDB) in LID discharges Center fueling by repetitive pellet injection + strong edge pumping by LID - achievement of superdense core plasma. formation of IDB - relatively high Te at the center. - strongly high pressure at the center ( ~0.1MPa ). - relatively low edge ne prevents radiation collapse keV 1019/m3 Superdense Core (SDC) Plasma * ne(0) ~ 5x1020 m-3, * Te(0) ~ 0.85 keV, * P(0) ~ 130kPa * Wdia ~ 980kJ * (0) ~ 4.2%, * highest fusion triple product nT ~ 4.4 x1019 keVsm-3 - large Shafranov shift (more than a/2) ne Te Formation of IDB & SDC plasma depends on magnetic axis position.

Promising results from HD experiment IDB-SDC plasma has been revealed that it is not specific to LID. Necessary condition (not sufficient) for IDB-SDC Plasma Center fueling by pellet injection + proper pumping by pumps and wall pump ne (1019/m3) HD LID Te (keV) Exhaustive wall conditioning w/o LID --> similar peaked profiles to LID Formation of IDB-SDC plasma depends on magnetic axis position, same as LID.

Effect of edge neutral pressure n0 on IDB formation Sequential operation degrades IDB  radiation collapse Edge ne high n0 high edge ne degradation or termination of discharge (no IDB) Edge Te radiation power Suppression of the neutral pressure in the edge region is a key parameter for IDB formation Stored energy

OUTLINE Introduction Edge plasma control for high density operation in LHD Superdense core plasma with Internal Diffusion Barrier (IDB) Characteristics of the edge plasma in IDB-SDC discharges Edge magnetic field structure and plasma profiles Divertor plasma properties Impurity behavior Summary

Edge magnetic structure LCFS (vacuum) Magnetic axis (vacuum) High central pressure induces large Shafranov shift 3D equilibrium  HINT code Strong edge modification - increase of ergodicity - increase of thickness of ergodic layer

Edge density and temperature profiles in the ergodic region DIII-D Exp. LHD exp. Edge Te begins to increase in the ergodic region, whereas the ne profile is flat. Similar trend is seen in DIII-D exp. T.E. Evans et al., Nucl. Fusion These results seems to be not consistent with the R-R model which predicts large c in ergodic layer.

OUTLINE Introduction Edge plasma control for high density operation in LHD Superdense core plasma with Internal Diffusion Barrier (IDB) Characteristics of the edge plasma in IDB -SDC discharges Edge magnetic field structure and plasma profiles Divertor plasma properties Impurity behavior Summary

Time evolutions of particle flux profiles on divertor plates Private region Connection length profile on the divertor plate at the time of maximum center pressure P0. Divertor flux profiles on the divertor plates drastically change during IDB discharge due to the modification of edge magnetic structure.

Modification of the particle deposition profile on the divertor induced by high central pressure D is assumed to be about 1 m2/s LHD divertor plate array w/o IDB IDB Numerical calculation simulating diffusing particles from core to divertor plates shows little difference between normal and SDC discharge. Divertor instruments (target plates, baffle, pump) for the IDB -SDC discharge is compatible with low density discharge.

Particle flux to divertor plates and neutral pressure Difference of ne_bar is ~10 times, but that of divertor flux is < ~3 times Divertor flux and neutral pressure are insensitive to the core plasma density, but well correlated to the edge plasma density as those during w/o IDB discharges.

OUTLINE Introduction Edge plasma control for high density operation in LHD Superdense core plasma with Internal Diffusion Barrier (IDB) Characteristics of the edge plasma in IDB-SDC discharges Edge magnetic field structure and plasma profiles Divertor plasma properties Impurity behavior Summary

Impurity behavior during SDC-IDB discharges Neoclassical ambipolar diffusion  ion root  negative radial electric field  impurity accumulation ? Harmful impurity accumulation has not been observed in IDB-SDC discharge. Implication of a possible mechanism of impurity screening in the edge region. negative Er : verified by CXRS

Impurity screening by friction force in the ergodic region Thermal force dominant friction force by plasma flow divertor core plasma nLCMS=2×1019 m-3 1018 m-3 thermal force 1.0 //-B field Increase ne //-impurity velocity 3×1019 0.1 friction force by plasma flow thermal force (Z-independent) Condition for impurity retention  Vz// > 0 0.01 4×1019 Friction force dominant The more collisional, the more effective screening in the ergodic layer Carbon density distribution (EMC3-EIRENE)

Experimental evidence of the screening Ionization potential : CIII (C+2) : 48 eV CIV (C+3) : 65 eV CV (C+4) : 392 eV CVI (C+5) : 490 eV big gap Clear separation of profile between C+1~C+3 & C+4~C+6 In the case of impurity screening : C+1,C+2,C+3 C+4,C+5,C+6 Ratio (CV+CVI) / (CIII+CIV) as a measure of carbon screening. Spectroscopic measurement suggests that impurity screening appear in the ergodic layer. O-03: M. Kobayashi

Summary IDB-SDC plasma has been obtained in LHD with the central fueling and proper edge pumping. 2) In the ergodic region, ne profile is relatively flat. On the other hand, Te profile has a steep gradient, which is not consistent with the classical model. 3) In spite of the strong edge modification by the large Shafranov shift in the IDB- SDC discharge, the PSI-related phenomena is not so different from those in the discharges without IDB. It is attributed to the similar edge density and temperature in IDB-SDC plasma to those in discharges without IDB. 4) Harmful impurity accumulation has not been observed in IDB-SDC discharges. Impurities are considered to be well shielded in the ergodic region by friction force. Results obtained in this experiments encourage us to study the new approach to the reactor plasma with IDB-SDC regime.

IDB-SDC discharge Scenario high density core + low density mantle high ne at core high Te at pedestal low recycling (lown0, edge ne ) central fueling high density core + low density mantle Edge Plasma Control by pumps * particle recycling Center fueling with pellet injection * 1.5-2.0x2021 atoms/pellet * 1000-1200m/s

Principle of LID LID is a divertor that uses an m/n = 1/1 island. heat & particle fluxes Closed geometry Outward heat and particle fluxes crossing island separatrix flow along field lines to backside of the island. High pumping efficiency of  50% Technical ease of pumping is the advantage of LID over closed full helical divertor because recycling is toroidally localized. Highly efficient pumping, combined with core fueling, is the key to improve plasma confinement.

Sustaining IDB using repetitive pellet injection