Edge plasma physics

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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
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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

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Introduction

• One of the characteristics of heliotron/stellarato
  r 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.
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Divertors in LHD
Local Island Divertor (LID)
Helical Divertor (HD)
Intrinsic double-null divertor in the
heliotron-type magnetic configuration. Ergodic
boundary Now, it is OPEN type. Closure plan is
under way.
Utilize m/n1/1 island gerenated by perturbation
coils LID is a CLOSED divertor High pumping
efficiency(gt 50 ) has been achieved.
P2-02 M. Shoji
Demonstrate a high performance of a
reactor-relavant helical plasma
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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.
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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 --gt
similar peaked profiles to LID
Formation of IDB-SDC plasma depends on magnetic
axis position, same as LID.
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Effect of edge neutral pressure n0on IDB
formation
Neutral pressure
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
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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

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Edge magnetic structure
High central pressure induces large Shafranov
shift
3D equilibrium ? HINT code
Strong edge modification - increase of
ergodicity - increase of thickness of ergodic
layer
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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.
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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

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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.
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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
IDB
w/o 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.
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Particle flux to divertor plates and neutral
pressure
Difference of ne_bar is 10 times, but that of
divertor flux is lt 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.
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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

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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
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Impurity screening by friction force in the
ergodic region
Thermal force dominant
friction force by plasma flow
divertor
core plasma
nLCMS21019 m-3
1018 m-3
thermal force
1.0
//-B field
Increase ne
//-impurity velocity
31019
0.1
friction force by plasma flow
thermal force (Z-independent)
Condition for impurity retention
? Vz// gt 0
0.01
41019
Friction force dominant
The more collisional, the more effective
screening in the ergodic layer
Carbon density distribution (EMC3-EIRENE)
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Experimental evidence of the screening
Ionization potential CIII (C2) 48 eV CIV
(C3) 65 eV CV (C4) 392 eV CVI (C5)
490 eV
big gap
Clear separation of profile between C1C3
C4C6
In the case of impurity screening C1,C2,C3
C4,C5,C6
Ratio (CVCVI) / (CIIICIV) as a measure of
carbon screening.
Spectroscopic measurement suggests that impurity
screening appear in the ergodic layer.
O-03 M. Kobayashi
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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.

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IDB-SDC discharge
Scenario
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
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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.
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Sustaining IDB using repetitive pellet injection