1. 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

2. 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

3. 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.

4. 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

5. 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.

6. 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.

7. 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

8. 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

9. 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

10. 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.

11. 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

12. 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.

13. 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.

14. 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.

15. 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

16. 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

17. 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)

18. Experimental evidence of the screening
Ionization potential :
CIII (C+2) : eV
CIV (C+3) : 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 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

19. 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.

20. 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
* x2021 atoms/pellet
* m/s

21. 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.

22. Sustaining IDB using repetitive pellet injection