density regime of complete detachment and operational density limit in lhd

1/14

21st IAEA Fusion Energy Conference, Chengdu, China (16 - 21 Oct., 2006)EX/3-2

Density Regime of Complete Detachment and Operational Density Limit in LHDJ. Miyazawa1), R. Sakamoto1), S. Masuzaki1), B.J. Peterson1), N. Tamura1), M. Goto1), M. Shoji1), M. Kobayashi1), H. Arimoto2), K. Kondo2), S. Murakami3), H. Funaba1), I. Yamada1), K. Narihara1), S. Sakakibara1), K. Tanaka1), M. Osakabe1), S. Morita1), H. Yamada1), N. Ohyabu1), A. Komori1), O. Motojima1), and the LHD Experimental Group

1) National Institute for Fusion Science, Toki, Gifu 509-5292, Japan2) Graduate School of Energy Science, Kyoto University, Uji, Kyoto 611-0011, Japan3) Department of Nuclear Engineering, Kyoto University, Kyoto 606-8501, Japan

2/14


High-density operation in fusion reactor

Future fusion rector will operate in a density range of order 1020 m-3.

Higher density is more favorable, since the fusion reaction rate increases with densit

y squared.

Reduction of divertor heat load by detachment is expected at high-density.

High-density experiments in existing devices

High-density plasmas of order 1020 m-3 have been studied in medium devices.

-Alcator C-Mod tokamak (C-Mod): R = 0.68 m, a = 0.22 m, B 8 T.

-Frascati Tokamak Upgrade (FTU): R = 0.935 m, a = 0.31 m, B 8 T.

-Wendelstein 7-AS stellarator (W7-AS): R = 2 m, a 0.16 m, B 2.5 T.

Ex) LHD: R = 3.6 m, a = 0.64 m, B 2.75 T (inward-shifted configuration).

Power density in LHD (0.5 MW/m3), is much smaller than in W7-AS ( 4 MW/m3)

where volume-averaged density of 4 1020 m-3 was attained with detachment.

Introduction

3/14


Density limit of net current free helical plasmas

Sudo density limit scaling (derived from H-E, H-DR, W7A, and L2):

ncSudo = 2.5 (Ptot B / (a2 R) )0.5 (units: 1019 m-3, MW, T, and m).

e.g. Greenwald Limit : ncGW (1020 m-3) = Ip/(a2) = (5B)/(qaR),

… Since the qa scarcely changes in net-current-free plasmas, ncGW is roughly a constant at

a given set of B and R (ncGW ~ 1.8 1020 m3, for B = 2.71 T, R = 3.65 m, and qa ~ 0.7).

It has been considered that the power dependence in the Sudo scaling is resulted fro

m the power balance between the heating power and the radiation loss that is propo

rtional to ne2, however,

- Radiative collapse is often triggered at a small radiation loss fraction of 30 %.

-At complete detachment, the radiation loss fraction ranges from 30 – 100 % wit

hout radiative collapse.

Strongly peaked density profile is not within the scope of the Sudo scaling.

Density limit prediction

4/14


Radiative Collapse

Complete DetachmentPlasma column shrinks and Wp

dia decreases.

Isat decreases at all the measured divertor tiles.

Detachment in LHD

Transient Partial Detachment

Localized in the gas puff port.

Without high recycling.

Wpdia slightly decreases.

Serpens ModeSustainable complete detachment.

A helical radiation belt is formed inside of the LCFS: serpentThe serpent rotates in the EB direction.

MarfeToroidally axisymmetric

radiation belt.

Sustainable in W7-AS.

DensityHot plasma boundary: 100eV

Radial position where Te = 100±50 eV.

Line radiations from right impurities increase at Te< 100 eV.

100eV > 1

100eV < 1

100eV ~ 0.9

100eV < 0.8

5/14


LCFSSerpent

Complete detachment in gas-fueled plasmas (Transient partial detachment)

Isat decreases only in the gas puff port.

(Complete detachment) The hot plasma boundary shrinks below the

LCFS (100eV < 1) and Isat decreases at all the

measured divertor tiles. The density ramp up rate increases even tho

ugh the gas puff rate is unchanged.

Fueling efficiency is improved.

(Serpens mode)

100eV is sustained at ~0.9

The serpent appears. Serpent Marfe

Hydrogen volume recombination

Observed Observed

Radial position On/Inside LCFS On/Inside LCFS

Shape Helical Axisymmetric

Rotation E B Toroidal (W7-AS)

6/14


Density regime of complete detachment

Gas-fueled:

Threshold for complete detachment

Threshold for the Serpens mode

During the Serpens mode

Pellet-fueled:

Attach

Detach

Complete detachment regime

Attachment regime

Collapse regime

Density regime of complete detachment is surrounded by the threshold density for complete detachment ( ) and the Serpens mode data ( ).

Radiative collapse takes place above the complete detachment regime.

High-densities in the collapse regime are achieved by applying pellet injection.

7/14


ne reaches 3 1020 m-3, in spite of small absorbed powe

r density in LHD.

The record ne0 in helical plasmas of 5 1020 m-3 has been

achieved in LHD.

A superdense-core (SDC) is formed inside of the internal diffusion barrier (IDB) and the central plasma pressure reaches 1 atm. EX/8-1 N. Ohyabu (on Friday)

These have been achieved in pellet-fueled plasmas with strongly peaked density profiles.

Maximum density in pellet-fueled plasmas

8/14


Edge densities are similar!

(Attached data) Even in the pellet-fueled plasma with a

strongly peaked density profile, ne100eV is

similar to that of the gas-fueled plasma at the threshold for complete detachment.

(Detached data) ne

100eV stays unchanged at various core

density.

Local densities, ne100eV, at 100eV, are

similar for each of attached and detached datasets.

9/14


<ne> linearly increases with the peaking factor (Attached data)

In both of gas-fueled and pellet-fueled plasmas, ne

100eV are well appro

ximated by 0.8 ncSudo.

Large ne in pellet-fueled data

is due to the strongly peaked density profile.

Attached data (100eV ~ 1):

Gas-fueled

Pellet-fueled

10/14


Critical edge density increase with P 0.5

Critical edge densities for complete detachment and radiative collapse incr

ease with the square root of heating power.

This is also expressed in the Sudo scaling: ncSudo = 2.5 (Ptot B / (a2 R) )0.5.

Gas-fueled:

Threshold for complete detachment

Threshold for the Serpens mode

During the Serpens mode

Pellet-fueled:

Attach

Detach

Detachment threshold

Collapse threshold

11/14


Te at the LCFS is well fitted by (Ptot0.5/ne)2/3, as long as Te > 100 eV.

The critical LCFS density that results in the critical LCFS t

emperature of 100 eV increases with Ptot0.5.

Parameter dependence of the edge temperature

Critical edge temperature

Complete detachment regime

Attachment regime

12/14


Edge density at a fixed , ne(), increases as the hot plasma column shrinks and

100eV decreases, as long as < 100eV.

Outside 100eV ( > 100eV), ne() decreases with 100eV.

ne100eV is a good representative of the maximum of ne() at each .

100eV is the radial position inside which one can increase the density by fueling.

Evolution of the edge density

13/14


ne100eV approximates the maximum local density and increases with Ptot

0.5 in the edg

e region. A plot of ne

100eV / Ptot0.5 versus 100eV corresponds to the radial profile of ma

ximum density in the edge region. ne

100eV / Ptot0.5 in attached plasmas reach the maximum (~ 0.8 nc

Sudo) at 100eV ~ 1.

ne100eV / Ptot

0.5 increases as 100eV decreases and saturates to ~ 1.5 ncSudo.

Maximum edge density

14/14


The highest central density in helical plasmas of 5 1020 m-3 has been achieved in LHD.

- In pellet-fueled plasmas with strongly peaked density profile.

- The volume-averaged density reaches 3 1020 m-3, in spite of small heating power density of < 0.5 MW/m3 and the magnetic field of < 3 T.

Even in these high-density pellet-fueled plasmas, edge densities are similar to those in gas-fueled plasmas with flat or hollow density profiles.

Complete detachment takes place when the edge temperature at LCFS decrea

ses to a critical value of ~100eV (100eV = 1).

In the edge region, the electron temperature is a function of the square root of heating power divided by the electron density.

The critical LCFS density for complete detachment is ~ 0.8 ncSudo.

High edge density of ~ 1.5 ncSudo is sustainable in the Serpens mode plasmas, w

here the volume-averaged density reaches ~ 2.2 ncSudo .

Summary

15/14


The End

16/14


Radiation loss

At the Serpens mode, Prad and the impurity

irradiation such as CIII increase.

However, these do not necessarily trigger the transition to the Serpens mode, as seen in the unstable detachment discharge (blue lines in the right figure).

i.e. the unstable detachment discharge does not enter the Serpens mode even though Prad and

the CIII intensity exceed the values in the

Serpens mode discharge (shown by red lines).

In the unstable detachment discharge, the electron density is lower than the Serpens mode discharge.

Electron density is more important than the total radiation loss.

17/14


The neutral pressure, p0, increases with the edge density in attached plasmas.

At complete detachment, p0 decreases even though gas puffing is continued and the edge dens

ity increases. In the Serpens mode after gas puff turned off, p0 decreases to ~1/3 of that during gas puffing.

Under a low recycling condition, p0 decreases further and reattachment takes place.

Fueling and recycling control is a key to achieve the Serpens mode.

Neutral Pressure

18/14


Maximum <ne> in LHD

The volume averaged electron density (ne) exceeds 3 1020 m-3, in spite of small

absorbed power density in LHD (< 0.5 MW/m3) compared with W7-AS ( 4 MW/m3) where ne = 4 1020 m-3 was attained with detachment.

- At the inward shifted configuration (R = 3.65 m).

- Attached plasma.

- Hollow temperature profile (transient).

19/14


At complete detachment, the hot plasma boundary shrinks inside the LCFS. After the transition to the Serpens mode, complete detachment is sustained with a rotating

helical radiation belt, named the serpent.

LCFS

Complete detachment and the Serpens mode

Serpent

20/14


During the Serpens mode, the ratio of

H/ H increases to 3 – 5 times of that

in the attached phase.

Similar ratio is observed in the

detached divertor region and the

Marfe radiation belt in W7-AS.

The H signal is fluctuating as the H

signal.

Each of the peaks in H and H

fluctuations appears as the serpent

passes by the measurements.

Hydrogen volume recombination in

the serpent is suggested.

In this respect, the serpent in LHD

and the Marfe in W7-AS resemble

each other.

Hydrogen recombination

density regime of complete detachment and operational density limit in lhd

Documents