Density Regime of Complete Detachment and Operational Density Limit in LHD

1
Density Regime of Complete Detachment and
Operational Density Limit in LHD
J. 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, Japan 2)
Graduate School of Energy Science, Kyoto
University, Uji, Kyoto 611-0011, Japan 3)
Department of Nuclear Engineering, Kyoto
University, Kyoto 606-8501, Japan
2
Introduction

• 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 density
  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.

3
Density limit prediction

• 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 from the power
  balance between the heating power and the
  radiation loss that is proportional 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 without radiative
  collapse.
• Strongly peaked density profile is not within the
  scope of the Sudo scaling.

4
Detachment in LHD
Radiative Collapse
?100eV lt 0.8

• Complete Detachment
• Plasma column shrinks and Wpdia decreases.
• Isat decreases at all the measured divertor
  tiles.

• Marfe
• Toroidally axisymmetric radiation belt.
• Sustainable in W7-AS.

?100eV lt 1

• Serpens Mode
• Sustainable complete detachment.
• A helical radiation belt is formed inside of the
  LCFS serpent
• The serpent rotates in the E?B direction.

?100eV 0.9

• Transient Partial Detachment
• Localized in the gas puff port.
• Without high recycling.
• Wpdia slightly decreases.

• Hot plasma boundary ?100eV
• Radial position where Te 10050 eV.
• Line radiations from right impurities increase at
  Telt 100 eV.

?100eV gt 1
5
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
  (r100eV lt 1) and Isat decreases at all the
  measured divertor tiles.
• The density ramp up rate increases even though
  the gas puff rate is unchanged.
• ? Fueling efficiency is improved.
• (Serpens mode)
• r100eV 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
Density regime of complete detachment
Collapse regime
Complete detachment regime
Attachment regime
7
Maximum density in pellet-fueled plasmas

• ltnegt reaches 3 ? 1020 m-3, in spite of small
  absorbed power 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.

8
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)
• ne100eV stays unchanged at various core density.
• ? Local densities, ne100eV, at r100eV, are
  similar for each of attached and detached
  datasets.

9
ltnegt linearly increases with the peaking factor

• (Attached data)
• In both of gas-fueled and pellet-fueled plasmas,
  ne100eV are well approximated by 0.8 ncSudo.
• Large ltnegt in pellet-fueled data is due to the
  strongly peaked density profile.

10
Critical edge density increase with P 0.5
Collapse threshold
Detachment threshold

• Critical edge densities for complete detachment
  and radiative collapse increase with the square
  root of heating power.
• This is also expressed in the Sudo scaling
  ncSudo 2.5 (Ptot B / (a2 R) )0.5.

11
Parameter dependence of the edge temperature
Attachment regime
Critical edge temperature
Complete detachment regime

• Te at the LCFS is well fitted by (Ptot0.5/ne)2/3,
  as long as Te gt 100 eV.
• The critical LCFS density that results in the
  critical LCFS temperature of 100 eV increases
  with Ptot0.5.

12
Evolution of the edge density

• Edge density at a fixed ?, ne(?), increases as
  the hot plasma column shrinks and
• ?100eV decreases, as long as ? lt ?100eV.
• Outside ?100eV (? gt ?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.

13
Maximum edge density

• ne100eV approximates the maximum local density
  and increases with Ptot0.5 in the edge region.
• A plot of ne100eV / Ptot0.5 versus ?100eV
  corresponds to the radial profile of maximum
  density in the edge region.
• ne100eV / Ptot0.5 in attached plasmas reach the
  maximum ( 0.8 ncSudo) at ?100eV 1.
• ne100eV / Ptot0.5 increases as ?100eV decreases
  and saturates to 1.5 ncSudo.

14
Summary

• 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 lt 0.5
  MW/m3 and the magnetic field of lt 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 decreases 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, where the
  volume-averaged density reaches 2.2 ncSudo .

15
The End
16
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
Neutral Pressure

• 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 density
  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.

18
Maximum ltnegt in LHD

• The volume averaged electron density (ltnegt)
  exceeds 3 ? 1020 m-3, in spite of small absorbed
  power density in LHD (lt 0.5 MW/m3) compared with
  W7-AS (? 4 MW/m3) where ltnegt 4 ? 1020 m-3 was
  attained with detachment.
• At the inward shifted configuration (R 3.65 m).
• Attached plasma.
• Hollow temperature profile (transient).

19
Complete detachment and the Serpens mode
LCFS

• 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.

20
Hydrogen recombination

• During the Serpens mode, the ratio of Hg / Ha
  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 Hg signal is fluctuating as the Ha signal.
• Each of the peaks in Ha and Hg 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.