Transcript MARFE Phenomena Observation on HT

HT-7
ASIPP
The study of MARFE during long
pulse discharges in the HT-7
tokamak
W.Gao, X.Gao, M.Asif,
Z.W.Wu, B.L.Ling, and J.G.Li
Institute of Plasma Physics, Chinese Academy of Sciences
HT-7
Outline
 Introduction
 The MARFE behavior with Mo Limiter
 The comparison of MARFE condition with Mo
limiter and C Limiter
 The LHCD discharge and relevant MARFE
Behavior
 Conclusion
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Introduction
• MARFE-----Multifaceted asymmetric radiation from the edge, is a
kind of instability. The formation of MARFE is greatly related to
the thermal instabilities. The observed sequence of MARFE can be
divided into several steps: (1) The radiation impurities in the edge
accumulate and form a poloidally uniform impurities band; (2) As
the density of the impurities increase, the band of impurities goes
stable and forms MARFE; (3) A further increase in density usually
leads to a disruptive collapse of the temperature profile, but under
certain conditions evolves into a cool poloidally symmetric edge
distribution in which virtually all of the plasma heating power is
radiated and the plasma is detached from the limiter.
• MARFE usually occurs under high Ne or high Zeff conditions in the
HT-7 tokamak. It is often observed at the early stage of the
discharge campaign, before wall conditioning, or high density
discharge, and also observed under long pulse discharge.
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How to observe the MARFE
•
To observe MARFE, an effective method is to find the sudden
modifications of radiation and optical spectrum signals. It is poloidally
asymmetric in spatial distribution, because some optical spectrum signals
changed while others do not.
•
MARFE can also be observed through inverse of XUV or H-alpha Array
signals .
•
An important parameter is summarized to analyze the occurrence of
1/ 2
f GW , where f GW  ne / nGW
MARFE: Z eff
Diagnostics of MARFE study for different limiters:
molybdenum and graphite
Mo: molybdenum limiters
C: graphite limiters
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The occurrence condition of MARFE with Mo limiter
•
MARFE often occurs in the
early stage omhic discharge of
the campaign. The plasma
current is about 100kA, loop
voltage is 3V,
Bt=1.5T, Zeff=6-7,Te=400600eV,Ti=200-300eV.
In the image, a sudden
modification of Da(ch.2), XUV,
Bresms signals can be seen,
while The Da(ch.34) signal
remain unchanged, which
comes a conclusion that it is a
asymmetrical emission. Also,
there is no MHD behavior and
alteration of Vloop.
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Sequence of the formation of MARFE
(1) An increase of impurities and plasma density before MARFE occurs;
(2) The precursor of the increased Brems(Zeff) signal at 251ms;
(3) The onset of MARFE is observed at 254.4ms.
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•
•
A dependence of critical value of the Zeffne(0.71a) just before the
MARFEs onset and the input ohmic power on HT-7.
MARFE occurs at values of Zeff½f in the 0.5~0.7 where f = pa2ne/Ip
(1020 MA-1m-1) is Greenwald factor.
GW
GW
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+
•
The improved confinement plasma: (a) central chord average
density, (b) central vertical chord soft X ray intensity, (c)
central chord region Da emission, (d) OII line emission along a
line of sight crossing the plasma center (to see Fig.1), (e) CIII
emission from channel 10.
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The new C limiter and a typical MARFE
Comparison between Mo and C limiters
Molybdenum limiter: the critical factor of MARFE onset is 0.5 ~ 0.7;
Graphite limiter: the critical factor of MARFE onset is 0.89 ~ 1.14; Under same
injected power, the critical density with C limiter is much higher than with Mo
limiter
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The long pulse discharge on HT-7
•
Long-pulse discharge study is one of the purposes of HT-7 tokamak and has
been carried out. Up to 2005 Autumn campaign, the long-pulse discharge
duration reached to 5 min based on the understanding of plasma wall
interactions.
•
Lower hybrid current drive (LHCD) experiments have been carried out to
achieve high performance for long pulse operation in the HT-7 superconducting tokamak. Multifaceted asymmetric radiation from the edge
(MARFE) phenomena, is observed in LHCD plasmas when PLH> 160 kW with
high edge safety factor q(a).
•
It shows that the critical density of MARFE onset is observed in the region of
zeff1/2 fGW = 0.6-0.9.
•
These MARFEs generally appear to have the same characteristics as high
density MARFEs and are positionally stable throughout the LHCD pulse.
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A typical MARFE in long-pulse discharge
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The critical parameter for
LHCD discharge MARFE
(1)
(2)
A dependence of critical value of the Zeffne(0.71a) just before the
MARFEs onset and the input ohmic power and LHCDPower on
HT-7.
The MARFE occurs at values of Zeff½f in the 0.6~0.9
GW
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(1) The asymmetric density profile of the MARFE onset during
LHCD plasma in the HT-7 tokamak.
(2) Radiation power profile evolution during MARFE formation.
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Conclusions
• The MARFE instability is correlated with both density and the Zeff.
The MARFE can cause plasma disruption, or sometime improve the
confinement on HT-7 tokamak.
• With Mo limiter, MARFE usually occurs upon the critical factor
Zeff1/2 fGW=0.5-0.7, and Zeff = 6-7, and Greenwald factor f GW > 25%.
• With C limiter, the condition that MAFE occur changes a lot, the
critical factor Zeff1/2 fGW =0.9-1.2, and Zeff = 1-4, and Greenwald
factor f GW = 50-90%, which shows a more stable edge environment.
• MARFE can often terminate the long-pulse discharge in LHCD
plasmas when PLH> 160 kW. The asymmetric radiation power
profile and density profile during the MARFE formation were
studied in detail on the HT-7 long-pulse discharges.
Thank you!