The study of MARFE during long pulse discharges in the HT-7 toka

makW.Gao, X.Gao, M.Asif,

Z.W.Wu, B.L.Ling, and J.G.Li

Institute of Plasma Physics, Chinese Academy of Sciences

ASIPP

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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 MARFE: , where GWeff fZ 2/1

GWeGW nnf /

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Diagnostics of MARFE study for different limiters: molybdenum and graphite

Mo: molybdenum limiters C: graphite limiters

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=400-600eV,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 MARF

Es onset and the input ohmic power on HT-7.

• MARFE occurs at values of Zeff½fGW in the 0.5~0.7 where fGW= a2ne/Ip (1

020 MA-1m-1) is Greenwald factor.

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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 D 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

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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

Comparison between Mo and C limiters

The long pulse discharge on HT-7• Long-pulse discharge study is one of the purposes of HT-7 tokamak and has b

een 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 super-conducting 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 zeff

1/2 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

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(1) 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.

(2) The MARFE occurs at values of Zeff½fGW in the 0.6~0.9

(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 Zef

f1/2 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 Zeff

1/2 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.

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Thank you!