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Operation region with Operation region with new water-cooled new water-cooled graphite limiters on the HT-7graphite limiters on the HT-7

superconducting tokamaksuperconducting tokamak

M.Asif, X.Gao andM.Asif, X.Gao and the HT-7 Team the HT-7 Team

Institute of Plasma Physics, Chinese Academy of Sciences, Institute of Plasma Physics, Chinese Academy of Sciences, P.O.Box 1126, Hefei, Anhui 230031, P.R.ChinaP.O.Box 1126, Hefei, Anhui 230031, P.R.China

(e-mail: asif@ipp.ac.cn)(e-mail: asif@ipp.ac.cn)

ASIPP

HT-7

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New graphite limiters Extended operational region

MARFE phenomena  

MHD behaviours on high density operation

Summary

OutlinesOutlines

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HT-7 Superconducting TokamakHT-7 Superconducting Tokamak

R = 1.22m, a = 0.27m

Ip = 100~250 kA (250)

BT = 1~2.5T(2.5)

ne = 1~8x1013cm-3 (6.5)

Te = 1~5 KeV (4)

Ti = 0.2~1.5K eV (1.5)

ICRF:

f = 15~30MHz, P=0.3MW(0.35)

f = 30~110MHz, P = 1.5MW

LHCD:

f = 2.45GHz, P = 1.2MW(0.8)

Fuelling:

Pellet injector

Supersonic beam injection

Main Goal: Advanced Steady state operation and related physics

Based on the understanding of plasma surface interactions, a new GBST1308 (1%B, 2.5%Si, 7.5%Ti) doped graphite was used as limiter material, which is expected to take the important rule for advanced steady state operation on the HT-7 superconducting tokamak.

-high thermal conductivity up to 210 W/m.K-good thermal shock resistance can withstand 6 MW/m2 -20% of first wall was covered by the graphite material (~ 2.34 m2)

All carbon titles were coated with 100 m SiC functional gradient coating.

New graphite limitersNew graphite limiters

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-One of its main objectives is to study the properties of steady-state plasma discharges.

-Before 2003 experimental campaion, HT-7 was equipped with one high field side belt limiter and two poloidal graphite limiters with active coolant stainless steel heat sink.

-The longest discharge pulse with 63.95 seconds has been successfully obtained after boronization in the 2003 spring campaign.

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-But in many plasma discharges which were longer than 30s, the temperatures measured at the graphite tile surface were over 1000OC.

-Abundant impurities were induced by chemical sputter, therefore long pulse-length plasma discharges couldn’t be sustained

-To promote plasma duration, and to provide a reference to design easy maintenance high performance PFCs toroidal limiters were designed in the last year and mounted in the spring of 2004

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-The basic functions of the toroidal limiter are:

Providing a heat removal capacity of 5 times higher than that of previous the HT-7 poloidal limiter.

Reducing the metal impurities produced by energetic particles

Reduce the average heat flux with large limiter surface

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Up to 240 s of long pulse plasmas have been achieved in Up to 240 s of long pulse plasmas have been achieved in the HT-7 superconducting tokamak on 2004the HT-7 superconducting tokamak on 2004

A new set of actively cooled toroidal double-ring graphite limiters were developed at bottom and top of the vacuum vessel on HT-7 tokamak in 2004

Long pulse operation on HT-7

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1994 1996 1998 2000 2002 2004Year

Puls

e du

rati

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s)

Di schargeMax- l ength

1994:First plasma1996:Development of plasmacontrol and diagnostics system1998:Feedback control forplasma current, displacementand density1999:RF wall condition2000:Upgrade data acquisition& diagnostics2001:Ferritic steel to reduceripple loss; New LHCDantenna system2002:Water cooling poloidaland belt graphite limiters2003:Water cooling toroidaldouble-ring graphite limiters

Poloidal limiter

Toroidal limiter

Belt limiter

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New graphite limiters Extended operational region

MARFE phenomena  

MHD behaviours on high density operation

Summary

OutlinesOutlines

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1997: the operation region of the HT-7 ohmic discharges were limited by the wall condition and the HT-7 controlling system;

1998: during the Spring experimental campaign in 1998, a feedback control system for plasma current, density and displacement was developed and put into daily operation;

1999: RF boronization and the RF siliconization leads the extension of operational parameter space of HT-7.

Hugill plot of the data shows the experiment limit corresponding to an inverse slope neRqa/B

= 1.61x1020 Wb-1: this line also represents the Greenwald limit ne/J = 1, where ne and J are

measured in units of 1020 m-3 and MA/m2, respectively.

Operational regionOperational region

Metallic wall:Molybdenum limiter and stainless steel liner

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Toroidal double-ring graphite limiters extended the operational space on HT-7

Hugill plot with new actively cooled toroidal double-ring graphite limiters in 2004 and poloidal molybdenum limiter in 1999 on HT-7 tokamak

Extended region with new graphiteExtended region with new graphite limiters

molybdenum limiters in 1999

graphite limiters in 2004

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Density behaviors for molybdenum and Density behaviors for molybdenum and graphitegraphite limiters

Mo: molybdenum limiters C: graphite limiters

Mo limiters: the peaking factor of electron density profile is strongly proportional to the line average densityC limiters: the lower peaking factor, the achieved density <ne> up to 8E13 cm-3

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C: graphite limitersMo: molybdenum limiters

Mo limiters: high density plasma appeared only in very narrow region of lower plasma current Ip = 120 ~ 140 kAC limiters: high density operational region was extended to high current region

Higher density was achieved on high plasma Higher density was achieved on high plasma current discharges by new graphitecurrent discharges by new graphite limiters

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New graphite limiters Extended operational region

MARFE phenomena  

MHD behaviours on high density operation

Summary

OutlinesOutlines

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•In HT-7 tokamak, the onset of MARFE usually appeared beyond a critical density.

•The occurrence and location of a MARFE are identified by different diagnostic system in the HT-7.

•Different location of MARFE is observed for different limiters.

MARFE phenomenaMARFE phenomena

Mo: molybdenum limiters C: graphite limiters

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The critical density of MARFE onset is observed in the region of 0.8 ~ 1.02 Greenwald factor with C limiter in the HT-7

Observation of MARFE formation with C limiter

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Line and section averaged densitySpace-time distribution of averaged electron density

Density profile during MARFE formation

MARFE appeared at t = 220 ms (66361#)

Asymmetric and peaked density profile is observed

MARFE onsetMARFE onset

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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

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New graphite limiters Extended operational region

MARFE phenomena  

MHD behaviours on high density operation

Summary

OutlinesOutlines

In the HT-7, the most dangerous MHD instability is the m/n = 2/1 resistive tearing mode, which is driven by the plasma current density gradient.

Disruption appears to be the production of a strong MHD instability in the high density operation.

The precursor of m = 2 mode is often observed before the disruption.

MHD behaviours on high density operation

21The MHD activity (m=2 mode ) on high density operation

MHD m=2 Mode is observed near Greenwald limit which leads to a disruption

Special structure of

the mode (m=2) development

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Summary• The operational region and density behaviours were studied with new s

et of actively cooled toroidal double-ring graphite (C) limiters, and compared with molybdenum (Mo) limiter. The progress on the extension of the HT-7 discharge operation region was reported.

• Extended high-density region at the high-current, low-qa were achieved

with new C limiters.

• The different behaviors of electron density profiles were studied between C and Mo limiters. The peaking factor of electron density profile is weakly proportional to the line average density with C limiters.

• The critical density of MARFE onset is observed in the region of 0.8 ~ 1.02 Greenwald factor with C limiter in the HT-7.

• Under same injected power, the critical density of MARFE onset with C limiter is much higher than with Mo limiter

• The precursor of MHD activity, m = 2 mode is often observed before the disruption on high density operation in the HT-7.

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Thanks!Thanks!

HT-7