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

1. HT-7 ASIPP Operation region with new water-cooled graphite limiters on the HT-7 superconducting tokamak M.Asif, X.Gao and the HT-7 Team Institute of Plasma Physics, Chinese Academy of Sciences, P.O.Box 1126, Hefei, Anhui 230031, P.R.China (e-mail: asif@ipp.ac.cn)

2. Outlines New graphite limiters Extended operational region MARFE phenomena MHD behaviours on high density operation Summary

3. HT-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

4. New graphite limiters 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 mm SiC functional gradient coating.

5. -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.

6. -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

7. -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

8. Up to 240 s of long pulse plasmas have been achieved in the 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

9. Outlines New graphite limiters Extended operational region MARFE phenomena MHD behaviours on high density operation Summary

10. Operational region 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. Metallic wall: Molybdenum limiter and stainless steel liner

11. Extended region with new graphite limiters Toroidal double-ring graphite limiters extended the operational space on HT-7 graphite limiters in 2004 molybdenum limiters in 1999 Hugill plot with new actively cooled toroidal double-ring graphite limiters in 2004 and poloidalmolybdenum limiter in 1999 on HT-7 tokamak

12. Density behaviors for molybdenum and graphite limiters Mo: molybdenum limiters C: graphite limiters Mo limiters: the peaking factor of electron density profile is strongly proportional to the line average density C limiters: the lower peaking factor, the achieved density <ne> up to 8E13 cm-3

13. Higher density was achieved on high plasma current discharges by new graphite limiters Mo: molybdenum limiters C: graphite limiters Mo limiters: high density plasma appeared only in very narrow region of lower plasma current Ip = 120 ~ 140 kA C limiters: high density operational region was extended to high current region

14. Outlines New graphite limiters Extended operational region MARFE phenomena MHD behaviours on high density operation Summary

15. MARFE phenomena • 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. Mo: molybdenum limiters C: graphite limiters

16. Observation of MARFE formation with C limiter 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

17. Density profile during MARFE formation MARFE appeared at t = 220 ms (66361#) MARFE onset MARFE onset Space-time distribution of averaged electron density Line and section averaged density Asymmetric and peaked density profile is observed

18. 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

19. Outlines New graphite limiters Extended operational region MARFE phenomena MHD behaviours on high density operation Summary

20. MHD behaviours on high density operation 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.

21. MHD m=2 Mode is observed near Greenwald limit which leads to a disruption Special structure of the mode (m=2) development The MHD activity (m=2 mode ) on high density operation

22. Summary • The operational region and density behaviours were studied with new set 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.

23. HT-7 Thanks!