Internal collapse of the plasma during the long pulse discharge and its influence on the plasma performance HT-7 ASIPP ASIPP L.Q. Hu and HT-7 Team Institute of Plasma Physics, Chinese Academy of Sciences P.O.Box 1126, Hefei, Anhui , P.R.China ( 17th international conference on Plasma Surface Interactions in controlled fusion devices (Hefei, P.R. China, May 22-26, 2006 )

Abstract Challenge for approaching advanced SSO on HT-7 Facilities and technology for long pulse discharge on HT-7 Recent technical modifications Internal collapse during long pulse plasma discharges Internal collapse during long duration discharge by synergy of LHCD and IBW Summary Outlines HT-7 ASIPP

Abstract During the operation scenario of long pulse discharge, internal collapse of the plasma, exhibiting apparently on the radiation signals of the soft x-ray (SXR), was observed. During the event, the plasma was cooled down, accompanying strong plasma and wall interaction in the appearance of plasma radiation increase (XUV, CIII, and bremmstralung), finally the plasma thermal quench happened. There is no apparent precursor except the peaking of the SXR radiation. Such event normally causes the termination of the long pulse plasma discharge, which could be avoided or postponed by careful adjustment of the plasma displacement to ameliorate heat load of the wall. The similar phenomena were also observed in the other operation scenario of synergy discharge of lower hybrid wave current (LHCD) and ion Bernstein wave (IBW), which was in different wall state. Their common issue is peaking of the radiation before the event on the SXR radiation signals. In this paper, the phenomena are analyzed in detail and the result is discussed for two operation scenarios. HT-7 ASIPP ASIPP

HT-7 Superconducting Tokamak R = 1.22m, a = 0.27m Ip = 100~250 kA (250) B T = 1~2.5T (2.5) n e = 1~8  cm -3 (6.5) Te = 1~5 keV (4) Ti = 0.2~1.5 keV (1.5) LHCD:P=1.2MW(0.8)(2.45GHz), CW ICRF: P=1.5MW (30~110MHz), CW IBW: P=0.3MW (0.35), CW (15~30 MHz) Fuelling: 8-barrel Pellet Injector Supersonic beam injection Main Objective: High-performance plasma operation under steady-state conditions and relevant physics HT-7 ASIPP

LHCD system HT-7 *1.2MW, f=2.45GHz, 12 klystrons, CW *A grill in 3×16 multi-junction and coated with TiN film *The parallel refractive index N // =  0.3, *Response time of phase regulation: 1 ms Current drive efficiency ×10 19 A·m -2 ·W -1 ASIPP

ICRF: 1.5MW/CW/30-110MHz/HFS/LFS antennas IBW: 0.3MW/CW/15-30MHz/LFS antenna Quadruple T-type with a central feeder and short ends n // : peaked at 8 (f=30 MHz), low fraction of lower n // component Central conductor TiN coating Graphite shielding and side protector (Doped graphite with 100μm SiC gradient coating) HT-7 Reducing impurity Faraday screen Two stub tunners favorable for e-heating ICRF & IBW System ASIPP

Key issues: Heat exhaust and removal, particle control, and MHD instability ----HP PFCs (PFMs), wall recycling, plasma control, edge transport and edge turbulence Problems involved: non-inductive current drive, effective plasma heating, real-time control of plasma position, wall conditioning, overheating of PFCs, maintenance of low impurity, outgassing, and uncontrolled density…. HT-7 Challenge for approaching advanced HT-7 SSO ASIPP

HT-7 Problems in 2003 impurity outgassing uncontrollable ne ………….. impurity outgassing uncontrolled n e Thermal instabilitie s Limiter Limitation of long pulse discharge by LHCD T>1000 ℃ ASIPP

* PFCs: New toroidal limiter with new heat sink and special design (2004) IR camera and 40 embedded thermocouples (NiCr-NiSi type) on PFCs * LHCD: cryopump and guard limiter for antenna enhanced water cooling for wave source *Steady-state poloidal field power and feedback control system (2005) * Additional Plasma control loop: the magnetic flux feedback control of the transformer (b y adjusting LHW power roughly and finely) *Avoidance or suppression of MHD instability redistributing j(r) profile through active external control of LHCD or IBW  Dynamic stabilization of MHD by Ip modulation, LH or IBW P & f  Variation of electron pressure profile by movement of IBW resonant layer HT-7 Technical modifications for steady state operation since 2004 ASIPP

HT-7 Technical improvements on PFCs Heat sink Water coatArray bars Water coverage SS tube lower toroidal Upper toroidal [J. Hu, et al., Fus. Eng. Des., 2005] * Toroidal double-ring graphite limiter, in 2004 spring campaign, main Larger facing area: S~1.7m 2, total~2.4m 2 ; strong water-cooling: 20 (40) ton/h, 10MPa * Configuration: bolted structure, Heat sink, flexible graphite sheet * New design for water cooling channel Heat removal capacity: increased by a factor of 5 Capability: ~ 2MW/m 2 for SSO and at Pin~300kW Copper-chromium alloy (nominal Cu-0.5%Cr)

HT-7 ASIPP ASIPP Internal collapse Ip=60kA Tesxs(0)~1.0keV n e (0)= ×10 19 m -3 Internal collapse in the long pulse discharge

HT-7 ASIPP ASIPP Internal collapse in the the long pulse discharge

HT-7 ASIPP ASIPP #71514 Electron density evolution after Abel inversion Evolution of soft-x-ray intensity profile #71514 Before the event

HT-7 ASIPP ASIPP #71514 Electron density evolution after Abel inversion during the internal collapse Evolution of soft-x-ray intensity profile during the internal collapse #71514 During the event

HT-7 ASIPP ASIPP Internal collapse in the synergetic shot of LHCD and IBW #83838

HT-7 ASIPP ASIPP #83838 Expanded traces of the internal collapse for the shot

HT-7 ASIPP ASIPP Evolution of soft-x-ray intensity profile during the internal collapse Evolution of electron density profile during the internal collapse During the event

18 HT-7 ASIPP ASIPP

HT-7

Summary For the long pulse discharge, internal collapse of the plasma normally causes the termination of the long pulse plasma discharge, which could only be avoided or postponed by careful adjustment of the plasma displacement to ameliorate heat load of the wall. For the synergy discharge of LHCD and IBW, which was in different wall state. Their common issue is peaking of the radiation before the event on the SXR radiation signals. In this paper, the phenomena are analyzed in detail and the result is discussed for two operation scenarios. HT-7 ASIPP ASIPP

HT-7