9th TTF Spain September 11, 2002 B. J. Peterson, NIFS, Japan page 1 Radiative Collapse and Density Limit in the Large Helical Device B. J. Peterson (for the LHD Experiment Group) with major contributions from Yuhong Xu, H. Yamada, N. Ashikawa, K. Yamazaki, T. Tokuzawa and K. Nishimura 9th Transport Task Force Workshop September 11, 2002 Cordoba, Spain National Institute for Fusion Science, Toki-shi , JAPAN

9th TTF Spain September 11, 2002 B. J. Peterson, NIFS, Japan page 2 Introduction to LHD Comparison of Radiative Collapse with Normal Termination Characteristics of Asymmetric Radiative Collapse Evolution of Asymmetric Radiative Collapse Asymmetric Radiative Collapse and Density Limit Relation of improved confinement to increased density limit Conclusions Outline

9th TTF Spain September 11, 2002 B. J. Peterson, NIFS, Japan page 3 Large Helical Device National Institute for Fusion Science, Toki, Japan  Heliotron l=2, m=10  Superconducting coils B = 3 T  Large scale R/a = 3.9 m/0.6 m  V = 30m 3 Three-dimensional magnetic field, plasma and vacuum vessel

9th TTF Spain September 11, 2002 B. J. Peterson, NIFS, Japan page 4 Comparison of Normal Termination and Radiative Collapse

9th TTF Spain September 11, 2002 B. J. Peterson, NIFS, Japan page 5 Comparison of discharge waveforms for TD(thermal decay) and RC (radiative collapse) W p, ECH, NBI timings Line-averaged density, average T e and gas puff P rad, transport loss power and deposited NBI power dW p /dt =P dep –P rad –P tran Emission intensity of O V and C III in/outboard bolometer channel’s brightness n imp L(T e ) = P rad /n e TD shot #5368RC shot #3574

9th TTF Spain September 11, 2002 B. J. Peterson, NIFS, Japan page 6 Rapid increase of P rad / indicates radiative thermal instability leading to collapse (a), (c) The mean plasma temperature (b), (d) The ratio of P rad /   n z L z (T e )  L z (T e ) No TI onset TI onset

9th TTF Spain September 11, 2002 B. J. Peterson, NIFS, Japan page 7 Clear difference is shown in the total radiation profiles between the Thermal Decay and Radiative Collapse (a) Contour plot of the time evolution of chord-integrated radiation brightness of TD discharge. (b) Contour plot of the time evolution of chord-integrated radiation brightness of RC discharge. (c) Expanded time trace for (b). symmetric thermal decay discharge asymmetric radiative collapse discharge

9th TTF Spain September 11, 2002 B. J. Peterson, NIFS, Japan page 8 Characteristics of Asymmetric Radiative Collapse

9th TTF Spain September 11, 2002 B. J. Peterson, NIFS, Japan page 9 Brightness profiles show inboard asymmetry Vertically elongated cross-section Horizontally elongated cross-section LHD has a helical magnetic field and a helical vacuum vessel What is toroidal variation of asymmetric radiative collapse? Symmetric at horizontally elongated cross-section Peaking inboard, center or outboard? Try to answer in future with imaging bolometers

9th TTF Spain September 11, 2002 B. J. Peterson, NIFS, Japan page 10 Imaging Bolometer’s FoVs at mid-plane Upper FOV IRVB Tangential FOV IRVB LCFS and Ergodic Edge

9th TTF Spain September 11, 2002 B. J. Peterson, NIFS, Japan page 11 Tangential view Top view Imaging Bolometers show axisymmetric collapse Experimental data Symmetric Hollow Profile Experimental data Asymmetric Collapse Reconstructed Image S(  ) Reconstructed Image (    )

9th TTF Spain September 11, 2002 B. J. Peterson, NIFS, Japan page 12 Symmetric radiation with gas puff AXUV Diode Arrays in semi- tangential cross-section Reconstructed 2-D Images (tomography courtesy Y.Liu, SWIPP, China) Asymmetric collapse 2-D AXUVD arrays also shows inboard collapse outin out in out Gas Puff

9th TTF Spain September 11, 2002 B. J. Peterson, NIFS, Japan page 13 Coincidence between asymmetric radiation and FIR signal degradation like in MARFE Radiation asymmetric on inboard side Corresponds spatially and temporally to degradation of FIR interferometer signal Degradation presumably due to beam refraction due to high density gradients Similar to phenomenon observed in MARFE

9th TTF Spain September 11, 2002 B. J. Peterson, NIFS, Japan page 14 Evolution of Asymmetric Radiative Collapse

9th TTF Spain September 11, 2002 B. J. Peterson, NIFS, Japan page 15 asymmetry preceded by thermal instabilty, not triggered by recycling Steady state phase I Followed by thermal instability phase II Followed by asymmetric radiation phase III H  increases rapidly after asymmetry in radiation indicating accelerating but not triggering role for H Phase I II III

9th TTF Spain September 11, 2002 B. J. Peterson, NIFS, Japan page 16 Phase I before TI onset Te decreases Phase II between TI and AR occurrences Plasma shrinks Phase III after AR onset Asymmetry appears Time evolution of temperature profiles across the three collapse phases (I, II, III) in the RC discharge

9th TTF Spain September 11, 2002 B. J. Peterson, NIFS, Japan page 17 Prior to the AR onset During the AR phase Development of the temperature asymmetry in AR phase of the RC discharge. K par ~ T e 7/2 T e emfp T e asymmetry

9th TTF Spain September 11, 2002 B. J. Peterson, NIFS, Japan page 18 Asymmetric Radiative Collapse and Density Limit

9th TTF Spain September 11, 2002 B. J. Peterson, NIFS, Japan page 19 RC occurs at density limit similar to other helical devices Plot of as a function of volume-averaged absorbed power and magnetic field, (P abs B/V) 0.5, for both TD (solid circles) and RC (open triangles) discharges. The data are taken at the time of maximum plasma stored energy (no pellet injection). Upper limit of density for RC shots, (10 20 m -3 )  1.8(P abs B/V) 0.5 Scaling plot of Heliotron-E, (10 20 m -3 )  1.1(P abs B/V) 0.5 W7-AS: (10 20 m -3 )  1.46 (P abs /V) 0.48 B 0.54

9th TTF Spain September 11, 2002 B. J. Peterson, NIFS, Japan page 20 Collapse occurs at higher P rad /P abs Collapse No collapse mixed However higher fractions with no collapse have been observed in discharges with significant high Z core radiation

9th TTF Spain September 11, 2002 B. J. Peterson, NIFS, Japan page 21 Improved Confinement and Density Limit

9th TTF Spain September 11, 2002 B. J. Peterson, NIFS, Japan page 22 Reference : International Stellarator Scaling 95 derived from medium-sized helical experiments (ATF, Heliotron-E, CHS, W7-AS) Near gyro-Bohm Similar to ITER ELMy-H scaling Improved confinement with inward shift of axis Energy Confinement in LHD Courtesy H. Yamada

9th TTF Spain September 11, 2002 B. J. Peterson, NIFS, Japan page 23 Clear dependence on power Pressure (nT) can be increased by increase of density as well as heating power.  unlike tokamak H-mode Pressure (nT) is not limited.  MHD limit has not been observed Edge temperature is systematically high at R ax =3.6m.  Geometrical effect can be seen. Improvement of transport by inward shifted configuration R ax =3.6 m is found in edge temperature. Qualitatively, dependence on n and T can be seen in outward shifted cases. Confinement improvement seen in T e at edge Courtesy H. Yamada

9th TTF Spain September 11, 2002 B. J. Peterson, NIFS, Japan page 24 Dependence of density limit on R axis Data shows higher density with inward shift Consistent with observations of improved confinement Higher edge temperature prevents thermal instabilty R ax = 3.6 m R ax > 3.7 m Courtesy K. Nishimura

9th TTF Spain September 11, 2002 B. J. Peterson, NIFS, Japan page 25 Asymmetric Radiative Collapse terminates LHD plasmas: at higher P rad /P abs At a density limit 60% higher than stellarator scaling With n e, T e asymmetry Preceded by thermal instability Accelerated but not triggered by recycling 2 x P rad, 2 x IRVB, n e, T e, 2-D AXUVD show axisymmetry asymmetry is MARFE - like, but not sustainable at a higher limit for the inward shifted case –consistent with higher edge temperature and improved confinement Conclusions