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Results from Helical Axis Stellarators Boyd Blackwell, H-1 National Facility Australian National University Thanks to: Enrique Ascasibar and TJ-II Group Prof. Obiki and Heliotron-J Group David Anderson and HSX Crew and the H-1 Team

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Spitzer 1951 Spitzer figure-8 stellarator “spatial axis” which produces rotational transform magnetic hill unstable to interchange Koenig 1955 Koenig helical winding/axis: = 1 single pair of helices Spitzer 1956 Johnson et al 1958 Spitzer 1956 possibility of shear stabilization for higher order windings = 2,3 demonstrated theoretically (resistivity 0) Johnson et al 1958 Furth, Killeen, Rosenbluth 1963 Furth, Killeen, Rosenbluth 1963 found resistive interchange instability possible even at low resistivity for small scale lengths several configurations proposed with magnetic well (average minimum B) found including heliac (straight). Exploitation of avg. min B regions of bad curvature possible ballooning instability Development of Helical Axis Stellarators -I +I = 1

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Nagao 1977 Nagao 1977 Asperator NP: toroidal helical axis stellarator (+extra helical windings) Yoshikawa Yoshikawa toroidal heliac HX-1 proposal Blackwell, Hamberger Blackwell, Hamberger SHEILA prototype heliac (0.2M, 0.2T, m 3 ) Tohoku, H-1 and TJ-II and heliacs proposed - and HBTX linear heliac UW - Operation in 1987 (Tohoku, Sendai) 1992 (H-1) and 1996(TJ-II, Spain) 1988 Nuhrenberg and Zille Nuhrenberg and Zille - quasi-helical symmetry - restore outstanding features of straight heliac. [transport, beta limit(Monticello et. al 1983)] Heliotron-J - combine heliotron/torsatron with advances in transport (optimise bumpy cpt, quasi-isodynamic) 1999 H elically S ymmetric E X periment 1999 H elically S ymmetric E X periment first quasi-symmetric experiment exploit high iota, N-m scaling Development of Helical Axis Stellarators II

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Canberra, Australia external vacuum vessel CIEMAT, Madrid internal vessel, upgrade to NBI IAE Kyoto “ inverted heliac” bumpy field cpt TSL, Madisoncontrolled “spoiling” of symmetry. Device Type Aspect Iota H-1 Heliac H-1 Heliac 3 period heliac, toroidal > helical 5 TJ-II Heliac TJ-II Heliac 4 period heliac, helical > toroidal 7 Heliotron J Heliotron J helical axis heliotron (TFC + =1) HSX HSX modular coils, helical symmetry Helical Axis Stellarators 2000

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Summary - Future Confinement in heliacs ~ISS95 or better (2keV, ~5ms). Ion beam probe to elucidate role of Eradial in improved confinement New configurations with improved neoclassical transport initial results promising, await full analysis HSX/H-J can compare similar configurations with vastly different neoclassical transport predictions. Confinement transitions possible at low power, many similarities with large devices/powers. Investigate effect of E-field imposed by localised ECH. No serious impurity accumulation problems yet. Real test when the ions are strongly heated No fatal instabilities observed yet. Several devices should have the heating capacity to test ballooning limits, at least in degraded configurations (consequence of flexibility).

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