The hybrid scenario [1] is a candidate for stationary high - fusion gain tokamak operation in the DT phase of JET, ITER and DEMO. To obtain such performance, the normalized pressure ? N must be maxi mized, which requires operating near or above ideal MHD no - wall limits. New experimental findings show how these limits can affect hybrid operation. Even if hybrids are mainly limited by neoclassical tearing modes, proximity to the no - wall limit lea ds to 3 D field amplification that affect s plasma profiles, e.g. rotation braking is observed in AUG throughout the plasma and peaks in the core. As a result , even the small AUG error fields [2] are amplified and their effect s become visible. To quantify such effe cts, AUG measured the response to 3D fields applied by 8x2 non - axisymmetric coils as ? N approaches the no - wall limit. The full n=1 response profile and poloidal structure are measured by a suite of diagnostics and agree well with linear MHD simulations , re vealing a characteristic feature of hybrids: while the edge response is d ue to a marginally - stable external kink, as in other types of high - ? N plasmas, hybrids additionally have large core response due to q 0 being just above 1. A helical core distortion of a few cm forms and causes the observed core rotation braking. In similar experiments, DIII - D measure d the effect of this helical core on the internal current profile, providing useful information to understand the physics of "magnetic flux pumping", i.e. anomalous current redistribution by MHD modes that keeps q 0 >1 [3]. Thanks to flux pumping, a broad current profile is maintained even with large on - axis current drive, enabling fully non - inductive operation with ? N up to 4 [4]. In summary, 3D fields can affect high - ? hybrid plasmas in various ways and should be controlled to optimize this scenario. Th ese experiments also provide crucial data to validate MHD codes and understand flux pumping physics, necessary step s to e xtrapolat e the hybrid scenario to future devices.
Impact of ideal MHD stability limits on high-beta hybrid operation
Piovesan P;Marrelli L;Terranova D;
2016
Abstract
The hybrid scenario [1] is a candidate for stationary high - fusion gain tokamak operation in the DT phase of JET, ITER and DEMO. To obtain such performance, the normalized pressure ? N must be maxi mized, which requires operating near or above ideal MHD no - wall limits. New experimental findings show how these limits can affect hybrid operation. Even if hybrids are mainly limited by neoclassical tearing modes, proximity to the no - wall limit lea ds to 3 D field amplification that affect s plasma profiles, e.g. rotation braking is observed in AUG throughout the plasma and peaks in the core. As a result , even the small AUG error fields [2] are amplified and their effect s become visible. To quantify such effe cts, AUG measured the response to 3D fields applied by 8x2 non - axisymmetric coils as ? N approaches the no - wall limit. The full n=1 response profile and poloidal structure are measured by a suite of diagnostics and agree well with linear MHD simulations , re vealing a characteristic feature of hybrids: while the edge response is d ue to a marginally - stable external kink, as in other types of high - ? N plasmas, hybrids additionally have large core response due to q 0 being just above 1. A helical core distortion of a few cm forms and causes the observed core rotation braking. In similar experiments, DIII - D measure d the effect of this helical core on the internal current profile, providing useful information to understand the physics of "magnetic flux pumping", i.e. anomalous current redistribution by MHD modes that keeps q 0 >1 [3]. Thanks to flux pumping, a broad current profile is maintained even with large on - axis current drive, enabling fully non - inductive operation with ? N up to 4 [4]. In summary, 3D fields can affect high - ? hybrid plasmas in various ways and should be controlled to optimize this scenario. Th ese experiments also provide crucial data to validate MHD codes and understand flux pumping physics, necessary step s to e xtrapolat e the hybrid scenario to future devices.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
