Edge turbulence approaching the density limit in RFX-mod experiment

Abstract

The boundary region of the RFX-mod reversed field pinch experiment is strongly influenced by the value of the electron density. With increasing density the magnetic topology in the plasma core and in the plasma edge changes, together with the statistical properties of the edge turbulent fluctuations. Using the density normalized to the Greenwald density (n/n G ), two density regimes are identified: the low density regime at n/n G <0.3-0.4, and the high density regime for n/n G >0.4-0.5, without a clear threshold between them. At low density the plasma is dominated by the (m, n)=(1, −7) magnetic mode, which gives the same shape to the magnetic boundary; at higher density a strong (0, 1) mode rises. When increasing n/n G , edge fluctuations develop strong non-Gaussian tails, turbulent structures become larger and their radial velocity become measurable with the gas puff imaging diagnostic when n/n G >0.3-0.4. This phenomenology is described in detail for the two density regimes of RFX-mod.

RFX-mod2 as a flexible device for reversed-field-pinch and low-field tokamak research

“…Indeed, the improved magnetic boundary foreseen on RFX-mod2 might have an impact as well on edge electrostatic transport: despite the high level of magnetic fluctuations, a relevant fraction of particle and energy transport in a RFP can still be ascribed to general convective filamentary transport [23][24][25][26].…”

RFX-mod2 diagnostic capability enhancements for the exploration of multi-magnetic-configurations

“…Two toroidal arrays of 71 probes each (one on the high field side and one on the low field side), along with four poloidal arrays (1 of 28, 1 of 36 and 2 of 17 elements) will be installed. Such a large amount of sensors is needed to better characterize the phenomena observed in the RFP, where toroidally localized bulging due to many modes phase locking can induce localized plasma-wall interaction phenomena at high electron density [11] and the self-organized single-axis state, which emerge at high plasma current levels, are found to induce significant modulation of the edge electrostatic turbulence even at small scales (of the order of few cm). Moreover, as mentioned, RFX-mod2 is planned to be operated in a variety of shaped tokamak configurations (circular, single null, double null), where the position of X-point will be moved along the poloidal direction.…”