Density turbulence during Wendelstein 7-X confinement transitions as measured with phase contrast imaging

In high performance plasmas at the Wendelstein 7-X stellarator, turbulence plays a large and often dominant role in determining plasma confinement, limiting achievable plasma profile gradients for given heating and fueling sources and magnetic configuration. Plasma performance is maximized by tailoring these sources to reach record values of ion temperature, plasma beta or confined energy. Confinement is improved chiefly when density gradients are increased after cryogenic pellet or neutral beam injection and particle transport is reduced. Additional electron cyclotron heating is then more effective at raising the ion temperature, though rising particle transport often quickly forces the density gradient back to the pre-injection state.

Confinement quality is strongly correlated with fluctuations in the kHz to MHz range observed by PCI, which measures continuous, absolutely calibrated and poloidally resolved density fluctuations. Broadband fluctuations from the ion temperature gradient region are anti-correlated with density peaking, providing evidence that reduction of ITG turbulence is responsible for increased performance. A higher velocity mode is present whenever the density gradient decays, hinting at additional turbulence modes responsible for strong particle transport.