Interior microturbulence characteristics during H- and VH-mode in the DIII-D tokamak

Measurements of interior microturbulence in the DIII-D tokamak have indicated that during double null discharges with low radiated power and strong neutral beam injection power, interior microturbulence is observed to reduce prior to the onset of the so-called VH-mode, when the peak energy confinement time is greater than two times that predicted by the JET/DIII-D H-mode scaling law. The core broadband turbulence level begins to modulate at frequencies of hundreds of Hertz, and over 100-200 ms following the L → H transition, the modulation period increases while the average fluctuation level reduces. The question arises as to whether shear flow stabilization might be reducing core turbulence in VH-mode and reducing the fluctuation driven energy transport. In recent experiments, it has been shown that independently reducing Er in the core limits the fluctuation reduction and prevents access to the VH-mode regime. (author)