In H-mode (high confinement mode), electrostatic turbulence is largely suppressed. This does not mean that there is no turbulence in H-mode. The transport is just much smaller in relation to the significantly steeper gradients. The area of steep gradients is called the pedestal. Its occurrence is explained by a transport barrier. This transport barrier is caused by suppression of turbulence due to a strong shear flow. This strong shear flow is generated by the steep ion pressure gradient.

The transport in the plasma core is stiff. That is, the gradients are predetermined by critical gradients of the microinstabilities and hardly change with the heating power. Thus, it is in particular the increased densities and temperatures at the edge, which are achieved by the occurrence of the transport barrier, that lead to the better performance of the H-mode.  

The gradients in the H-mode become so steep that MHD instabilities, so-called ELMs (edge localized modes), occur. The ELMs lead to a periodic relaxation of the gradients at the edge. This is associated with strongly increased heat fluxes to the plasma wall. In a fusion reactor this may be hardly tolerable. Therefore, ELM-free regimes are currently being investigated very intensively. One example of an ELM-free regime with increased plasma confinement is the I-mode, another would be the EDA H-mode regime. Perhaps more interesting is the QCE (quasi-continuous exhaust) regime. Here, small ELMs occur continuously, keeping the gradient just below the critical condition for large ELMs to occur.

We ask ourselves

•     What type of turbulence occurs in the H-mode?
•     What is the impact of turbulence on the operational limits in the H-mode regime?
•     How does turbulence affect power exhaust?