32 / 2021-04-09 21:47:50

A Mean Field Model of the L-H Transition in a Stochastic Magnetic Field

stochastic field, turbulence, poloidal flow, toroidal flow, particle flux , mean field

The resonant magnetic perturbation (RMP) is one of powerful means to mitigate or control ELMs and thus eliminate transient substantial heat loads on plasma facing components. However, one tradeoff for this benefit is that the RMP probably causes a higher power threshold for the LàH transition. Motivated by this, we present a new mean field model including stochastic magnetic field effect which may explain the increased LàH power threshold.

        The model consists of coupled equations for mean radial electric field, poloidal rotation, toroidal rotation, density ion temperature and turbulence intensity. Novel features include a Maxwell stress on poloidal rotation Vθ induced by the stochastic magnetic field. This tends to work against Vθ, since it has the same phase as, but sign opposite to, the Reynolds stress. The effect of Maxwell stress on Vφ can induce force through radial current across separatrix and reverse the toroidal rotation on the stochastic layers. Stochastic magnetic fields induce a non-diffusive, parallel flow gradient driven particle flux, which may explain RMP-induced pump-out. In addition, stochastic magnetic fields can degrade the coherence of Vr and Vθ in the Reynolds stress, thus weakening the LàH trigger mechanism. Finally, stochastic fields necessarily carry a portion of the ion heat flux.

        Results so far indicate that brbθ≠0 breaks ambipolarity, so both amplitude and profile of br2 are significant, Jr (induced by brbθ) drives an intrinsic toroidal torque, V'E ensures that brbθ opposes VrVθ, and that br2 can modify Ti and n profiles. The dimensionless parameter that quantifies the increment in power threshold is identified as α≡[br2q]/(ρ*2βϵ) and this is used to assess the impact of stochastic field on the L-H transition. Results indicate that the routine RMP strength of br~10-4 is sufficient to inhibit the transition. Ongoing work is concerned with quantifying power threshold dependencies. Both a 0-D and 1-D numerical solutions of the model are ongoing.