A simple, low-dimensional model of edge localized mode (ELM) phenomena
is presented. ELM dynamics are determined by the interaction of few b
asic processes at the edge of tokamak plasma; these include the evolut
ion of magnetohydrodynamic (MHD) pressure gradient driven instabilitie
s, the low-high (L-H) transition (mediated by electric field shear ind
uced suppression and generation), the fueling of the edge by neutral p
articles, and edge heating by thermal flux from the core plasma. In th
e parameter regime characteristic of an H-mode plasma, the model exhib
its a transition to stationary relaxation oscillations (i.e., stable l
imit cycle behavior) corresponding to ELMs. The dependence of ELM freq
uency, amplitude, etc. on the heating power P-in and other control par
ameters is studied. The transition from giant to grassy ELM behavior a
ppears as a consequence of increasingly marginal stability to ideal ba
llooning as P-in increases. Several details of the dynamics of the ELM
onset and behavior close to the L-H transition threshold are revealed
, as well. (C) 1995 American Institute of Physics.