title: Nonlinear Excitation of Subcritical Instabilities in a Toroidal Plasma.
abstract: In a collisionless plasma, it is known that linearly stable modes can be destabilized (subcritically) by the presence of structures in phase space. However, nonlinear growth requires the presence of a seed structure with a relatively large threshold in amplitude. We demonstrate that, in the presence of another, linearly unstable (supercritical) mode, wave-wave coupling can provide a seed, which is significantly below the threshold, but can still grow by (and only by) the collaboration of fluid and kinetic nonlinearities. By modeling the subcritical mode kinetically, and the impact of the supercritical mode by simple wave-wave coupling equations, it is shown that this new kind of subcritical instability can be triggered, even when the frequency of the supercritical mode is rapidly sweeping. The model is applied to the bursty onset of geodesic acoustic modes in a LHD experiment. The model recovers several key features such as relative amplitude, time scales, and phase relations. It suggests that the strongest bursts are subcritical instabilities, driven by this mechanism of combined fluid and kinetic nonlinearities.
authors.affiliation: Research Institute for Applied Mechanics, Kyushu University, Kasuga 816-8580, Japan.
authors.forename: M
authors.initials: M
authors.lastname: Lesur
authors.affiliation: National Institute for Fusion Science, Toki 509-5292, Japan.|Research Center for Plasma Turbulence, Kyushu University, Kasuga 816-8580, Japan.
authors.forename: K
authors.initials: K
authors.lastname: Itoh
authors.affiliation: National Institute for Fusion Science, Toki 509-5292, Japan.
authors.forename: T
authors.initials: T
authors.lastname: Ido
authors.affiliation: National Institute for Fusion Science, Toki 509-5292, Japan.|SOKENDAI (The Graduate University for Advanced Studies), Toki 509-5292, Japan.
authors.forename: M
authors.initials: M
authors.lastname: Osakabe
authors.affiliation: National Institute for Fusion Science, Toki 509-5292, Japan.|SOKENDAI (The Graduate University for Advanced Studies), Toki 509-5292, Japan.
authors.forename: K
authors.initials: K
authors.lastname: Ogawa
authors.affiliation: National Institute for Fusion Science, Toki 509-5292, Japan.
authors.forename: A
authors.initials: A
authors.lastname: Shimizu
authors.affiliation: Research Institute for Applied Mechanics, Kyushu University, Kasuga 816-8580, Japan.|Research Center for Plasma Turbulence, Kyushu University, Kasuga 816-8580, Japan.
authors.forename: M
authors.initials: M
authors.lastname: Sasaki
authors.affiliation: National Institute for Fusion Science, Toki 509-5292, Japan.|SOKENDAI (The Graduate University for Advanced Studies), Toki 509-5292, Japan.
authors.forename: K
authors.initials: K
authors.lastname: Ida
authors.affiliation: Research Institute for Applied Mechanics, Kyushu University, Kasuga 816-8580, Japan.|Research Center for Plasma Turbulence, Kyushu University, Kasuga 816-8580, Japan.
authors.forename: S
authors.initials: S
authors.lastname: Inagaki
authors.affiliation: Research Institute for Applied Mechanics, Kyushu University, Kasuga 816-8580, Japan.|Research Center for Plasma Turbulence, Kyushu University, Kasuga 816-8580, Japan.
authors.forename: S-I
authors.initials: SI
authors.lastname: Itoh
authors.affiliation: National Institute for Fusion Science, Toki 509-5292, Japan.