3-DIMENSIONAL KINETIC SIMULATION OF THE NONLINEAR EVOLUTION OF LOWER-HYBRID PUMP WAVES

Nonlinear evolution of lower hybrid (LH) waves is studied by means of a fully three-dimensional parallel particle-in-cell (PPIC) code. The p lasma is driven by a monochromatic LH pump wave, which drives secondar y LH and low-frequency waves having a broad frequency spectrum from om ega<Omega(i) to omega similar to omega(0) > omega(lh), where Omega(i), omega(0) and omega(lh) are the ion cyclotron, pump and LH resonance f requencies, respectively. The temporal variations in the electric fiel d components show both amplitude and phase modulations. In a plasma wi th equal electron and ion temperatures the dominant amplitude modulati on occurs at the ion cyclotron timescale tau(ci). The pondermotive for ce associated with the vector nonlinearity arising from the (E) under bar x (B) under bar drift of electrons is seen to generate both densit y depletions and enhancements depending on the time-varying phase diff erence between the orthogonal electric field components E-x and E-y tr ansverse to the ambient magnetic field B-0 in the z direction. Despite the use of quite strong pump wave amplitude, wave collapse in density cavities alone is not seen; instead, equally strong density cavities (cavitons) and enhancements (pilons) occur quasiperiodically both in t ime and space. The phase difference between E-x and E-y and its evolut ion a rotating transverse electric field vector with hodograms of E-x and E-y changing with time. The temporal evolution of the parallel acc eleration of electrons and transverse heating of ions are discussed. F or relatively slow pumps the electron acceleration is predominantly un idirectional parallel to the pump phase velocity V-p parallel to 0 pil e. On the other hand, for sufficiently large pump phase velocities the acceleration becomes bidirectional. The parallel electron acceleratio n up to V-parallel to max similar to 2V(p parallel to 0) is common, an d the transverse ion acceleration occurs up to V-perpendicular to max congruent to(m/M)(1/2) V-p parallel to 0 the observations on LH, where m and M are the electron and ion mass, respectively. The relevance of the above waves and their role in electron and ion accelerations is d iscussed.