Observation of Nonlinear Frequency-Sweeping Suppression with rf Diffusion

Abstract

Nonlinear frequency sweeping of unstable waves in a laboratory plasma is suppressed upon application of rf fields. Frequency sweeping is driven by a population of energetic electrons trapped in a magnetic dipole field that excite drift-resonant potential fluctuations and create coherent structures in phase space. Self-consistent numerical simulation reproduces the suppression and suggests an explanation due to rf scattering of energetic electrons that destroys the phase-space structures.

Weak effect of ion cyclotron acceleration on rapidly chirping beam-driven instabilities in the National Spherical Torus Experiment

“…Fast-ion populations created by intense ion-cyclotron heating can also excite rapid chirping [12,13]. Qualitatively similar phenomena are driven by intense fast-electron populations in conventional tokamaks [14][15][16] and in a dipole experiment [17,18]. The widespread nature of the phenomenon in systems with different instabilities and different energetic particle populations suggests that a generic theoretical model of the coupled wave-particle system might describe the essential features.…”

“…The experiment reported here is motivated by the dipole experiment that used cyclotron heating to suppress frequency chirping [17,18]. The basic idea of the experiment is to use neutral beam injection to create chirping instabilities, then use high harmonic fast wave (HHFW) heating to scatter the fast ions that drive the instabilities out of resonance, thereby suppressing the frequency chirping.…”

“…Modes in MAST that chirp upwards in frequency, downwards, or in both directions can also be explained [33], as can upward frequency chirping of geodesic acoustic modes in JET [34]. A detailed test of the BB model was performed in a dipole experiment [17,18]. A population of energetic electrons drove a strongly chirping interchange instablity.…”

Impact of energetic particle orbits on long range frequency chirping of BGK modes

“…Similar to subsection 2.2, we multiply equation (19) by cos [n (k p z − φ(t))] and integrate over one wave-length. We also write all the physical quantities in the fast particle term in terms of the new action-angle variables (θ,J).…”

“…However, experimental evidence exists for mode activities in which the frequency shifts are as large as the initial eigenfrequency itself [18,19,20]. As the mode amplitude saturates due to flattening of the distribution function of the energetic particles, the physical picture of each evolving phase-space structure is a BGK mode whose frequency changes in time and its structure is notably affected by the frequency shift.…”