Speaker
Description
The parallel motion and the perpendicular gyro-averaging operation are the most fundamental description of guiding centre (GC) dynamics in magnetic fusion plasmas. Though look straightforward, they contain two interesting aspects to be explored in the context of energetic particle dynamics, including potential applications. One is the appearance of the parallel GC drift (Ud) and the velocity-dependent effective magnetic field (B||∗) and the other is the gyro-averaging operation when the particle gyro-radius is larger than the wavelength of background fluctuations. Here, we first derive an expression of a gyro-averaging operator (G) that is applicable to electrostatic fluctuations in a strongly inhomogeneous magnetic field. Specifically, we calculate an asymptotic expression for G in the high wavenumber limit using the method of stationary phase. We find that the operator naturally involves the fractional integration whose expression could be given in terms of the Riemann-Liouville or Riesz forms. Discussions are made of a potential impact of this asymptotic expression in the high wavenumber limit in relation to the effect of energetic particles on micro-turbulence. Secondly, we elucidate the physics origin of Ud and B||∗ appearing in the Hamiltonian dynamics in curved magnetic field configuration. The magnetic twist term is shown to be responsible for both of them. The parallel drift stems from the change in the effective parallel velocity due to the combined effect of the initial perpendicular motion and the magnetic twist. The effective magnetic field arises from the combined effect of the change in perpendicular velocity due to its initial parallel GC motion and the conservation of magnetic moment. From this physics picture, we find that the oft-quoted singularity in GC phase space appearing at high energy is to be an apparent one.
| Presentation type | Oral |
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