Investigation of Single-particle Motion in the X-point of Two-wire Model*

A numerical analysis with the single-particle simulation is performed to investigate various phenomena of single-particle motion in the two-wire model (TWM). TWM is the configuration of magnetic field generated by two parallel current-carrying wires [1]. The field lines form an ideal figure ‘8’ shape (Cassini oval), which models simple single-null diverted tokamak configuration, that can be analytically analyzed. The wire current and the distance between the wires shape the profile of the TWM field. And two parameters mainly determine the single-particle motion of charged particles; they are canonical momentum or constant flux surface value, and total energy, which are both invariant. Magnetic moment, an adiabatic invariant, may change if field gradient is not small enough [2]. We investigate this phenomenon, the evolution and the resultant distribution of magnetic moment of particle motion near X-point null region. Also, due to finite gyro-radius, particles with constant flux surface inside separatrix may cross the X-point region to migrate to the other side of the TWM field. This phenomenon makes it possible for particles inside separatrix to reach divertor without collisional transport. We derive this crossing condition and the crossing confinement time of particles. In addition, we show that, with the single-particle simulation including Monte-Carlo collision, particles prefer to reside closer to the separatrix for relatively longer times. And finally, with the analyses of the aforementioned phenomena, we predict the expected profiles of collisionless magnetized plasmas in TWM.

[1] A. Reiman, Physics of Plasmas, 3, 906 (1996) [2] C. Stephens, Physics of Plasmas, 24, 102517 (207)