Plasma Potential Formation Due to Local ECR

Plasma potential formation is investigated in a fully-ionized collisionless plasma flow in the presence of an electron cyclotron resonance (ECR) point under simple mirror configurations of magnetic field. The magnetic-well, converging-field and diverging-field configurations are generated in the single-ended Q machine. When ECR takes place at the magnetic-well bottom, a potential strucature is observed to consist of a negative potential depression around the ECR point and a subsequent positive potential peak. This potential structure gradually collapses in the time scale determined by the ion-flow speed, during which low-frequency drift and flute instabilities are observed to grow intensively. In the case of a converging magnetic field such a potential structure is observed to persist in the steady state when the ECR point is located in the region of good magnetic-field curvature. But, this structure is formed only transiently when the ECR point is located in the region of bad magnetic-field curvature. Thus, the formation of an ion plug and electron thermal-barrier potential is demonstrated in the presence of the ECR point under simple configurations of magnetic mirror field, being different from the original scenario for the tandem-mirror plasma confinement. When ECR takes place in a diverging magnetic field, on the other hand, there appears a strong potential drop along the field lines, which results from a field-aligned electron acceleration, being accompanied by an effective ion acceleration. The potential formations described above are explained by the selective electron-deceleration/acceleration parallel to the field lines due to ECR and electrostatic ion-deceleration/acceleration self-consistently organized due to the condition of quasineutrality.