The effects of edge magnetic structure on particle fueling and exhaust in the HSX and W7-X stellarators

Author / Creator

Stephey, Laurie, author

Available as

devicesOnline

menu_bookPhysical

Summary

In this study, both experimental and numerical techniques have been used to study particle behavior in response to changes in the edge magnetic field (i.e.~inserting and removing edge islands) at t...

In this study, both experimental and numerical techniques have been used to study particle behavior in response to changes in the edge magnetic field (i.e.~inserting and removing edge islands) at the Helically Symmetric eXperiment (HSX) and Wendelstein-7X (W7-X) stellarators. At HSX, the two magnetic configurations studied were the Edge-Islands and the No-Edge-Islands configurations. Measurements of the effective hydrogen particle confinement time, $\tau_{p,H}^*$, and the effective helium particle confinement time, $\tau_{p,He-II}^*$, were a factor of 1.5 and 1.6 longer, respectively, in the Edge-Islands configuration. A complete hydrogen particle balance yielded particle confinement times $\tau_{p,H}$ a factor of 1.3 longer in the Edge-Islands configuration. EMC3-EIRENE fueling efficiency calculations yielded values of 0.70 in the Edge-Islands configuration and 0.87 in the No-Edge-Islands configuration, a factor of 1.2 difference. At W7-X, the two magnetic configurations studied were the Increased Iota and the Standard configurations. Measurements of $\tau_{p,He-I}^*$ and $\tau_{p,He-II}^*$ were a factor of 1.5 and 1.2 longer, respectively, in the Increased Iota configuration. A complete hydrogen particle balance yielded values of $\tau_{p,H}$ that were approximately the same in both configurations. Similarly, EMC3-EIRENE fueling efficiency calculations yielded values approximately the same in both configurations. These results indicate that the edge magnetic structure can have a significant impact on the particle fueling and exhaust, but only if the change in edge structure is made in the ionization source region. In HSX, when the 8/7 island chain was inserted in the peak ionization source region in the Edge-Islands configuration, the 8/7 islands were able to decouple the plasma source from the confinement region through locally peaked temperatures at the island X-point (creating a private flux region in the same way that a divertor does). This led to reduced fueling efficiencies and increased global particle confinement times. For this same reason, moving the islands in W7-X did not result in major changes in particle fueling and exhaust, because the islands were not located in the ionization source region.

Show More