OPTEMIST: A neutral beam for probing quasi-omnigeneous fast ion behavior in Wendelstein 7-X

A new neutral beam system is ex- plored which can directly demonstrate the quasi-omnigeneous nature of the Wendelstien 7-X stellarator. The Wendelstein 7-X (W7-X) stellarator high mirror magnetic configuration is predicted to have improved fast ion confinement at finite beta due to the quasi-omnigeneous (QO) nature of the device. As the magnetic field becomes more QO in nature, the drift orbits of the toroidally trapped particles drift poloidally which has the effect of averaging out the effects of bad curvature. It is shown that the existing neutral beam system does not directly populate these orbits. Additionally, the energy of the neutral beam fast ions is not high enough to be considered collisionless in W7-X discharges. A new 150 kV neutral beam is envisioned which directly populates the deeply trapped orbits with collisionless particles, satisfying the two necessary conditions to demonstrate the QO optimization. Simulations of beam injection and fast ion confinement with the BEAMS3D code demonstrate the ability of such a beam to demonstrate the QO behavior. Synthetic diagnostic signals as modeled by FIDASIM help to gauge the minimum beam line parameters and diagnostic set needed. The use of the ion cyclotron resonance heating system is also considered in this presentation.