Plasma Jet Diagnostic for Runaway Electron Beam-Plasma Interaction

FAR-TECH's recently developed C$_{60}$/C plasma jet has the potential to rapidly and significantly increase electron density, deep into tokamak plasma, hence to change the `critical electric field' as well as the runaway electrons (REs) collisional drag, during different phases of REs dynamics. Suitably chosen visible/UV lines emitted by the injected C ions can then be used for line intensity quantitative spectroscopy, allowing the diagnostic of the RE beam-plasma interaction. The C$_{60}$ delivered in $\sim1$ ms by the prototype plasma jet system, estimated to be $\sim75$ mg, carries $\sim4\times10^{21}$ C atoms and $\sim2.4\times10^{22}$ electrons, and would lead to an electron density $n_{e}\sim2.4\times10^{21}$ m$^{-3}$, i.e. $\simeq60$ times larger than typical DIII-D pre-disruption value ($n_{e0}\approx 4\times10^{19}$ m$^{-3}$). While the prototype's C$_{60}$/C plasma jet mass is not sufficient to achieve the Rosenbluth electron density in DIII-D, it delivers a total number of electrons $\sim5$ times larger than that of the Ar pellet, with the advantage of a much faster response and precisely chosen delivery time. We will present several proposed diagnostic schemes using rapid C$_{60}$/C plasma jet injection capability in different phases of the discharge in DIII-D.