Penetration of supersonic gas jets into a tokamak

The injection of high-pressure supersonic jets into the tokamak plasma is considered a promising method of future thermonuclear reactor fuelling and as a tool for disruption mitigation. Successful experiments were correspondingly performed on Tore Supra and DIII-D. In the present paper the evolution of such a jet is analysed. The jet expansion, deceleration of the ambient electrons and ions by the jet, self-consistent electric field, elementary processes, radiation and adiabatic cooling of the ambient plasma are taken into account. The jet is simulated by a MHD code, which is similar to the code previously used for pellets. It is demonstrated that the ionization degree of the jet strongly depends on the jet parameters. Several simulations were performed for the range of parameters typical for DIII-D. The jet of initial density 4 × 1024 m−3 remains almost neutral, and only the outer regions are ionized. When the initial jet density is reduced by a factor of 2 or more the main part of the jet becomes ionized rather fast. It is demonstrated that ionization at the jet edge in the poloidal (perpendicular to the magnetic field) direction of the jet is sufficient to stop poloidal expansion of the jet by force. The final poloidal size of the jet remains of the order of its initial poloidal dimension (of the order of ten centimetres). The jet motion in the radial direction (direction of the injection) is provided by the polarization poloidal electric field and the corresponding drift. In the paper two mechanisms of polarization reduction are considered: Alfvén conductivity of the ambient plasma and the ∇B-induced drift. It is shown that an almost neutral jet can penetrate deep into the tokamak while a modest ionization degree should prevent its penetration for the case of low field side injection.