(EX/P3-18) High Kinetic Energy Jet Injection into Globus-M Spherical Tokamak

Abstract.  Progress in theoretical and experimental development of the plasma jet source and injection of hydrogen plasma and neutral gas jets into the Globus-M spherical tokamak is discussed. There are: (i) injection of plasma jets with higher densities than reached in earlier study; (ii) injection of high-density neutral gas jets with a directed energy of the particles of few tens of eV; (iii) plasma startup in tokamak with the help of the plasma gun in conditions of double swing CS operation regime. Injection of pure, highly ionized hydrogen plasma jet with a density up to 2×10²² m^-3, total number of accelerated particles (1-5)  ×10¹⁹ and a flow velocity of ∼110 km/s was used as instrument for the density control. First numerical simulations showed that due to the large initial velocity such jet should penetrate deep (10-20 cm) in the Globus-M plasma. The density raise recorded by interferometer and Thomson scattering is consistent with simulations. The fuelling by high velocity plasma jet might be preferable than Laval nozzle technique, especially for injection from high field side. The gas and plasma jets were injected into Globus-M and penetrated efficiently into the magnetic field. The density rise time of ∼2.5 ms (for gas jet) observed is shorter than that achieved customarily with conventional gas puffing (4-5 ms) while being much longer than that of characteristic density raise time by plasma jet injection (<0.5 ms). It increased plasma particle inventory in Globus-M by ∼50% (from 0.65×10¹⁹ to 1×10¹⁹) in a single shot without plasma parameter degradation. Fast density increase in all spatial points of the plasma column including the plasma central region confirmed density rise during the time less than 1 ms, which is the signature of deep jet penetration. The plasma discharge, initiated by means of plasma jet demonstrated faster heating of the discharge as compared to RF pre-ionisation.

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