The injection of high-pressure supersonic jets into the tokamak plasma is considered as a promising way of future thermonuclear reactor fueling and as a tool for disruption mitigation. Successful experiments were performed on Tore Supra and DIII-D, correspondingly. In the present paper the evolution of such a jet is analyzed. 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 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 poloidal (perpendicular to the magnetic field) direction of the jet is sufficient to stop poloidal expansion of the jet by -3 24 m 10 4 ⋅ B j r r × 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 correspoding B E r r × 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 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 (LFS) injection.

S (a) 1.V.Kurchatov Inmtitute, Momcow, USSR ( b ) Central Inmtitute of Electrun Phymicm, Academy of S c i m c e m o f the GDR, Berlin 1. Introduction It im u e l l known that in the edge plamaa of tokarakm m t r o n g d m m i t y and potential fluctuations exist which can lead to a n o m a l w s tranmport p h m o r e n a /I/. There are mreamonm to suppome that the level o f theme fluctuationm im c o n n u t e d with neutral particle fluxes penetrating into the plamaa due to gampufting or recycling procesmem. The dimmipative drift d e i m one o f the momt important rodem in the collimion dominated ed9r plamma /2-41. We investigate the influence o f neutral particlcm on the bchavioClr o f theme nodem. We s h o w that their mtability criteria may br changed mignificantly by taking into conmidrration the ionization o f the n wtral particle. and charge-exchange proccmmes. It i m knoun that in the abmence o f neutralm the ragnetic m h e a r mtabilirem the dimmipative drift modem in the mlab approximation /3/. Our calculationm m h o u that theme modem may become unstable owing to ionization procermem if the concentration o f the neutral particlrm e x c w d m m o m s critical value, wheream the charge-exchange doem not affect the mtability. In the toroidal geometry theme nodem conserve their main proprtiem, but the value of the critical neutral denmity i m mlightly m o d i f i d . On the other hand, mome unmtable molutionm of the eigenvalucequation f o r the drift waves exist in the a b m m c e of neutral prrticlem, which arc connected with the mtrong toroidal rode coupling. They dirmappear in the plamma mlab limit / 4 / . The incr-t o f t h s e nodes is modified significantly both by ionization and charge-exchange. The i'onization of the neutral particles leads always to an increasing of the increment. The influence of charge-exchange procesmem on the time b e h a v i w r of the drift waves dependm on their frequcnciem. A C o n t r i b . Plasma Phys. 30 (1990)1. 31-35 /1/ P.C.Li-r, Nuc1.Fusion 23 (1985) 543 /2/ B.B.Kadoatsev, 0.P.Pogutse. "Voprosy teorii plasma-, vyp. 3 Atomizdat,'Moscou 1971

Current decays after disruptions as well as after noble gas injections in tokamak are examined. The thermal balance is supposed to be determined by Ohmic heating and radiative losses. Zero dimensional model for radiation losses and temperature distribution over minor radius is used. Plasma current evolution is simulated with DIMRUN and DINA codes. As it is shown, the cooled plasmas at the stage of current decay are opaque for radiation in lines giving the main impact into total thermal losses. Impurity distribution over ionization states is calculated from the time-dependent set of differential equations. The opacity effects are found to be most important for simulation of JET disruption experiments with beryllium seeded plasmas. Using the coronal model for radiation one can find jumps in temperature and extremely short decay times. If one takes into account opacity effects, the calculated current decays smoothly in agreement with JET experiments. The decay times are also close to the experimental values. Current decay in argon seeded and carbon seeded plasmas for ITER parameters are simulated. The temperature after thermal quench is shown to be twice higher in comparison with the coronal model. The effect for carbon is significantly higher. The smooth time dependence of the toroidal current for argon seeded plasmas is demonstrated in contrast to the behavior in carbon seeded ones.

Plasma radiation plays important role in temperature balance and stability of plasmas at the edge of fusion devices like tokamaks and stellarators. For example, Microfaceted Asymmetric Radiation From the Edge (MARFE), and detached plasma regimes are associated with radiation. Analogical situations appear in current quench after disruptions as well during noble gas injection proposed for current event mitigation in ITER. As it well known, radiation-condensation instability determining MARFE formation may be observed. The sound branch also may be destabilized by radiation. The basic properties of radiative plasmas are discussed in the current presentation. Radiation losses in pure fully ionized plasmas are small in comparison with flows associated with heat conductivity. However, radiation of multi-electron impurity ions is significant, even if the impurity concentration is small. It depends strongly on the Impurity Distribution Over Ionization States (IDOIS). In many theoretical papers the impurity behavior is simplified drastically. (See, for example, the review paper [1]). Particularly, the coronal equilibrium is assumed, and thermal forces are ignored. The current presentation is devoted to recent investigations showing these assumptions to be not valid for a most part of situations in experiments and nature. (See review paper [2]). Many interesting effects taking into account the finite relaxation time of IDOIS and thermal forces have been found. In particular, the anomalous sound damping due to the internal friction, decompression shocks (Ref [3]), slow thermal waves (Ref. [4]), and selfsustained thermal oscillation are discussed in the current presentation. Usually, opacity effects are negligible. However, plasmas are opaque in clouds surrounding pellets and noble gas jets. As a consequence, radiation is trapped inside the cloud; the temperature of the cloud is significantly higher than the temperature estimated with the assumption of plasma transparency. Experimental results have been explained taking into account opacity effects (Ref. [5]). Opacity effects in processes of current decay after noble gas injection and disruption also are discussed in the current presentation.

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