One-dimensional simulation on stability of detached plasma in a tokamak divertor

Abstract

The stability of the radiation front in the scrape-off layer (SOL) of a tokamak is studied with a one-dimensional fluid code; the time-dependent transport equations are solved in the direction parallel to a magnetic field line. As the energy from the core into the SOL plasma is reduced, stable attached solutions change into stable detached solutions. Whenever such stable detached states are attained, the strong radiation front is in contact with or at a small distance from the divertor target. When the input energy is decreased further and total losses become larger than the input power, the radiation front starts to move towards the X-point, cooling the SOL plasma. In such a case, no solutions for the radiation front resting in the divertor channel are observed in our parameter space. This qualitatively corresponds to the results of tokamak divertor experiments which show the movement of the radiation front.

Development of the Detachment under the Influence of the Supersonic Flow in the Divertor Region

“…The transition to the supersonic flow occurs if the density at the target and, consequently, the upstream density increases or the input power decreases to a certain level. It is important to note a significant difference with previous studies where the minimum value of the Mach number at the target plate allowed by the Bohm criterion, M t = 1, has been prescribed, see, e.g., [15]. There no steady state solution could be obtained at a low input power and a moving front of detached plasma was observed.…”

Modeling of supersonic plasma flow in the scrape-off layer

“…[25] where the minimum Mach number at the target plate, M t = 1, has been adopted. For this comparison impurity radiation has been included into the present model by adding the term S I = Ànn 2-L I (T) to the energy loss S E , Eq.…”

“…In addition as in Ref. [25] the SOL has been assumed fully opaque for neutrals by taken s n = 1 in Eq. (7).…”

“…As a consequence, the mean free path of neutral particles is longer than the characteristic length of the change of the plasma parameters. Nevertheless, for our qualitative analysis we use a fluid diffusion approximation, which has been often applied earlier [4,25] and which gives a relatively good agreement with the experimental measurements, see Ref. [26]: d dz ÀD n dn n dz ¼ Àk i n n n þ k r n 2 À n n s n hðx d À xÞ:…”

Effects of Radial Losses of Particle and Energy on the Stability of Detachment Front in a Divertor Plasma

“…The particle, momentum and energy source/sink terms are represented by S , M and Q, respectively, in which several kinds of atomic processes are taken into account. For the details of each term, see [8,9,12].…”