Review of L-2M stellarator recent results

Stellarator L-2M is a medium size classical stellarator with the multipolarity l=2, the total number of magnetic field periods N = 14 and major radius R = 100cm. So called 'standard' configuration has the rotational transform μ(0) = 0.18 at the magnetic axis and μ(ap) = 0.78 at the separatrix with the average radius ap = 11.5cm. All the experiments are performed with a small β∼(0.1 0.2)% electron cyclotron heated (ECH) plasmas. The more recent experiments were definitely predictable on the base of magnetohydrodynamic (MHD) theory and transport calculations. We start with the results of the 'standard' configuration experiments that has no such a simple explanation. Interesting transport phenomena occur in ideal MHD stable plasmas at sufficiently high densities. They have a threshold in heating power and start with sudden (with duration of ∼200μsec) small drop of total energy. After the event plasma energy slowly monotonically increase up to the value ∼ (5-10)% larger the value before the event. Peculiarities of the transition to stationary phase from the beginning of the discharge are also presented. It is shown that the transition may be slow or fast depending on the value of heating power and plasma density, but universally is accompanied (caused?) by the appearance of high frequency (40-100 kHz) electromagnetic oscillations measured by Mirnov probes. For clarifying the situation we present analysis based on the model of coupling of drift, Alfven and acoustic modes. In the next section of the work question is raised as to whether the plasma confinement is degraded with the decrease in μ(a/ap - 2/3) value following International Stellarator Scaling (ISS95) with the law ∼ {μ(a/ap = 2/3)}0.4. Decreasing the current in the helical windings with four equal steps to the half of initial value we obtain five different configurations where μ can be changed by a factor four that is the largest possible value among the existing stellarators. Properties of plasma confined in these configurations are briefly outlined. It is shown that the decrease of the total energy with the rotational transform value is significantly more pessimistic than that following from the ISS95. Another possibility to change the configuration is the use of co/contra ohmic current. Changing the rotational transform ohmic current does not participate practically in the heating of ECH plasmas. With the value of co injected current confinement initially slightly improves. Higher positive currents lead to definite degradation of the confinement connected with strong MHD activity. Contra injected (negative) currents lead to visible decrease of electron temperature at given average density and ECH power. MHD activity in this case is much smaller than that of co injected current. Calculations demonstrate that with the increase of the absolute value of negative current topological structures with several nodal points may appear in the central part of the plasma. Having no possibilities of the direct experimental identification of the magnetic surface topology we analyze the experimental data and adduce arguments for and against this hypothesis. (author)