The 3.7 GHz lower hybrid current drive system for the Tokamak de Varennes

A lower hybrid heating and current drive system is presently being constructed for the Tokamak de Varennes (TdeV). This medium sized fusion experiment has parameters R = 0.86 m, a ≤ 0.27 m, BT = 1.5 T and Ip ≤ 300 kA. It has recently been fitted with a double-null, closed divertor. A major part of the scientific program of the tokamak is the study of impurity control and plasma-surface interactions under near steady-state conditions. To this end, the magnetic coils are capable of producing 30 second pulses. Non-inductive current drive is necessary to extend the plasma discharge duration from its 1-2 second length in the present ohmic regime. The lower hybrid current drive system will operate at a frequency of 3.7 GHz to avoid possible density limitations. Up to 1.l MW will be injected into plasma, giving a high power density of 1 MW/m3. Due to the low magnetic field, the waves must be launched at a high N to remain accessible to the center at moderate densities. The preliminary technical design of the RF system has been presented elsewhere. Its main features, including some recent modifications, are summarized below. In section 3 the predicted performance of the system, based both on empirical scaling and theoretical simulations, is discussed. Section 4 outlines some of the planned lower hybrid experiments. (author) 7 refs., 3 figs