An ITER-relevant passive active multijunction launcher for lower hybrid current drive in JET-grade plasmas

Abstract

Lower hybrid current drive (LHCD) has been demonstrated to be an important actuator for controlling the current profiles of advanced tokamak plasmas, either in fully non-inductive or in hybrid regimes. To continue such scenario development in JET, and most notably towards extrapolations for ITER, an upgrade of its LH launcher is required to deliver more power (>5 MW) and to deal with a broad array of density situations at the plasma edge more consistently. These may range from steep gradients to ELM activity; extend from as low as the cut-off density nec to large and fast density variations and give rise to strong and rapid reflections of RF power. The passive active multijunction (PAM), advocated as the LH launcher concept for ITER, is claimed to be more resilient to such conditions, run with increased efficiency near nec and even with a small vacuum gap, being also specially appropriate for long pulse operation—all conditions hampering traditional designs. In addition, some of this leverage has been confirmed experimentally at the ITER-relevant FTU with the first PAM ever deployed, and more recently with Tore Supra's PAM. The JET version addressed in this study, with the latter focusing on the physics that drives and supports the adopted technical solutions, allows the injection of up to 5.2 MW at 3.7 GHz and N||peak = 1.9. With directivities better than 70%, reflection coefficients below 2% even for ∼nec and power densities comparable to those foreseen for ITER at 5 GHz, it would validate this concept in ITER-relevant scenarios.