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.

A high field side multijunction launcher with aperture impedance matching for lower hybrid current drive in DIII-D advanced tokamak plasmas

“…Multijunctions consist of a power divider formed by partitioning a feeding waveguide with internal septa phase shifted to launch a specified n || [22]. Power reflected from the plasma recirculates between multijunction arms and destructively interferes when returning to the feed waveguide, resulting in low reflected power to the RF source.…”

Survey of heating and current drive for K-DEMO

“…The frequency, 5 GHz, is chosen to balance the minimization of LH power absorbed on fast alpha particles [49][50][51][52] against the need to keep the waveguide width sufficiently large so it can be manufactured [53]. The K-DEMO conditions require only modest extrapolation from the multi-parameter survey calculations of alpha particle absorption in [52].…”

Optimization of a lower hybrid current drive launcher for ITER

“…What is more, this concept was already validated experimentally, first on FTU [11] and, more recently, in Tore Supra, where it has been successfully in operation since 2009 [12,13]. A detailed design of a PAM launcher for JET, operating at 3.7 GHz, has also been proposed [14].…”