In order to explain the results of the non-inductive current produced in the lower hybrid current drive (LHCD)
experiments, a broadening of the radiofrequency (RF) power spectrum coupled to tokamak plasma needs to occur.
The presented modelling, supported by diagnostic measurements, shows that the parametric instability (PI) driven
by ion sound quasimodes, which occur in the scrape-off plasma layer located near the antenna mouth, produces a
significant broadening of the launchedLHspectrum. Considering the parameters ofLHCDexperiments of JET (Joint
European Torus), and other machines as well, the PI growth rate is high enough for producing the compensation of the
convective losses and, consequently, the broadening of a small fraction (of the order of 10%) of the launched power
spectrum. Such a phenomenon is identified to be intrinsic to the RF power coupling in theLHCDexperiments. As the
principal implication of considering such spectral broadening in modelling the LH deposition profile, experiments
of LHCD-sustained internal transport barriers in JET were successfully interpreted, which evidenced the effects
of a well-defined LH deposition profile. The present work is important for addressing the long-lasting debate on
the problem of the so-called spectral gap in LHCD. The design of LHCD scenarios relevant to the modern fusion
research programme, an important requirement of which is the control of the plasma current profile in the outer half
of plasma, can be properly achieved by considering PI-induced spectral broadening.
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