High Te, low collisional plasma confinement characteristics in LHD

High electron temperature (Te) plasmas of more than 15 keV were achieved due to the upgraded electron cyclotron resonance heating system with newly installed 77 GHz-over 1 MW gyrotrons in the Large Helical Device. The value of Te greatly exceeds ∼10 keV, which was obtained in previous experiments. The configuration dependence of high Te plasma performance was investigated under the magnetic field strength of ∼2.7 T and the optimum configuration for achieving high Te plasma was observed. These high Te plasmas accompanied an electron internal transport barrier (e-ITB). Clear threshold of density-normalized ECRH power for an e-ITB formation was found. The foot point of an e-ITB moved outward during ECRH injection and the flat Te profile gradually healed with the propagation of the e-ITB foot point. The foot point finally reached to a lower order rational surface of ι/2π=0.5. (author)