Journal Article Effect of magnetic structure on RF-induced breakdown in QUEST

Yoneda, R.  ,  Hanada, K.  ,  Nakamura, K.  ,  Idei, H.  ,  Yoshida, N.  ,  Hasegawa, M.  ,  Onchi, T.  ,  Kuroda, K.  ,  Kawasaki, S.  ,  Higashijima, A.  ,  Nagata, T.  ,  Isayama, A.  ,  Mitarai, O.  ,  Fukuyama, A.  ,  Y.Takase

24 ( 6 )  , pp.062513-1 - 062513-10 , 2017-06
In tokamak operations, breakdown of plasma is the first step of the plasma build-up. In this paper, we present a combinative investigation of radio frequency (RF)-induced breakdown experiments in QUEST (Q-shu University Experiment with Steady-State Spherical Tokamak) and a one-point model of hydrogen ionization. Experimental results with two different frequencies of 2.45 GHz and 8.2 GHz showed that the clear threshold on connection length, L , existed for breakdown with a negative n- index configuration n =(dR/dBv)/(dBv/dR) , where R is the major radius and Bv the is vertical magnetic field. In contrast, breakdown was always obtained with positive n -index when changing L. It indicates that a lifetime of an incubated electron plays a significant role in the plasma breakdown. According to one-point model calculation, the experimental threshold of L is well predicted by the lifetime of the incubated electron estimated by employing the loss term along with L . The model calculation also describes the requirement of the minimum electron temperature Te for RF-induced breakdown to realize an avalanche of electrons in the tokamak magnetic structure.

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