Description
In the Low Aspect ratio Torus Experiment (LATE) device, non-inductive startup of spherical Tokamak by electron cyclotron wave (ECW) and/or electron Bernstein wave (EBW) using 2.45 GHz microwaves has been explored, where all the processes including breakdown, plasma current initiation and ramp-up are carried out with EC and/or EB waves.
In such EC startup of Tokamaks, initial closed flux surfaces are generated via a current jump by injecting microwaves into the vacuum vessel with prefilled hydrogen gas under a weak steady vertical field. One possible cause of the current jump is cross-field passing (CFP) electrons, which are a forward energetic part of electrons in the velocity space. Since their orbits take typically elongated banana orbits, the currents carried by CFP electrons could be affected by the vertical space of vacuum vessel. To investigate the dependence on the vertical space of vacuum vessel, movable limiters have been installed in the top and bottom region of LATE, which can vary the aspect ratio of poloidal cross section in the range of Kvv = 1.7 - 2.3 (Kvv is vacuum vessel elongation which is the ratio of vertical length to horizontal one of plasma production region).
The results show that high Kvv is advantageous. With Kvv = 2.3, closed flux surfaces can be formed in a wide range of vertical field decay index and injected microwave power. With Kvv = 1.7, on the other hand, the parameter area in vertical field decay index and microwave power that can make closed flux surfaces become quite small, although the formation of closed flux surfaces is still possible under a sufficient vertical field decay index and microwave power.
