Experiments on the current rise stages of the LT-3 tokamak

Measurement of the plasma parameters during the first two milliseconds (Isub(phi)<10 kA) of the LT-3 discharge indicates that the plasma passes through three distinctive phases. The first phase is a low-βsub(theta) phase during which the electron density is small. The second phase is ionization phase which often features hollow current profiles. The third phase is an impurity-influx phase which closely follows the completion of the hydrogen ionization. It is found that inclusion of some Z- and theta-pinching during these stages would produce hotter discharges. However, Z-pinching often induces extra impurity influx and theta-pinching reduces the current channel significantly. The theta-pinching often causes sudden partial filling of the hollow current channel, accompanied by positive voltage spikes. (author)