It is promising that injection of compact torus (CT) plasma is useful way for starting up initial plasma, for putting on fuel particle and for sustaining of main tokamak plasma and spherical tokamak. A CT plasma can be injected deeply the interior of main tokamak. Comparing with other sustainiment methods the CT plasma injection supplies all torus plasma components of main tokamak plasma. The CT plasma itself has all plasma parameters and compornents of main tokamak plasma.
For effective fueling and sustainment, single pulsed injection of small CT plasma is not sufficient in many cases. Therefore injection of the CT plasma with high repetition rate is required.
We have experimented on the CT plasma injection with high repetition rate and studied the effects of increasing repetition rate. The CT plasma was generted by a small magnetized coaxial gun (be also called as Coaxial Helicity Injector: CHI). Using same polarity oscillating current as a power source, the magnetized coaxial gun can generate a series of the CT plasma wifth same helicity sign in high repetition rate.
A series of the generated CT plasma was injected into a drift tube. A metal (SUS) made drift tube was used for measurement of magnetic field structure of the injected CT plasmas. A transparent drift tube that was made of pyrex glass was used for observation of cross sectional view of the plasma in the drift tube by a fast-gated image intensifier camera.
The first plasma that was generated by the first current pulse of gun moved into the tube. According to the decreasing of the first gun current, the injected first plasma decayed. The second gun current pulse generated the next CT plasma. The CT plasma moved into the tube and interacted with the former decayed CT plasma.
Therefor we observed the accumulation effects by the interaction between the second plasma and the first one, though the experiments were conducted in the drift tube. The repetition frequency of the CT injection was 50KHz and we examined the effects of increasing this frequency. The repetition frequency of the CT injection can increase by decrasing the value of a capacitor. Using half of the capacitor, the repetition rate was increased by 1.4 times to 70KHz.
The effects of accumulation were estimated by the ratio of the magnetic flux change of first plasma to the second one. In the case of 50 KHz of repetition frequency, this ratio was 0.8. The ratio remakably increased to 1.8 in the case of 70 KHz.
Increasing the repetition frequency of CT plasma injection was very effective way for fueling and for sustainment of main tokamak plasma and spherical tokamak plasma.