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Abstract
Recent experimental and theoretical research has revealed that radial electric fields ($E\sb{r}$) play a crucial role in the tokamak plasma transition into an improved confinement regime. Motivated by these results, two novel techniques utilizing injection of hot electrons were developed and tested in this research. The objective is to control the transition to H-modes in large scale future devices by inducing $E\sb{r}$ via external means.
The electron ripple injection (ERI) technique has been developed on CDX-U to fulfill this in a non-intrusive way. This technique utilizes externally provided local magnetic field ripple and electron cyclotron resonance heating (ECRH) to trap edge electrons and to cause them to drift towards the plasma center so that they charge up flux surfaces negatively. The temperature anisotropy of resonant electrons in a tokamak plasma was calculated to study the effects of ECRH on electrons. The behavior of energetic electrons and the generation of $E\sb{r}$ in the presence of ripple fields were investigated using a guiding-center electron orbit code. Examples for CDX-U and ITER parameters are given.
Preliminary experiments have been performed on CDX-U. The results show the existence of drifts, and combined effects of electron trapping and heating. The electron temperature anisotropy produced by ECRH was also measured. Since both modeling and preliminary experiments gave encouraging results, further experiments are warranted.
Another novel approach has been successfully taken using electron injection: an emissive limiter biasing experiment on CCT involving an injection of hot electrons at the plasma edge from the limiter. The emissive capability appears to be important for operating at lower bias voltage ($V\sb{bias}\leq-150\ V$) compared to biasing a cold limiter ($V\sb{bias}\leq-450\ V$) for inducing H-mode transition. The results shown in this experiment suggest that the performance of the limiter or divertor bias experiments can be significantly improved by simply adding emissive capability.