The electron cyclotron resonance (ECR) plasma is characterized by low working pressure and high dissociation rate, and it has important applications in the deuterium negative ion \mathrmD^- source technology. In this paper, the Yacora collisional-radiative model is applied to the emission spectrum diagnosis of \mathrmD^- in ECR deuterium plasma. The \mathrmD^- density is estimated by using the I_\mathrmD_\mathrm\alpha /I_\mathrmD_\mathrm\beta ratio and the relative intensity of other deuterium molecular lines, thereby avoiding complex calibration procedure of absolute intensity. The spatial structure of \mathrmD^- is studied by the multichannel emission spectrum measured in the source region and diffusion region.

The experiments are conducted on a 2.45-GHz ECR plasma source at a deuterium gas pressure of 1 Pa and microwave power of 660 W. The Balmer series of atomic deuterium ( \mathrmD_\mathrm\alpha , \mathrmD_\mathrm\beta , \mathrmD_\mathrm\gamma , \mathrmD_\mathrm\delta ) and the Fulcher band Q-branches of molecular deuterium are measured in the source region and expanding region of the ECR plasma. It is found that the intensity of \mathrmD_\mathrm\alpha in the source region is much higher than that of \mathrmD_\mathrm\beta , specifically, the I_\mathrmD_\mathrm\alpha /I_\mathrmD_\mathrm\beta ratio reaches as high as 23, indicating a selective enhancement of Balmer lines due to the mutual neutralization process of \mathrmD^- . Furthermore, \mathrmD^- density in the source region is estimated to be about 3.6\times 10^15\;\mathrmm^-3 , and the \mathrmD^- density in the expanding region decreases significantly. In the ECR plasma source region, the plasma-wall interaction is strong due to the small volume of the cavity. The recombination desorption process produces more vibrationally excited molecules, thereby further enhancing the dissociation attachment reaction, which is beneficial to the generation of deuterium negative ions. On the other hand, the axial electric field within the ECR plasma inhibits the axial transport of \mathrmD^- , suggesting that the production and loss of \mathrmD^- are both localized. These characteristics of the ECR plasma source contribute to the formation of a large gradient of \mathrmD^- density between the source region and the expanding region.