Pure volume production of D− negative ions is studied in a rectangular arc chamber. Production and control of D2 plasmas are tested by varying the intensity of the magnetic filter MF. The values of electron temperature Te and electron density ne in D2 plasmas are slightly higher than those in H2 plasmas. Te in D2 plasmas cannot be decreased below 1 eV in the extraction region with the same MF conditions for optimizing H2 plasmas. A stronger MF field is required for controlling Te in D2 plasmas. Therefore, the transport in or the production of H2 and D2 plasmas is different, i.e. there is an isotope effect, a consequence of which is that the H− and D− densities have different spatial distributions. Extracted H− and D− currents are primarily determined by H− and D− densities in front of the extraction hole, respectively. According to the estimated rate coefficients and collision frequencies of main collision processes, it is reconfirmed that Te in the extraction region should be maintained below 1 eV with ne kept high for the enhancement of H− and D− production. For the enhancement of D− production, preliminary results on VUV emission measurement are presented and discussed briefly.