H^- and D^- production efficiency in a multi-dipole ECR-plasma source as a function of gas pressure

Abstract

The electron cyclotron resonance (ECR) negative ion source “Prometheus I” is operated either with high purity H2 (> 99.999%) or D2 (> 99.8%) to probe H- and D- ions, respectively, and examine the isotope effect within a wide range of gas pressure. These ions are predominantly formed in the bulk plasma by dissociative attachment (DA) of low-energy (cold) electrons to highly ro-vibrationally excited molecules. The latter result mainly from the radiative decay and excitation (EV) process sustained by high-energy (hot) electrons heated in the ECR zones. Langmuir probe and laser photo-detachment measurements are realized within the pressure range 0.27 to 2.67 Pa under constant microwave power (0.9 kW). It is revealed that: (i) the plasma potential, cold electron temperature, and cold electron density tend to be higher in deuterium; (ii) no pronounced difference in the hot electron density and temperature is found between the two plasmas; and (iii) overall a similar H- and D- negative ion yield (up to 6×109 cm-3; under the present conditions) is achieved. However, for equal plasma densities an isotope effect is exhibited showing higher H- density over the entire pressure range. Finally, the nH- / n ratio is constantly higher than the nD- / ne one and they both peak around 1.33 Pa.

ECR–Driven Negative Ion Sources Operating with Hydrogen and Deuterium

“…In Fig. 12.15(b) the relative negative ion density is given, unveiling a strong isotope effect in support of the H − ions and a sharp optimum around 1.33 Pa in both gases [Mitrou et al 2021]. On the top of that, Fig.…”

“…On the other hand, Fig. 12.15(a) makes a direct comparison between the negative ion yields achieved in the two gases [Mitrou et al 2021]. At a first glance, H − and D − densities have a qualitatively and quantitatively similar evolution as a function of the operating pressure, i.e., a sharp increase is followed by a quasi saturated phase.…”

“…On the top of that, Fig. 12.16(a) compares the promote H − and D − densities under equal plasma density (cold electrons), following pressure elimination [Mitrou et al 2021]. Higher values for the H − ion density, are observed.…”

“…12.14 gives the absolute density values of these populations, unveiling a difference of about two orders of magnitude in favor of the cold population. These measurements are carried out 65 mm downstream of the ECR zone middle planes [Mitrou et al 2021]. The hot population downstream of the ECR zones is a fingerprint of the energetic electrons generated in these zones, whereas the cold population refers to the electrons escaping these zones [Lagarde et al 2001].…”