Effect of Highly Ordered Pyrolytic Graphite Surfaces on the Production of H⁻ Negative Ions in ECR–Driven Plasmas

The use of Cs in neutral beam injection (NBI) systems for international tokamak experimental reactor (ITER) and DEMO tokamaks to reduce the work function of the tungsten surface could be problematic as it can potentially escape from the surfaces onto which it is deposited and be among the factors which promote aberration. Hence, Cs-free ion sources could be a conceivable solution to produce a more uniform beam of
$\text{H}_{0} / \text{D}_{0}$
neutral atoms, formed from negative ions by stripping. Accordingly, this work focuses on the production of negative ions by dissociative attachment (DA) in the plasma volume when highly rovibrationally excited molecules (
$v^{\prime \prime } > 5$
) in their electronic ground state
$X^{1}{\Sigma }_{g}^{+}(v^{\prime \prime }, J^{\prime \prime })$
react with cold electrons (
$kT_{e} < 1$
eV). Such molecules are created first by collision between hot electrons (
$kT_{e} > $
15 eV) and molecules in the ground state (
$v^{\prime \prime } =0$
); second through recombinative desorption, i.e., when an atom produced in the plasma volume reacts with an atom previously absorbed on the surface. Quantum dynamics calculations on rigid surfaces of graphite have shown that the vibrational levels of molecules created in such a manner have an average final level
$v''$
higher than 5. This possibility is here evaluated by testing highly ordered pyrolytic graphite (HOPG) surfaces facing an electron cyclotron resonance (ECR) plasma. The DA-induced products, i.e., the negative ions, are probed and their absolute density is measured by means of a laser photo-detachment technique. The results show an enhancement by a factor of three of the negative ion density when graphite partially covers the inner quartz surface of the plasma chamber. At the same time, the other plasma features remain practically unchanged. The role of the position of the HOPG surface with respect to the ECR coupling zone on the negative ion yield is stressed by the results.