Generation of negative ions in tandem high-density hydrogen discharges

Abstract

An optimized tandem two-chamber negative-ion source system is discussed. In the first chamber high-energy (E>20 eV) electron collisions provide for H2 vibrational excitation, while in the second chamber negative ions are formed by dissociative attachment. The gas density, electron density, and system scale length are varied as independent parameters. The extracted negative ion current density passes through a maximum as electron and gas densities are varied. This maximum scales inversely with system scale length R. The optimum extracted current densities occur for electron densities nR=1013 electrons cm−2 and gas densities N2R in the range 1014–1015 molecules cm−2. The extracted current densities are sensitive to the atomic concentration in the discharge. The atomic concentration is parametrized by the wall recombination coefficient γ and scale length R. As γ ranges from 0.1 to 1.0 and for system scale lengths of 1 cm, extracted current densities range from 8.0 to 80 mA cm−2. The relative negative-ion yields from single-chamber and tandem two-chamber systems are compared. Estimates are made for the rates of polar dissociation of H2 molecules and H+3 ions, and these rates are compared with the dissociative attachment rates.

From cold to fusion plasmas: spectroscopy, molecular dynamics and kinetic considerations

“…In [6,41] the distributions of H (n) arizing from non-equilibrium ground VDFs have been investigated. In figures 15 the translational energy distribution functions (TEDFs) of atoms with principal quantum numbers n =3 and 4, for two different electron density values, corresponding to non-equilbrium VDFs [42] (see figure 14), are reported [6,41]. Results were obtained considering the cross section vibrational dependence well described by Franck-Condon density factors.…”

“…Figure 14. Non-equilibrium vibrational distributionas a function of the vibrational quantum number for different electron densities [42].…”

Efficient production of negative hydrogen ions in RF plasma by using a self-biased grid electrode

“…In the first stage, high energy electrons e ℎ with e more than 20 eV excite H 2 molecules to generate vibrationally excited molecules H 2 * (ν) via the reaction H 2 + e ℎ → H 2 * + e . In the second stage, low energy electrons e with e~1 eV collide with H 2 * (ν) for the production of H − via dissociative attachment collisions reaction H 2 * + e → H − + H [15]. In the second stage, however, very low energy electrons with e < 0.5 eV have too small energy to cause the above reaction effectively [16].…”

Relaxation behavior of rovibrationally excited H2 in a rarefied expansion

“…[17][18][19][20] In such sources rovibrationally excited hydrogen plays a key role, since the negative hydrogen ions is formed in the dissociative attachment ͑DA͒ reaction. And this reaction becomes five orders of magnitude more efficient when the rovibrational energy of the hydrogen molecule is increased to around 2.5 eV.…”