The Formation of Plasma by Injection and Dissociation of Accelerated Hydrogen ''Granions''

The INGRA experiment is an experimental study of the formation of a hot-ion plasma (Ti ≃ 1.5 keV) by the injection, dissociation and capture of a beam of accelerated hydrogen ''granions'' in a magnetic configuration. The granions are agglomerates of hydrogen atoms with one or more positive electric charges. The accelerator is described in the paper. The neutral agglomerates (granules) produced in a supersonic jet of hydrogen, previously cooled to the region of the liquefying temperature, are charged by electron bombardment and accelerated to 600 kV by an electrostatic potential difference. Mass analysis of the accelerated beam is carried out by means of a magnetic separator, which can be used for analysing mass/charge numbers N/Z up to 10 000 at 600 keV. Using an 0.3 x 4 mm capillary nozzle, one can obtain in the continuous régime a beam of the order of 70 mA equivalent (4 x 1017 atoms/s), whose mass spectrum is very broad and hasa maximum N/Z close to 100, corresponding at 600 keV to an energy of 6 keV/atom. A definite increase in the beam intensity can be obtained by increasing the pressure upstream of the expansion at the same time as the capillary temperature. There is also an increase in the granule size; the electron density in the ionizer must then be increased to bring the mean mass of the granions to the values of interest (N/2 ≃ 100 to 500). To take into account the reduced mean mass in the ionizer, the authors propose a model which assumes that when an electron ionizes a granion that has already been singly charged, the granion dissociates spontaneously by electrostatic repulsion into two granions of complementary mass, each with a charge, in a process of the following type: H+N + e -> HN++ + H+(1-f)N + 2e with 0 < f < 0.5 With each value of f is associated a probability p(f). The calculations based on this model for different forms of p(f) with an iom'zation cross-section proportional to N2/3 are in good agreement with experimental results. (author). (author)