The travel of plasma flow produced by a dc arc through a transport system based on a curved magnetic field was studied. The characteristics of the system were the absence of a curved metallic plasma guiding duct ('open architecture') and the fact that the magnetic field coils were non-coaxial to the plasma flow. By means of Langmuir probe measurements it was shown that both shape and position of the cathode plasma flow at the exit of the transport system were governed by variation of currents of the magnetic coils as well as by biasing of a special electrode inserted into the plasma flow. It was found that with parameters of the transport system held constant, the plasma ions with lower m/Z were deflected more, e.g. Al ions were deflected more than Ti ions. For an arc with a composite cathode, consisting of mainly Cr-Fe-Ni, the profile of atoms of these elements at the exit of the transport system was measured by x-ray fluorescence spectrometry. The results obtained were consistent with the probe measurements, hence the transport system, in principle, may be used for spatial separation of a multi-component (in masses) plasma flow.
The propagation of a metal plasma flow in a transport system with a curvilinear magnetic field was studied experimentally. The flow was generated by a pulsed vacuum arc discharge with a composite (W + Fe) cathode. The ion energy measurements at the transport system output showed that all ion components were accelerated up to equal energies per charge unit, about 150 eV and 320 eV in the outer and inner areas of the curved plasma flow, respectively. The spatial separation of the atoms of the cathode material was measured at the system output by x-ray fluorescence spectrometry. The ions of the lighter element (Fe) were concentrated in the inner part of the cathodic plasma flow deflected by the magnetic field while the distribution of the heavy element (W) was substantially shifted toward the outer area of the flow. The maximum mass separation efficiency reached 45, the effective value being 7.7. Such a system is promising for use in plasma technology for reprocessing spent nuclear fuel, namely for the separation of the heavy radioactive fission product from nuclear waste.
Detailed measurements of ion energy distributions (IEDs) originating at different stages of a pulsed vacuum arc are studied. It is shown that for a variety of cathode materials (Al, Mg, Ti and Zr), the directed energies per charge unit of ion originating at the initial stage of the arc, i.e. in 25 µs after ignition, are close to each other and are, approximately, as much as Edir/Z ≈ 70 eV. A non-Maxwellian shape of these IEDs that is due to the presence of significant ‘tails’ of ions accelerated up to energies of a few hundred eletronvolts is found. In 100 µs after the arc ignition the directed energies relax to values that are close, principally, to values that have been measured earlier elsewhere. It is found that the ‘anomalously’ accelerated ions propagate within a narrow angle that is as much as, approximately, ±15° in relation to the plasma flux axis. These characteristics suggest that beyond the commonly adopted gas-dynamic mechanism of ion acceleration in cathode micro-jets, at the initial stage of a pulsed arc the mechanism of additional ion acceleration is presented, which is due, obviously, to a self-consistent electric field arising in front of the plasma macro-jet.
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