Observations of the interaction of a plasma stream withneutral gas: evidence of plasma loss through molecular-activatedrecombination

We describe an experiment, the UMIST Linear System (ULS), in which a hydrogenplasma stream, guided by a longitudinal magnetic field, is injected through adiaphragm containing an orifice into a separately-pumped target chamber inwhich the neutral hydrogen pressure can be raised to a maximum of 8 mTorr. Thestream is about 6 mm in diameter, has an electron temperature of up to 15 eVand an ion flux of 3×1018 s-1; it is supersonic with Machnumber up to M≈3. We have studied both the passage of the streamthrough the orifice and the interaction of the supersonic plasma with neutralhydrogen in the target chamber. We find that transmission is incomplete evenwhen the orifice diameter is five times that of the plasma; we attribute thisto the presence of ion trajectories which extend well outside the visibleplasma and are intercepted by the diaphragm. In the target chamber, the streamdoes not broaden, but the ion flux decreases approximately exponentially withdistance, with a scale length of the order of the mean free path for momentumtransfer in ion-neutral collisions, and much less than that expected for otherprocesses, such as charge exchange or electron-ion recombination. Elasticcollisions alone cannot decrease the flux, but would lead to a largeaccumulation of slow ions in thermal equilibrium with the neutral gas, whichmust be limited by some other loss process: collisional diffusion andelectron-ion recombination are too slow, leading to a density approaching1020 m-3. The observed density is of the order of 1018 m-3, requiring a process with a rate of 10-100 times faster. Calculatedrates for molecular-activated recombination (MAR) of the slow ions are of thisorder, and the predicted density agrees with our observations to order ofmagnitude.