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We present a simple technique for obtaining the time-resolved ion energy distribution function (IEDF) at a boundary in pulsed plasmas using a commercial quadrupole mass energy analyser. In this technique, ions are extracted from the plasma at selected parts of the pulse cycle, through the synchronized electrical biasing of a grid assembly attached to the barrel of the instrument, forming an electrostatic shutter. This sampling method has the advantage over the normal technique of electronically gating the detected ion signal to achieve time resolution, since the IEDFs can be obtained even when the ion flight time through the instrument (typically 100 µs) is greater than the pulse period or the characteristic time of transients in the plasma under investigation. The arrangement allows us therefore to diagnose plasmas pulsed at high frequencies (≥10 kHz). Presently, a time resolution of 4 µs can be obtained, limited only by the driving electronics design. The technique has been tested on a DC magnetron discharge operated in argon. The plasma was pulsed at a low frequency of 2 kHz, but with a discharge voltage waveform containing fast transients (on the µs time scale or faster). The results show clearly the evolution of the IEDFs during the pulse, responding to these fast transients with a significant number of ions created at plasma potentials above 140 V. These time-evolved IEDFs cannot be obtained using the conventional, manufacturer’s time-resolved method for this instrument. However, the new technique does introduce a small distortion in the measured IEDFs at energies above 120 eV, which is always observed, irrespective of the position of the shutter window during the pulse. This is due to the transient nature of the discriminating grid bias used in the electrostatic shuttering. Its effects and possible elimination are discussed.

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We describe an experiment, the UMIST Linear System (ULS), in which a hydrogen plasma stream, guided by a longitudinal magnetic field, is injected through a diaphragm containing an orifice into a separately-pumped target chamber in which the neutral hydrogen pressure can be raised to a maximum of 8 mTorr. The stream is about 6 mm in diameter, has an electron temperature of up to 15 eV and an ion flux of 3 × 10 18 s −1 ; it is supersonic with Mach number up to M ≈ 3. We have studied both the passage of the stream through the orifice and the interaction of the supersonic plasma with neutral hydrogen in the target chamber. We find that transmission is incomplete even when the orifice diameter is five times that of the plasma; we attribute this to the presence of ion trajectories which extend well outside the visible plasma and are intercepted by the diaphragm. In the target chamber, the stream does not broaden, but the ion flux decreases approximately exponentially with distance, with a scale length of the order of the mean free path for momentum transfer in ion-neutral collisions, and much less than that expected for other processes, such as charge exchange or electron-ion recombination. Elastic collisions alone cannot decrease the flux, but would lead to a large accumulation of slow ions in thermal equilibrium with the neutral gas, which must be limited by some other loss process: collisional diffusion and electron-ion recombination are too slow, leading to a density approaching 10 20 m −3 . The observed density is of the order of 10 18 m −3 , requiring a process with a rate of 10-100 times faster. Calculated rates for molecular-activated recombination (MAR) of the slow ions are of this order, and the predicted density agrees with our observations to order of magnitude.

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