Ion heating in the presheath

Abstract

A one-dimensional model of a small plasma ion source ͑10 cm long͒ is studied. A hybrid simulation where ions are treated as particles and electrons as a fluid obeying the Boltzmann relation is used to investigate ion heating in the plasma presheath. At low pressure ͑below a few mTorr͒, the ion velocity distribution is Maxwellian in the bulk and becomes a drifting Maxwellian distribution while transiting the presheath. The distribution remains essentially isotropic as the ions are accelerated through the presheath to satisfy the Bohm criterion. At intermediate pressures ͑around 10 mTorr͒, ion-neutral collisions scatter a significant part of the ion kinetic energy from the parallel direction to the perpendicular direction, leading to a net heating of the ions. In addition, the ion velocity distribution becomes distinctly anisotropic. At higher pressure ͑above a few tens of mTorr͒, ion heating is still observed, but yields isotropic ion velocity distributions.

Collisionless “thermalization” in the sheath of an argon discharge

“…Indeed, the potential drop across the presheath is a sharply increasing function of the neutral pressure, as was shown in Ref. 28. Our results are a clear indication that we are working in the low-collisionality regime, so that the method of Claire et al to arrive at the potential profiles is indeed justified not only in the sheath but also in the presheath region close to the sheath entrance.…”

Experimental studies of transverse metastable ion velocity distribution functions in the presheath of a weakly collisional argon plasma

“…On the face of it, our experimental results seem to differ with that of Claire et al, and with the PIC code simulation results of Meige et al 12 It is important to observe that while the neutral pressures of the two experiments were nominally the same ͑both ϳ0.3 mTorr͒, the electron temperatures differed ͑0.76 eV and ϳ2 eV, respectively͒, and both differed from the electron temperature used in the simulation, which was arbitrarily set at 3 eV for all pressures considered from 0.1 to 100 mTorr. In real experiments, the electron temperature is affected by details of the plasma source, so that even two dc discharge plasmas of different size may have different electron temperatures at the same neutral pressure.…”

“…The isotropic cross section increases as ion energy decreases ͑see Fig. 1 in Meige et al 12 ͒. Ion energies throughout the presheath are affected by the electric field, which in turn depend on the electron temperature.…”

“…The isotropic ion-neutral collision cross section is the one that gives rise directly to transverse ion heating as described in the work of Meige et al, 12 and this depends on ion energy. The isotropic cross section increases as ion energy decreases ͑see Fig.…”

“…Doubt arises because of the paucity of measured ion-neutral collision cross sections for highly excited metastable state ions, whereas the collision cross sections involving the ground state ion are well known. The simulation of Meige et al, 12 ground state ion. However, one reason to suppose that the metastable state ions typically used to measure IVDFs are indeed good proxies for ground state ions is that they have been observed to satisfy the Bohm criterion in a variety of low pressure, low temperature, weakly collisional pure argon, and pure xenon plasmas.…”