Eunsuk Ko

Presheath environment in weakly ionized single and multispecies plasmas

The presheath located near boundaries in weakly ionized plasmas is a rich environment in which charge exchange, and ion-ion streaming instabilities combine to establish the electric fields that accelerate ions to close to the Bohm velocity at the sheath/presheath boundary. Charge exchange sets the presheath scale length in weakly collisional plasma, in which ionization can be neglected. The transition of mobility limited ion flow near the bulk plasma to free fall motion close to the plate for single species plasmas is explored. Measurements in argon-helium multidipole plasmas of plasma potential with emissive probes and ion energy distribution functions with laser induced fluorescence are presented. These data show that the argon ions are speeded up by the presheath electric fields, argon ions are heated, and ion-ion instability is present as ions approach the boundary.

Presheath environment in weakly ionized multi-species plasmas

Summary form only given. The presheath located near boundaries in weakly ionized multi-species plasmas is a rich environment in which charge exchange, ionization and ion-ion streaming instabilities compete to establish the electric fields that accelerate ions to close to the Bohm velocity at the sheath/presheath boundary. In weakly collisional plasmas, charge exchange and ionization set the presheath scale length and are essential to the creation of the presheath. Measurements in argon-helium multi-dipole plasmas of plasma potential with emissive probes, wave propagation with Langmuir probes, drift velocities with Mach probes and IEDfs with LIF are presented. These data show that ions are heated both perpendicular and parallel to the boundary, the argon ions are speeded up, and the ion-ion instability grows and saturates. The transition of mobility limited ion flow near the bulk plasma to free fall motion close to the plate for single species plasmas and the role of instabilities in multi-species plasmas in altering ion velocities near the presheath/sheath boundary are explored.

The measurement of ion flow velocity by Mach probes in unmagnetized plasmas

Summary form only given. The measurement of ion flow in unmagnetized plasmas is performed using two Mach probes, one is a planar Mach probe and the other is spherical. The planar Mach probe consists of two single sided planar Langmuir probes back to back with an insulator between them. The spherical Mach probe, which was inspired by Hutchinsons numerical calculation, consists of a conducting sphere that has two conducting probe tips, insulated from the sphere and mounted at /spl theta/=0/spl deg/ and 180/spl deg/ with respect to the flow direction. The experiments are performed in a double-plasma device for ion flow velocity v/sub f/ up to 4.5 c/sub s/, where c/sub s/ is the sound speed, and along a presheath in a multi-dipole DC plasma for v/sub f/ < 1.0 c/sub s/ with Argon pressure ranging from 0.1 to 1 mTorr and plasma density range of 10/sup 7//spl sim/10/sup 10/ cm/sup -3/. The laboratory plasma in the, presheath was limited to T/sub i/, <0.1 T/sub e/ and v/sub f//spl les/1.0 c/sub s/. The measured super-sonic ion flow velocities are compared to the velocity from ion energy using an ion energy analyzer and single sided probes.

The study of presheath instabilities in two ion species plasmas

Summary form only given. The presheath is a region of weak electric field that accelerates ions into the sheath at the plasma boundary. When the plasma consists of two ion species of different masses, ion-ion two-stream instability can be excited by two ion streams with different drift velocities that are created in the presheath. The experiments were performed in multi-dipole DC plasmas with He-Ar gas mixtures (P/sub total//spl ges/1.0 mTorr, n/sub e//spl ges/1/spl times/10/sup 9/ cm/sup -3/, T/sub e//spl les/2 eV). The ion concentration of each species was determined by measuring ion acoustic wave phase velocity and electron temperature in the bulk region. Instabilities changing with different partial pressure, positions and discharge current were observed by using a cylindrical probe and a spectrum analyzer. Since charge exchange dominated in our presheath, the ion temperature increased approaching the sheath-presheath boundary. A fluid and a kinetic dispersion relation taking the ion temperature into account are applied and compared to describe these instabilities.

Presheaths in weakly collisional plasmas with two ion species

Summary form only given, as follows. In plasmas with two or more ion species, it is usually assumed that ions leave plasma at their individual Bohm velocities. In collisionless plasmas, this result can be achieved if ions fall through a presheath with a plasma potential drop of T/sub e//2e. If ionization is ignored, such a potential drop cannot exist. If ionization is included with the same profile for each species, a solution can exist, but the presheath length will equal the chamber length and the potential drop must exceed T/sub e//2e. With weakly collisional plasmas, mobility limited flow can be assumed for weak electric fields but not for velocities close to the individual species Bohm velocities. Experimental data from emissive probes, Mach probes and LIF in an argon/helium plasma suggest another solution to the problem. It appears that the plasma potential drop exhibits two consecutive presheaths and that neither ion leaves at their individual Bohm velocity.